Lidé

RNDr. Vojtěch Šimon, Ph.D.

Všechny publikace

Soft X-ray observing the cosmic sources by the ESA–CAS satellite SMILE

  • DOI: 10.1016/j.jheap.2024.02.005
  • Odkaz: https://doi.org/10.1016/j.jheap.2024.02.005
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    This paper describes the scientific potential of a Soft X-ray Imager (SXI) onboard the ESA–CAS satellite SMILE for investigating cosmic X-ray sources of various types. We show that this instrument, albeit designed for X-ray imaging of solar wind Charge eXchange of the magnetosheath and the cusps, is also essential for astrophysics because it is able to provide wide-field imaging of the sky in the soft X-ray region. Regarding sufficiently luminous X-ray sources with continuous spectra to be observable by the X-ray monitor MAXI/ISS to assess the object types and their light curves that are expected to be detected by SXI/SMILE, the compact sources accreting matter are promising targets for evaluating the possibilities of SXI. We assumed only the cosmic objects located in the planned fields to be observed by SXI. We used the 2–3 keV band flux of MAXI/ISS covering at least part of the expected band of the SXI/SMILE telescope. We used the data obtained by MAXI/ISS to assess the object types and their light curves expected to be detected by SXI/SMILE. We show the long-term activity of the examples of X-ray binary types located in this region. We present the typical features of such objects' activity on long timescales (e.g., outbursts and state transitions) and include physical interpretations of these phenomena. Furthermore, we discuss how SXI can contribute to this branch.

Development of an operation plan for observing the Galactic center region with the lobster-eye monitor

  • DOI: 10.31577/caosp.2023.53.4.191
  • Odkaz: https://doi.org/10.31577/caosp.2023.53.4.191
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    Using a CubeSat-like satellite, we show the lobster-eye (LE) monitor's perspectives and development of an observing plan. This instrument is important because it is able to provide wide-field X-ray imaging. We present the possibilities of monitoring the Galactic center region in the soft X-ray energy (a few keV) bands. The reason is that many X-ray binaries concentrate in the bulge surrounding the center of our Galaxy. Several such binaries are ex-pected to be present in the monitor's field of view (typically a square of about 5 x 5 degrees or more) at the same time. We used the planned LE telescope's 3-6 keV and 6-9 keV band flux. To compare the expected results, we used the data obtained by MAXI/ISS and accommodated these energy bands to assess the object types and their light curves expected to be detected by this LE. We show how such data of GX 5-1 can contribute to this branch.

Relation between Long-term Activity and Luminosity of the Pre- and Post-novae CT Ser and V446 Her

  • DOI: 10.3847/1538-3881/acac87
  • Odkaz: https://doi.org/10.3847/1538-3881/acac87
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    We show a relation of the pre- and post-nova activity of CT Ser (nova 1948) and V446 Her (nova 1960), known to have remarkably similar orbital periods P (orb). We use both photographic and CCD photometric observations. Post-nova CT Ser shows a nova-like high-state activity with only a slight decrease of absolute magnitude M (opt). In the interpretation, its M (opt) is above the region of the thermal-viscous instability (TVI) regime, so the disk is ionized out to its outer rim, similarly to before its classical nova (CN) outburst. On the contrary, V446 Her started its CN outburst from the TVI regime. Extensive changes in activity type in years after CN outburst were observed in V446 Her. The features consistent with the dwarf-nova outbursts appeared at most three decades after the end of the CN outburst. It suggests that its disk transitioned into the TVI region in that it was before its CN outburst. The pre-nova and post-nova states of activity did not change significantly for a given system, except for several decades after the CN outburst. Even pre-novae and post-novae with mutually similar P (orb), hence secondary star (donor) spectral types and dimensions and disk radii, can show largely discrepant M (opt) and disk states in the similar epochs (several decades) surrounding the CN outburst.

X-ray astrophysics as secondary science with ESA/China SMILE satellite

  • DOI: 10.31577/caosp.2023.53.4.136
  • Odkaz: https://doi.org/10.31577/caosp.2023.53.4.136
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    We present and discuss the feasibility of X-ray astrophysics with ESA-China SMILE satellite designed to investigate the Earth's magnetosphere's dynamic response to the solar wind's impact. We plan to study celestial X-ray targets as a secondary science for the onboard wide-field X-ray telescope SXI and related Czech participation in this mission. The Solar Wind Magneto-sphere Ionosphere Link Explorer, or SMILE, is a joint mission between the European Space Agency (ESA) and the Chinese Academy of Sciences (CAS). We show that SXI can also provide wide-field imaging of the sky in the soft X-ray region (E = 0.15 - 3 keV) and observe the long-term activity of cosmic X-ray sources. We show the stability of the orientation of the field of view of the SXI telescope; it will undergo only minor changes during several years of the planned operation. Therefore, observing the long-term soft X-ray activity of cosmic sources, e.g., those in the Magellanic Clouds, will be possible.

Activity of the post-nova V1363 Cygni on long timescales

  • Autoři: RNDr. Vojtěch Šimon, Ph.D.,
  • Publikace: Publications of the Astronomical Society of Japan. 2022, 74(3), 569-577. ISSN 2053-051X.
  • Rok: 2022
  • DOI: 10.1093/pasj/psac017
  • Odkaz: https://doi.org/10.1093/pasj/psac017
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    V1363 Cyg is a cataclysmic variable (CV) and a post-nova. Our analysis of its long-term optical activity used the archival data from the AAVSO database and literature. We showed that the accretion disk of V1363 Cyg is exposed to the thermal-viscous instability (TVI) for at least part of the time. The time fraction spent in the high state or the outbursts dramatically changed on the timescale of decades. The highly variable brightness of V1363 Cyg displayed several episodes of a strong brightening (bumps in the light curve) from a cool disk in the TVI zone. This can be interpreted to mean their vastly discrepant decay rates show that only some of these bumps can be attributed to the dwarf nova outbursts without strong irradiation of the disk by the hot white dwarf. The Bailey relation of the decay rate, if ascribed to a DN outburst of V1363 Cyg, speaks in favor of its orbital period P-orb being very long for a CV, about 20-40 h. A dominant cycle length of about 435 d was present in the brightness changes all the time, even when the disk was well inside the TVI zone. We interpret it as modulation of the companion's mass outflow by differential rotation of the active region(s).

Perspectives of the LOBSTER-EYE monitor in the soft X-ray observing the Galactic center region

  • DOI: 10.1016/j.jheap.2022.06.001
  • Odkaz: https://doi.org/10.1016/j.jheap.2022.06.001
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    We show the lobster-eye (LE) monitor's perspectives and observing plan based on a small LE telescope on a small (CubeSat-like) satellite platform. This instrument is important because it is able to provide wide-field X-ray imaging. We present the possibilities of monitoring the Galactic center region in the soft X-ray energy band. The reason is that many X-ray binaries, especially those with a low-mass lobe-filling secondary and mostly the neutron star accretor, concentrate in the bulge surrounding the center of our Galaxy. Several such binaries are expected to be present in our monitor's field of view (a square of about degrees). We show the long-term activity of the examples of X-ray binaries located in this region. We present the typical features of such objects' long-term X-ray activity (e.g., outbursts and state transitions) and discuss how the lobster-eye monitor can contribute. Observing in the soft X-ray band is the most promising because their X-ray intensity is the highest in this band.

The activity of the dwarf nova RU Pegasi with rapidly changing outburst types

  • Autoři: RNDr. Vojtěch Šimon, Ph.D.,
  • Publikace: Monthly Notices of the Royal Astronomical Society. 2022, 510(3), 3430-3436. ISSN 0035-8711.
  • Rok: 2022
  • DOI: 10.1093/mnras/stab3577
  • Odkaz: https://doi.org/10.1093/mnras/stab3577
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    RU Pegasi is a dwarf nova (DN) of the U Geminorum type. This analysis of its long-term optical activity uses data from the data base of the American Association of Variable Star Observers (AAVSO). It concentrates on investigating the properties of the individual outbursts and the time evolution of the ensemble of these events. No significant irradiation of the disc by the white dwarf was detected. In the interpretation, a variable steepness of the rising branches of the individual outbursts shows that the start of outbursts of RU Peg can occur at various distances from the disc centre without remarkable changes of the outburst recurrence time TC. The disc overflow of the inflowing mass stream, varying with time, could contribute to the changes in the starting position of the heating front, and hence the variations of the outburst types. A typical length of TC was 90 d. The segments of the relatively stable length of TC were accompanied by the primarily little variable and small values of the fluence (the energy radiated in the optical band in the individual outbursts). Jumps of TC, accompanied by the big scatter of the fluences, sometimes replaced them. In the interpretation, a combination of variations of TC with the unstable properties of the outbursts, including an unstable mass transfer rate between the components, shows the influence of several mechanisms on the state of the disc in various time segments

A study of the long-term activity of five intermediate polars with accretion discs

  • Autoři: RNDr. Vojtěch Šimon, Ph.D.,
  • Publikace: Monthly Notices of the Royal Astronomical Society. 2021, 505(1), 161-171. ISSN 0035-8711.
  • Rok: 2021
  • DOI: 10.1093/mnras/stab739
  • Odkaz: https://doi.org/10.1093/mnras/stab739
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    Intermediate polars (IPs) are cataclysmic variables with mildly magnetized white dwarfs (WDs). This analysis of the long-term optical activity of five examples of IPs with accretion discs used data from the Catalina Real-time Transient Survey, Digital Access to a Sky Century @ Harvard (DASCH) and the American Association of Variable Star Observers (AAVSO). It is shown that each of these IPs had their most preferred value of absolute magnitude Mopt, even if it significantly varied on the superorbital time-scale. The values of Mopt of these IPs were in the zone of thermal-viscous instability (TVI) of the disc most of the time. The properties of a series of outbursts of V426 Oph can be explained by an intermittently operating TVI. The activity of TV Col and DW Cnc is interpreted as caused by a gradually variable mass inflow rate from the secondary to a cool disc. The mass transfer rate from the secondary varied on a well-determined time-scale. It is shown that Mopt of EI UMa, close to the peaks of outbursts of non-magnetic dwarf novae, fluctuated on the time-scale of days; it also produced short flares, ascribed to the bursts of matter from the donor. HY Leo, with a presumably cool disc, fluctuated between its high and low states. A temporary brightening from an extended low state is ascribed to a short, intense burst of matter from the donor to the remaining cool disc or torus.

ESA THESEUS and cataclysmic variables

  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    The capabilities of the considered space mission THESEUS for investigation of cataclysmic variables (CVs) are discussed

Lobster eye X-ray monitors: Recent status

  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    The capabilities of the considered space mission LOBSTER for investigation of cataclysmic variables (CVs) are discussed

Long-term observations of the classical nova X Serand the activity of post-novae

  • Autoři: RNDr. Vojtěch Šimon, Ph.D.,
  • Publikace: 5th Golden Age of Cataclysmic Variables and Related Objects. Trieste: Proceedings of Science, 2021. ISSN 1824-8039.
  • Rok: 2021
  • DOI: 10.22323/1.368.0037
  • Odkaz: https://doi.org/10.22323/1.368.0037
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    We show a relation of the classical nova outbursts and the activity of post-novae. A variety of theoptical activities of post-novae exists. Also large changes of the type of activity in the decadesafter the classical nova outburst are observed in some of them. Although the ensemble of systemsin this analysis is incomplete, the similarities in the decaying branches of the outbursts in somepost-novae are the features favoring the dwarf nova outbursts. The features consistent with thedwarf nova outbursts appear mainly in the later phases (years, decades) of the gradual decay ofthe mean luminosity of some post-novae. This suggests that they contain the accretion diskswhich were able to switch from the hot (ionized) state to the thermal-viscous instability (TVI)regime. Such a TVI of the disk can appear in some post-novae even during the first century afterthe classical nova outburst (often sooner after the classical nova outburst in cataclysmic variableswith longer orbital periods). Activity of the mass-donating star in the brightness variations ofsome post-novae can be caused by the appearance and changes of the position of the active regionson the donor.

4U 1608-52 as a quasi-persistent X-ray source

  • Autoři: RNDr. Vojtěch Šimon, Ph.D.,
  • Publikace: Publications of the Astronomical Society of Japan. 2020, 72(6), ISSN 0004-6264.
  • Rok: 2020
  • DOI: 10.1093/pasj/psaa099
  • Odkaz: https://doi.org/10.1093/pasj/psaa099
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    4U 1608–52 is a soft X-ray transient. The analysis presented here of a particular part of its X-ray activity uses observations of RXTE/ASM and Swift/BAT. We show a time segment (MJD 54262–MJD 55090) (828 d) in which 4U 1608–52 behaved as a quasi-persistent X-ray source with a series of bumps, with a complicated relation between the evolution of fluxes in the soft (1.5–12 keV) and the hard (15–50 keV) X-ray regions. We ascribe these bumps to a series of propagations of heating and cooling fronts over the inner disk region without any transitions to the true quiescence. 4U 1608–52 oscillated around the boundary between the dominance of the Comptonized component and the dominance of the multicolor accretion disk in its luminosity. Only some of the bumps in this series were accompanied by a transition from the hard to the soft state; if it occurred, it displayed a strong hysteresis effect. The hard-band emission with the dominant Comptonized component was present for most of this active state and showed a cycle of about 40 d. We argue that the cyclic variations of flux come from the inner disk region, not, e.g., from a jet. We also discuss the observed behavior of 4U 1608–52 in the context of other quasi-persistent low-mass X-ray binaries.

ESA THESEUS and Czech participation

  • DOI: 10.1002/asna.202013787
  • Odkaz: https://doi.org/10.1002/asna.202013787
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    The recent status of the European Space Agency's (ESA) THESEUS (Transient High-Energy Sources and Early Universe Surveyor) project and the recent and expected Czech participation in this project are briefly presented and discussed. THESEUS is a mission concept proposed in response to the ESA's call for medium-size mission (M5) within the Cosmic Vision Programme and selected by ESA on May 7, 2018, to enter an assessment phase A/0 study. The expected Czech participation is on the Soft X-ray Imager (SXI) and a set of four lobster-eye (LE) telescopes, covering a total field of view (FOV) of 1 sr with source location accuracy better than 1-2 arcmin and energy coverage 0.3-6 keV. We also show the prospects of detecting and observing cataclysmic variables (CVs) with the hard X-ray imaging spectrometer (XGIS) and SXI. Since the X-ray spectrum largely determines what emission components and mechanisms can be observed, a combination of these instruments will be very useful. Also if we concentrate on the long-term activity of CVs, it will even be meaningful to use the observations of CVs located in the wide FOVs of XGIS and SXI when searching for and observing other types of events such as gamma-ray bursts.

European Space Agency SMILE and Czech participation

  • DOI: 10.1002/asna.202013785
  • Odkaz: https://doi.org/10.1002/asna.202013785
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    SMILE (Solar wind Magnetosphere Ionosphere Link Explorer) is a space mission which aims to measure Earth's global system responses to solar wind and geomagnetic variations with innovative instrumentation, for example, wide-field X-ray telescope of Lobster-Eye type, on board. It is a collaborative project of the European Space Agency and the Chinese Academy of Sciences. The SMILE international consortium involves the Czech Technical University in Prague and the Czech teams are expected to contribute to the project, mainly to the science and software related to the X-ray telescope. The aim of this short paper is to provide an outline of SMILE and in particular of the Czech participation to this mission, with emphasis on observation of celestial X-ray sources as SMILE secondary science.

Outbursts of the black hole X-ray transient KV UMa (XTE J1118+480) in the optical band

  • Autoři: RNDr. Vojtěch Šimon, Ph.D.,
  • Publikace: Publications of the Astronomical Society of Australia. 2020, 37 ISSN 1323-3580.
  • Rok: 2020
  • DOI: 10.1017/pasa.2019.47
  • Odkaz: https://doi.org/10.1017/pasa.2019.47
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    KV UMa (XTE J1118+480) is an X-ray binary that is known to undergo outbursts in 2000 and 2005. This paper presents the discovery of a large outburst starting in 1927 on the archival photographic plates and an analysis of the long-term optical activity of this system. We used the photographic data from DASCH (Digital Access to a Sky Century @ Harvard). We placed the 1927 outburst in the context of the observed outbursts of KV UMa. We show that it is a double event, with a precursor similar to the one of the outbursts in 2000. We find a big difference between the 1927 and 2000 outbursts as regards the length of the gap between the precursor and the main outburst. It is more than 250 d in 1927, whereas it is about 20 d in 2000, although the brightnesses of all peaks are mutually comparable. We also show that the individual optical outbursts of KV UMa differ from each other by the duration of the stage of a slow decline of brightness (sometimes roughly a plateau). This determines the length of the entire main outburst. Both the peak magnitude and the brightness of the outburst when the slow decline transitions to a steep final decaying branch plausibly reproduce in all three outbursts. In the interpretation, the short duration of the precursor is caused by the fact that only the thermal-viscous instability operated in the accretion disk while also the tidal instability of the disk contributed in the subsequent main outburst.

The long-term optical activity of the propellers AE Aquarii and AR Scorpii

  • Autoři: RNDr. Vojtěch Šimon, Ph.D.,
  • Publikace: Publications of the Astronomical Society of Japan. 2020, 72(2), ISSN 0004-6264.
  • Rok: 2020
  • DOI: 10.1093/pasj/psaa012
  • Odkaz: https://doi.org/10.1093/pasj/psaa012
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    This analysis of the long-term optical activity of the propellers AE Aqr and AR Sco uses data from the Catalina Real-time Transient Survey, DASCH, and AAVSO. The site and character of the emissions from the phenomena caused by the magnetic field of the white dwarf (WD) vary from system to system. The histogram of intensities of the ensemble of flares of AE Aqr suggests that the long-term activity consists of a large variety of the peak magnitudes of the flares, with the probability of their detection gradually decreasing with increasing intensity. Any increase of activity only leads to an increase of the number of blobs of the transferring matter. We also detected a season with a transient decrease or even a cessation of the mass outflow from the donor to the lobe of the WD. The very strong orbital modulation of AR Sco is most stable in the phases of the extrema of brightness for about a century; its minor changes suggest that the trailing side of the synchrotron-emitting region is more unstable than the leading side.

Long-term activity of a little known dwarf nova DT Octantis

  • DOI: 10.1134/S199034131904014X
  • Odkaz: https://doi.org/10.1134/S199034131904014X
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    We analyze the long-term activity of a little known dwarf nova (DN) DT Oct. We use photographic observations from the Bamberg Observatory (Southern Patrol Photographic Sky Survey), CCD data from ASAS-3 survey, and AAVSO data (both CCD and visual). We determined the recurrence times of the superoutbursts, $T_{\rm Cso}$, and of the normal outbursts, $T_{\rm Cno}$, to be approximately 311\,d and about 30\,d, respectively. Although the length of $T_{\rm Cso}$ evolves gradually and undergoes only small variations (increases and decreases) of the nearby or neighbouring events (on the timescale of a few epochs), the evolution of $T_{\rm Cso}$ displays considerably larger changes on the longer timescales. They suggest a dominant trend of a decrease of $T_{\rm Cso}$ on the timescale of tens of years. The possibly magnetized white dwarf does not prevent DT Oct to show the long-term activity of the SU UMa systems. We place the time evolution of the superoutbursts of DT Oct in the context of DNe of various sub-types.

Complex long-term activity of the post-nova XSerpentis

  • DOI: 10.1051/0004-6361/201731308
  • Odkaz: https://doi.org/10.1051/0004-6361/201731308
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    Aims. X Ser is a cataclysmic variable (CV) which erupted as a classical nova in 1903. In this work we use over 100 years of photometry to characterize the long-term light curve of XSer, with the aim of interpreting the post-nova activity in XSer in the context of behaviors in other old novae.

Investigation of the long-term activity of the binary X-ray sources with the planned satellites

  • Autoři: RNDr. Vojtěch Šimon, Ph.D.,
  • Publikace: Contributions of the Astronomical Observatory Skalnaté Pleso. 2018, 48(3), 421-429. ISSN 1335-1842.
  • Rok: 2018
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    Monitors of X-ray emission are important instruments for observing the long-term activity of cosmic sources on the long timescales (e.g. years). We show the perspectives and possibilities of observing the selected types of X-ray binary sources with the planned satellites THESEUS and eXTP. The hardness of the X-ray spectrum largely determines the observability of a given object (or a type of objects) with a monitor operating in the specific band. We show the perspectives of detecting and observing outbursts in the soft X-ray transients with the hard X-ray monitors eXTP/WFM and THESEUS/XGIS. Their broad energy bands and energy resolution will enable to detect the outbursts and the complex spectral changes during these events. We also show the perspectives of observing the binary supersoft X-ray sources with the planned THESEUS/SXI. These sources sometimes have the luminosity close to the Eddington limit but their X-ray emission is usually below the band used by the monitors.

Long-term activity of cataclysmic variables

  • Autoři: RNDr. Vojtěch Šimon, Ph.D.,
  • Publikace: Frontier Research in Astrophysics – III (FRAPWS2018). Trieste: SISSA/ISAS, 2018. Proceedings of Science (PoS). vol. 331. ISSN 1824-8039.
  • Rok: 2018
  • DOI: 10.22323/1.331.0058
  • Odkaz: https://doi.org/10.22323/1.331.0058
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    We show examples of the long-term optical activity of various types of cataclysmic variables. Episodic events are the common properties of this activity which often consists of the discrete levels of brightness and/or discrete phenomena (e.g. outbursts, low state episodes). The length of the recurrence time $T_{\rm C}$ of the outbursts in dwarf novae (DNe) displays complicated time changes. This suggests that the individual outbursts of a given DN depend on each other, and only a small fraction of the disk matter is accreted in a given outburst. The processes in the cool disk (between the outbursts) play a big role in DNe. The low states in novalikes and polars often occur in clusters. The state transitions sometimes occur in intermittently present cycles. In the interpretation, changes of the position of the active regions on the donor with respect to the L1 point by a differential rotation of the donor are suggested. The short high states, surrounded by the long low states, can be explained by isolated bursts of mass transfer from the donor. EI\,UMa is an example of the intermediate polars whose brightness can largely fluctuate on the timescale of days and also produce flares. Large changes of the mode of accretion (flares, shalow low states) occur in a similar brightness of EI\,UMa, hence in a similar mass accretion rate onto the WD. The long-term activity of propellers depends on the configuration of the magnetic field of the white dwarf; this activity of AR\,Sco, with the luminosity comparable to the deep and very long (at least several decades) low states of novalikes, is dominated by a strong orbital modulation of synchrotron emission.

Perspectives of monitoring of cataclysmic variables in the very soft X-ray band

  • DOI: 10.1002/asna.201813363
  • Odkaz: https://doi.org/10.1002/asna.201813363
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    Some components and important regions of X-ray spectra of cataclysmic variables (CVs) are present only in the very soft band with energy E<1.5keV, which is beyond the region of sensitivity of most available X-ray monitors. We show that a population study of a large ensemble of such systems with various states of activity that will be observable by eROSITA (on board the SRG satellite) will be a suitable method of investigation even with its very sampled data. We demonstrate that the hardness ratios of such X-ray spectra show specific positions and patterns of the individual types of CVs in the hardness vs. hardness diagrams. Quiescent dwarf novae and intermediate polars constitute a long tail extending from a group of the high-state polars. Interstellar absorption plays only a marginal role in this respect. A complicated distribution of CVs in these diagrams thus mainly reflects the complexity of the emitting regions, the absorbing regions intrinsic to the source, and the emission mechanisms in various CVs and CV types. Although this is a case study using incomplete data as examples, it demonstrates serendipitous scientific potentials of the eROSITA archives.

X-ray outbursts and high-state episodes of HETE J1900.1-2455

  • Autoři: RNDr. Vojtěch Šimon, Ph.D.,
  • Publikace: Monthly Notices of the Royal Astronomical Society. 2018, 477(1), 67-73. ISSN 0035-8711.
  • Rok: 2018
  • DOI: 10.1093/mnras/sty575
  • Odkaz: https://doi.org/10.1093/mnras/sty575
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    HETEJ1900.1-2455 is an ultra-compact low-mass X-ray binary that underwent a long-lasting (about 10 yr) active state. The analysis presented here of its activity uses the observations of RXTE/ASM, Swift/BAT, and ISS/MAXI for investigating this active state and the relation of time evolution of fluxes in the hard and medium X-ray bands. We show that the variations of the flux of HETEJ1900.1-2455 on the time-scales of days and weeks have the form both of the outbursts and occasional high-state episodes. These outbursts are accompanied by the large changes of the hardness of the spectrum in the surroundings of the peaks of their soft X-ray flux. The very strong peaks of these outbursts occur in the soft X-ray band (2-4 keV) and are accompanied by a large depression in the 15-50 keV band flux. We interpret these events as an occasional occurrence of a thermal viscous instability of the accretion disc that gives rise to the outbursts similar to those in the soft X-ray transients. On the other hand, the 2-4 and the 15-50 keV band fluxes are mutually correlated in the high-state episodes, much longer than the outbursts. In the interpretation, the episodes of the X-ray high states of HETEJ1900.1-2455 during the active state bear some analogy with the standstills in the Z Cam type of cataclysmic variables.

CVs Astrophysics with LOBSTER and ESA LOFT

Long-term activity of cataclysmic variables

  • Autoři: RNDr. Vojtěch Šimon, Ph.D.,
  • Publikace: Proceedings of the 48th Conference on Variable Stars Research. Praha: Česká astronomická společnost, 2017. p. 51-56. OEJV Publications. vol. 180. ISSN 1801-5964.
  • Rok: 2017
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    Cataclysmic variables are objects in which matter transfers onto the white dwarf from a close companion (usually low-mass main sequence star). The orbital period of such systems is typically several hours. This mass transfer is the reason of the very large variability of these systems on many timescales. We show the examples when it is possible to observe this strong long-term activity like outbursts or transitions between the high and low states even with the commonly accessible instruments. We also show examples of the relation of this activity to the activity in X-rays.

Long-term activity of QU Car

  • DOI: 10.1002/asna.201713316
  • Odkaz: https://doi.org/10.1002/asna.201713316
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    This paper presents an analysis of the long-term activity of the cataclysmic variable QU Car using CCD V-band ASAS data, obtained between 2001 and 2009. I show that the evolution of brightness on the timescale of weeks and months (not from night-to-hight) dominates the light curve during this mapped time. The histograms of brightness are not consistent with the normal distribution and show that the character of the light curve changes for the individual years, although the mean brightness remains similar. One episode of an exceptionally bright state displayed a return accompanied by a series of strong fluctuations of brightness. This shows that the amplitude of the brightness variations changes from time to time, and does not always depend on the mean level of brightness. This analysis of QU Car thus shows the type of the long-term activity in a CV with a very high mass transfer rate.

LONG-TERM OPTICAL ACTIVITY OF THE HARD X-RAY FLARING STAR DG CVN

  • Autoři: RNDr. Vojtěch Šimon, Ph.D.,
  • Publikace: REVISTA MEXICANA DE ASTRONOMIA Y ASTROFISICA. 2017, 53(1), 59-66. ISSN 0185-1101.
  • Rok: 2017
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    DG CVn is a young late-type star which displayed an X-ray and optical super flare in 2014. This paper presents an analysis of the long-term activity of this object in the optical band. I used the photographic data from DASCH (Digital Access to a Sky Century @ Harvard). These measurements from the years 1895-1989 cover the blue spectral region. CCD V-band ASAS data were used for several UV Cettype stars to place the activity of DC CVn in the context of flaring stars. I show that three large brightenings (flares) of DG CVn by more than 1 mag were detected on the DASCH plates. The character of the long-term activity (regarding the histogram of brightness) of DG CVn is compatible with those of flaring stars UV Cet and V371 Ori. The flares brighter than similar to 0.4 mag represent less than 1 percent of the observed data in all three objects

Perspectives of observing the color indices of optical afterglows of gamma-ray bursts with ESA Gaia

  • DOI: 10.1007/s10686-017-9550-5
  • Odkaz: https://doi.org/10.1007/s10686-017-9550-5
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    We propose a strategy for detecting and analyzing optical afterglows (OAs) of long gamma-ray bursts (GRBs) without the need to obtain their light curves. This approach is useful for the Gaia satellite, which provides sampled optical ultra-low-dispersion spectroscopic observations of the sky. For this purpose, we show that most OAs of long GRBs display specific values of some of their color indices, representing synchrotron emission of the jet. They are stable in time during the event. These indices, which can be determined from the spectra, are very similar for the ensemble of OAs with redshift z < 3.5 and display a strong clustering in some color-color diagrams. These indices also enable to constrain the properties of the local interstellar medium of GRBs. The long-lasting mapping of the sky with the Gaia instruments also gives us a hope to search for the so-called orphan afterglows, which, according to some authors, can be considerably more numerous than OAs of the observed GRBs. We also show how to resolve OAs from other transients in the Gaia data. The color indices and the properties of the quiescent sources (host galaxies of OAs detectable later by the large ground-based telescopes at the co-ordinates of the OA determined by Gaia) would tell us which one, among transients detected by Gaia, is a GRB OA.

Perspectives of the lobster-eye telescope: The promising types of cosmic X-ray sources

  • Autoři: RNDr. Vojtěch Šimon, Ph.D.,
  • Publikace: Contributions of the Astronomical Observatory Skalnaté Pleso. 2017, 47(2), 184-191. ISSN 1335-1842.
  • Rok: 2017
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    We show the astrophysical aspects of observing the X-ray sky with the planned lobster-eye telescope. This instrument is important because it is able to provide wide-field X-ray imaging. For the testing observations, we propose to include also X-ray binaries in which matter transfers onto the compact object (mostly the neutron star). We show the typical features of the long-term X-ray activity of such objects. Observing in the soft X-ray band is the most promising because their X-ray intensity is the highest in this band. Since these X-ray sources tend to concentrate toward the center of our Galaxy, several of them can be present in the field of view of the tested instrument.

Study of Cataclysmic Variables with LDS Sky Surveys and ESA Gaia

  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    The goal of this paper is to discuss the capabilities of ESA satellite missions Gaia and of the photographic LDS surveys for investigation of cataclysmic variables (CVs). We show that both of them can contribute to the study of CVs and related objects. Spectrophotometry and lowdispersion spectroscopy are the most important for CV analyses with the observations of Gaia. We present the possible strategies for investigation of CVs in the sampled photometric and spectroscopic data provided by Gaia. E.g. statistical properties of the long-term activity of various types of CVs can be determined from them.

The long-term activity of dwarf novae and soft X-ray transients

  • Autoři: RNDr. Vojtěch Šimon, Ph.D.,
  • Publikace: Golden Age of Cataclysmic Variables and Related Objects – IV. Trieste: SISSA/ISAS, 2017. PoS. vol. 315. ISSN 1824-8039.
  • Rok: 2017
  • DOI: 10.22323/1.315.0035
  • Odkaz: https://doi.org/10.22323/1.315.0035
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    We show the activity of dwarf novae (DNe) and soft X-ray transients (SXTs) which were observed to undergo series of relatively frequent outbursts. The length of the recurrence time of these events, TC, is usually less than a year (at most about 4 years). We show that the energy output (e.g. fluence) sometimes largely differs even for the neighboring outbursts both of DNe and SXTs. This energy output is related to the fraction of the disk mass, accreted during an outburst, but this difference of the energy output does not influence the length of TC in a given object. The length of TC in a given DN or SXT is variable, but the occurrence of these outbursts is not dramatically irregular in most cases. The long-term trends can be clearly resolved in the O–C diagrams of TC but they show only small epoch-to-epoch fluctuations. In DNe, the disk zone where the outburst starts can change even in a given binary for the individual outbursts. The decaying branch of the outburst is governed by propagation of cooling front and the observations show that this part of the light curve reproduces well for the ensemble of outbursts in a given DN. As for SXTs, the final phase of the decay in the 1.5–12 keV band reproduces well for the individual outbursts while the preceding phase can vary for the individual events. We also show an analogy of the Z Cam-type behavior in an SXT (transitions between the segments of a hot thermally stable (in standstills) and unstable disk (outbursts)).

Astrophysics of "extreme" solar-like stars

  • Autoři: Caballero-García, M.D., Castro-Tirado, A.J., Claret, A., Gazeas, K., RNDr. Vojtěch Šimon, Ph.D., Jelínek, M., Cwiek, A., Zarnecki, A.F., Oates, S., Jeong, S., prof. RNDr. René Hudec, CSc.,
  • Publikace: Revista Mexicana de Astronomia y Astrofisica: Serie de Conferencias. 2016, 48 59-63. ISSN 1405-2059.
  • Rok: 2016
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    Only a few red dwarf flaring stars in the solar neighbourhood have undergone exceptional events called superflares. They have been detected with high-energy satellites (Swift) and have been proven to be powerful events (both in intensity and energy) and potentially hazardous for any extraterrestial life. The physics of these events can be understood as an extrapolation of the (much) weaker activity already occurring in the most powerful solar flares occurring in the Sun. Nevertheless, the origin (why?) of these superflares occur is currently unknown. A recent study presents the optical and X-ray long-term evolution of the emission by the super-flare from the red-dwarf star DG CVn undertaken in 2 014. In that paper we comment on the context of these observations and on the properties that can be derived through the analysis of them.

Cataclysmic variables - A research laboratory for accretion processes

  • Autoři: RNDr. Vojtěch Šimon, Ph.D.,
  • Publikace: 2nd Frontier Research in Astrophysics, FRAPWS2016. Trieste: SISSA/ISAS, 2016. Proceedings of Science. vol. 2016-May. ISSN 1824-8039.
  • Rok: 2016
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    We show that cataclysmic variables (CVs) are important laboratories for investigation of accretion processes. It is important that a CV binary works as a system strongly variable with time. The character of the long-term activity (and operation of the emission processes) largely depend on the mass inflow into the lobe of the WD, and on the strength of the magnetic field of this WD. Strong structural changes of the emitting regions with time often accompany the transitions between the states of activity. Cycles of this activity (often on timescale of months to years) are common. Episodic events are the common properties of this activity which often consists of the discrete levels of intensity and/or discrete phenomena (e.g. outbursts, low state episodes). We also show that classical nova explosion has the large consequences for the type of activity (and even its change) after returning to quiescence in some CVs. Investigation of interplay of various emitting regions (and emission processes) in various states of activity is very important.

Evolution of the optical and hard X-ray activity of AM Her in a season dominated by the high states

  • DOI: 10.1007/s10509-016-2825-x
  • Odkaz: https://doi.org/10.1007/s10509-016-2825-x
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    The long-term activity of AM Her, the prototype of polars, consists of interchanging high and low states. This analysis uses the data from the BAT/ Swift, MAXI/ ISS, and AAVSO data archives (including long CCD V-band night series) for investigating the relation of the time evolution of intensities in the hard X-ray, medium/hard X-ray, and the optical bands on super-orbital timescale in a season dominated by the long high-state episodes. The observations mapped the relation of the cyclotron and bremsstrahlung luminosities in such episodes. Although an increase of intensity of the cyclotron emission is generally accompanied by a brightening of the bremsstrahlung component, this relation differs for the individual high-state episodes. These variations were accompanied by the large changes of the optical modulation. In my view, all of these variations suggest variations of the dimensions and structure of the cyclotron emitting region(s), not only the changes of the mass accretion rate. The activity during a decline (much less steep than the state transition) from an initial very bright peak can be explained by a gradual evolution of the conditions in stratified shock regions in a single high state episode. The observed behavior in the neighboring high-state episodes is also important for explaining the mechanisms which cause the low state between them. In the interpretation, some low-state episodes cause that AM Her develops a different accretion mode than the one before entering the low state. Each high-state episode is a complex phenomenon in the history of accretion in this system.

Properties of the long-term optical activity of the prototype polar AM Herculis

  • Autoři: RNDr. Vojtěch Šimon, Ph.D.,
  • Publikace: Monthly Notices of the Royal Astronomical Society. 2016, 463(2), 1342-1351. ISSN 0035-8711.
  • Rok: 2016
  • DOI: 10.1093/mnras/stw1964
  • Odkaz: https://doi.org/10.1093/mnras/stw1964
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    AM Her displays strong long-term activity with the high and low states. This investigation uses AAVSO optical data for a time series analysis of the long-term variations. Rapid changes of brightness (e.g. the orbital modulation) were smoothed out to emphasize the activity on super-orbital time-scale. I show that the character of this activity changed considerably on time-scales of years, which is reflected in a large evolution of the complicated histogram of the optical brightness. The high states are not the well-defined, narrow levels of brightness. I also show that AM Her displays transitions between the high and low states with the intermittently existing cycles. The longest uninterrupted series of transitions from the high to low state consists of seven episodes (about 6 yr). The existence of this series can be controlled by the lifetime of the active regions on the donor, which modulates the mass transfer rate. I show that the episodes of the high and low states accumulate in clusters, which produces an additional cycle after smoothing by the moving averages. The cycles of activity of the donor can explain this modulation. A single isolated short episode of the low state does not imply a break of this cycle. I also argue that the specific properties of star-spots and their migration caused by the differential rotation of the donor would be needed to explain the complex activity of AM Her.

Activity of the Polar AM Her (RX J1816.2+4952): A Short Review

  • Autoři: RNDr. Vojtěch Šimon, Ph.D., Henden, A.
  • Publikace: THE GOLDEN AGE OF CATACLYSMIC VARIABLES AND RELATED OBJECTS - II. Praha: České vysoké učení technické v Praze, 2015. pp. 81-85. Acta Polytechnica Proceedings. ISSN 2336-5382. ISBN 978-80-01-05688-2.
  • Rok: 2015
  • DOI: 10.14311/APP.2015.02.0081
  • Odkaz: https://doi.org/10.14311/APP.2015.02.0081
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    We show that AM Her displays the transitions between the high and low states with an intermittently existing dominant cycle with length between 400 and 800 days. Moreover, these transitions accumulate in clusters, which produces an additional long cycle after smoothing; a single isolated short episode of the low state does not suggest a break of this cycle. The seasons of existence of the cycle can be controlled by the lifetime of the active regions (e.g. prominences, spots) on the donor. In some high-state episodes, a higher luminosity of the bremsstrahlung emission is not accompanied by a higher optical (cyclotron+stream) emission. Part of the bremsstrahlung emission can be buried in some episodes. Changes of the structure of the accretion region(s) are necessary to explain the variations of the optical and activity in the high-state episodes of AM Her.

Early optical follow-up of the nearby active star DG CVn during its 2014 superflare

  • DOI: 10.1093/mnras/stv1565
  • Odkaz: https://doi.org/10.1093/mnras/stv1565
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    DG Canum Venaticorum (DG CVn) is a binary system in which one of the components is an M-type dwarf ultrafast rotator, only three of which are known in the solar neighbourhood. Observations of DG CVn by the Swift satellite and several ground-based observatories during its superflare event on 2014 allowed us to perform a complete hard X-ray-optical follow-up of a superflare from the red-dwarf star. The observations support the fact that the superflare can be explained by the presence of (a) large active region(s) on the surface of the star. Such activity is similar to the most extreme solar flaring events. This points towards a plausible extrapolation between the behaviour from the most active red-dwarf stars and the processes occurring in the Sun.

Evolution of the complicated decaying branch of the very long outburst in XTE J1701-462

  • DOI: 10.1016/j.newast.2014.11.005
  • Odkaz: https://doi.org/10.1016/j.newast.2014.11.005
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    I analyze the evolution of the features which occurred on the timescale of weeks in the very long (∼600 d) X-ray outburst of the neutron star low-mass X-ray binary XTE J1701-462. I used RXTE/ASM (1.5-12 keV) and Swift/BAT (15-50 keV) observations for a time-series analysis of the long-term variations. I utilized the means (usually one-day) of intensity I to increase the signal-to-noise ratio. I applied weighted wavelet Z-transform to investigate the cyclic modulation of I. Using the combined data from both satellites, I also calculated the hardness ratio HR = I(15-50 keV)/I(1.5-12 keV) and investigated its evolution throughout the outburst. Despite the largely different profiles of the light curves in the ASM and the BAT bands especially in the first fourth of the outburst, the profiles of the measures of their scatter (the σresASM and the σresBAT curves) display mutual similarities. Both the σresASM and σresBAT curves achieved their maxima in the primary peak of the light curve which was present only in the ASM band. The formation of the quasi-periodic variations occurred and their approximate cycle-length PC was decided already in the surrounding of this peak when the bolometric luminosity was close to or even exceeding the Eddington limit. The modulation with PC is present in some time segments only in the ASM band, while the intensity variations are always chaotic in the BAT band. Using the results of spectroscopy of Lin et al., I suggests that only multicolor disk, dominant in the ASM band, was involved in this cycle. The existence of this cycle did not depend on the exact type of the Z, so it was independent of the exact conditions in the inner disk region. The length of PC displayed a large and gradual increase from ∼16 to ∼26 days with the progress of the outburst. Using models of Foulkes et al., I interpret these variations as a superorbital cycle, caused by tilting and warping of the irradiated disk.

GLORIA and Study of Cataclysmic Variables

  • Autoři: prof. RNDr. René Hudec, CSc., RNDr. Vojtěch Šimon, Ph.D.,
  • Publikace: THE GOLDEN AGE OF CATACLYSMIC VARIABLES AND RELATED OBJECTS - II. Praha: České vysoké učení technické v Praze, 2015. pp. 303-307. Acta Polytechnica Proceedings. ISSN 2336-5382. ISBN 978-80-01-05688-2.
  • Rok: 2015
  • DOI: 10.14311/APP.2015.02.0303
  • Odkaz: https://doi.org/10.14311/APP.2015.02.0303
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    We report here on the ongoing EU FP7 Project GLORIA (GLObal Robotic-telescopes Intelligent Array) with emphasis on possibility of investigation of cataclysmic variables by users. GLORIA will enable the first free and open-access network of robotic telescopes in the world. We show several examples of the not often used topics (but suitable for GLORIA) for the studies of activity of cataclysmic variables, e.g. search for outbursts in intermediate polars and the fluctuations of brightness in their quiescence, and investigation of the optical counterparts of supersoft X-ray sources.

Lobster-Eye All-Sky Soft X-ray Monitors

  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    The idea of a satellite providing monitoring of specified sky areas in soft X-rays is presented. Observation plan could include also e.g. searching for X-ray afterglows of gamma-ray bursts. The spacecraft could be of the nano-satellite class. Using of Schmidt lobster optics is proposed. Design study of the optics is presented. The results of experimental tests of the specimen of the optics indicate that the mission is feasible.

Long gamma-ray bursts in the Swift era: the color indices of their optical afterglows

  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    We present an analysis of optical afterglows (OAs) of long GRBs which were mostly discovered by NASA Swift. We used the method of the color indices that uses the commonly available multiband photometry. We show that for most OAs, in the observer frame, these colors, corrected for the reddening inside our Galaxy, display prominent clustering in some color-color diagrams.

Long-term evolution of the Main-On states in Hercules X-1

  • DOI: 10.1088/0004-6256/150/1/3
  • Odkaz: https://doi.org/10.1088/0004-6256/150/1/3
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    Rossi X-ray Timing Explorer All-Sky Monitor (RXTE ASM) observations and American Association of Variable Star Observers (AAVSO) optical data were used for a time-series analysis of the long-term activity of Her X-1. The problem of measuring the long-term evolution of the X-ray intensity in the light curve which consists only of separated intense spikes was addressed. For this purpose, the fluence of each Main-On state was determined. The fluences vary generally (not only in the transitions into/from anomalous low states (ALSs)) by tens of percent on the timescales of at least several epochs of the cycle-length of the superorbital cycle (but without any stable cycle) while irradiation of the donor producing the optical modulation remains considerably more stable. ALS1 and ALS2 are extensions of the tail in the statistical distribution of these fluences. In this interpretation, the variations of the fluences are caused by the changes of the structure of the inner disk region, which produces variable obscuration of the beams (emitting in the ASM band) at the neutron star. A small change of obscuration of these beams by the inner disk region suffices to change the fluences largely. However, the irradiation of the donor is changed significantly less because this inner disk region (which emits beyond the ASM band and acts as the occulting region of the beams) also irradiates the donor.

Preliminary design of optics for nano-satellite monitor

  • Autoři: Tichý, V., prof. RNDr. René Hudec, CSc., RNDr. Vojtěch Šimon, Ph.D.,
  • Publikace: EUV and X-ray Optics: Synergy between Laboratory and Space IV. Prague: SPIE - The International Society for Optical Engineering, 2015. Proceedings of SPIE. ISSN 0277-786X. ISBN 9781628416312.
  • Rok: 2015
  • DOI: 10.1117/12.2178683
  • Odkaz: https://doi.org/10.1117/12.2178683
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    Schmidt lobster eye multi-foil optics allows high field of view and it can have small mass and dimensions. It makes the optic usable on small low-cost satellite mission that would permanently monitor selected sky area(s). In the paper, possible optical designs are presented. Presented designs are derived of existing optics specimen, therefore they should be technically feasible.

Study of Cataclysmic Variables with the Satellites LOFT and Gaia

  • Autoři: prof. RNDr. René Hudec, CSc., RNDr. Vojtěch Šimon, Ph.D.,
  • Publikace: THE GOLDEN AGE OF CATACLYSMIC VARIABLES AND RELATED OBJECTS - II. Praha: České vysoké učení technické v Praze, 2015. pp. 297-302. Acta Polytechnica Proceedings. ISSN 2336-5382. ISBN 978-80-01-05688-2.
  • Rok: 2015
  • DOI: 10.14311/APP.2015.02.0297
  • Odkaz: https://doi.org/10.14311/APP.2015.02.0297
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    The goal of this paper is to discuss the capabilities of ESA satellite missions Gaia (already in space) and LOFT (considered for the ESA M4 slot) for investigation of cataclysmic variables (CVs). Both Gaia and LOFT can contribute to study of CVs and related objects. Spectrophotometry and low dispersion spectroscopy are the most important for CV analyses with the observations of Gaia. We present the possible strategies for investigation of CVs in the sampled photometric and spectroscopic data provided by Gaia. E.g. statistical properties of the long-term activity of various types of CVs can be determined from them. LOFT can be a promising satellite to provide a sensitive X-ray monitor which will enable to investigate the little studied long-term activity of various types of CVs (especially the magnetic ones) in the X-ray band.

The intermediate polar GK Persei: An unstable relation of the X-ray and the optical intensities in a series of outbursts

  • DOI: 10.1051/0004-6361/201424910
  • Odkaz: https://doi.org/10.1051/0004-6361/201424910
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    GK Per is an intermediate polar that has been displaying dwarf nova outbursts since the middle of the twentieth century. Aims. I analyzed a series of such outbursts in the optical and X-ray bands. I pay attention to the relation of intensities of the optical and X-ray emissions, and its reproducibility in a series of these consecutive outbursts. Methods. This analysis uses the data from the BAT/Swift, ASM/RXTE, AAVSO, and AFOEV databases. It investigates the relation of the time evolution of the profiles of outbursts in the individual bands (hard X-ray, medium/hard X-ray, and optical). Results. This analysis shows that the X-ray intensity steeply rises only in the start of the optical outburst and steeply declines only when the optical outburst comes to its end. However, the 1.5-50 keV band intensity saturates and balances on a plateau during the outburst. (The longer the outburst, the longer its plateau.) The peak X-ray intensities of this series display a significantly narrower range than the optical ones (a factor of about two versus a factor of about eight). This implies a discrepancy between the mass flow through the disk and the production of the X-ray emission via bremsstrahlung at the polar caps of the white dwarf. This discrepancy is the largest in the time of the peak optical intensity when the whole disk (or at least its inner part) is in the hot state and the flow of matter through the disk is the greatest. This study shows that a series of outbursts constitutes more general properties of this discrepancy. I argue that the saturation of the X-ray luminosity in outburst cannot be caused by a dominant increase in X-ray absorption. In the interpretation, large structural changes of the accreting regions at the magnetic poles of the white dwarf occur during the outburst. A buried shock proposed by some authors for polars is also promising for explaining the X-ray light curves of outbursts of GK Per.

Changing patterns of the long-term activity of the intermediate polar V1223 Sgr

  • DOI: 10.1016/j.newast.2014.04.005
  • Odkaz: https://doi.org/10.1016/j.newast.2014.04.005
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    High and low states represent a large variety of features in the optical activity of some types of cataclysmic variables. Here, we examine them in the intermediate polar V1223 Sgr. We use the photographic observations from the Bamberg Observatory and from Garnavich and Szkody, and the CCD ASAS-3 and NSVS data. One plate or CCD image was usually obtained per night. These data span 78 years. We use the histograms of brightness to set the separation between the high and low states in V1223 Sgr at the level of 13.9 mag(B). The profile of this statistical distribution is dominated by a broad bump with the peak-to-peak amplitude of more than 1 mag(B). This is the result of co-adding various episodes of the high states, sometimes with the brightness varying on a timescale of months even during a single episode. The high state is not any uniquely defined level of brightness. Groups of the low states represent tails from such bumps rather than forming specific levels of brightness. The fuzzy boundaries of the bright side of the statistical distributions of brightness show that the system cannot find equilibrium of the mass transfer even in the high state. We show that V1223 Sgr underwent a large change of its high/low state activity; we ascribe it to the changes of the stellar activity of the donor. The episodes of the low states with mutually similar depth and profile were observed to recur with a cycle of 1092 days in the data mapping the years from 1999 to 2009; this behavior was discordant with the typical recurrence times present before. In our scenario, this cycle is driven by the changing aspect of some structure (e.g. a sequence of star spots migrating across the L1 point) on the donor. Because of the differential rotation of the donor, this configuration cyclically influences the mass flow to the accretion disk.

Investigation of binary X-ray sources with photographic plates

  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    We summarize how the archival photographic plates can be used for a study of the long-term activity of various types of X-ray binaries in the optical band. We show examples of the unpredictable phenomena (flares, outbursts, transitions between the states) which are easily recognizable on the plates. Such plates are very helpful in extending the investigated time intervals into the past, especially because some of these systems underwent dramatic changes of their activity during the last decades, sometimes even before they were discovered as the variable objects and/or the X-ray sources.

Low-mass binary X-Ray sources: Monitoring with various satellites

  • DOI: 10.14311/AP.2014.54.0248
  • Odkaz: https://doi.org/10.14311/AP.2014.54.0248
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    We show the importance of observing low-mass binary (LMXB) X-ray sources with the X-ray monitors onboard satellites. This enables us to study the physical processes governing the longterm activity of LMXBs. They are excellent targets for monitoring because of their strong activity. We recall that various physical processes operating in LMXBs produce specific large-amplitude variations of X-ray luminosity which can be investigated even in a single-band X-ray light curve as often provided by the monitors. We also emphasize the role of the spectral region of the X-ray monitor. Choosing the spectral region often strongly influences the profile of the observed intensity curve because different emission components may dominate in different X-ray energy ranges. We show several examples of LMXBs which undergo various types of unpredictable activity (e.g. outbursts, superorbital cycles).

Scientific astrophysical payloads for pico and nano-satellites

  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    The picosatellites, recently in development at many institutes and universities (e.g. Schmidt et al. 2009 and Schilling 2006), require a miniaturized payload. We present and discuss an idea for a suitable scientific payload, namely a miniature Lobster Eye X-ray telescope.

Simultaneous monitoring of binary X-ray sources in the optical and X-ray bands++++++++

  • Autoři: RNDr. Vojtěch Šimon, Ph.D.,
  • Publikace: III Workshop on Robotic Autonomous Observatories. México: Universidad Nacional Autónoma de México, 2014. p. 51-52. Revista Mexicana de Astronomia y Astrofisica-Serie de Conferencias. ISSN 1405-2059.
  • Rok: 2014
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    We discuss the importance of the simultaneous monitoring of binary X-ray sources in the optical and X-ray bands. Monitoring is important for investigating the processes that operate on the timescales of years, especially as regards the unpredictable events like the high/low state transitions and outbursts. The strategy discussed here can shed more light on the accretion processes since the optical and X-ray radiations are often emitted from different regions of the binary. We show some examples of the binary systems with the accreting compact objects of various kinds where the observations provided by the X-ray monitors onboard satellites (RXTE, Swift) are combined with a long, simultaneous dense series of data from various groundbased telescopes.

The X-Ray Binary KS 1731-260: Possible Analogy with Her X-1

  • Autoři: RNDr. Vojtěch Šimon, Ph.D.,
  • Publikace: Acta Polytechnica CTU Proceedings Vol 1, No 1 (2014): FRASCATI WORKSHOP 2013. Praha: Czech Technical University in Prague, 2014. p. 246-250. Acta Polytechnica CTU Proceedings. ISSN 2336-5382.
  • Rok: 2014
  • DOI: 10.14311/APP.2014.01.0246
  • Odkaz: https://doi.org/10.14311/APP.2014.01.0246
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    The X-ray binary with the neutron star (NS), KS 1731-260, displays superorbital cycle similar to that in Her X-1. The accretion disk had the memory of the cycle-length even when this modulation sometimes disappeared in the main outburst of KS 1731-260, and during anomalous low state in Her X-1. The disk still existed during such seasons. Although irradiation of the disk by X-rays is a viable explanation for the disk precession and warping (see model of Foulkes et al.), the mechanisms which give rise to the observed X-ray modulation are quite different for each of these systems. Variable absorption can explain this cycle only in Her X-1. We propose a variable mass accretion rate onto the NS in KS 1731-260 due to a highly variable impact of the inflowing mass stream with the changing phase of the cycle.

Color indices of optical afterglows of long GRBs in the Swift era

  • Autoři: RNDr. Vojtěch Šimon, Ph.D., Pizzichini, G., prof. RNDr. René Hudec, CSc.,
  • Publikace: Gamma-ray Bursts: 15 Years of GRB Afterglows – Progenitors, Environments and Host Galaxies from the Nearby to the Early Universe. Les Ulis: EDP Sciences, 2013. pp. 271-273. EAS Publications Series. ISSN 1633-4760. ISBN 978-2-7598-1002-4.
  • Rok: 2013
  • DOI: 10.1051/eas/1361043
  • Odkaz: https://doi.org/10.1051/eas/1361043
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    We show the power of the method of the color indices that uses the commonly available multiband photometry of optical afterglows (OAs) of long GRBs detected by Swift. Our study shows that for most OAs, in the observer frame, these colors, corrected for the Galactic reddening, display prominent clustering in the color-color diagrams. The color indices enable us to distinguish OAs from other types of objects even without available gamma-ray emission. The color-color diagrams can be used to assess the extinction inside the host galaxy. Our approach is also generally important for investigating the numerous faint OAs for which obtaining spectra with good S/N ratio is often impossible.

GRB INVESTIGATIONS BY ESA GAIA AND LOFT

  • DOI: 10.14311/AP.2013.53.0799
  • Odkaz: https://doi.org/10.14311/AP.2013.53.0799
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    The possibility of studying GRBs with the ESA Gaia and LOFT missions is briefly addressed. The ESA Gaia satellite to be launched in November 2013 will focus on high precision astrometry of stars and all objects down to limiting magnitude 20. The satellite will also provide photometric and spectral information and hence important inputs for various branches of astrophysics, including the study of GRBs and related optical afterglows (OAs) and optical transients (OTs). The strength of Gaia in GRB analyses will be the fine spectral resolution (spectro-photometry and ultra-low dispersion spectroscopy), which will allow the correct classication of related triggers. An interesting feature of Gaia BP and RP instruments will be the study of highly redshifted triggers. Similarly, the low dispersion spectroscopy provided by various plate surveys can also supply valuable data for investigations of high-energy sources. The ESA LOFT candidate mission, now in the assessment study phase, will also be able to detect and be used in the study of GRBs, with emphasis on low-energy (X-ray) emission

GRB Potential of ESA Gaia

  • Autoři: prof. RNDr. René Hudec, CSc., RNDr. Vojtěch Šimon, Ph.D.,
  • Publikace: Gamma-ray Bursts: 15 Years of GRB Afterglows – Progenitors, Environments and Host Galaxies from the Nearby to the Early Universe. Les Ulis: EDP Sciences, 2013. pp. 639-643. EAS Publications Series. ISSN 1633-4760. ISBN 978-2-7598-1002-4.
  • Rok: 2013

NANOX: proposed Nano-Satellite X-Ray Mission

  • Autoři: Tichý, V., RNDr. Vojtěch Šimon, Ph.D., prof. RNDr. René Hudec, CSc., Inneman, A., Burrows, D.N.
  • Publikace: Proceedings of SPIE Vol. 8777 Damage to VUV, EUV, and X-ray Optics IV; and EUV and X-ray Optics: Synergy between Laboratory and Space III. Bellingham (stát Washington): SPIE, 2013. Proceedings of SPIE. ISSN 0277-786X. ISBN 978-0-8194-9579-2.
  • Rok: 2013
  • DOI: 10.1117/12.2017612
  • Odkaz: https://doi.org/10.1117/12.2017612
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    We present the idea of a low-cost satellite providing permanent monitoring of X-ray binaries. These systems contain a compact object (a neutron star or a black hole) accreting matter from a donor companion. They concentrate in the vicinity of the Galactic plane and toward the center of the Galaxy. It therefore appears very advantageous to point the telescope toward the Galactic center. The strong activity of X-ray binaries with non-predictable episodes of brightening suggests that we can obtain meaningful and physically important information even by a study using a small, inexpensive satellite. The proposed spacecraft can be of the nano-satellite class. We propose Schmidt lobster optics for this satellite. The results of experimental tests of the specimen of such optics show that the mission is feasible.

NON-THERMAL EMISSION FROM CATACLYSMIC VARIABLES: IMPLICATIONS ON ASTROPARTICLE PHYSICS

  • DOI: 10.14311/AP.2013.53.0595
  • Odkaz: https://doi.org/10.14311/AP.2013.53.0595
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    We review the lines of evidence that some cataclysmic variables (CVs) are the sources of non-thermal radiation. It was really observed in some dwarf novae in outburst, a novalike CV in the high state, an intermediate polar, polars, and classical novae (CNe) during outburst. The detection of this radiation suggests the presence of highly energetic particles in these CVs. The conditions for the observability of this emission depend on the state of activity, and the system parameters. We review the processes and conditions that lead to the production of this radiation in various spectral bands, from gamma-rays including TeV emission to radio. Synchrotron and cyclotron emissions suggest the presence of strong magnetic fields in CV. In some CVs, e.g. during some dwarf nova outbursts, the magnetic field generated in the accretion disk leads to the synchrotron jets radiating in radio. The propeller effect or a shock in the case of the magnetized white dwarf (WD) can lead to a strong acceleration of the particles that produce gamma-ray emission via pi0 decay; even Cherenkov radiation is possible. In addition, a gamma-ray production via pi0 decay was observed in the ejecta of an outburst of a symbiotic CN. Nuclear reactions during thermonuclear runaway in the outer layer of the WD undergoing CN outburst lead to the production of radioactive isotopes; their decay is the source of gamma-ray emission. The production of accelerated particles in CVs often has episodic character with a very small duty cycle; this makes their detection and establishing the relation of the behavior in various bands difficult.

Activity of the mass-accreting compact objects - prospects for the optical monitoring

  • Autoři: RNDr. Vojtěch Šimon, Ph.D.,
  • Publikace: Second Workshop on Robotic Autonomous Observatories. Bengaluru: Astronomical Society of India, 2012. pp. 287-295. Astronomical Society of India Conference Series. ISBN 978-81-922926-5-6.
  • Rok: 2012
  • Pracoviště: Katedra radioelektroniky
  • Anotace:
    We discuss the perspectives of investigation of the mass-accreting compact objects in the binary systems, for example cataclysmic variables (CVs), with monitors. Especially the processes that operate on timescales of years are little studied, hence little understood. However, the profiles of the light curves can be significantly affected by the sampling of data from the monitors. We show an approximation of how the profiles of these light curves change after a heavy sampling of the data. We find that histograms of brightness (in magnitudes) provide us with a representative description of the properties of the long-term activity of a given type of CV even in the sampled data. We also discuss the strategy of monitoring of these systems, particularly as regards the spatial distribution of these systems in the Galaxy and their peak brightness.

Za stránku zodpovídá: Ing. Mgr. Radovan Suk