Vojtech Simon

Colors and luminosities of the optical afterglows of the γ-ray bursts

Results of the study of the color indices and luminosities of 17 optical afterglows (OAs) of GRBs are presented. We show that the color variations during the decline of OAs (except for GRB000131) are relatively small during t T0 < 10 days and allow a comparison among them, even for the less densely sampled OAs. The colors in the observer frame, corrected for the Galactic reddening, concentrate at (V R)0 =0 :40 0:13, (R I)0 =0 :46 0:18, (B V )0 =0 :47 0:17. The color evolution of the OAs is negligible although their brightness declines by several magnitudes during the considered time interval. Such a strong concentration of the color indices also suggests that the intrinsic reddening (inside their host galaxies) must be quite similar and relatively small for all these events. The absolute brightness of OAs in the observer frame, corrected for the host galaxy, lies within MR0 = 26: 5t o 22:2 for (t T0)rest =0 :25 days. This spread of MR0 is not signicantly influenced by the shifts of , caused by the dierent redshift z of the respective OAs. The general decline rate of the OA sample considered here seems to be independent of the absolute optical brightness of the OA, measured at some t T0 identical for all OAs, and the light curves of all events are almost parallel, when corrected for the redshift-induced time dilation.

On the recurrence time and outburst properties of the soft X-ray transient Aquila X-1

The analysis of the recurrence time TC of outbursts in Aql X-1, using the method of O–C residuals, revealed that the character of the O–C curves bears a striking similarity to that of dwarf novae. It means that variations of TC are large, but generally not chaotic and long-term trends in the O–C curves can be resolved. The prevailing value of TC is 211 days but the evolution of TC shows several large jumps. The mean TC varies by more than 1:2 and the switches occur within just a single epoch. A comprehensive study of the morphology of the light curves from ASM/RXTE revealed that although the respective outbursts in Aql X-1 differ in both their duration and the maximum intensity, their rising and decay branches display remarkable similarities. The rising branches are exponential. The slope of the decay is broken in some events, with the final phase having a steeper slope and being roughly linear. We argue that just the inner disk is irradiated during outburst and that viscous heating plays a big role. The evolution of the hardness ratios HR1 and HR2 displays common characteristics for the respective outbursts observed by ASM/RXTE. HR1 at the peak intensity of the more intense outbursts tends to be harder. Similar behaviour is also apparent for HR2. An asymmetric course (hysteresis) of HR2 with respect to the moment of maximum intensity is apparent for some outbursts, the largest value of HR2 occurring on the rising branch. The observed behaviour is discussed in terms of the thermal instability model and variations of the thermal and power-law spectral components.

Colors of Optical Afterglows of GRBs and Their Time Evolution

The study of the color indices of 23 optical afterglows (OAs) of GRBs shows that the color variations during the decline of the OAs are very small during t − T0 < 10 days. The colors in the observer frame concentrate at (B − V)0 = 0.45 ± 0.15, (V − R)0 = 0.41 ± 0.09, (R − I)0 = 0.46 ± 0.13, (I − J)0 = 0.88 ± 0.15, but are more scattered in (U − B)0. These findings entirely confirm our previous ones, determined from a smaller ensemble of the OAs with a smaller range of the redshifts z. The strong concentration of the color indices of most OAs also suggests that the intrinsic reddening (i.e. inside their host galaxies) must be quite similar and relatively small for these events. The maximum reddening which can be allowed by the scatter of the colors of the individual OAs is about EB−V = 0.2. This suggests that they are not deep inside the star‐forming regions or, alternatively, that the dust is destroyed by the energetic initial flash.

Evolution of the color indices in SN 2006aj associated with GRB 060218

Aims. We analyze the photometric properties of SN 2006aj associated with GRB 060218. We also investigate this event in relation to other supernovae. Methods. We determined the color indices from multiband photometry and investigated their time evolution by taking their time stretch factors into account. We used both the ground-based and UVOT/Swift data to ensure a coverage from the cosmic UV region (UVW1 (2510 A), UVM2 (2170 A), UVW2 (1880 A)) to the I band. Results. We find a large difference between the values and time evolution of the color indices of the early phase (t −T0 ≤ 2. 5d ) of the optical afterglow (OA) and the subsequent SN 2006aj. The clustering and small evolution of most color indices in UV in t −T0 ≤ 2. 5d resemble the behavior seen in the ensemble of OAs of long GRBs. In our interpretation, the light of the early OA comes from far above the photosphere of the progenitor and its wind. The outer layers of SN 2006aj underwent the strongest evolution among the considered supernovae of various types. We find that the adjustment of time evolution of various color indices by stretching is very helpful in searching for the proper supernova type. We attribute this evolution of SN 2006aj to the exceptionally low mass of its ejecta and the structural changes of the region producing the GRB. In our interpretation, we observed the different regions simultaneously in the complicated, asymmetric shape of SN 2006aj. The colors in the UV band can be explained if line blanketing undergoes only small variations and the UV emitting area tends to shrink during the evolution of SN 2006aj over the time interval investigated. Our approach is also of general importance for investigating supernovae in OAs.

Echo outbursts in KS 1731-260

Context. The thermal-viscous instability of the accretion disk is thought to play an important role in the activity of low-mass X-ray binaries (LMXBs). It should also appear when a (quasi)persistent LMXB goes into its low state. Aims. We analyze the long-term X-ray activity of the neutron star LMXB KS 1731–260 during a part of its long main outburst and a complicated transition back to the low state (quiescence). We pay special attention to the variations during the state transition. Methods. We make use of the ASM/RXTE observations for a timeseries analysis of the longterm variations and investigation of the X-ray color changes in the 1.5–12 keV passband. Results. In our interpretation, the mass transfer rate u mtr between the donor and neutron star significantly increased during the main outburst of KS 1731–260, which prolonged this event by keeping the disk ionized to its outer rim. Later on, a considerable fraction of the outer part of the disk was rapidly brought out of steady-state when u mtr decreased again. Only a slow decrease of u mtr continued even after the end of the main outburst. This lead to the formation of the central disk region, which was kept thermally stable, ionized by irradiation, and was surrounded by the outer, thermally unstable disk region. This configuration triggered the series of the echo outbursts. These outbursts are outside-in, caused by the still continuing u mtr via the mass stream impinging the outer rim of the disk without overflow. We argue that introducing any strong bursts of matter from the donor to the disk as the cause of the echo outbursts is superfluous. The spectral evolution suggests that the disk returns to steady-state during the maxima of the echo outbursts. The X-ray emission between the echo outbursts is much higher than in the true quiescence and can be due to the ongoing u mtr. We place the peculiar long-term activity of KS 1731–260 in the context of the systems with the echo outbursts, both soft X-ray transients and dwarf novae.

Complicated variations in the early optical afterglow of GRB 090726

Aims. We report on the detection of an early rising phase of optical afterglow (OA) of a long GRB 090726. We resolve a complicated profile of the optical light curve. We also investigate the relation of the optical and X-ray emission of this event. Methods. We made use of the optical photometry of this OA obtained by the 0.5 m telescope of AI AS CR, supplemented by the data obtained by other observers, and the X-ray Swift/XRT data. Results. The optical emission peaked at ∼17. 5m ag(R )a tt −T0 ≈ 500 s. We find a complex profile of the light curve during the early phase of this OA: an approximately power-law rise, a rapid transition to a plateau, a weak flare superimposed on the center of this plateau, and a slowly steepening early decline followed by a power-law decay. We discuss several possibilities for explaining the short flare on the flat top of the optical light curve at t −T0 ≈ 500 s. Activity of the central engine is favored, although reverse shock cannot be ruled out. We show that power-law outflow with Θobs/Θc > 2.5 is the best case for the OA of GRB 090726. The initial Lorentz factor is Γ0 ≈ 230−530 in the case of propagation of the blast wave in a homogeneous medium, while propagation of this wave in a wind environment gives Γ0 ≈ 80−300. The value of Γ0 in GRB 090726 thus falls into the lower half of the range observed in GRBs and it may even lie on the lower end. We also show that both the optical and X-ray emission decayed simultaneously and that the spectral profile from X-ray to the optical band did not vary. This is true for both the time periods before and after the break in the X-ray light curve. This break can be regarded as achromatic. The available data show that neither the dust nor the gaseous component of the circumburst medium underwent any evolution during the decay of this OA, that is, after t − T0 < 3000 s. We also show that this OA belongs to the least luminous ones in the phase of its power-law decay, corresponding to what is observed for the ensemble of OAs of long GRBs.

Long-term color variations of the peculiar X-ray binary V Sagittae

We present an analysis of the color variations of the X-ray binary V Sge during high, medium and low state of its activity over the years 1995–1997, using UBV data. We resolved the track of V Sge in the color diagrams (B−V and U−B versus mag(V )) during transitions between the states and show that the color variations on the orbital time scale are significantly smaller than the changes caused by the long-term activity. The mean B − V decreases by 0.15 mag during the upper part of the transition (brightness higher than 11.5 mag(V )) from the low to the high state but stays almost constant below this level. On the contrary, U −B does not change significantly during the whole transition. Comparison of our data with those of Herbig et al. (1965) shows that although the character of the long-term activity in V Sge changed significantly during the last decades (Šimon & Mattei 1999) the colors and their variations with the brightness level remained similar.

Cycles in the cataclysmic variable V795 Herculis

Aims. We investigate the photometric activity of the cataclysmic variable (CV) V795 Her. We pay attention to the time evolution of the length of its superhump cycle Psh. We investigate how well the superhump profile is reproduced in the individual phases Φsh of Psh. We also analyze the mutual relation of the intensity of the superhump light and the superimposed rapid intensity variations. Methods. We used the photometric V-band CCD observations obtained between 2008 and 2010. The observing run on a given night consisted of a dense series of V-band images (exp. time of 20 s), which enabled us to investigate the superhump profile and the superimposed rapid changes. Results. We identified several time segments characterized by a relatively stable Psh and mean intensity level. We find that inside such a segment, the smoothed profile of the intensity curve folded with Psh displays the largest scatter of the residuals of the mean profile in Φsh of the peak intensity and during the decay from this peak. A zone of quiescence occurs at Φsh ≈ 0.6−0.7. The profile of these residuals can be explained if the strength and luminosity of the spiral arms undergo large variations on a timescale much shorter than the segment’s duration, as predicted in a previously developed model, while the conditions for the disk precession remain much more stable. The formation of these arms is considerably faster than their fading. We find that the emission of the superimposed short cycle (∼0.01–0.02 d) in this CV depends on Φsh. We conclude that it does not come from pulsations of the white dwarf or, generally, from the close vicinity of this accretor. We argue that it is flickering with the site in the disk region that produces the superhump. This flickering is not located in the impact of the stream on the disk. V795 Her is a borderline case in the sense that its disk is close to the switch to the cool state. We explain the suppression of the thermal-viscous instability of the disk in V795 Her as caused by the tidal force that is also the reason for the superhumps. This tidal heating can also help to keep the accretion disks in other permanent superhumpers in the hot state.

Astrophysics with LOBSTER

We report on astrophysical aspects of fully innovative very wide-field X-ray telescopes with high sensitivity. The prototypes are very promising, allowing the proposals for space projects with very wide-field Lobster-eye X-ray optics to be considered. The novel telescopes will monitor the sky with unprecedented sensitivity and angular resolution of order of 1 arcmin. They are expected to contribute essentially to study of various astrophysical objects such as AGN, SNe, Gamma-ray bursts (GRBs), X-ray flashes (XRFs), galactic binary sources, stars, CVs, X-ray novae, various transient sources, etc.

Characteristics of the photometric modulation of HZ Her/Her X-1 in active states over a very long time interval

Abstract The properties of the photometric orbital modulation of the X-ray binary HZ Her / Her X-1 and its variations over a long time interval (decades) are studied by the statistical methods using photographic plates of Sonneberg Observatory. The moving averages of the light curve in the long-lasting active state (1959–1993) show that both the smoothed orbital light curve and the smoothed scatter of its residuals σ mag display striking asymmetries with respect to the orbital phase φ orb =0.5; they are apparent both in the whole data set and in the segments of the 35 day cycle. The light curve near the orbital phase 0.5 can be characterized by the maximum either flat-topped and symmetric or sharp and lagging behind φ orb =0.5. The course of σ mag is largely asymmetric with respect to the primary minimum, usually with a smooth decrease of σ mag within φ orb =0.2–0.6 and a steep rise within φ orb =0.6–0.85. All these facts suggest the presence of an additional source of variations, in addition to the geometric effects of the precessing disk. The role of the interaction of the mass stream with the precessing warped disk is suggested as a possible explanation. Our data for the extended active state within the years 1959–1993 revealed that the mean brightness at all orbital phases remains stable and displays at most marginal secular trends. The brightness during the short active states (1934–1937 and 1941–1949) is shown to be lower than in the active state within 1959–1993 and is attributed to a lower degree of heating of the secondary star.