Cyril Polasek

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.

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.

BART: The Czech Autonomous Observatory

The High Energy Astrophysics group of the stellar department of the Astronomical Institute in Ondřejov operates two small aperture robotic telescopes called BART and D50. Both telescopes are capable of making automatic followup observation of gamma-ray burst optical counterparts. This paper deals with the smaller telescope BART.

Cycles and their instability in the cataclysmic variable V795 Her

We analyze the cycles in the possible intermediate polar V795 Her, the optical counterpart of 2RXP J171256.0+333121, using the photometric V band CCD observations obtained within 5 days in 2008. We find that the times of the minimum intensity of the known cycle of ∼2.8028 hours are more stable than those of the peak intensity. We show that while the smoothed profile of the folded intensity curve is single‐peaked, the profile of its residuals is double‐peaked. We resolve the variations of the superimposed cycle of ∼0.01 d during a single night; this cycle is thus not caused by spin modulation of the white dwarf. We discuss our observations in terms of superhumps.We analyze the cycles in the possible intermediate polar V795 Her, the optical counterpart of 2RXP J171256.0+333121, using the photometric V band CCD observations obtained within 5 days in 2008. We find that the times of the minimum intensity of the known cycle of ∼2.8028 hours are more stable than those of the peak intensity. We show that while the smoothed profile of the folded intensity curve is single‐peaked, the profile of its residuals is double‐peaked. We resolve the variations of the superimposed cycle of ∼0.01 d during a single night; this cycle is thus not caused by spin modulation of the white dwarf. We discuss our observations in terms of superhumps.

Operating a global network of autonomous observatories

We discuss our experiences operating a heterogeneous global network of autonomous observatories. The observatories are presently situated on four continents, with a fifth expected during the summer of 2010. The network nodes are small to intermediate diameter telescopes (<= 150 cm) owned by different institutions but running the same observatory control software. We report on the experience gained during construction, commissioning and operation of the observatories, as well as future plans. Problems encountered in the construction and operation of the nodes are summarised. Operational statistics as well as scientific results from the observatories are also presented.