No abstract

We report on two small aperture robotic telescopes called BART and D50 operated in Ondřejov. Both telescopes are capable of automatic observation of gamma ray burst (GRB) optical afterglows. Coordinates of GRBs are taken from alerts distributed via Internet. Telescopes observe other interesting high energy sources when there is not any alert. The smaller telescope BART has aperture D = 254 mm. The bigger telescope D50 has a primary mirror of diameter D = 500 mm. Both telescopes are controlled by free software package RTS2 and are accessible through Internet. We describe the two telescopes and related software and show some results such as our first observed optical counterpart of GRB.

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.

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 P sh . We investigate how well the superhump profile is reproduced in the individual phases Φ sh of P sh . 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 P sh and mean intensity level. We find that inside such a segment, the smoothed profile of the intensity curve folded with P sh displays the largest scatter of the residuals of the mean profile in Φ sh of the peak intensity and during the decay from this peak. A zone of quiescence occurs at Φ sh ≈ 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 Φ sh . 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.

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 ∼17.5 mag(R) at t − T 0 ≈ 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 − T 0 ≈ 500 s. Activity of the central engine is favored, although reverse shock cannot be ruled out. We show that power-law outflow with Θ obs /Θ c > 2.5 is the best case for the OA of GRB 090726. The initial Lorentz factor is Γ 0 ≈ 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 Γ 0 ≈ 80−300. The value of Γ 0 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 − T 0 < 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.