K. A. Stoyanov

Short-term flux and colour variations in low-energy peaked blazars

We have measured multi-band optical flux and colour variations for a sample of 12 low energy peaked blazars on short, day-to-month, timescales. Our sample contains six BL Lacertae objects and six flat spectrum radio quasars. These photometric observations, made during September 2008 to June 2009, used five optical telescopes, one in India and four in Bulgaria. We detected short term flux variations in eleven of these blazars and colour variability in eight of them. Our data indicate that six blazars (3C 66A, AO 0235+164, S5 0716+714, PKS 0735+178, OJ 287 and 3C 454.3) were observed in preor post-outburst states, that five (PKS 0420 014, 4C 29.45, 3C 279, PKS 1510 089 and BL Lac) were in a low state, while one (3C 273) was in an essentially steady state. The duty cycles for flux and colour variations on short timescales in these low energy peaked blazars are �92 per cent and � 33 per cent, respectively. The colour vs magnitude correlations seen here support the hypothesis that BL Lac objects tend to become bluer with increase in brightness; however, flat spectrum radio quasars may show the opposite trend, and there are exceptions to these trends in both categories of blazar. We briefly discuss emission models for active galactic nuclei that might explain our results.

The awakening of BL Lacertae: observations by Fermi, Swift and the GASP-WEBT

Since the launch of the Fermi satellite, BL Lacertae has been moderately active at ?-rays and optical frequencies until 2011 May, when the source started a series of strong flares. The exceptional optical sampling achieved by the GLAST–AGILE Support Program of the Whole Earth Blazar Telescope in collaboration with the Steward Observatory allows us to perform a detailed comparison with the daily ?-ray observations by Fermi. Discrete correlation analysis between the optical and ?-ray emission reveals correlation with a time lag of 0 ± 1 d, which suggests cospatiality of the corresponding jet emitting regions. A better definition of the time lag is hindered by the daily gaps in the sampling of the extremely fast flux variations. In general, optical flares present more structure and develop on longer time-scales than corresponding ?-ray flares. Observations at X-rays and at millimetre wavelengths reveal a common trend, which suggests that the region producing the mm and X-ray radiation is located downstream from the optical and ?-ray-emitting zone in the jet. The mean optical degree of polarization slightly decreases over the considered period and in general it is higher when the flux is lower. The optical electric vector polarization angle (EVPA) shows a preferred orientation of about 15°, nearly aligned with the radio core EVPA and mean jet direction. Oscillations around it increase during the 2011–2012 outburst. We investigate the effects of a geometrical interpretation of the long-term flux variability on the polarization. A helical magnetic field model predicts an evolution of the mean polarization that is in reasonable agreement with the observations. These can be fully explained by introducing slight variations in the compression factor in a transverse shock waves model.

UBVRI observations of the flickering of RS Ophiuchi at quiescence

We report observations of the flickering variability of the recurrent nova RS Oph at quiescence on the basis of simultaneous observations in five bands (UBV RI). RS Oph has a flickering source with (U - B) o = -0.62 ± 0.07, (B - V) o = 0.15 ± 0.10 and (V - R) o = 0.25 ± 0.05. We find for the flickering source a temperature T fl ≈ 9500 ± 500 K, and luminosity L fl ~ 50-150 L ☉ (using a distance of d = 1.6 kpc). We also find that on a (U - B) versus (B - V) diagram, the flickering of the symbiotic stars differs from that of the cataclysmic variables. The possible source of the flickering is discussed. The data are available upon request from the authors.

Hα observations of the γ-ray-emitting Be/X-ray binary LSI+61 ◦ 303: orbital modulation, disk truncation, and long-term variability ,

We report 138 spectral observations of the Hα emission line of the radio- andγ-ray-emitting Be/X-ray binary LSI+61 ◦ 303 obtained during the period of September 1998 ‐ January 2013. From measuring various Hα parameters, we found that the orbital modulation of the Hα is best visible in the equivalent width ratio EW(B)/EW(R), the equivalent width of the blue hump, and in the radial velocity of the central dip. The periodogram analysis confirmed that t he Hα emission is modulated with the orbital and superorbital periods. For the past 20 years the radius of the circumstellar disk is s imilar to the Roche lobe size at the periastron. It is probabl y truncated by a 6:1 resonance. The orbital maximum of the equivalent width of Hα emission peaks after the periastron and coincides on average with the X-ray andγ-ray maxima. All the spectra are available upon request from the authors and through the CDS.

Optical flickering of the recurrent nova RS Ophiuchi: amplitude–flux relation

We report observations of the flickering variability of the symbiotic recurrent nova RS Oph at quiescence in five bands (UBVRI). We find evidence of correlation between the peak-to-peak flickering amplitude (�F) and the average flux of the hot component (Fav). The correlation is highly significant with correlation coefficient 0.85 and p value � 10 20 . Combining the data from all wavebands, we find a dependence of

Optical spectroscopy of Be/gamma-ray binaries

We report optical spectroscopic observations of the Be/gamma-ray binaries LSI+61303, MWC 148 and MWC 656. The peak separation and equivalent widths of prominent emission lines (H-alpha, H-beta, H-gamma, HeI, and FeII) are measured. We estimated the circumstellar disc size, compared it with separation between the components, and discussed the disc truncation. We find that in LSI+61303 the compact object comes into contact with the outer parts of the circumstellar disc at periastron, in MWC 148 the compact object goes deeply into the disc during the periastron passage, and in MWC 656 the black hole is accreting from the outer parts of the circumstellar disc along the entire orbit. The interstellar extinction was estimated using interstellar lines. The rotation of the mass donors appears to be similar to the rotation of the mass donors in Be/X-ray binaries. We suggest that X-ray/optical periodicity of about 1 day deserves to be searched for.

Active phases and flickering of a symbiotic recurrent nova T CrB

T CrB is a symbiotic recurrent nova known to exhibit active phases, characterised by apparent increases in the hot component temperature and the appearance of flickering, i.e. changes in the observed flux on the time-scale of minutes. Historical UV observations have ruled out orbital variability as an explanation for flickering and instead suggest flickering is caused by variable mass transfer. We have analysed optical and X-ray observations to investigate the nature of the flickering as well as the active phases in T CrB. The spectroscopic and photometric observations confirm that the active phases follow two periods of ~1000d and ~5000d. Flickering in the X-rays is detected and follows an amplitude-flux relationship similar to that observed in the optical. The flickering is most prominent at harder X-ray energies, suggesting that it originates in the boundary layer between the accretion disc and the white dwarf. The X-ray radiation from the boundary layer is then reprocessed by a thick accretion disc or a nebula into UV radiation. A more detailed understanding of flickering would benefit from long-term simultaneous X-ray and optical monitoring of the phenomena in symbiotic recurrent novae and related systems such as Z And type symbiotic stars.

Connection between orbital modulation of Hα and gamma-rays in the Be/X-ray binary LS I+61°303

We studied the average orbital modulation of various parameters (γ-ray flux, Hα emission line, optical V band brightness) of the radioand γ-ray emitting Be/ X-ray binary LS I+61 ◦ 303. Using the Spearman rank correlation test, we found highly significant correlations between the orbital variability of the equivalent width of the blue hump of the Hα and Fermi-LAT flux with a Spearman p-value ∼2 × 10 −5 , and the equivalent widths ratio EWB/EWR and Fermi-LAT flux with p-value ∼9 × 10 −5 . We also found a significant anticorrelation between Fermi-LAT flux and V band magnitude with a p-value ∼7 × 10 −4 . All these correlations refer to the average orbital variability, and we conclude that the Hα and γ-ray emission processes in LS I+61 ◦ 303

Orbital parameters of the high-mass X-ray binary 4U 2206+54†

We present new radial velocities of the high-mass X-ray binary star 4U 2206+54 based on optical spectra obtained with the Coude spectrograph at the 2 m RCC telescope of the Rozhen National Astronomical Observatory, Bulgaria in the period November 2011–July 2013. The radial velocity curve of the He I δ6678 A line is modeled with an orbital period Porb = 9.568 d and an eccentricity of e = 0.3. These new measurements of the radial velocity resolve the disagreements of the orbital period discussions. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

AG Pegasi – now a classical symbiotic star in outburst?

Optical spectroscopy study of the recent AG Peg outburst observed during the second half of 2015 is presented. Considerable variations of the intensity and the shape of the spectral features as well as the changes of the hot component parameters, caused by the outburst, are discussed and certain similarities between the outburst of AG Peg and the outburst of a classical symbiotic stars are shown. It seems that after the end of the symbiotic nova phase, AG Peg became a member of the classical symbiotic stars group.