G. Latev

Optical flux and spectral variability of blazars

We report the results of optical monitoring for a sample of 11 blazars including 10 BL Lacertae objects (BL Lacs) and one flat spectrum radio quasar (FSRQ). We have measured the multiband optical flux and colour variations in these blazars on intraday and short-term time-scales of months and have limited data for two more blazars. These photometric observations were made during 2009–2011, using six optical telescopes, four in Bulgaria, one in Greece and one in India. On short-term time-scales we found significant flux variations in nine of the sources and colour variations in three of them. Intraday variability was detected on six nights for two sources out of the 18 nights and four sources for which we collected such data. These new optical observations of these blazars plus data from our previous published papers (for three more blazars) were used to analyse their spectral flux distributions in the optical frequency range. Our full sample for this purpose includes six high-synchrotron-frequency-peaked BL Lacs (HSPs), three intermediate-synchrotron-frequency-peaked BL Lacs (ISPs) and six low-synchrotron-frequency-peaked BL Lacs (LSPs; including both BL Lacs and FSRQs). We also investigated the spectral slope variability and found that the average spectral slopes of LSPs show a good accordance with the synchrotron self-Compton loss dominated model. Our analysis supports previous studies that found that the spectra of the HSPs and FSRQs have significant additional emission components. The spectra of all these HSPs and LSPs get flatter when they become brighter, while for FSRQs the opposite appears to hold. This supports the hypothesis that there is a significant thermal contribution to the optical spectrum for FSRQs.

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

Multiband optical variability of the blazar OJ 287 during its outbursts in 2015 – 2016

We present recent optical photometric observations of the blazar OJ 287 taken during 2015 September-2016 May. Our intense observations of the blazar started in 2015 November and continued until 201 ...

Discovery of optical flickering from the symbiotic star EF Aquilae

We report optical charge-coupled device (CCD) photometry of the recently identified symbiotic star EF Aquilae (EF Aql). Our observations in the Johnson V and B bands clearly show the presence of stochastic light variations with an amplitude of about 0.2 mag on a time scale of minutes. The observations point toward a white dwarf (WD) as the hot component in the system. It is the 11th object among more than 200 symbiotic stars known with detected optical flickering. Estimates of the mass accretion rate onto the WD and the mass loss rate in the wind of the Mira secondary star lead to the conclusion that less than 1% of the wind is captured by the WD. Eight further candidates for the detection of flickering in similar systems are suggested.

Difference between the optical flickering colours of cataclysmic variables and symbiotic recurrent novae

Institute of Astronomy and National Astronomical Observatory, Bulgarian Academy of Sciences, 72 Tsarigradsko shose,BG-1784, Sofia, BulgariaReceived 2 September 2014, accepted 2014 Dec 04Published online 2015 Mar 01Key words Stars: binaries – novae, cataclysmic variables – binaries: symbiotic – stars: individual: T CrB, RS OphWe performed simultaneous observations in 3 bands (UBV)of the flickering variability of the recurrent novae RS Ophand T CrB at quiescence. Using new and published data, we compare the colours of the flickering in cataclysmic variablesand symbiotic recurrent novae. We find a difference between t he colours of the flickering source in these two types ofaccreting white dwarfs. The detected difference is highly significant with p−value≈2×10

The recurrent nova RS Oph: simultaneous B- and V- band observationsof the flickering variability

We performed 48.6 hours (in 28 nights) of simultaneous B and V band observations of the flickering variability of the recurrent nova RS Oph in quiescence. During the time of our observations the brightness of the system varied between 13.2 > B > 11.1 and the colour in the range 0.86 < B-V < 1.33 . We find that RS~Oph becomes more blue, as it becomes brighter, however the hot component becomes more red as it becomes brighter (assuming that the red giant is non-variable). During all the runs RS Oph exhibits flickering with amplitude 0.16 - 0.59 mag in B band. For the flickering source we find that it has colour -0.14 < B-V < 0.40, temperature in the range 7200 < T_fl < 18900, and average radius 1.1 < R_fl < 6.7 R_sun. We do not find a correlation between the temperature of the flickering and the brightness. However, we do find a strong correlation (correlation coefficient 0.81, significance 1.1x10^{-7} ) between B band magnitude and the average radius of the flickering source - as the brightness of the system increases the size of the flickering source also increases. The estimated temperature is similar to that of the bright spot of cataclysmic variables. The persistent presence of flickering indicates that the white dwarf is actively accreting material for the next outburst.

The cataclysmic variable AE Aquarii: B–V color of the flares

We report simultaneous observations of the flaring behavior of the cataclysmic variable star AE Aquarii (AE Aqr). The observations are in Johnson B and V bands. The color–magnitude diagrams (B–Vvs. V and B–Vvs. B) show that the star becomes more blue as it becomes brighter. In our model, AE Aqrs behavior can be explained with flares (fireballs) with 0.03⩽B–V⩽0.30, and temperature in the interval 8000⩽T⩽12000.

Simultaneous UBVRI Observations of the AE Aquarii Blobs

We summarize the results of our study of the cataclysmic variable AE Aqr on the basis of simultaneous UBV RI observations. For the flares, we estimated the average risetime of about 300 sec, and colours (U−B) 0 ~1.1 and (B−V ) 0 ~0.1. We also calculated temperatures, sizes, masses and expansion velocities of a few individual fireballs. In a single night (16.08.2012), we detected ~ 8 min quasi-periodic modulation.