V. Fallah Ramazani

Primary Black Hole Spin in OJ 287 as Determined by the General Relativity Centenary Flare

OJ 287 is a quasi-periodic quasar with roughly 12 year optical cycles. It displays prominent outbursts that are predictable in a binary black hole model. The model predicted a major optical outburst in 2015 December. We found that the outburst did occur within the expected time range, peaking on 2015 December 5 at magnitude 12.9 in the optical R-band. Based on Swift/XRT satellite measurements and optical polarization data, we find that it included a major thermal component. Its timing provides an accurate estimate for the spin of the primary black hole,

Gamma-ray and optical oscillations of 0716+714, MRK 421, and BL Lacertae

\chi =0.313\pm 0.01

Optical and radio variability of the northern VHE gamma-ray emitting BL Lacertae objects

. The present outburst also confirms the established general relativistic properties of the system such as the loss of orbital energy to gravitational radiation at the 2% accuracy level, and it opens up the possibility of testing the black hole no-hair theorem with 10% accuracy during the present decade.

Optical polarization of high-energy BL Lacertae objects

We examine the 2008–2016 γ-ray and optical light curves of three bright BL Lac objects, 0716+714, MRK 421, BL Lac, which exhibit large structured variability. We searched for periodicities by using a fully Bayesian approach. For two out of three sources investigated, no significant periodic variability was found. In the case of BL Lac, we detected a periodicity of ~680 days. Although the signal related to this is modest, the coincidence of the periods in both gamma and optical bands is indicative of a physical relevance. Taking into consideration previous literature results, possibly related γ-ray and optical periodicities of about one year time scale are proposed in four bright γ-ray blazars out of the ten examined in detail. Compared with results from periodicity search of optical archives of quasars, the presence of quasi-periodicities in blazars may be more frequent by a large factor. This suggests the intriguing possibility that the basic conditions for their observability are related to the relativistic jet in the observer direction, but the overall picture remains uncertain.Key words: gamma rays: galaxies / BL Lacertae objects: general / galaxies: jets / BL Lacertae objects: individual: PKS 0716+71 / BL Lacertae objects: individual: MRK 421 / BL Lacertae objects: individual: BL Lac

Empirical multi-wavelength prediction method for very high energy gamma-ray emitting BL Lacertae objects

We compare the variability properties of very high energy gamma-ray emitting BL Lac objects in the optical and radio bands. We use the variability information to distinguish multiple emission components in the jet, to be used as a guidance for spectral energy distribution modelling. Our sample includes 32 objects in the Northern sky that have data for at least 2 years in both bands. We use optical R-band data from the Tuorla blazar monitoring program and 15 GHz radio data from the Owens Valley Radio Observatory blazer monitoring program. We estimate the variability amplitudes using the intrinsic modulation index, and study the time-domain connection by cross-correlating the optical and radio light curves assuming power law power spectral density. Our sample objects are in general more variable in the optical than radio. We find correlated flares in about half of the objects, and correlated long-term trends in more than 40% of the objects. In these objects we estimate that at least 10%-50% of the optical emission originates in the same emission region as the radio, while the other half is due to faster variations not seen in the radio. This implies that simple single-zone spectral energy distribution models are not adequate for many of these objects.

A search for QPOs in the blazar OJ287: preliminary results from the 2015/2016 observing campaign

Context. We investigate the optical polarization properties of high-energy BL Lac objects using data from the RoboPol blazar monitoring program and the Nordic Optical Telescope. Aims. We wish to understand if there are differences between the BL Lac objects that have been detected with the current-generation TeV instruments and those objects that have not yet been detected. Methods. We used a maximum-likelihood method to investigate the optical polarization fraction and its variability in these sources. In order to study the polarization position angle variability, we calculated the time derivative of the electric vector position angle (EVPA) change. We also studied the spread in the Stokes Q / I − U / I plane and rotations in the polarization plane. Results. The mean polarization fraction of the TeV-detected BL Lacs is 5%, while the non-TeV sources show a higher mean polarization fraction of 7%. This difference in polarization fraction disappears when the dilution by the unpolarized light of the host galaxy is accounted for. The TeV sources show somewhat lower fractional polarization variability amplitudes than the non-TeV sources. Also the fraction of sources with a smaller spread in the Q / I − U / I plane and a clumped distribution of points away from the origin, possibly indicating a preferred polarization angle, is larger in the TeV than in the non-TeV sources. These differences between TeV and non-TeV samples seem to arise from differences between intermediate and high spectral peaking sources instead of the TeV detection. When the EVPA variations are studied, the rate of EVPA change is similar in both samples. We detect significant EVPA rotations in both TeV and non-TeV sources, showing that rotations can occur in high spectral peaking BL Lac objects when the monitoring cadence is dense enough. Our simulations show that we cannot exclude a random walk origin for these rotations. Conclusions. These results indicate that there are no intrinsic differences in the polarization properties of the TeV-detected and non-TeV-detected high-energy BL Lac objects. This suggests that the polarization properties are not directly related to the TeV-detection, but instead the TeV loudness is connected to the general flaring activity, redshift, and the synchrotron peak location.

EBL constraints using a sample of TeV gamma-ray emitters measured with the MAGIC telescopes

Aim: We have collected the most complete multi-wavelength (6.0−6.0 × 10 -18 cm) dataset of very high energy (VHE) γ -ray emitting (TeV) BL Lacs, which are the most numerous extragalactic VHE sources. Using significant correlations between different bands, we aim to identify the best TeV BL Lac candidates that can be discovered by the current and next generation of imaging air Cherenkov telescopes. Methods: We formed five datasets from lower energy data, i.e. radio, mid-infrared, optical, X-rays, and GeV γ -ray, and five VHE γ -ray datasets to perform a correlation study between different bands and to construct the prediction method. The low energy datasets were averaged for individual sources, while the VHE γ -ray data were divided into subsets according to the flux state of the source. We then looked for significant correlations and determined their best-fit parameters. Using the best-fit parameters we predicted the level of VHE γ -ray flux for a sample of 182 BL Lacs, which have not been detected at TeV energies. We identified the most promising TeV BL Lac candidates based on the predicted VHE γ -ray flux for each source. Results: We found 14 significant correlations between radio, mid-infrared, optical, γ -ray, and VHE γ -ray bands. The correlation between optical and VHE γ -ray luminosity is established for the first time. We attribute this to the more complete sample and more accurate handling of host galaxy flux in our work. We found nine BL Lac candidates whose predicted VHE γ -ray flux is high enough for detection in less than 25 h with current imaging air Cherenkov telescopes.