arXiv subjects

R. Bachev

Publications and source records attributed to R. Bachev.

At least 37 records · Page 2Linked to original sources

arXiv2017

Multi-Band Intra-Night Optical Variability of BL Lacertae

We monitored BL Lacertae frequently during 2014 - 2016 when it was generally in a high state. We searched for intra-day variability for 43 nights using quasi-simultaneous measurements in the B, V, R, and I bands (totaling 143 light curves); the typical sampling interval was about eight minutes. On hour-like timescales, BL Lac exhibited significant variations during 13 nights in various optical bands. Significant spectral variations are seen during most of these nights such that the optical spectrum becomes bluer when brighter. The amplitude of variability is usually greater for longer observations but is lower when BL Lac is brighter. No evidence for periodicities or characteristic variability time-scales in the light curves was found. The color variations are mildly chromatic on long timescales.

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arXiv2017

Multi-wavelength temporal and spectral variability of the blazar OJ 287 during and after the December 2015 flare: a major accretion disc contribution

We present a multi-wavelength spectral and temporal analysis of the blazar OJ 287 during its recent activity between December 2015 -- May 2016, showing strong variability in the near-infrared (NIR) to X-ray energies with detection at $γ$-ray energies as well. Most of the optical flux variations exhibit strong changes in polarization angle and degree. All the inter-band time lags are consistent with simultaneous emissions. Interestingly, on days with excellent data coverage in the NIR--UV bands, the spectral energy distributions (SEDs) show signatures of bumps in the visible--UV bands, never seen before in this source. The optical bump can be explained as accretion-disk emission associated with the primary black hole of mass $\sim \rm 1.8 \times10^{10} M_{\odot}$ while the little bump feature in the optical-UV appears consistent with line emission. Further, the broadband SEDs extracted during the first flare and during a quiescent period during this span show very different $γ$-ray spectra compared to previously observed flare or quiescent spectra. The probable thermal bump in the visible seems to have been clearly present since May 2013, as found by examining all available NIR-optical observations, and favors the binary super-massive black hole model. The simultaneous multi-wavelength variability and relatively weak $γ$-ray emission that shows a shift in the SED peak is consistent with $γ$-ray emission originating from inverse Compton scattering of photons from the line emission that apparently contributes to the little blue bump.

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arXiv2017

A peculiar multi-wavelength flare in the Blazar 3C 454.3

The blazar 3C454.3 exhibited a strong flare seen in gamma-rays, X-rays, and optical/NIR bands during 3--12 December 2009. Emission in the V and J bands rose more gradually than did the gamma-rays and soft X-rays, though all peaked at nearly the same time. Optical polarization measurements showed dramatic changes during the flare, with a strong anti-correlation between optical flux and degree of polarization (which rose from ~ 3% to ~ 20%) during the declining phase of the flare. The flare was accompanied by large rapid swings in polarization angle of ~ 170 degree. This combination of behaviors appear to be unique. We have cm-band radio data during the same period but they show no correlation with variations at higher frequencies. Such peculiar behavior may be explained using jet models incorporating fully relativistic effects with a dominant source region moving along a helical path or by a shock-in-jet model incorporating three-dimensional radiation transfer if there is a dominant helical magnetic field. We find that spectral energy distributions at different times during the flare can be fit using modified one-zone models where only the magnetic field strength and particle break frequencies and normalizations need change. An optical spectrum taken at nearly the same time provides an estimate for the central black hole mass of ~ 2.3 * 10^9 M_sun. We also consider two weaker flares seen during the $\sim 200$ d span over which multi-band data are available. In one of them, the V and J bands appear to lead the $γ$-ray and X-ray bands by a few days; in the other, all variations are simultaneous.

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arXiv2017

Intra-night variability of the blazar CTA 102 during its 2012 and 2016 giant outbursts

We obtained and analyzed more than 100 hours of multicolour optical time series of the blazar CTA 102 during its 2012 and 2016 outbursts. The object reached almost 11-th mag at the end of 2016, which is perhaps the brightest blazar state ever observed! During both outbursts, CTA 102 showed significant and rapid variability on intra-night time scales, reaching up to 0.2 mag for 30 min on some occasions. The "rms-flux" relation, built for all datasets, shows a large scatter and no apparent saturation on the magnitude scale. The ensemble structure function of the light curves can be fitted well with a straight line of a slope of $\sim$0.4. The time lags between the different optical bands appear to be consistent with zero, taking into account our time resolution. We discuss different variability scenarios and favor the changing Doppler factor of the emitting blobs as the most plausible one to account for the observed intra-night variability.

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arXiv2017

Discovery of optical flickering from the symbiotic star EF Aquilae

We report optical CCD photometry of the recently identified symbiotic star EF Aql. Our observations in 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 11-th 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 per cent of the wind is captured by the WD. Eight further candidates for the detection of flickering in similar systems are suggested.

astro-ph.SR↗

arXiv2017

Gamma-ray and Optical Oscillations of 0716+714, MRK 421, and BL Lac

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. Considering previous literature results, possibly related $γ$-ray and optical periodicities of about one year time scale are proposed in 4 bright $γ$-ray blazars out of the 10 examined in detail. Comparing with results from periodicity search of optical archives of quasars, the presence of quasi-periodicities in blazars might 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.

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arXiv2016

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 September 2015 -- May 2016. Our intense observations of the blazar started in November 2015 and continued until May 2016 and included detection of the large optical outburst in December 2016 that was predicted using the binary black hole model for OJ 287. For our observing campaign, we used a total of 9 ground based optical telescopes of which one is in Japan, one is in India, three are in Bulgaria, one is in Serbia, one is in Georgia, and two are in the USA. These observations were carried out in 102 nights with a total of ~ 1000 image frames in BVRI bands, though the majority were in the R band. We detected a second comparably strong flare in March 2016. In addition, we investigated multi-band flux variations, colour variations, and spectral changes in the blazar on diverse timescales as they are useful in understanding the emission mechanisms. We briefly discuss the possible physical mechanisms most likely responsible for the observed flux, colour and spectral variability.

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arXiv2016

Multifrequency Photo-polarimetric WEBT Observation Campaign on the Blazar S5 0716+714: Source Microvariability and Search for Characteristic Timescales

Here we report on the results of the WEBT photo-polarimetric campaign targeting the blazar S5~0716+71, organized in March 2014 to monitor the source simultaneously in BVRI and near IR filters. The campaign resulted in an unprecedented dataset spanning $\sim 110$\,h of nearly continuous, multi-band observations, including two sets of densely sampled polarimetric data mainly in R filter. During the campaign, the source displayed pronounced variability with peak-to-peak variations of about $30\%$ and "bluer-when-brighter" spectral evolution, consisting of a day-timescale modulation with superimposed hourlong microflares characterized by $\sim 0.1$\,mag flux changes. We performed an in-depth search for quasi-periodicities in the source light curve; hints for the presence of oscillations on timescales of $\sim 3$\,h and $\sim 5$\,h do not represent highly significant departures from a pure red-noise power spectrum. We observed that, at a certain configuration of the optical polarization angle relative to the positional angle of the innermost radio jet in the source, changes in the polarization degree led the total flux variability by about 2\,h; meanwhile, when the relative configuration of the polarization and jet angles altered, no such lag could be noted. The microflaring events, when analyzed as separate pulse emission components, were found to be characterized by a very high polarization degree ($> 30\%$) and polarization angles which differed substantially from the polarization angle of the underlying background component, or from the radio jet positional angle. We discuss the results in the general context of blazar emission and energy dissipation models.

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arXiv2016

Exceptional outburst of the blazar CTA 102 in 2012: The GASP-WEBT campaign and its extension

After several years of quiescence, the blazar CTA 102 underwent an exceptional outburst in 2012 September-October. The flare was tracked from gamma-ray to near-infrared frequencies, including Fermi and Swift data as well as photometric and polarimetric data from several observatories. An intensive GASP-WEBT collaboration campaign in optical and NIR bands, with an addition of previously unpublished archival data and extension through fall 2015, allows comparison of this outburst with the previous activity period of this blazar in 2004-2005. We find remarkable similarity between the optical and gamma-ray behaviour of CTA 102 during the outburst, with a time lag between the two light curves of ~1 hour, indicative of co-spatiality of the optical and gamma-ray emission regions. The relation between the gamma-ray and optical fluxes is consistent with the SSC mechanism, with a quadratic dependence of the SSC gamma-ray flux on the synchrotron optical flux evident in the post-outburst stage. However, the gamma-ray/optical relationship is linear during the outburst; we attribute this to changes in the Doppler factor. A strong harder-when-brighter spectral dependence is seen both the in gamma-ray and optical non-thermal emission. This hardening can be explained by convexity of the UV-NIR spectrum that moves to higher frequencies owing to an increased Doppler shift as the viewing angle decreases during the outburst stage. The overall pattern of Stokes parameter variations agrees with a model of a radiating blob or shock wave that moves along a helical path down the jet.

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arXiv2016

Multi-band optical variability of three TeV Blazars on Diverse Timescales

We present our optical photometric observations of three TeV blazars, PKS 1510-089, PG 1553+113 and Mrk 501 taken using two telescopes in India, one in Bulgaria, one in Greece and one in Serbia during 2012 - 2014. These observations covered a total of 95 nights with a total of 202 B filter frames, 247 images in V band, 817 in R band while 229 images were taken in the I filter. This work is focused on multi-band flux and colour variability studies of these blazars on diverse timescales which are useful in understanding the emission mechanisms. We studied the variability characteristics of above three blazars and found all to be active over our entire observational campaigns. We also searched for any correlation between the brightness of the sources and their colour indices. During the times of variability, no significant evidence for the sources to display spectral changes correlated with magnitude was found on timescales of a few months. We briefly discuss the possible physical mechanisms most likely responsible for the observed flux variability.

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arXiv2015

Kepler light curve analysis of the blazar W2R 1926+42

We study the long term Kepler light curve of the blazar W2R 1926+42 ($\sim$ 1.6 years) which indicates a variety of variability properties during different intervals of observation. The normalized excess variance, $F_{\rm var}$ ranges from 1.8 % in the quiescent phase and 43.3 % in the outburst phase. We find no significant deviation from linearity in the $F_{\rm var}$-flux relation. Time series analysis is conducted using the Fourier power spectrum and the wavelet analysis methods to study the power spectral density (PSD) shape, infer characteristic timescales and statistically significant quasi-periodic oscillations (QPOs). A bending power law with an associated timescale of $T_B = 6.2^{+6.4}_{-3.1}$ hours is inferred in the PSD analysis. We obtain a black hole mass of $M_\bullet = (1.5 - 5.9) \times 10^7 M_\odot$ for the first time using $F_{\rm var}$ and the bend timescale for this source. From a mean outburst lifetime of days, we infer a distance from the jet base $r \leq 1.75$ pc indicating that the outburst originates due to a shock. A possible QPO peaked at 9.1 days and lasting 3.4 cycles is inferred from the wavelet analysis. Assuming that the QPO is a true feature, $r = (152 - 378)~ G M_\bullet/c^2$ and supported by the other timing analysis products such as a weighted mean PSD slope of $-1.5 \pm 0.2$ from the PSD analysis, we argue that the observed variability and the weak and short duration QPO could be due to jet based processes including orbital features in a relativistic helical jet and others such as shocks and turbulence.

astro-ph.HE↗

arXiv2015

Multi-band optical variability of the Blazar S5 0716+714 in outburst state during 2014-2015

We analyzed the multi-band optical behaviour of the BL Lacertae object, S5 0716+714, during its outburst state from 2014 November - 2015 March. We took data on 23 nights at three observatories, one in India and two in Bulgaria, making quasi-simultaneous observations in B, V, R, and I bands. We measured multi-band optical fluxes, colour and spectral variations for this blazar on intraday and short timescales. The source was in a flaring state during the period analyzed and displayed intense variability in all wavelengths. R band magnitude of 11.6 was attained by the target on 18 Jan 2015, which is the brightest value ever recorded for S5 0716+714. The discrete correlation function method yielded good correlation between the bands with no measurable time lags, implying that radiation in these bands originate from the same region and by the same mechanism. We also used the structure function technique to look for characteristic timescales in the light curves. During the times of rapid variability, no evidence for the source to display spectral changes with magnitude was found on either of the timescales. The amplitude of variations tends to increase with increasing frequency with a maximum of $\sim$ 22% seen during flaring states in B band. A mild trend of larger variability amplitude as the source brightens was also found. We found the duty cycle of our source during the analyzed period to be $\sim$ 90%. We also investigated the optical spectral energy distribution of S5 0716+714 using B, V, R, and I data points for 21 nights. We briefly discuss physical mechanisms most likely responsible for its flux and spectral variations.

astro-ph.HE↗

arXiv2015

Optical and Radio Variability of BL Lacertae

We observed the prototype blazar, BL Lacertae, extensively in optical and radio bands during an active phase in the period 2010--2013 when the source showed several prominent outbursts. We searched for possible correlations and time lags between the optical and radio band flux variations using multifrequency data to learn about the mechanisms producing variability. During an active phase of BL Lacertae, we searched for possible correlations and time lags between multifrequency light curves of several optical and radio bands. We tried to estimate any possible variability timescales and inter-band lags in these bands. We performed optical observations in B, V, R and I bands from seven telescopes in Bulgaria, Georgia, Greece and India and obtained radio data at 36.8, 22.2, 14.5, 8 and 4.8 GHz frequencies from three telescopes in Ukraine, Finland and USA. Significant cross-correlations between optical and radio bands are found in our observations with a delay of cm-fluxes with respect to optical ones of ~250 days. The optical and radio light curves do not show any significant timescales of variability. BL Lacertae showed many optical 'mini-flares' on short time-scales. Variations on longer term timescales are mildly chromatic with superposition of many strong optical outbursts. In radio bands, the amplitude of variability is frequency dependent. Flux variations at higher radio frequencies lead the lower frequencies by days or weeks. The optical variations are consistent with being dominated by a geometric scenario where a region of emitting plasma moves along a helical path in a relativistic jet. The frequency dependence of the variability amplitude supports an origin of the observed variations intrinsic to the source.

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arXiv2015

Nature of Intra-night Optical Variability of BL Lacertae

We present the results of extensive multi-band intra-night optical monitoring of BL Lacertae during 2010--2012. BL Lacertae was very active in this period and showed intense variability in almost all wavelengths. We extensively observed it for a total for 38 nights; on 26 of them observations were done quasi-simultaneously in B, V, R and I bands (totaling 113 light curves), with an average sampling interval of around 8 minutes. BL Lacertae showed significant variations on hour-like timescales in a total of 19 nights in different optical bands. We did not find any evidence for periodicities or characteristic variability time-scales in the light curves. The intranight variability amplitude is generally greater at higher frequencies and decreases as the source flux increases. We found spectral variations in BL Lacertae in the sense that the optical spectrum becomes flatter as the flux increases but in several flaring states deviates from the linear trend suggesting different jet components contributing to the emission at different times.

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arXiv2015

Multi-band optical-NIR variability of blazars on diverse timescales

To search for optical variability on a wide range of timescales, we have carried out photometric monitoring of two flat spectrum radio quasars, 3C 454.3 and 3C 279, plus one BL Lac, S5 0716+714, all of which have been exhibiting remarkably high activity and pronounced variability at all wavelengths. CCD magnitudes in B, V, R and I pass-bands were determined for $\sim$ 7000 new optical observations from 114 nights made during 2011 - 2014, with an average length of $\sim$ 4 h each, at seven optical telescopes: four in Bulgaria, one in Greece, and two in India. We measured multiband optical flux and colour variations on diverse timescales. Discrete correlation functions were computed among B, V, R, and I observations, to search for any time delays. We found weak correlations in some cases with no significant time lags. The structure function method was used to estimate any characteristic time-scales of variability. We also investigated the spectral energy distribution of the three blazars using B, V, R, I, J and K pass-band data. We found that the sources almost always follows a bluer-when-brighter trend. We discuss possible physical causes of the observed spectral variability.

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arXiv2015

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 a correlation between the peak-to-peak flickering amplitude ($ΔF$) and the average flux of the hot component ($F_{\rm av}$). The correlation is highly significant, with a correlation coefficient of 0.85 and a $p$-value of~$\sim 10^{-20}$. Combining the data from all wavebands, we find a dependence of the type $ΔF \propto F^k_{\rm av}$, with power-law index $k = 1.02 \pm 0.04$ for the $UBVRI$ flickering of RS~Oph. Thus, the relationship between the amplitude of variability and the average flux of the hot component is consistent with linearity. The rms amplitude of flickering is on average 8 per cent ($\pm2$ per cent) of $F_{\rm av}$. The detected correlation is similar to that found in accreting black holes/neutron stars and cataclysmic variables. The possible reasons are briefly discussed. The data are available upon request from the authors.

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arXiv2014

UV spectral diagnostics for low redshift quasars: estimating physical conditions and radius of the Broad Line Region

The UV spectral range (1100 - 3000 A) contains the strongest resonance lines observed in active galactic nuclei (AGN). Analysis of UV line intensity ratios and profile shapes in quasar spectra provide diagnostics of physical and dynamical conditions in the broad line emitting region. This paper discusses properties of UV lines in type-1 AGN spectra, and how they lead an estimate of ionizing photon flux, chemical abundances, radius of the broad line emitting region and central black hole mass. These estimates are meaningfully contextualised through the 4D "eigenvector-1" (4DE1) formalism.

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arXiv2014

Photometric reverberation mapping of Markarian 279

By using standard broad-band VRI photometry we were able to discriminate the variations of the broad hydrogen alpha line from the continuum variations for the active galaxy Mkn 279. Cross-correlating both light curves enabled us to determine the time lag of the broad line variations behind the continuum and thus to determine the BLR size (about 8 light days). Our preliminary results are rather consistent with the spectroscopic reverberation mapping results (about 12/17 days). This study is a part of an ambitious program to perform photometric reverberation mapping and determine BLR sizes (respectively - the central black hole masses) for more that 100 nearby AGN.

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