Multi-wavelength observations of 3C 279 during the extremely bright gamma-ray flare in 2014 March-April

The well studied blazar 3C 279 underwent a giant $\gamma$-ray outburst in 2014 March-April. The measured $\gamma$-ray flux (1.21 $\pm$ 0.10 $\times$ 10$^{-5}$ ph cm$^{-2}$ s$^{-1}$ in 0.1-300 GeV energy range) is the highest detected from 3C 279 by Fermi Large Area Telescope. Hour scale $\gamma$-ray flux variability are observed, with a flux doubling time as short as 1.19 $\pm$ 0.36 hours detected during one flare. The $\gamma$-ray spectrum is found to be curved at peak of the flare suggesting low probability of detecting very high energy (VHE; E $>$ 100 GeV) emission, which is further confirmed by the Very Energetic Radiation Imaging Telescope Array System observations. The $\gamma$-ray flux increased by more than an order in comparison to low activity state and the flare consists of multiple sub-structures having fast rise and slow decay profile. The flux enhancement is seen in all the wavebands though at a lesser extent compared to $\gamma$-rays. During the flare, a considerable amount of the kinetic jet power gets converted to $\gamma$-rays and the jet becomes radiatively efficient. A one zone leptonic emission model is used to reproduce the flare and we find increase in the bulk Lorentz factor as a major cause of the outburst. From the observed fast variability, lack of VHE detection, and the curved $\gamma$-ray spectrum, we conclude that the location of the emission region cannot be far out from the broad line region (BLR) and contributions from both BLR and torus photons are required to explain the observed $\gamma$-ray spectrum.Comment: 42 pages, 8 figures, 7 tables, to appear in the Astrophysical journa

Clear Evidence for Intranight Optical Variability in Radio-quiet Quasars

We present new clues to the problem of the radio loudness dichotomy arising from an extensive search for intranight optical variability in seven sets of optically luminous radio-quiet quasars and (radio-loud) BL Lacertae objects, which are matched in optical luminosity and redshift. Our monitoring of radio-quiet quasars has for the first time clearly detected such intranight variability, with peak-to-peak amplitudes ~1%, occurring with a duty cycle of ~ 1/6. The matched BL Lacs have both higher variability amplitudes and duty cycles when observed in the same fashion. We show that the much less pronounced intranight variability of the radio-quiet quasars relative to BL Lacs can be understood in terms of a modest misalignment of the jets in radio-quiet quasars from the line-of-sight. We thus infer that relativistic particle jets may well also emerge from radio-quiet quasars, but while traversing the short optical-emitting distances, they could be snuffed out, possibly through inverse Compton losses in the nuclear region.Comment: 13 pages, 3 figures, in press in ApJ Letters (20 March 2003

Improved characterisation of intra-night optical variability of prominent AGN classes

The incidence of intra-night optical variability (INOV) is known to to differ significantly among different classes of powerful active galactic nuclei (AGN). A number of statistical methods have been employed in the literature for testing the presence of INOV in the light curves, sometimes leading to discordant results. In this paper we compare the INOV characteristics of six prominent classes of AGN, as evaluated using three commonly used statistical tests, namely the $\chi^2-$test, the modified $C-$test and the $F-$test, which has recently begun to gain popularity. The AGN classes considered are: radio-quiet quasars (RQQs), radio-intermediate quasars (RIQs), lobe-dominated quasars (LDQs), low optical polarization core-dominated quasars (LPCDQs), high optical polarization core-dominated quasars (HPCDQs), and TeV blazars. Our analysis is based on a large body of AGN monitoring data, involving 262 sessions of intra-night monitoring of a total 77 AGN, using 1-2 metre class optical telescopes located in India. In order to compare the usefulness of the statistical tests, we have also subjected them to a `sanity check' by comparing the number of false positives yielded by each test with the corresponding statistical prediction. The present analysis is intended to serve as a benchmark for future INOV studies of AGN of different classes.Comment: 26 pages, 2 tables, Accepted for publication in MNRA

Violent Hard X-ray Variability of Mrk 421 Observed by NuSTAR in 2013 April

The well studied blazar Markarian 421 (Mrk 421, $z$=0.031) was the subject of an intensive multi-wavelength campaign when it flared in 2013 April. The recorded X-ray and very high energy (VHE, E$>$100 GeV) $\gamma$-ray fluxes are the highest ever measured from this object. At the peak of the activity, it was monitored by the hard X-ray focusing telescope {\it Nuclear Spectroscopic Telescope Array} ({\it NuSTAR}) and {\it Swift} X-Ray Telescope (XRT). In this work, we present a detailed variability analysis of {\it NuSTAR} and {\it Swift}-XRT observations of Mrk 421 during this flaring episode. We obtained the shortest flux doubling time of 14.01$\pm$5.03 minutes, which is the shortest hard X-ray (3$-$79 keV) variability ever recorded from Mrk 421 and is on the order of the light crossing time of the black hole's event horizon. A pattern of extremely fast variability events superposed on slowly varying flares is found in most of the {\it NuSTAR} observations. We suggest that these peculiar variability patterns may be explained by magnetic energy dissipation and reconnection in a fast moving compact emission region within the jet. Based on the fast variability, we derive a lower limit on the magnetic field strength of $B \ge 0.73 \delta_1^{-2/3} \, \nu_{19}^{1/3}$~G, where $\delta_1$ is the Doppler factor in units of 10, and $\nu_{19}$ is the characteristic X-ray synchrotron frequency in units of $10^{19}$~Hz.Comment: 23 pages, 5 figures, 2 tables, to appear in the Astrophysical Journa

Multi-epoch intra-night optical monitoring of 8 radio-quiet BL Lac candidates

For a new sample of 8 weak-line-quasars (WLQs) we report a sensitive search in 20 intranight monitoring sessions, for blazar-like optical flux variations on hour-like and longer time scale (day/month/year$-$like). The sample consists exclusively of the WLQs that are not radio$-$loud and have either been classified as `radio-weak probable BL Lac candidates' and/or are known to have exhibited at least one episode of large, blazar$-$like optical variability. Whereas only a hint of intra$-$night variability is seen for two of these WLQs, J104833.5$+$620305.0(z = 0.219) and J133219.6$+$622715.9 (z = 3.15), statistically significant inter$-$night variability at a few per cent level is detected for three of the sources, including the radio-intermediate WLQ J133219.6$+$622715.9 (z = 3.15) and the well known bona$-$fide radio$-$quiet WLQs J121221.5$+$534128.0 (z = 3.10) and WLQ J153259.9$-$003944.1 (z = 4.62). In the rest$-$frame, this variability is intra-day and in the far$-$UV band. On the time scale of a decade, we find for three of the WLQs large brightness changes, amounting to 1.655$\pm$0.009, 0.163$\pm$0.010 and 0.144$\pm$0.018 mag, for J104833.5$+$620305.0, J123743.1$+$630144.9 and J232428.4$+$144324.4, respectively. Whereas the latter two are confirmed radio-quiet WLQs, the extragalactic nature of J104833.5$+$620305.0 remains to be well established, thanks to the absence of any feature(s) in its available optical spectra. The present study forms a part of our ongoing campaign of intranight optical monitoring of radio quiet weak-line quasars, in order to improve the understanding of this enigmatic class of Active Galactic Nuclei and to look among them for a possible tiny, elusive population of radio-quiet BL Lacs.Comment: Accepted to MNRAS. 12 pages, 1 figure, 4 Tabl