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Register a new userExploring the Nature of the 2016 {gamma}-ray Emission in the Blazar 1749+096
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Recent Fermi-Large Area Telescope (LAT) light curves indicate an active $gamma$-ray state spanning about five months from 2016 June to 2016 October in the BL Lac object 1749+096 (OT 081). During this period, we find two notable $gamma$-ray events: an exceptionally strong outburst followed by a significant enhancement (local peak). In this study, we analyze multi-waveband light curves (radio, optical, X-ray, and $gamma$-ray) plus very-long baseline interferometry (VLBI) data to investigate the nature of the $gamma$-ray events. The $gamma$-ray outburst coincides with flux maxima at longer wavelengths. We find a spectral hardening of the $gamma$-ray photon index during the $gamma$-ray outburst. The photon index shows a transition from a softer-when-brighter to a harder-when-brighter trend at around 1.8 $times$ $10^{-7}$ ph cm$^{-2}$ s$^{-1}$. We see indication that both the $gamma$-ray outburst and the subsequent enhancement precede the propagation of a polarized knot in a region near the VLBI core. The highest polarized intensity, 230,mJy, and an electric vector position angle rotation, by $sim$32$^{circ}$, are detected about 12 days after the $gamma$-ray outburst. We conclude that both $gamma$-ray events are caused by the propagation of a disturbance in the mm-wave core.
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We report on the variation in the optical polarization of the blazar PKS 1749+096 observed in 2008--2015. The degree of polarization (PD) tends to increase in short flares having a time-scale of a few days. The object favors a polarization angle (PA) of $40^circ$--$50^circ$ at the flare maxima, which is close to the position angle of the jet ($20^circ$--$40^circ$). Three clear polarization rotations were detected in the negative PA direction associated with flares. In addition, a rapid and large decrease in the PA was observed in the other two flares, while another two flares showed no large PA variation. The light curve maxima of the flares possibly tend to lag behind the PD maxima and color-index minima. The PA became $-50^circ$ to $-20^circ$ in the decay phase of active states, which is almost perpendicular to the jet position angle. We propose a scenario to explain these observational features, where transverse shocks propagate along curved trajectories. The favored PA at the flare maxima suggests that the observed variations were governed by the variations in the Doppler factor, $delta$. Based on this scenario, the minimum viewing angle of the source, $theta_mathrm{min}=4.8^circ$--$6.6^circ$, and the location of the source, $Delta rgtrsim 0.1$pc, from the central black hole were estimated. In addition, the acceleration of electrons by the shock and synchrotron cooling would have a time-scale similar to that of the change in $delta$. The combined effect of the variation in $delta$ and acceleration/cooling of electrons is probably responsible for the observed diversity of the polarization variations in the flares.
PKS 1749+096 is a BL Lac object showing weak extended jet emission to the northeast of the compact VLBI core on parsec scales. We aim at better understanding the jet kinematics and variability of this source and finding clues that may applicable to other BL Lac objects. The jet was studied with multi-epoch multi-frequency high-resolution VLBI observations. The jet is characterized by a one-sided curved morphology at all epochs and all frequencies. The VLBI core, located at the southern end of the jet, was identified based on its spectral properties. The equipartition magnetic field of the core was investigated, through which we derived a Doppler factor of 5, largely consistent with that derived from kinematics (component C5). The study of the detailed jet kinematics at 22 and 15 GHz, spanning a period of more than 10 years, indicates the possible existence of a bimodal distribution of the jet apparent speed. Ballistic and non-ballistic components are found to coexist in the jet. Superluminal motions in the range of 5-21 c were measured in 11 distinct components. We estimated the physical jet parameters with the minimum Lorentz factor of 10.2 and Doppler factors in the range of 10.2-20.4 (component C5). The coincidence in time of the components ejection and flares supports the idea that, at least in PKS 1749+096, ejection of new jet components is connected with major outbursts in flux density. For the best-traced component (C5) we found that the flux density decays rapidly as it travels downstream the jet, accompanied by a steepening of its spectra, which argues in favor of a contribution of inverse Compton cooling. These properties make PKS 1749+096 a suitable target for an intensive monitoring to decipher the variability phenomenon of BL Lac objects.
We report results of a multiband monitoring campaign of the flat spectrum radio quasar TXS 0536+145 at redshift 2.69. This source was detected during a very high gamma-ray activity state in 2012 March by the Large Area Telescope on board Fermi, becoming the gamma-ray flaring blazar at the highest redshift detected so far. At the peak of the flare the source reached an apparent isotropic gamma-ray luminosity of 6.6 x 10^49 erg/s which is comparable to the values achieved by the most luminous blazars. This activity triggered radio-to-X-rays monitoring observations by Swift, Very Long Baseline Array, European VLBI Network, and Medicina single-dish telescope. Significant variability was observed from radio to X-rays supporting the identification of the gamma-ray source with TXS 0536+145. Both the radio and gamma-ray light curves show a similar behaviour, with the gamma-rays leading the radio variability with a time lag of about 4-6 months. The luminosity increase is associated with a flattening of the radio spectrum. No new superluminal component associated with the flare was detected in high resolution parsec-scale radio images. During the flare the gamma-ray spectrum seems to deviate from a power law, showing a curvature that was not present during the average activity state. The gamma-ray properties of TXS 0536+145 are consistent with those shown by the high-redshift gamma-ray blazar population.
A comparison of AGN detected at gamma ray energies by EGRET with flat-spectrum radio sources observed in surveys for intraday variability reveals that a remarkably high fraction of EGRET blazars show significant interstellar scintillation at centimetre wavelengths. Scintillating AGN will therefore be targets of interest for GLAST, scheduled for launch in early 2008. We suggest that the variable, scintillating flat-spectrum radio source PMN J1326-5256 is associated with the unidentified EGRET source 3EG J1316-5244. We describe the properties of PMN J1326-5256 and present recent results of monitoring with the ATCA and Ceduna radio telescopes.
The quasar 3C454.3 underwent a uniquely-structured multi-frequency outburst in June 2016. The blazar was observed in the optical $R$ band by several ground-based telescopes in photometric and polarimetric modes, at $gamma$-ray frequencies by the emph{Fermi} Large Area Telescope, and at 43 GHz with the Very Long Baseline Array. The maximum flux density was observed on 2016 June 24 at both optical and $gamma$-ray frequencies, reaching $S^mathrm{max}_mathrm{opt}=18.91pm0.08$ mJy and $S_gamma^mathrm{max} =22.20pm0.18times10^{-6}$ ph cm$^{-2}$ s$^{-1}$, respectively. The June 2016 outburst possessed a precipitous decay at both $gamma$-ray and optical frequencies, with the source decreasing in flux density by a factor of 4 over a 24-hour period in $R$ band. Intraday variability was observed throughout the outburst, with flux density changes between 1 and 5 mJy over the course of a night. The precipitous decay featured statistically significant quasi-periodic micro-variability oscillations with an amplitude of $sim 2$-$3%$ about the mean trend and a characteristic period of 36 minutes. The optical degree of polarization jumped from $sim3%$ to nearly 20% during the outburst, while the position angle varied by $sim120degr$. A knot was ejected from the 43 GHz core on 2016 Feb 25, moving at an apparent speed $v_mathrm{app}=20.3cpm0.8c$. From the observed minimum timescale of variability $tau_mathrm{opt}^mathrm{min}approx2$ hr and derived Doppler factor $delta=22.6$, we find a size of the emission region $rlesssim2.6times10^{15}$ cm. If the quasi-periodic micro-variability oscillations are caused by periodic variations of the Doppler factor of emission from a turbulent vortex, we derive a rotational speed of the vortex $sim0.2c$.
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