We present results from monitoring the multi-waveband flux, linear polarization, and parsec-scale structure of the quasar PKS 1510 - 089, concentrating on eight major {gamma}-ray flares that occurred ...during the interval 2009.0-2009.5. The {gamma}-ray peaks were essentially simultaneous with maxima at optical wavelengths, although the flux ratio of the two wave bands varied by an order of magnitude. The optical polarization vector rotated by 720 deg. during a five-day period encompassing six of these flares. This culminated in a very bright, {approx}1 day, optical and {gamma}-ray flare as a bright knot of emission passed through the highest-intensity, stationary feature (the 'core') seen in 43 GHz Very Long Baseline Array images. The knot continued to propagate down the jet at an apparent speed of 22c and emit strongly at {gamma}-ray energies as a months-long X-ray/radio outburst intensified. We interpret these events as the result of the knot following a spiral path through a mainly toroidal magnetic field pattern in the acceleration and collimation zone of the jet, after which it passes through a standing shock in the 43 GHz core and then continues downstream. In this picture, the rapid {gamma}-ray flares result from scattering of infrared seed photons from a relatively slow sheath of the jet as well as from optical synchrotron radiation in the faster spine. The 2006-2009.7 radio and X-ray flux variations are correlated at very high significance; we conclude that the X-rays are mainly from inverse Compton scattering of infrared seed photons by 20-40 MeV electrons.
Blazars are the most extreme active galactic nuclei. They possess oppositely directed plasma jets emanating at near light speeds from accreting supermassive black holes. According to theoretical ...models, such jets are propelled by magnetic fields twisted by differential rotation of the black hole's accretion disk or inertial-frame-dragging ergosphere. The flow velocity increases outward along the jet in an acceleration and collimation zone containing a coiled magnetic field. Detailed observations of outbursts of electromagnetic radiation, for which blazars are famous, can potentially probe the zone. It has hitherto not been possible to either specify the location of the outbursts or verify the general picture of jet formation. Here we report sequences of high-resolution radio images and optical polarization measurements of the blazar BL Lacertae. The data reveal a bright feature in the jet that causes a double flare of radiation from optical frequencies to TeV gamma-ray energies, as well as a delayed outburst at radio wavelengths. We conclude that the event starts in a region with a helical magnetic field that we identify with the acceleration and collimation zone predicted by the theories. The feature brightens again when it crosses a standing shock wave corresponding to the bright 'core' seen on the images.
Blazars are the most extreme active galactic nuclei. They possess oppositely directed plasma jets emanating at near light speeds from accreting supermassive black holes. According to theoretical ...models, such jets are propelled by magnetic fields twisted by differential rotation of the black hole's accretion disk or inertial-frame-dragging ergosphere. The flow velocity increases outward along the jet in an acceleration and collimation zone containing a coiled magnetic field. Detailed observations of outbursts of electromagnetic radiation, for which blazars are famous, can potentially probe the zone. It has hitherto not been possible to either specify the location of the outbursts or verify the general picture of jet formation. Here we report sequences of high-resolution radio images and optical polarization measurements of the blazar BL Lacertae. The data reveal a bright feature in the jet that causes a double flare of radiation from optical frequencies to TeV -ray energies, as well as a delayed outburst at radio wavelengths. We conclude that the event starts in a region with a helical magnetic field that we identify with the acceleration and collimation zone predicted by the theories. The feature brightens again when it crosses a standing shock wave corresponding to the bright 'core' seen on the images.
We present multi-epoch optical spectra of the \(\gamma\)-ray bright blazar 1156+295 (4C +29.45, Ton 599) obtained with the 4.3~m Lowell Discovery Telescope. During a multi-wavelength outburst in late ...2017, when the \(\gamma\)-ray flux increased to \(2.5\times 10^{-6} \; \rm phot\; cm^{-2}\; s^{-1}\) and the quasar was first detected at energies \(\geq100\) GeV, the flux of the Mg II \(\lambda 2798\) emission line changed, as did that of the Fe emission complex at shorter wavelengths. These emission line fluxes increased along with the highly polarized optical continuum flux, which is presumably synchrotron radiation from the relativistic jet, with a relative time delay of \(\lesssim2\) weeks. This implies that the line-emitting clouds lie near the jet, which points almost directly toward the line of sight. The emission-line radiation from such clouds, which are located outside the canonical accretion-disk related broad-line region, may be a primary source of seed photons that are up-scattered to \(\gamma\)-ray energies by relativistic electrons in the jet.
We present the results of flux density, spectral index, and polarization intra-night monitoring studies of a sample of eight optically bright blazars, carried out by employing several small to ...moderate aperture (0.4\,m to 1.5\,m diameter) telescopes fitted with CCDs and polarimeters located in Europe, India, and Japan. The duty cycle of flux variability for the targets is found to be \(\sim 45\) percent, similar to that reported in earlier studies. The computed two-point spectral indices are found to be between 0.65 to 1.87 for our sample, comprised of low- and intermediate frequency peaked blazars, with one exception; they are also found to be statistically variable for about half the instances where `confirmed' variability is detected in flux density. In the analysis of the spectral evolution of the targets on hourly timescale, a counter-clockwise loop (soft-lagging) is noted in the flux-spectral index plane on two occasions, and in one case a clear spectral flattening with the decreasing flux is observed. In our data set, we also observe a variety of flux-polarization degree variability patterns, including instances with a relatively straightforward anti-correlation, correlation, or counter-clockwise looping. These changes are typically reflected in the flux-polarization angle plane: the anti-correlation between the flux and polarization degree is accompanied by an anti-correlation between the polarization angle and flux, while the counter-clockwise flux-PD looping behaviour is accompanied by a clockwise looping in the flux-polarization angle representation. We discuss our findings in the framework of the internal shock scenario for blazar sources.
We analyze the linear polarization of the relativistic jet in BL Lacertae object OJ~287 as revealed by multi-epoch Very Long Baseline Array (VLBA) images at 43 GHz and monitoring observations at ...optical bands. The electric-vector position angle (EVPA) of the optical polarization matches that at 43 GHz at locations that are often in the compact millimeter-wave "core" or, at other epochs, coincident with a bright, quasi-stationary emission feature \(\sim0.2\)~milliarcsec (\(\sim\)0.9~pc projected on the sky) downstream from the core. This implies that electrons with high enough energies to emit optical synchrotron and \(\gamma\)-ray inverse Compton radiation are accelerated both in the core and at the downstream feature, the latter of which lies \(\geq10\)~pc from the central engine. The polarization vector in the stationary feature is nearly parallel to the jet axis, as expected for a conical standing shock capable of accelerating electrons to GeV energies.
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 \(S^\mathrm{max}_\mathrm{opt}=18.91\pm0.08\) mJy and \(S_\gamma^\mathrm{max} =22.20\pm0.18\times10^{-6}\) 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 \(\sim120\degr\). A knot was ejected from the 43 GHz core on 2016 Feb 25, moving at an apparent speed \(v_\mathrm{app}=20.3c\pm0.8c\). 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 \(r\lesssim2.6\times10^{15}\) 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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