Through The Ring Of Fire: $gamma$-Ray Variability In Blazars By A Moving Plasmoid Passing A Local Source Of Seed Photons

Blazars exhibit flares across the electromagnetic spectrum. Many $gamma$-ray flares are highly correlated with flares detected at optical wavelengths; however, a small subset appears to occur in isolation, with little or no variability detected at longer wavelengths. These orphan $gamma$-ray flares challenge current models of blazar variability, most of which are unable to reproduce this type of behavior. We present numerical calculations of the time-variable emission of a blazar based on a proposal by Marscher et al. (2010) to explain such events. In this model, a plasmoid (blob) propagates relativistically along the spine of a blazar jet and passes through a synchrotron-emitting ring of electrons representing a shocked portion of the jet sheath. This ring supplies a source of seed photons that are inverse-Compton scattered by the electrons in the moving blob. The model includes the effects of radiative cooling, a spatially varying magnetic field, and acceleration of the blobs bulk velocity. Synthetic light curves produced by our model are compared to the observed light curves from an orphan flare that was coincident with the passage of a superluminal knot through the inner jet of the blazar PKS 1510$-$089. In addition, we present Very Long Baseline Array polarimetric observations that point to the existence of a jet sheath in PKS 1510$-$089, thus providing further observational support for the plausibility of our model. An estimate of the bolometric luminosity of the sheath within PKS 1510$-$089 is made, yielding $L_{rm sh} sim 3 times 10^{45} ~ rm erg ~ rm s^{-1}$. This indicates that the sheath within PKS 1510$-$089 is potentially a very important source of seed photons.

Comprehensive Monitoring of Gamma-ray Bright Blazars. I. Statistical Study of Optical, X-ray, and Gamma-ray Spectral Slopes

We present $gamma$-ray, X-ray, ultraviolet, optical, and near-infrared light curves of 33 $gamma$-ray bright blazars over four years that we have been monitoring since 2008 August with multiple optical, ground-based telescopes and the Swift satellite, and augmented by data from the Fermi Gamma-ray Space Telescope and other publicly available data from Swift. The sample consists of 21 flat-spectrum radio quasars (FSRQs) and 12 BL Lac objects (BL Lacs). We identify quiescent and active states of the sources based on their $gamma$-ray behavior. We derive $gamma$-ray, X-ray, and optical spectral indices, $alpha_gamma$, $alpha_X$, and $alpha_o$, respectively ($F_ upropto u^alpha$), and construct spectral energy distributions (SEDs) during quiescent and active states. We analyze the relationships between different spectral indices, blazar classes, and activity states. We find (i) significantly steeper $gamma$-ray spectra of FSRQs than for BL Lacs during quiescent states, but a flattening of the spectra for FSRQs during active states while the BL Lacs show no significant change; (ii) a small difference of $alpha_X$ within each class between states, with BL Lac X-ray spectra significantly steeper than in FSRQs; (iii) a highly peaked distribution of X-ray spectral slopes of FSRQs at $sim-$0.60, but a very broad distribution of $alpha_X$ of BL Lacs during active states; (iv) flattening of the optical spectra of FSRQs during quiescent states, but no statistically significant change of $alpha_o$ of BL Lacs between states; and (v) a positive correlation between optical and $gamma$-ray spectral slopes of BL Lacs, with similar values of the slopes. We discuss the findings with respect to the relative prominence of different components of high-energy and optical emission as the flux state changes.

A Tight Connection between Gamma-Ray Outbursts and Parsec-Scale Jet Activity in the Quasar 3C 454.3

We analyze the multifrequency behavior of the quasar 3C 454.3 during three prominent gamma-ray outbursts: 2009 Autumn, 2010 Spring, and 2010 Autumn. The data reveal a repeating pattern, including a triple flare structure, in the properties of each gamma-ray outburst, which implies similar mechanism(s) and location for all three events. The multi-frequency behavior indicates that the lower frequency events are co-spatial with the gamma-ray outbursts, although the gamma-ray emission varies on the shortest timescales. We determine that the variability from UV to IR wavelengths during an outburst results from a single synchrotron component whose properties do not change significantly over the different outbursts. Despite a general increase in the degree of optical linear polarization during an outburst, the polarization drops significantly at the peak of the gamma-ray event, which suggests that both shocks and turbulent processes are involved. We detect two disturbances (knots) with superluminal apparent speeds in the parsec-scale jet associated with the outbursts in 2009 Autumn and 2010 Autumn. The kinematic properties of the knots can explain the difference in amplitudes of the gamma-ray events, while their millimeter-wave polarization is related to the optical polarization during the outbursts. We interpret the multi-frequency behavior within models involving either a system of standing conical shocks or magnetic reconnection events located in the parsec-scale millimeter-wave core of the jet. We argue that gamma-ray outbursts with variability timescales as short as ~ 3 hr can occur on parsec scales if flares take place in localized regions such as turbulent cells.

An X-Ray View of the Jet-Cycle in the Radio Loud AGN 3C120

We present a study of the central engine in the broad-line radio galaxy 3C120 using a multi-epoch analysis of a deep XMM-Newton observation and two deep Suzaku pointings (in 2012). In order to place our spectral data into the context of the disk-disruption/jet-ejection cycles displayed by this object, we monitor the source in the UV/X-ray bands, and in the radio band. We find three statistically acceptable spectral models, a disk-reflection model, a jet-model and a jet+disk model. Despite being good descriptions of the data, the disk-reflection model violates the radio constraints on the inclination, and the jet-model has a fine-tuning problem, requiring a jet contribution exceeding that expected. Thus, we argue for a composite jet+disk model. Within the context of this model, we verify the basic predictions of the jet-cycle paradigm, finding a truncated/refilling disk during the Suzaku observations and a complete disk extending down to the innermost stable circular orbit (ISCO) during the XMM-Newton observation. The idea of a refilling disk is further supported by the detection of the ejection of a new jet knot approximately one month after the Suzaku pointings. We also discover a step-like event in one of the Suzaku pointings in which the soft band lags the hard band. We suggest that we are witnessing the propagation of a disturbance from the disk into the jet on a timescale set by the magnetic field.

Locating the Gamma-ray Flaring Emission of Blazar AO 0235+164 in the Jet at Parsec Scales Through Multi Spectral Range Monitoring

We present observations of a major outburst at centimeter, millimeter, optical, X-ray, and gamma-ray wavelengths of the BL Lacertae object AO 0235+164 in 2008. We analyze the timing of multi-waveband variations in the flux and linear polarization, as well as changes in Very Long Baseline Array (VLBA) images at 7mm with ~0.15 milliarcsecond resolution. The association of the events at different wavebands is confirmed at high statistical significance by probability arguments and Monte-Carlo simulations. A series of sharp peaks in optical linear polarization, as well as a pronounced maximum in the 7mm polarization of a superluminal jet knot, indicate rapid fluctuations in the degree of ordering of the magnetic field. These results lead us to conclude that the outburst occurred in the jet both in the quasi-stationary core and in the superluminal knot, both at >12 parsecs downstream of the supermassive black hole. We interpret the outburst as a consequence of the propagation of a disturbance, elongated along the line of sight by light-travel time delays, that passes through a standing recollimation shock in the core and propagates down the jet to create the superluminal knot. The multi-wavelength light curves vary together on long time-scales (months/years), but the correspondence is poorer on shorter time-scales. This, as well as the variability of the polarization and the dual location of the outburst, agrees with the expectations of a multi-zone emission model in which turbulence plays a major role in modulating the synchrotron and inverse Compton fluxes.

Multiwavelength Variations of 3C 454.3 during the November 2010 to January 2011 Outburst

We present multiwavelength data of the blazar 3C 454.3 obtained during an extremely bright outburst from November 2010 through January 2011. These include flux density measurements with the Herschel Space Observatory at five submillimeter-wave and far-infrared bands, the Fermi Large Area Telescope at gamma-ray energies, Swift at X-ray, ultraviolet (UV), and optical frequencies, and the Submillimeter Array at 1.3 mm. From this dataset, we form a series of 52 spectral energy distributions (SEDs) spanning nearly two months that are unprecedented in time coverage and breadth of frequency. Discrete correlation anlaysis of the millimeter, far-infrared, and gamma-ray light curves show that the variations were essentially simultaneous, indicative of co-spatiality of the emission, at these wavebands. In contrast, differences in short-term fluctuations at various wavelengths imply the presence of inhomegeneities in physical conditions across the source. We locate the site of the outburst in the parsec-scale core, whose flux density as measured on 7 mm Very Long Baseline Array images increased by 70 percent during the first five weeks of the outburst. Based on these considerations and guided by the SEDs, we propose a model in which turbulent plasma crosses a conical standing shock in the parsec-scale region of the jet. Here, the high-energy emission in the model is produced by inverse Compton scattering of seed photons supplied by either nonthermal radiation from a Mach disk, thermal emission from hot dust, or (for X-rays) synchrotron radiation from plasma that crosses the standing shock. For the two dates on which we fitted the model SED to the data, the model corresponds very well to the observations at all bands except at X-ray energies, where the spectrum is flatter than observed.

Location of the Gamma-Ray Flaring Emission in the Parse-Scale Jet of the BL Lac Object AO 0235+164

We locate the gamma-ray and lower frequency emission in flares of the BL Lac object AO 0235+164 at >12pc in the jet of the source from the central engine. We employ time-dependent multi-spectral-range flux and linear polarization monitoring observations, as well as ultra-high resolution (~0.15 milliarcsecond) imaging of the jet structure at lambda=7mm. The time coincidence in the end of 2008 of the propagation of the brightest superluminal feature detected in AO 0235+164 (Qs) with an extreme multi-spectral-range (gamma-ray to radio) outburst, and an extremely high optical and 7mm (for Qs) polarization degree provides strong evidence supporting that all these events are related. This is confirmed at high significance by probability arguments and Monte-Carlo simulations. These simulations show the unambiguous correlation of the gamma-ray flaring state in the end of 2008 with those in the optical, millimeter, and radio regime, as well as the connection of a prominent X-ray flare in October 2008, and of a series of optical linear polarization peaks, with the set of events in the end of 2008. The observations are interpreted as the propagation of an extended moving perturbation through a re-collimation structure at the end of the jets acceleration and collimation zone.

Gamma-ray emission region located in the parsec scale jet of OJ287

We report on the location of the gamma-ray emission region in flares of the BL Lacertae object OJ287 at >14pc from the central supermassive black hole. We employ data from multi-spectral range (total flux and linear polarization) monitoring programs combined with sequences of ultra-high-resolution 7mm VLBA images. The correlation between the brightest gamma-ray and mm flares is found to be statistically significant. The two gamma-ray peaks, detected by Fermi-LAT, that we report here happened at the rising phase of two exceptionally bright mm flares accompanied by sharp linear polarization peaks. The VLBA images show that these mm flares in total flux and polarization degree occurred in a jet region at >14pc from the innermost jet region. The time coincidence of the brighter gamma-ray flare and its corresponding mm linear polarization peak evidences that both the gamma-ray and mm outbursts occur >14pc from the central black hole. We find two sharp optical flares occurring at the peak times of the two reported gamma-ray flares. This is interpreted as the gamma-ray flares being produced by synchrotron self-Compton scattering of optical photons from the flares triggered by the interaction of moving knots with a stationary conical shock in the jet.

On the Location of the Gamma-ray Emission in the 2008 Outburst in the BL Lacertae Object AO 0235+164 through Observations across the Electromagnetic Spectrum

We present observations of a major outburst at centimeter, millimeter, optical, X-ray, and gamma-ray wavelengths of the BL Lacertae object AO 0235+164. We analyze the timing of multi-waveband variations in the flux and linear polarization, as well as changes in Very Long Baseline Array (VLBA) images at 7mm with 0.15 milliarcsecond resolution. The association of the events at different wavebands is confirmed at high statistical significance by probability arguments and Monte-Carlo simulations. A series of sharp peaks in optical linear polarization, as well as a pronounced maximum in the 7 mm polarization of a superluminal jet knot, indicate rapid fluctuations in the degree of ordering of the magnetic field. These results lead us to conclude that the outburst occurred in the jet both in the quasi-stationary core and in the superluminal knot, both parsecs downstream of the supermassive black hole. We interpret the outburst as a consequence of the propagation of a disturbance, elongated along the line of sight by light-travel time delays, that passes through a standing recollimation shock in the core and propagates down the jet to create the superluminal knot. The multi-wavelength light curves vary together on long time-scales (months/years), but the correspondence is poorer on shorter time-scales. This, as well as the variability of the polarization and the dual location of the outburst, agrees with the expectations of a multi-zone emission model in which turbulence plays a major role in modulating the synchrotron and inverse Compton fluxes.

Multi-waveband Emission Maps of Blazars

We are leading a comprehensive multi-waveband monitoring program of 34 gamma-ray bright blazars designed to locate the emission regions of blazars from radio to gamma-ray frequencies. The maps are anchored by sequences of images in both total and polarized intensity obtained with the VLBA at an angular resolution of ~ 0.1 milliarcseconds. The time-variable linear polarization at radio to optical wavelengths and radio to gamma-ray light curves allow us to specify the locations of flares relative to bright stationary features seen in the images and to infer the geometry of the magnetic field in different regions of the jet. Our data reveal that some flares occur simultaneously at different wavebands and others are only seen at some of the frequencies. The flares are often triggered by a superluminal knot passing through the stationary core on the VLBA images. Other flares occur upstream or even parsecs downstream of the core.