arXiv subjects

Alan P. Marscher

Publications and source records attributed to Alan P. Marscher.

At least 145 records · Page 8Linked to original sources

arXiv2010

Flaring Behavior of the Quasar 3C~454.3 across the Electromagnetic Spectrum

We analyze the behavior of the parsec-scale jet of the quasar 3C~454.3 during pronounced flaring activity in 2005-2008. Three major disturbances propagated down the jet along different trajectories with Lorentz factors $Γ>$10. The disturbances show a clear connection with millimeter-wave outbursts, in 2005 May/June, 2007 July, and 2007 December. High-amplitude optical events in the $R$-band light curve precede peaks of the millimeter-wave outbursts by 15-50 days. Each optical outburst is accompanied by an increase in X-ray activity. We associate the optical outbursts with propagation of the superluminal knots and derive the location of sites of energy dissipation in the form of radiation. The most prominent and long-lasting of these, in 2005 May, occurred closer to the black hole, while the outbursts with a shorter duration in 2005 Autumn and in 2007 might be connected with the passage of a disturbance through the millimeter-wave core of the jet. The optical outbursts, which coincide with the passage of superluminal radio knots through the core, are accompanied by systematic rotation of the position angle of optical linear polarization. Such rotation appears to be a common feature during the early stages of flares in blazars. We find correlations between optical variations and those at X-ray and $γ$-ray energies. We conclude that the emergence of a superluminal knot from the core yields a series of optical and high-energy outbursts, and that the mm-wave core lies at the end of the jet's acceleration and collimation zone.

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arXiv2010

Unexpected High Brightness Temperature 140 PC from the Core in the Jet of 3C 120

We present 1.7, 5, 15, 22 and 43 GHz polarimetric multi-epoch VLBA observations of the radio galaxy 3C 120. The higher frequency observations reveal a new component, not visible before April 2007, located 80 mas from the core (which corresponds to a deprojected distance of 140 pc), with a brightness temperature about 600 times higher than expected at such distances. This component (hereafter C80) is observed to remain stationary and to undergo small changes in its brightness temperature during more than two years of observations. A helical shocked jet model - and perhaps some flow acceleration - may explain the unusually high Tb of C80, but it seems unlikely that this corresponds to the usual shock that emerges from the core and travels downstream to the location of C80. It appears that some other intrinsic process in the jet, capable of providing a local burst in particle and/or magnetic field energy, may be responsible for the enhanced brightness temperature observed in C80, its sudden appearance in April 2007, and apparent stationarity.

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arXiv2010

The Inner Jet of the Quasar PKS 1510$-$089 as Revealed by Multi-waveband Monitoring

As part of our comprehensive long-term multi-waveband monitoring of 34 blazars, we followed the activity in the jet of the blazar PKS 1510-089 during major outbursts during the first half of 2009. The most revealing event was a two-month long outburst that featured a number of gamma-ray flares. During the outburst, the position angle of optical linear polarization rotated by about 720 degrees, which implies that a single emission feature was responsible for all of the flares during the outburst. At the end of the rotation, a new superluminal knot (~ 22c) passed through the "core" seen on 43 GHz VLBA images at essentially the same time as an extremely sharp, high-amplitude gamma-ray and optical flare occurred. We associate the entire multi-flare outburst with this knot. The ratio of gamma-ray to synchrotron integrated flux indicates that some of the gamma-ray flares resulted from inverse Compton scattering of seed photons outside the ultra-fast spine of the jet. Because many of the flares occurred over time scales of days or even hours, there must be a number of sources of IR-optical-UV seed photons -- probably synchrotron emission -- surrounding the spine, perhaps in a slower sheath of the jet.

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arXiv2010

Probing the Inner Jet of the Quasar PKS 1510-089 with Multi-waveband Monitoring during Strong Gamma-ray Activity

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 wavebands varied by an order of magnitude. The optical polarization vector rotated by 720 degrees during a 5-day period encompassing six of these flares. This culminated in a very bright, roughly 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.

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arXiv2009

Disk-Jet Connection in the Radio Galaxy 3C 120

We present the results of extensive multi-frequency monitoring of the radio galaxy 3C 120 between 2002 and 2007 at X-ray, optical, and radio wave bands, as well as imaging with the Very Long Baseline Array (VLBA). Over the 5 yr of observation, significant dips in the X-ray light curve are followed by ejections of bright superluminal knots in the VLBA images. Consistent with this, the X-ray flux and 37 GHz flux are anti-correlated with X-ray leading the radio variations. This implies that, in this radio galaxy, the radiative state of accretion disk plus corona system, where the X-rays are produced, has a direct effect on the events in the jet, where the radio emission originates. The X-ray power spectral density of 3C 120 shows a break, with steeper slope at shorter timescale and the break timescale is commensurate with the mass of the central black hole based on observations of Seyfert galaxies and black hole X-ray binaries. These findings provide support for the paradigm that black hole X-ray binaries and active galactic nuclei are fundamentally similar systems, with characteristic time and size scales linearly proportional to the mass of the central black hole. The X-ray and optical variations are strongly correlated in 3C 120, which implies that the optical emission in this object arises from the same general region as the X-rays, i.e., in the accretion disk-corona system. We numerically model multi-wavelength light curves of 3C 120 from such a system with the optical-UV emission produced in the disk and the X-rays generated by scattering of thermal photons by hot electrons in the corona. From the comparison of the temporal properties of the model light curves to that of the observed variability, we constrain the physical size of the corona and the distances of the emitting regions from the central BH.

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arXiv2009

Implications of the VHE Gamma-Ray Detection of the Quasar 3C279

The MAGIC collaboration recently reported the detection of the quasar 3C279 at > 100 GeV gamma-ray energies. Here we present simultaneous optical (BVRI) and X-ray (RXTE PCA) data from the day of the VHE detection and discuss the implications of the snap-shot spectral energy distribution for jet models of blazars. A one-zone synchrotron-self-Compton origin of the entire SED, including the VHE gamma-ray emission can be ruled out. The VHE emission could, in principle, be interpreted as Compton upscattering of external radiation (e.g., from the broad-line regions). However, such an interpretation would require either an unusually low magnetic field of B ~ 0.03 G or an unrealistically high Doppler factor of Gamma ~ 140. In addition, such a model fails to reproduce the observed X-ray flux. This as well as the lack of correlated variability in the optical with the VHE gamma-ray emission and the substantial gamma-gamma opacity of the BLR radiation field to VHE gamma-rays suggests a multi-zone model. In particular, an SSC model with an emission region far outside the BLR reproduces the simultaneous X-ray -- VHE gamma-ray spectrum of 3C279. Alternatively, a hadronic model is capable of reproducing the observed SED of 3C279 reasonably well. However, the hadronic model requires a rather extreme jet power of L_j ~ 10^{49} erg s^{-1}, compared to a requirement of L_j ~ 2 X 10^{47} erg s^{-1} for a multi-zone leptonic model.

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arXiv2009

Multi-wavelength Variability of the Broad Line Radio Galaxy 3C 120

We present results from a multi-year monitoring campaign of the broad-line radio galaxy 3C 120, using the Rossi X-ray Timing Explorer (RXTE) for nearly five years of observations. Additionally, we present coincident optical monitoring using data from several ground-based observatories. Both the X-ray and optical emission are highly variable and appear to be strongly correlated, with the X-ray emission leading the optical by 28 days. The X-ray power density spectrum is best fit by a broken power law, with a low-frequency slope of -1.2, breaking to a high-frequency slope of -2.1, and a break frequency of log nu_b=-5.75 Hz, or 6.5 days. This value agrees well with the value expected based on 3C 120's mass and accretion rate. We find no evidence for a second break in the power spectrum. Combined with a moderately soft X-ray spectrum (Gamma=1.8) and a moderately high accretion rate (mdot / mdot_Edd ~ 0.3), this indicates that 3C 120 fits in wellwith the high/soft variability state found in most other AGNs. Previous studies have shown that the spectrum has a strong Fe K-alpha line, which may be relativistically broadened. The presence of this line, combined with a power spectrum similar to that seen in Seyfert galaxies, suggests that the majority of the X-ray emission in this object arises in or near the disk, and not in the jet.

astro-ph.GA↗

arXiv2007

Multiwaveband Polarimetric Observations of 15 Active Galactic Nuclei at High Frequencies: Correlated Polarization Behavior

We report on multi-frequency linear polarization monitoring of 15 active galactic nuclei containing highly relativistic jets with apparent speeds from $\sim$4 $c$ to $>40c$. The measurements were obtained at optical, 1 mm, and 3 mm wavelengths, and at 7 mm with the Very Long Baseline Array. The data show a wide range in degree of linear polarization among the sources, from $<$1% to $>$30%, and interday polarization variability in individual sources. The polarization properties suggest separation of the sample into three groups with low, intermediate, and high variability of polarization in the core at 7 mm : LVP, IVP, and HVP, respectively. The groups are partially associated with the common classification of active galactic nuclei as radio galaxies and quasars with low optical polarization (LVP), BL Lacertae objects (IVP), and highly optically polarized quasars (HVP). Our study investigates correlations between total flux, fractional polarization, and polarization position angle at the different wavelengths. We interpret the polarization properties of the sources in the sample through models in which weak shocks compress turbulent plasma in the jet. The differences in the orientation of sources with respect to the observer, jet kinematics, and abundance of thermal matter external to the jet near the core can account for the diversity in the polarization properties. The results provide strong evidence that the optical polarized emission originates in shocks, most likely situated between the 3 mm and 7 mm VLBI cores. They also support the idea that the 1 mm core lies at the edge of the transition zone between electromagnetically dominated and turbulent hydrodynamical sections of the jet.

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arXiv2007

Rapid Multiwaveband Polarization Variability in the Quasar PKS 0420-014: Optical Emission from the Compact Radio Jet

An 11-day monitoring campaign in late 2005 reveals clear correlation in polarization between the optical emission and the region of the intensity peak (the "pseudocore") at the upstream end of the jet in 43 GHz Very Long Baseline Array images in the highly variable quasar PKS 0420-014. The electric-vector position angle (EVPA) of the pseudocore rotated by about 80 degrees in four VLBA observations over a period of nine days, matching the trend of the optical EVPA. In addition, the 43 GHz EVPAs agree well with the optical values when we correct the former for Faraday rotation. Fluctuations in the polarization at both wavebands are consistent with the variable emission arising from a standing conical shock wave that compresses magnetically turbulent plasma in the ambient jet. The volume of the variable component is the same at both wavebands, although only ~20% of the total 43 GHz emission arises from this site. The remainder of the 43 GHz flux density must originate in a separate region with very low polarization. If 0420-014 is a typical case, the nonthermal optical emission from blazars originates primarily in and near the pseudocore rather than closer to the central engine where the flow collimates and accelerates.

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arXiv2005

Polarimetric Observations of 15 Active Galactic Nuclei at High Frequencies: Jet Kinematics from Bimonthly Monitoring with the Very Long Baseline Array

We present total and polarized intensity images of 15 active galactic nuclei obtained with the Very Long Baseline Array at 7 mm at 17 epochs from 1998 March to 2001 April. At some epochs the images are accompanied by nearly simultaneous polarization measurements at 3 mm, 1.35/0.85 mm, and optical wavelengths. Here we analyze the 7 mm images to define the properties of the jets of two radio galaxies, five BL Lac objects, and eight quasars on angular scales $\gtrsim 0.1$ milliarcseconds. We determine the apparent velocities of 106 features in the jets; for many of the features we derive Doppler factors using a new method based on comparison of timescale of decline in flux density with the light-travel time across the emitting region. This allows us to estimate the Lorentz factors, intrinsic brightness temperatures, and viewing angles of 73 superluminal knots, as well as the opening angle of the jet for each source. We analyze the derived physical parameters of the jets. In nine sources we detect statistically meaningful deviations from ballistic motion, with the majority of components accelerating with distance from the core. In six sources we identify jet features with characteristics of trailing shocks that form behind the primary strong perturbations in jet simulations. The apparent speeds of these components increase with distance from the core suggesting an acceleration of the underlying jet.

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arXiv2005

External Compton Radiation from Rapid Nonthermal Flares in Blazars

In this paper we extend our approach to modeling multifrequency emission variability from blazars to include external-radiation Compton (ERC) emission and electron energy losses from inverse Compton scattering of seed photons originating outside the jet. We consider seed-photon emission from a dusty molecular torus and the broad line region (BLR) surrounding the central engine. We establish constraints on the properties of the molecular torus and BLR under which synchrotron self-Compton (SSC) emission dominates such that the results obtained in our previous paper are applicable. The focus of this study is on relative time delays between the light curves observed at different frequencies. For definiteness, we consider emission resulting from a collision between relativistic shocks, but the results apply more generally to conditions involving acceleration of relativistic electrons and/or magnetic field amplification at any type of front. Unlike SSC emission, ERC flares involving a constant field of seed photons are not delayed by light travel time of the seed photons. The main cause of delays is from radiative energy losses, which result in frequency stratification behind the front and quench the flare first at the highest frequencies, progressing to lower frequencies as time advances. However, if the spectrum of electrons injected at the shock front is characterized by a relatively high value of the minimum energy (a Lorentz factor $γ_{min}\sim100$ is sufficient), the ERC flare in the X-ray band can be delayed and may even peak after the injection has ceased. This effect is strongly frequency dependent, with a longer lag at lower frequencies and an X-ray spectral index that changes rapidly from positive (inverted spectrum) to steep values.

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arXiv2004

Synchrotron Self-Compton Model for Rapid Nonthermal Flares in Blazars with Frequency-Dependent Time Lags

We model rapid variability of multifrequency emission from blazars occurring across the electromagnetic spectrum (from radio to gamma-rays). Lower energy emission is produced by the synchrotron mechanism, whereas higher energy emission is due to inverse Compton scattering of the synchrotron emission. We take into account energy stratification established by particle acceleration at shock fronts and energy losses due to synchrotron emission. We also consider the effect of light travel delays for the synchrotron emission that supplies the seed photons for inverse Compton scattering. The production of a flare is caused by the collision between a relativistic shock wave and a stationary feature in the jet (e.g., a Mach disk). The collision leads to the formation of forward and reverse shocks, which confine two contiguous emission regions resulting in complex profiles of simulated flares. Simulations of multifrequency flares indicate that relative delays between the inverse Compton flares and their synchrotron counterparts are dominated by energy stratification and geometry of the emitting regions, resulting in both negative and positive time delays depending on the frequency of observation. Light travel effects of the seed photons may lead to a noticeable delay of the inverse Compton emission with respect to synchrotron variability if the line of sight is almost perfectly aligned with the jet. We apply the model to a flare in 3C 273 and derive the properties of shocked plasma responsible for the flare. We show that the pronounced negative time delay between the X-ray and IR light curves (X-rays peak after the maximum in the synchrotron emission) can be accounted for if both forward and reverse shocks are considered.

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arXiv2001

Monthly 43 GHz VLBA Polarimetric Monitoring of 3C120 over 16 Epochs: Evidence for Trailing Shocks in a Relativistic Jet

We present a 16-month sequence of monthly polarimetric 43 GHz VLBA images of the radio galaxy 3C 120. The images probe the inner regions of the radio jet of this relatively nearby superluminal radio galaxy at a linear resolution of 0.07 $h_{65}^{-1}$ pc ($H_o= 65 h_{65}$ km s$^{-1}$ Mpc$^{-1}$). We follow the motion of a number of features with apparent velocities between 4.01$\pm$0.08 and $5.82\pm 0.13 h_{65}^{-1} c$. A new superluminal knot, moving at $4.29\pm 0.16 h_{65}^{-1} c$, is observed to be ejected from the core at a time coincident with the largest flare ever observed for this source at millimeter wavelengths. Changes in the position angle of this component, as well as a progressive rotation of its magnetic polarization vector, suggest the presence of a twisted (resembling a helix in projection) configuration of the underlying jet magnetic field and jet geometry. We identify several knots that appear in the wake of the new superluminal component, moving at proper motions $\sim 4$ times slower than any of the other moving knots observed in 3C 120. These features have properties similar to those of the ``trailing'' shocks seen in relativistic, time-dependent, hydrodynamical and emission simulations of compact jets. Such trailing compressions are triggered by pinch-mode jet-body instabilities caused by the propagation of a strong perturbation, which we associate with the new strong superluminal component.

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arXiv2001

Multi-Epoch VLBA Observations of EGRET-Detected Quasars and BL Lac Objects: Connection between Superluminal Ejections and Gamma-Ray Flares in Blazars

We examine the coincidence of times of high $γ$-ray flux and ejections of superluminal components from the core in EGRET blazars based on a VLBA monitoring program at 22 and 43 GHz from November 1993 to July 1997. In 23 cases of $γ$-ray flares for which sufficient VLBA data exist, 10 of the flares (in 8 objects) fall within 1$σ$ uncertainties of the extrapolated epoch of zero separation from the core of a superluminal radio component. In each of two sources (0528+134 and 1730-130) two successive $γ$-ray flares were followed by the appearance of new superluminal components. We carried out statistical simulations which show that if the number of coincidences $\ge$ 10 the radio and $γ$-ray events are associated with each other at >99.999% confidence. Our analysis of the observed behavior, including variability of the polarized radio flux, of the sources before, during, and after the $γ$-ray flares suggests that the $γ$-ray events occur in the superluminal radio knots. This implies that the $γ$-ray flares are caused by inverse Compton scattering by relativistic electrons in the parsec-scale regions of the jet rather than closer to the central engine.

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arXiv2001

Multi-Epoch VLBA Observations of EGRET-Detected Quasars and BL Lac Objects: Superluminal Motion of Gamma-Ray Bright Blazars

We present the results of a program to monitor the structure of the radio emission in 42 $γ$-ray bright blazars (31 quasars and 11 BL Lac objects) with the VLBA at 43, 22, and occasionally 15 and 8.4 GHz, over the period from November 1993 to July 1997. We determine proper motions in 33 sources and find that the apparent superluminal motions in $γ$-ray sources are much faster than for the general population of bright compact radio sources. This follows the strong dependence of the $γ$-ray flux on the level of relativistic beaming for both external-radiation Compton and synchrotron self-Compton emission. There is a positive correlation (correlation coefficient $r$=0.45) between the flux density of the VLBI core and the $γ$-ray flux and a moderate correlation (partial correlation coefficient $r$=0.31) between $γ$-ray apparent luminosity and superluminal velocities of jet components, as expected if the $γ$-ray emission originates in a very compact region of the relativistic jet and is highly beamed. In 43% of the sources the jet bends by more than 20$^\circ$ on parsec scales, which is consistent with amplification by projection effects of modest actual changes in position angle. In 27 of the sources in the sample there is at least one non-core component that appears to be stationary during our observations. Different characteristics of stationary features close to and farther from the core lead us to suggest two different classes of stationary components: those within about 2 milliarcseconds (mas) of the core, probably associated with standing hydrodynamical compressions, and those farther down the jet, which tend to be associated with bends in the jet.

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arXiv1999

Space VLBI Observations of 3C371

We present the first space VLBI observations of 3C~371, carried out at a frequency of 4.8 GHz. The combination of the high resolution provided by the orbiting antenna Highly Advanced Laboratory for Communications and Astronomy (HALCA) and the high sensitivity of the VLBA allows imaging of the jet of 3C~371 with an angular resolution of approximately 0.26 mas, which for this relatively nearby source corresponds to $\sim$ 0.4 h$^{-1}$ pc. Comparison between two epochs separated by 66 days reveals no apparent motions in the inner 7 mas jet structure above an upper limit of $\sim 1.4 h^{-1}$ c. This value, the absence of detectable counterjet emission from the presumably symmetric jet, plus the presence of extended double-lobe structure, are consistent with the knots in the jet being stationary features such as standing shocks. The jet intensity declines with the angular distance from the core as $ϕ^{-1.68}$. This is more gradual than that derived for 3C~120, $ϕ^{-1.86}$, for which there is evidence for strong intereactions between the jet and ambient medium. This suggests that in 3C~371 there is a greater level of {\it in situ} acceleration of electrons and amplification of magnetic field. We interpret sharp bends in the jet at sites of off-center knots as further evidence for the interaction between the jet and external medium, which may also be responsible for the generation of standing recollimation shocks. These recollimation shocks may be responsible for the presumably stationary components. The radio properties of 3C~371 are intermediate between those of other radio galaxies with bright cores and those of BL Lacertae objects.

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arXiv1999

86, 43, and 22 GHz VLBI Observations of 3C120

We present the first 86 GHz VLBI observations of the radio galaxy 3C~120, together with contemporaneous 43 and 22 GHz polarimetric VLBA observations. The very high angular resolution obtained at 86 GHz provides an upper limit to the size of the core of 54 $μ$as (0.025 $h^{-1}$pc). This represents a direct determination of the base of the jet which is independent of variability arguments (which depend on uncertain estimates of the Doppler factor), and places it below approximately one light-month. Comparison with previous VLBA observations after a one-year interval shows pronounced changes in the structure and polarization of the jet. Most of the components are found to follow a curved path while undergoing a steepening of their spectra accompanied by a decrease in total and polarized emission. However, at least one component is observed to follow a quasi-ballistic motion, accompanied by a flattening of its spectrum, as well as an increase in total and polarized flux. This may be explained by its interaction with the external medium, resulting in a shock that enhances the emission and aligns the magnetic field perpendicular to the component motion, thereby producing an increase of the degree of polarization from undetected values to as high as 15%. A second strong component, with the highest degree of polarization (23%), is found to have experienced a displacement from the ridge line of the structural position angle of the jet as it moved downstream. We have found a mean swing to the south of the position angle of the innermost components of $\sim 6^{\circ}$ between late 1996 and 1997, which may be responsible for the jet curvature observed at parsec and kiloparsec scales.

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arXiv1999

Outburst in the Polarized Structure of the Compact Jet of 3C454.3

We present three-epoch polarimetric images of the quasar 3C 454.3 obtained with the Very Long Baseline Array at 22 and 43 GHz. 22 GHz polarized intensity images show a sudden change in the polarization structure of a bright eastern component (which we call the ``core,'' although it may be neither at the upstream end of the jet nor completely stationary) over a 41-day interval, coincident with the ejection of a new component from the core, as resolved in the corresponding 43 GHz images. This polarization outburst is also present at 43 GHz in both the core and the new component. This may represent a rapid change in the electric vector position angle of the ejected component from being orthogonal to almost parallel to that of the core. About seven months later, the new component, moving superluminally at 2.9 \pm 0.4 h^{-1} c$ (q_{\circ}=0.5) relative to the core and 3.9 \pm 0.4 h^{-1} c$ relative to a bright stationary component about 0.6 mas west of the core --- very low compared with previous measurements --- is found at 43 GHz to exhibit a further rotation of 90$^{\circ}$ in the orientation of its polarization. Opacity effects may account for the first rotation, but changes in the magnetic field of the component and/or that of the underlying jet in the inner milliarcsecond structure of 3C 454.3 are needed to account for the second. Polarized intensity images of the quasar 0420 - 014, used as a calibrator, are also presented. The polarization position angle of the core rotated between late 1994 and late 1996.

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