arXiv subjects

A. P. Marscher

Publications and source records attributed to A. P. Marscher.

At least 91 records · Page 5Linked to original sources

arXiv2009

AGILE detection of a rapid gamma-ray flare from the blazar PKS 1510-089 during the GASP-WEBT monitoring

We report the detection by the AGILE satellite of a rapid gamma-ray flare from the powerful gamma-ray quasar PKS 1510-089, during a pointing centered on the Galactic Center region from 1 March to 30 March 2008. This source has been continuosly monitored in the radio-to-optical bands by the GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope (WEBT). Moreover, the gamma-ray flaring episode triggered three ToO observations by the Swift satellite in three consecutive days, starting from 20 March 2008. In the period 1-16 March 2008, AGILE detected gamma-ray emission from PKS 1510-089 at a significance level of 6.2-sigma with an average flux over the entire period of (84 +/- 17) x 10^{-8} photons cm^{-2} s^{-1} for photon energies above 100 MeV. After a predefined satellite re-pointing, between 17 and 21 March 2008, AGILE detected the source at a significance level of 7.3-sigma, with an average flux (E > 100 MeV) of (134 +/- 29) x 10^{-8} photons cm^{-2} s^{-1} and a peak level of (281 +/- 68) x 10^{-8} photons cm^{-2} s^{-1} with daily integration. During the observing period January-April 2008, the source also showed an intense and variable optical activity, with several flaring episodes and a significant increase of the flux was observed at millimetric frequencies. Moreover, in the X-ray band the Swift/XRT observations seem to show an harder-when-brighter behaviour of the source spectrum. The spectral energy distribution of mid-March 2008 is modelled with a homogeneous one-zone synchrotron self Compton emission plus contributions from inverse Compton scattering of external photons from both the accretion disc and the broad line region. Indeed, some features in the optical-UV spectrum seem to indicate the presence of Seyfert-like components, such as the little blue bump and the big blue bump.

astro-ph.HE↗

arXiv2009

Jets in Active Galactic Nuclei

The jets of active galactic nuclei can carry a large fraction of the accreted power of the black-hole system into interstellar and even extragalactic space. They radiate profusely from radio to X-ray and gamma-ray frequencies. In the most extreme cases, the outward flow speeds correspond to high Lorentz factors that can reach 40 or more. This chapter displays images at various wavebands as well as light curves and continuum spectra that illustrate the variability with location, time, and frequency of the emission from compact, parsec- and subparsec-scale jets. It presents a physical framework for investigating many aspects of the structure and dynamical processes from such data.

astro-ph.HE↗

arXiv2009

Synchronous Optical and Radio Polarization Variability in the Blazar OJ287

We explore the variability and cross-frequency correlation of the flux density and polarization of the blazar OJ287, using imaging at 43 GHz with the Very Long Baseline Array, as well as optical and near-infrared polarimetry. The polarization and flux density in both the optical waveband and the 43 GHz compact core increased by a small amount in late 2005, and increased significantly along with the near-IR polarization and flux density over the course of 10 days in early 2006. Furthermore, the values of the electric vector position angle (EVPA) at the three wavebands are similar. At 43 GHz, the EVPA of the blazar core is perpendicular to the flow of the jet, while the EVPAs of emerging superluminal knots are aligned parallel to the jet axis. The core polarization is that expected if shear aligns the magnetic field at the boundary between flows of disparate velocities within the jet. Using variations in flux density, percentage polarization, and EVPA, we model the inner jet as a spine-sheath system. The model jet contains a turbulent spine of half-width 1.2 degrees and maximum Lorentz factor of 16.5, a turbulent sheath with Lorentz factor of 5, and a boundary region of sheared field between the spine and sheath. Transverse shocks propagating along the fast, turbulent spine can explain the superluminal knots. The observed flux density and polarization variations are then compatible with changes in the direction of the inner jet caused by a temporary change in the position of the core if the spine contains wiggles owing to an instability. In addition, we can explain a stable offset of optical and near-IR percentage polarization by a steepening of spectral index with frequency, as supported by the data.

astro-ph.CO↗

arXiv2009

Extragalactic Hard X-ray Surveys: From INTEGRAL to Simbol-X

We present some results of the deepest extragalactic survey performed by the INTEGRAL satellite. The fraction of very absorbed AGN is quite large. The sharp decrease in the absorption fraction with X-ray luminosity observed at lower-energy X-rays is not observed. The current lack of truly Compton-thick objects, with an upper limit of 14% to the size of this population, is just compatible with recent modeling of the cosmic X-ray background. We also study the prospects for a future hard X-ray serendipitous survey with Simbol-X. We show that Simbol-X will easily detect a large number of serendipitous AGN, allowing us to study the evolution of AGN up to redshifts about 2, opening the door to the cosmological study of hard X-ray selected AGN, which is barely possible with existing satellites like Swift and INTEGRAL.

astro-ph.GA↗

arXiv2008

Results of WEBT, VLBA and RXTE monitoring of 3C 279 during 2006-2007

We present radio-to-optical data taken by the WEBT, supplemented by VLBA and RXTE observations, of 3C 279. Our goal is to use this extensive database to draw inferences regarding the physics of the relativistic jet. We assemble multifrequency light curves with data from 30 ground-based observatories and the space-based instruments, along with linear polarization vs. time in the optical R band. In addition, we present a sequence of 22 images (with polarization vectors) at 43 GHz at resolution 0.15 milliarcsec, obtained with the VLBA. We analyse the light curves and polarization, as well as the spectral energy distributions at different epochs, corresponding to different brightness states. The IR-optical-UV continuum spectrum of the variable component corresponds to a power law with a constant slope of -1.6, while in the 2.4-10 keV X-ray band it varies in slope from -1.1 to -1.6. The steepest X-ray spectrum occurs at a flux minimum. During a decline in flux from maximum in late 2006, the optical and 43 GHz core polarization vectors rotate by ~300 degrees. The continuum spectrum agrees with steady injection of relativistic electrons with a power-law energy distribution of slope -3.2 that is steepened to -4.2 at high energies by radiative losses. The X-ray emission at flux minimum comes most likely from a new component that starts in an upstream section of the jet where inverse Compton scattering of seed photons from outside the jet is important. The rotation of the polarization vector implies that the jet contains a helical magnetic field that extends ~20 pc past the 43 GHz core.

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arXiv2008

Correlated Multi-Waveband Variability in the Blazar 3C~279 from 1996 to 2007

We present the results of extensive multi-waveband monitoring of the blazar 3C~279 between 1996 and 2007 at X-ray energies (2-10 keV), optical R band, and 14.5 GHz, as well as imaging with the Very Long Baseline Array (VLBA) at 43 GHz. In all bands the power spectral density corresponds to "red noise" that can be fit by a single power law over the sampled time scales. Variations in flux at all three wavebands are significantly correlated. The time delay between high and low frequency bands changes substantially on time scales of years. A major multi-frequency flare in 2001 coincided with a swing of the jet toward a more southerly direction, and in general the X-ray flux is modulated by changes in the position angle of the jet near the core. The flux density in the core at 43 GHz--increases in which indicate the appearance of new superluminal knots--is significantly correlated with the X-ray flux. We decompose the X-ray and optical light curves into individual flares, finding that X-ray leads optical variations (XO) in 6 flares, the reverse occurs in 3 flares (OX), and there is essentially zero lag in 4 flares. Upon comparing theoretical expectations with the data, we conclude that (1) XO flares can be explained by gradual acceleration of radiating electrons to the highest energies; (2) OX flares can result from either light-travel delays of the seed photons (synchrotron self-Compton scattering) or gradients in maximum electron energy behind shock fronts; and (3) events with similar X-ray and optical radiative energy output originate well upstream of the 43 GHz core, while those in which the optical radiative output dominates occur at or downstream of the core.

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arXiv2008

Stratification in polarization and Faraday rotation in the jet of 3C 120

Very long baseline interferometric observations of the radio galaxy 3C 120 show a systematic presence of gradients in Faraday rotation and degree of polarization across and along the jet. These are revealed by the passage of multiple superluminal components throughout the jet as they move out from the core in a sequence of 12 monthly polarimetric observations taken with the VLBA at 15, 22, and 43 GHz. The degree of polarization has an asymmetric profile in which the northern side of the jet is more highly polarized. The Faraday rotation measure is also stratified across the jet width, with larger values for the southern side. Superposed on this structure we find a localized region of high Faraday rotation measure (about 6000 rad/m^2) between approximately 3 and 4 mas from the core. This region of enhanced Faraday rotation may result from the interaction of the jet with the ambient medium, which may also explain the stratification in degree of polarization. The data are also consistent with a helical magnetic field in a two-fluid jet model, consisting of an inner emitting jet and a sheath of nonrelativistic electrons.

astro-ph↗

arXiv2008

Faraday rotation and polarization gradients in the jet of 3C~120: Interaction with the external medium and a helical magnetic field?

We present a sequence of 12 monthly polarimetric 15, 22, and 43 GHz VLBA observations of the radio galaxy 3C 120 revealing a systematic presence of gradients in Faraday rotation and degree of polarization across and along the jet. The degree of polarization increases with distance from the core and toward the jet edges, and has an asymmetric profile in which the northern side of the jet is more highly polarized. The Faraday rotation measure is also stratified across the jet width, with larger values for the southern side. We find a localized region of high Faraday rotation measure superposed on this structure between approximately 3 and 4 mas from the core, with a peak of about 6000 rad/m^2. Interaction of the jet with the external medium or a cloud would explain the confined region of enhanced Faraday rotation, as well as the stratification in degree of polarization and the flaring of superluminal knots when crossing this region. The data are also consistent with a helical field in a two-fluid jet model, consisting of an inner, emitting jet and a sheath containing nonrelativistic electrons. However, this helical magnetic field model cannot by itself explain the localized region of enhanced Faraday rotation. The polarization electric vectors, predominantly perpendicular to the jet axis once corrected for Faraday rotation, require a dominant component parallel to the jet axis (in the frame of the emitting plasma) for the magnetic field in the emitting region.

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arXiv2007

Superluminal non-ballistic jet swing in the quasar NRAO 150 revealed by mm-VLBI

NRAO 150 -a compact and bright radio to mm source showing core/jet structure- has been recently identified as a quasar at redshift z=1.52 through a near-IR spectral observation. To study the jet kinematics on the smallest accessible scales and to compute the first estimates of its basic physical properties, we have analysed the ultra-high-resolution images from a new monitoring program at 86 GHz and 43 GHz with the GMVA and the VLBA, respectively. An additional archival and calibration VLBA data set, covering from 1997 to 2007, has been used. Our data shows an extreme projected counter-clock-wise jet position angle swing at an angular rate of up to ~11 deg./yr within the inner ~31 pc of the jet, which is associated with a non-ballistic superluminal motion of the jet within this region. The results suggest that the magnetic field could play an important role in the dynamics of the jet in NRAO 150, which is supported by the large values of the magnetic field strength obtained from our first estimates. The extreme characteristics of the jet swing make NRAO 150 a prime source to study the jet wobbling phenomenon.

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arXiv2007

The Outburst of the Blazar AO 0235+164 in 2006 December: Shock-in-Jet Interpretation

We present the results of polarimetric ($R$ band) and multicolor photometric ($BVRIJHK$) observations of the blazar AO 0235+16 during an outburst in 2006 December. The data reveal a short timescale of variability (several hours), which increases from optical to near-IR wavelengths; even shorter variations are detected in polarization. The flux density correlates with the degree of polarization, and at maximum degree of polarization the electric vector tends to align with the parsec-scale jet direction. We find that a variable component with a steady power-law spectral energy distribution and very high optical polarization (30-50%) is responsible for the variability. We interpret these properties of the blazar withina model of a transverse shock propagating down the jet. In this case a small change in the viewing angle of the jet, by $\lesssim 1^o$, and a decrease in the shocked plasma compression by a factor of $\sim$1.5 are sufficient to account for the variability.

astro-ph↗

arXiv2007

How Compact are the Cores of AGN? Sub-Parsec Scale Imaging with VLBI at Millimeter Wavelengths

We study the most central regions of AGN jets with an angular resolution of tens of micro-arcseconds using VLBI at millimeter wavelengths (mm-VLBI). We present and discuss a new 86 GHz VLBI survey of compact radio sources. We show new high dynamic range images of two nearby radio galaxies (3C 120 and M87). In M87 the size of the compact VLBI core (the jet base) is < 15 Schwarzschild radii. Future mm-VLBI observations at 1 mm and shorter wavelengths should lead to images of galactic and extragalactic radio sources with a spatial resolution down to a few Schwarzschild radii of the central super massive black holes. To achieve this, the participation of large and sensitive millimeter and sub-millimeter telescopes in VLBI is essential. Owing to their high sensitivity existing and planned phased interferometers like the IRAM Plateau de Bure interferometer, CARMA, the SMA and ALMA could play an important role.

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arXiv2006

Detailed Structure of the X-ray Jet in 4C 19.44 (=PKS1354+195)

We investigate the variations of the magnetic field, Doppler factor, and relativistic particle density along the jet of a quasar at z=0.72. We chose 4C 19.44 for this study because of its length and straight morphology. The 18 arcsec length of the jet provides many independent resolution elements in the Chandra X-ray image. The straightness suggests that geometry factors, although uncertain, are almost constant along the jet. We assume the X-ray emission is from inverse Compton scattering of the cosmic microwave background. With the aid of assumptions about jet alignment, equipartition between magnetic-field and relativistic-particle energy, and filling factors, we find that the jet is in bulk relativistic motion with a Doppler factor about 6 at an angle no more than 10 degrees to the line of sight over deprojected distances about 150--600 kpc from the quasar, and with a magnetic field approximately 10 micro Gauss.

astro-ph↗

arXiv2006

The milliarcsecond-scale jet of PKS 0735+178 during quiescence

We present polarimetric 5 GHz to 43 GHz VLBI observations of the BL Lacertae object PKS 0735+178, spanning March 1996 to May 2000. Comparison with previous and later observations suggests that the overall kinematic and structural properties of the jet are greatly influenced by its activity. Time intervals of enhanced activity, as reported before 1993 and after 2000 by other studies, are followed by highly superluminal motion along a rectilinear jet. In contrast the less active state in which we performed our observations, shows subluminal or slow superluminal jet features propagating through a twisted jet with two sharp bends of about 90 deg. within the innermost three-milliarcsecond jet structure. Proper motion estimates from the data presented here allow us to constrain the jet viewing angle to values < 9 deg., and the bulk Lorentz factor to be between 2 and 4.

astro-ph↗

arXiv2005

Real vs. simulated relativistic jets

Intensive VLBI monitoring programs of jets in AGN are showing the existence of intricate emission patterns, such as upstream motions or slow moving and quasi-stationary componentes trailing superluminal features. Relativistic hydrodynamic and emission simulations of jets are in very good agreement with these observations, proving as a powerful tool for the understanding of the physical processes taking place in the jets of AGN, microquasars and GRBs. These simulations show that the variability of the jet emission is the result of a complex combination of phase motions, viewing angle selection effects, and non-linear interactions between perturbations and the underlying jet and/or ambient medium. Both observations and simulations suggest that shock-in-jet models may be an overly simplistic idealization when interpreting the emission structure observed in actual jets.

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arXiv2004

A Multiband Approach to AGN: Radioscopy & Radio Astronomy

Only in the radio-loud population of active galactic nuclei (AGN) does the production, collimation, and acceleration of powerful relativistic jets take place. We introduce here a concept of combined VLBI- and X-ray spectroscopic observations of sources with relativistic, broad iron lines. This approach has enormous potential to yield deep insights into the accretion/jet-production process in AGN. Better knowledge of the milliarcsecond-resolution radio structure of the nuclear radio cores in so-called ``radio-quiet'' broad-iron-line Seyfert galaxies is essential for future combined radio/X-ray studies of the different modes of radio-jet production in accreting black hole systems.

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arXiv2003

X-ray/Optical Variability of Akn 120 and 3C 120

Using the Rossi X-ray Timing Experiment (RXTE), we have observed both the Seyfert 1 galaxy Akn 120 and the broad-line radio galaxy 3C 120. Monitoring observations spanning 2 years plus intensive stares spanning several days have been obtained for both objects. These data have allowed us to construct the power density spectrum (PDS) for each object. Covering more than 4 decades in temporal frequency, the PDS for 3C 120 represents the first such measurement for a radio loud galaxy. The PDS for 3C 120 shows a steep power law at high frequencies, which turns over at low frequencies. Akn 120 also has a power law shape in its PDS. Using the PDS of Cygnus X-1, we estimate the masses for both of these objects. Optical data and cross correlation functions are also presented for Akn 120. The long-term light curve of Akn 120 shows a strong correlation with optical data, with zero time lag.

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arXiv2003

Multiwavelength Observations of Strong Flares From the TeV-Blazar 1ES 1959+650

Following the detection of strong TeV gamma-ray flares from the BL Lac object 1ES 1959+650 with the Whipple 10 m Cherenkov telescope on May 16 and 17, 2002, we performed intensive Target of Opportunity (ToO) radio, optical, X-ray and TeV gamma-ray observations from May 18, 2002 to August 14, 2002. Observations with the X-ray telescope RXTE and the Whipple and HEGRA gamma-ray telescopes revealed several strong flares, enabling us to sensitively test the X-ray/gamma-ray flux correlation properties. Although the X-ray and gamma-ray fluxes seemed to be correlated in general, we found an ``orphan'' gamma-ray flare that was not accompanied by an X-ray flare. After describing in detail the radio (UMRAO, VLA), optical (Boltwood, Abastumani), X-ray (RXTE) and gamma-ray (Whipple, HEGRA) light curves and Spectral Energy Distributions (SEDs) we present initial modeling of the SED with a simple Synchrotron Self-Compton (SSC) model. With the addition of another TeV blazar with good broadband data, we consider the set of all TeV blazars to begin to look for a connection of the jet properties to the properties of the central accreting black hole thought to drive the jet. Remarkably, the temporal and spectral X-ray and gamma-ray emission characteristics of TeV blazars are very similar, even though the masses estimates of their central black holes differ by up to one order of magnitude.

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arXiv2003

Parsec Scale Properties of Markarian 501

We present the results of a high angular resolution study of the BL Lac object Markarian 501 in the radio band. We consider data taken at 14 different epochs, ranging between 1.6 GHz and 22 GHz in frequency, and including new Space VLBI observations obtained on 2001 March 5 and 6 at 1.6 and 5 GHz. We study the kinematics of the parsec-scale jet and estimate its bulk velocity and orientation with respect to the line of sight. Limb brightened structure in the jet is clearly visible in our data and we discuss its possible origin in terms of velocity gradients in the jet. Quasi-simultaneous multi-wavelength observations allow us to map the spectral index distribution and to compare it to the jet morphology. Finally, we estimate the physical parameters of the parsec-scale jet.

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