arXiv subjects

M. L. Lister

Publications and source records attributed to M. L. Lister.

At least 73 records · Page 4Linked to original sources

arXiv2015

Studies of the Jet in BL Lacertae. II. Superluminal Alfvén Waves

We study the kinematics of ridge lines on the pc-scale jet of the active galactic nucleus BL Lac. We show that the ridge lines display transverse patterns that move superluminally downstream, and that the moving patterns are analogous to waves on a whip. Their apparent speeds $β_\mathrm{app}$ (units of $c$) range from 3.9 to 13.5, corresponding to $β_\mathrm{wave}^\mathrm{gal}= 0.981 - 0.998$ in the galaxy frame. We show that the magnetic field in the jet is well-ordered with a strong transverse component, and assume that it is helical and that the transverse patterns are Alfvén waves propagating downstream on the longitudinal component of the magnetic field. The wave-induced transverse speed of the jet is non-relativistic ($β_\mathrm{tr}^\mathrm{gal} \lesssim 0.09$). In 2010 the wave activity subsided and the jet then displayed a mild wiggle that had a complex oscillatory behaviour. The Alfvén waves appear to be excited by changes in the position angle of the recollimation shock, in analogy to exciting a wave on a whip by shaking the handle. A simple model of the system with plasma sound speed $β_\mathrm{s}=0.3$ and apparent speed of a slow MHD wave $β_\mathrm{app,S}=4$ yields Lorentz factor of the beam $Γ_\mathrm{beam} \sim 4.5$, pitch angle of the helix (in the beam frame) $α\sim 67^\circ$, Alfvén speed $β_\mathrm{A}\sim 0.64$, and magnetosonic Mach number $M_\mathrm{ms}\sim 4.7$. This describes a plasma in which the magnetic field is dominant and in a rather tight helix, and Alfvén waves are responsible for the moving transverse patterns.

astro-ph.HE↗

arXiv2014

The Parsec-scale Structure, Kinematics, and Polarization of Radio-Loud Narrow-Line Seyfert 1 Galaxies

Several narrow-line Seyfert 1 galaxies (NLS1s) have now been detected in gamma rays, providing firm evidence that at least some of this class of active galactic nuclei (AGN) produce relativistic jets. The presence of jets in NLS1s is surprising, as these sources are typified by comparatively small black hole masses and near- or super-Eddington accretion rates. This challenges the current understanding of the conditions necessary for jet production. Comparing the properties of the jets in NLS1s with those in more familiar jetted systems is thus essential to improve jet production models. We present early results from our campaign to monitor the kinematics and polarization of the parsec-scale jets in a sample of 15 NLS1s through multifrequency observations with the Very Long Baseline Array. These observations are complemented by fast-cadence 15 GHz monitoring with the Owens Valley Radio Observatory 40m telescope and optical spectroscopic monitoring with with the 2m class telescope at the Guillermo Haro Astrophysics Observatory in Cananea, Mexico.

astro-ph.GA↗

arXiv2014

MOJAVE XII: Acceleration and Collimation of Blazar Jets on Parsec Scales

We report on the acceleration properties of 329 features in 95 blazar jets from the MOJAVE VLBA program. Nearly half the features and three-quarters of the jets show significant changes in speed and/or direction. In general, apparent speed changes are distinctly larger than changes in direction, indicating that changes in the Lorentz factors of jet features dominate the observed speed changes rather than bends along the line of sight. Observed accelerations tend to increase the speed of features near the jet base, $\lesssim 10-20$ parsecs projected, and decrease their speed at longer distances. The range of apparent speeds at fixed distance in an individual jet can span a factor of a few, indicating that shock properties and geometry may influence the apparent motions; however, we suggest that the broad trend of jet features increasing their speed near the origin is due to an overall acceleration of the jet flow out to de-projected distances of order $10^2$ parsecs, beyond which the flow begins to decelerate or remains nearly constant in speed. We estimate intrinsic rates of change of the Lorentz factors in the galaxy frame of order $\dotΓ/Γ\simeq 10^{-3}$ to $10^{-2}$ per year which can lead to total Lorentz factor changes of a factor of a few on the length scales observed here. Finally, we also find evidence for jet collimation at projected distances of $\lesssim 10$ parsecs in the form of the non-radial motion and bending accelerations that tend to better align features with the inner jet.

astro-ph.HE↗

arXiv2014

Studies of the Jet in BL Lacertae I. Recollimation Shock and Moving Emission Features

Parsec-scale VLBA images of BL Lac at 15 GHz show that the jet contains a permanent quasi-stationary emission feature 0.26 mas (0.34 pc projected) from the core, along with numerous moving features. In projection, the tracks of the moving features cluster around an axis at position angle -166.6 deg that connects the core with the standing feature. The moving features appear to emanate from the standing feature in a manner strikingly similar to the results of numerical 2-D relativistic magneto-hydrodynamic (RMHD) simulations in which moving shocks are generated at a recollimation shock. Because of this, and the close analogy to the jet feature HST-1 in M87, we identify the standing feature in BL Lac as a recollimation shock. We assume that the magnetic field dominates the dynamics in the jet, and that the field is predominantly toroidal. From this we suggest that the moving features are compressions established by slow and fast mode magneto-acoustic MHD waves. We illustrate the situation with a simple model in which the slowest moving feature is a slow-mode wave, and the fastest feature is a fast-mode wave. In the model the beam has Lorentz factor about 3.5 in the frame of the host galaxy, and the fast mode wave has Lorentz factor about 1.6 in the frame of the beam. This gives a maximum apparent speed for the moving features 10c. In this model the Lorentz factor of the pattern in the galaxy frame is approximately 3 times larger than that of the beam itself.

astro-ph.HE↗

arXiv2014

MOJAVE: Monitoring of Jets in Active Galactic Nuclei with VLBA Experiments. XI. Spectral distributions

We have obtained milliarcsecond-scale spectral index distributions for a sample of 190 extragalactic radio jets through the MOJAVE project. The sources were observed in 2006 at 8.1, 8.4, 12.1, and 15.4 GHz, and we study the four-frequency spectrum in individual jet features. We performed detailed simulations to study the effects of image alignment and (u,v)-plane coverage on the spectral index maps to verify our results. The core spectral indices are on average flat, while the jet spectrum is in general steep with a mean index of -1.04. A simple power-law fit is often inadequate for the core regions, as expected if the cores are partially self-absorbed. The overall jet spectrum steepens at a rate of about -0.001 to -0.004 per deprojected parsec when moving further out from the core with flat spectrum radio quasars having significantly steeper spectra than the BL Lac objects. The spectrum in both types of objects flattens at the locations of the jet components indicating particle acceleration or density enhancements along the jet. The mean spectral index at the component locations of -0.81 corresponds to a power-law index of ~2.6 for the electron energy distribution. We find a significant trend that jet components with linear polarization parallel to the jet (magnetic field perpendicular to the jet) have flatter spectra, as expected for transverse shocks. Compared to quasars, BL Lacs have more jet components with perpendicular magnetic field alignment, which may explain their generally flatter spectra. The overall steepening of the spectra with distance can be explained with radiative losses if the jets are collimating or with the evolution of the high-energy cutoff in the electron spectrum if the jets are conical. This interpretation is supported by a significant correlation with the age of the component and the spectral index, with older components having steeper spectra. (abridged)

astro-ph.GA↗

arXiv2014

Two active states of the narrow-line gamma-ray-loud AGN GB 1310+487

Previously unremarkable, the extragalactic radio source GB 1310+487 showed a gamma-ray flare on 2009 November 18, reaching a daily flux of ~10^-6 photons/cm^2/s at energies E>100 MeV and becoming one of the brightest GeV sources for about two weeks. Its optical spectrum is not typical for a blazar, instead, it resembles those of narrow emission-line galaxies. We investigate changes of the object's radio-to-GeV spectral energy distribution (SED) during and after the prominent GeV flare with the aim to determine the nature of the object and constrain the origin of the variable high-energy emission. The data collected by the Fermi and AGILE satellites at gamma-ray energies, Swift at X-ray and ultraviolet, Kanata, NOT, and Keck telescopes at optical, OAGH and WISE at infrared, and IRAM 30m, OVRO 40m, Effelsberg 100m, RATAN-600, and VLBA at radio, are analysed together to trace the SED evolution on timescales of months. The gamma-ray/radio-loud narrow-line active galactic nucleus (AGN) is located at redshift z=0.638. It is shining through an unrelated foreground galaxy at z=0.500. The AGN light is likely amplified by a factor of a few because of gravitational lensing. The AGN SED shows a two-humped structure typical of blazars and gamma-ray-loud NLSy1 galaxies, with the high-energy (inverse-Compton) emission dominating by more than an order of magnitude over the low-energy (synchrotron) emission during gamma-ray flares. The difference between the two SED humps is smaller during the low-activity state. Fermi observations reveal a strong correlation between the gamma-ray flux and spectral index, with the hardest spectrum observed during the brightest gamma-ray state. If the gamma-ray flux is a mixture of synchrotron self-Compton (SSC) and external Compton (EC) emission, the observed GeV spectral variability may result from varying relative contributions of these two emission components.

astro-ph.HE↗

arXiv2013

Basic properties of Narrow-Line Seyfert 1 Galaxies with Relativistic Jets

We present the preliminary results of a survey performed with Swift to observe a sample of radio-loud Narrow-Line Seyfert 1 Galaxies (RLNLS1s). Optical-to-X-ray data from Swift are complemented with gamma-ray observations from Fermi/LAT and radio measurements available in the literature. The comparison with a sample of bright Fermi blazars indicates that RLNLS1s seem to be the low-power tail of the distribution.

astro-ph.HE↗

arXiv2013

Intrinsic brightness temperatures of blazar jets at 15 GHz

We have developed a new Bayesian Markov Chain Monte Carlo method to deconvolve light curves of blazars into individual flares, including proper estimation of the fit errors. We use the method to fit 15\,GHz light curves obtained within the OVRO 40-m blazar monitoring program where a large number of AGN have been monitored since 2008 in support of the Fermi Gamma-Ray Space Telescope mission. The time scales obtained from the fitted models are used to calculate the variability brightness temperature of the sources. Additionally, we have calculated brightness temperatures of a sample of these objects using Very Long Baseline Array data from the MOJAVE survey. Combining these two data sets enables us to study the intrinsic brightness temperature distribution in these blazars at 15 GHz. Our preliminary results indicate that the mean intrinsic brightness temperature in a sample of 14 sources is near the equipartition brightness temperature of $\sim 10^{11}$K.

astro-ph.CO↗

arXiv2013

Discovery of a new TeV gamma-ray source: VER J0521+211

We report the detection of a new TeV gamma-ray source, VER J0521+211, based on observations made with the VERITAS imaging atmospheric Cherenkov telescope array. These observations were motivated by the discovery of a cluster of >30GeV photons in the first year of Fermi-LAT observations. VER J0521+211 is relatively bright at TeV energies, with a mean photon flux of 1.93 +/- 0.13_stat +/- 0.78_sys 10^-11 cm-2 s-1 above 0.2 TeV during the period of the VERITAS observations. The source is strongly variable on a daily timescale across all wavebands, from optical to TeV, with a peak flux corresponding to ~0.3 times the steady Crab Nebula flux at TeV energies. Follow-up observations in the optical and X-ray bands classify the newly-discovered TeV source as a BL Lac-type blazar with uncertain redshift, although recent measurements suggest z=0.108. VER J0521+211 exhibits all the defining properties of blazars in radio, optical, X-ray, and gamma-ray wavelengths.

astro-ph.HE↗

arXiv2013

MOJAVE. X. Parsec-Scale Jet Orientation Variations and Superluminal Motion in AGN

We describe the parsec-scale kinematics of 200 AGN jets based on 15 GHz VLBA data obtained between 1994 Aug 31 and 2011 May 1. We present new VLBA 15 GHz images of these and 59 additional AGN from the MOJAVE and 2 cm Survey programs. Nearly all of the 60 most heavily observed jets show significant changes in their innermost position angle over a 12 to 16 year interval, ranging from 10 deg to 150 deg on the sky, corresponding to intrinsic variations of ~0.5 deg to ~2 deg. The BL Lac jets show smaller variations than quasars. Roughly half of the heavily observed jets show systematic position angle trends with time, and 20 show indications of oscillatory behavior. The time spans of the data sets are too short compared to the fitted periods (5 to 12 y), however, to reliably establish periodicity. The rapid changes and large jumps in position angle seen in many cases suggest that the superluminal AGN jet features occupy only a portion of the entire jet cross section, and may be energized portions of thin instability structures within the jet. We have derived vector proper motions for 887 moving features in 200 jets having at least five VLBA epochs. For 557 well-sampled features, there are sufficient data to additionally study possible accelerations. We find that the moving features are generally non-ballistic, with 70% of the well-sampled features showing either significant accelerations or non-radial motions. Inward motions are rare (2% of all features), are slow (<0.1 mas per y), are more prevalent in BL Lac jets, and are typically found within 1 mas of the unresolved core feature. There is a general trend of increasing apparent speed with distance down the jet for both radio galaxies and BL Lac objects. In most jets, the speeds of the features cluster around a characteristic value, yet there is a considerable dispersion in the distribution. (abridged)

astro-ph.CO↗

arXiv2013

VLBA observations of a rare multiple quasar imaging event caused by refraction in the interstellar medium

We report on the first detection of the theoretically-predicted rare phenomenon of multiple parsec-scale imaging of an active galactic nucleus induced by refractive effects due to localized foreground electron density enhancements, e.g., in an AU-scale plasma lens(es) in the ionized component of the Galactic interstellar medium. We detected multiple imaging in the low galactic latitude (b=-2 deg) quasar 2023+335 from the 15.4 GHz MOJAVE observations when the source was undergoing an ESE. While the parsec-scale jet of the source normally extends along PA -20 deg, in the 28 May 2009 and 23 July 2009 images a highly significant multi-component pattern of secondary images is stretched out nearly along the constant galactic latitude line with a local PA 40 deg, indicating that the direction of relative motion of the plasma lens is close to orbital. Weaker but still detectable imaging patterns at similar position angles are sporadically manifest at several other epochs. Modeling the ESE that occurred in early 2009 and lasted ~0.14 yr, we determined that the foreground screen has a double-lens structure, with proper motion (~6.8 mas/yr), and angular size (~0.27 mas). We also found that the angular separation between the two brightest sub-images roughly follows a wavelength-squared dependence expected from plasma scattering. Furthermore, by analyzing archival non-simultaneous VLBA observations covering a wide frequency range from 1.4 to 86 GHz, we found that the scattered angular size of the VLBI core follows a $ν^{-1.89}$ dependence, implying the presence of a turbulent, refractive dominated scattering screen that has a confined structure or is truncated transverse to the line of sight toward 2023+335.

astro-ph.CO↗

arXiv2013

MOJAVE: Monitoring of Jets in Active Galactic Nuclei with VLBA Experiments. VIII. Faraday rotation in parsec-scale AGN jets

We report observations of Faraday rotation measures (RMs) for a sample of 191 extragalactic radio jets observed within the MOJAVE program. Multifrequency VLBA observations were carried out over twelve epochs in 2006 at four frequencies between 8 and 15 GHz. We detect parsec-scale Faraday RMs in 149 sources and find the quasars to have larger RMs on average than BL Lac objects. The median core RMs are significantly higher than in the jet components. This is especially true for quasars where we detect a significant negative correlation between the magnitude of the RM and the de-projected distance from the core. We perform detailed simulations of the observational errors of total intensity, polarization and Faraday rotation, and concentrate on the errors of transverse Faraday RM gradients in unresolved jets. Our simulations show that the finite image restoring beam size has a significant effect on the observed RM gradients, and spurious gradients can occur due to noise in the data if the jet is less than two beams wide in polarization. We detect significant transverse RM gradients in four sources (0923+392, 1226+023, 2230+114 and 2251+158). In 1226+023 the RM is for the first time seen to change sign from positive to negative over the transverse cuts, which supports the presence of a helical magnetic field in the jet. In this source we also detect variations in the jet RM over a time scale of three months, which are difficult to explain with external Faraday screens and suggest internal Faraday rotation. By comparing fractional polarization changes in jet components between the four frequency bands to depolarization models we find that an external purely random Faraday screen viewed through only a few lines of sight can explain most of our polarization observations but in some sources, such as 1226+023 and 2251+158, internal Faraday rotation is needed.

astro-ph.CO↗

arXiv2012

Catching the radio flare in CTA 102: II. VLBI kinematic analysis

We performed multifrequency multiepoch Very Long Baseline Array (VLBA) observations of the blazar CTA 102 during its 2006 radio flare, the strongest ever reported for this source. These observations provide an excellent opportunity to investigate the evolution of the physical properties of blazars, especially during these flaring events. We want to study the kinematic changes in the source during the strong radio outburst in April 2006 and test the assumption of a shock-shock interaction. This assumption is based on the analysis and modeling of the single-dish observations of CTA\,102 (Paper I). In this paper we study the kinematics of CTA 102 at several frequencies using VLBI observations. From the modeled jet features we derived estimates for the evolution of the physical parameters, such as the particle density and the magnetic field. Furthermore, we combined our observations during the 2006 flare with long-term VLBA monitoring of the source at 15 GHz and 43 GHz. We cross-identified seven features throughout our entire multifrequency observations and find evidence of two possible recollimation shocks around 0.1 mas (deprojected 18 pc at a viewing angle of 2.6 degrees) and 6.0 mas (deprojected 1 kpc) from the core. The 43 GHz observations reveal a feature ejected at epoch $t_\mathrm{ej}=2005.9\pm0.2$, which could be connected to the 2006 April radio flare. Furthermore, this feature might be associated with the traveling component involved in the possible shock-shock interaction, which gives rise to the observed double peak structure in the single-dish light curves reported in Paper I.

astro-ph.HE↗

arXiv2012

Rapid TeV Gamma-Ray Flaring of BL Lacertae

We report on the detection of a very rapid TeV gamma-ray flare from BL Lacertae on 2011 June 28 with the Very Energetic Radiation Imaging Telescope Array System (VERITAS). The flaring activity was observed during a 34.6-minute exposure, when the integral flux above 200 GeV reached $(3.4\pm0.6) \times 10^{-6} \;\text{photons}\;\text{m}^{-2}\text{s}^{-1}$, roughly 125% of the Crab Nebula flux measured by VERITAS. The light curve indicates that the observations missed the rising phase of the flare but covered a significant portion of the decaying phase. The exponential decay time was determined to be $13\pm4$ minutes, making it one of the most rapid gamma-ray flares seen from a TeV blazar. The gamma-ray spectrum of BL Lacertae during the flare was soft, with a photon index of $3.6\pm 0.4$, which is in agreement with the measurement made previously by MAGIC in a lower flaring state. Contemporaneous radio observations of the source with the Very Long Baseline Array (VLBA) revealed the emergence of a new, superluminal component from the core around the time of the TeV gamma-ray flare, accompanied by changes in the optical polarization angle. Changes in flux also appear to have occurred at optical, UV, and GeV gamma-ray wavelengths at the time of the flare, although they are difficult to quantify precisely due to sparse coverage. A strong flare was seen at radio wavelengths roughly four months later, which might be related to the gamma-ray flaring activities. We discuss the implications of these multiwavelength results.

astro-ph.HE↗

arXiv2012

Radio-to-gamma-ray Monitoring of the Narrow-Line Seyfert 1 Galaxy PMN J0948+0022 from 2008 to 2011

We present more than three years of observations at different frequencies, from radio to high-energy gamma-rays, of the Narrow-Line Seyfert 1 (NLS1) Galaxy PMN J0948+0022 (z=0.585). This source is the first NLS1 detected at energies above 100 MeV and therefore can be considered the prototype of this emerging new class of gamma-ray emitting active galactic nuclei (AGN). The observations performed from 2008 August 1 to 2011 December 31 confirmed that PMN J0948+0022 generates a powerful relativistic jet, able to develop an isotropic luminosity at gamma-rays of the order of 10^48 erg s^-1, at the level of powerful quasars. The evolution of the radiation emission of this source in 2009 and 2010 followed the canonical expectations of relativistic jets, with correlated multiwavelength variability (gamma-rays followed by radio emission after a few months), but it was difficult to retrieve a similar pattern in the light curves of 2011. The comparison of gamma-ray spectra before and including 2011 data suggested that there was a softening of the high-energy spectral slope. We selected five specific epochs to be studied by modelling the broad-band spectrum, characterised by an outburst at gamma-rays or very low/high flux at other wavelengths. The observed variability can largely be explained either by changes in the injected power, the bulk Lorentz factor of the jet or the electron spectrum. The characteristic time scale of doubling/halving flux ranges from a few days to a few months, depending on the frequency and the sampling rate. The shortest doubling time scale at gamma-rays is 2.3+-0.5 days. These small values underline the need of highly-sampled multiwavelength campaigns to better understand the physics of these sources.

astro-ph.HE↗

arXiv2012

MOJAVE: Monitoring of Jets in Active Galactic Nuclei with VLBA Experiments. IX. Nuclear opacity

We have investigated a frequency-dependent shift in the absolute position of the optically thick apparent origin of parsec-scale jets ("core shift" effect) to probe physical conditions in ultra-compact relativistic outflows in AGN. We used multi-frequency Very Long Baseline Array (VLBA) observations of 191 sources carried out in 12 epochs in 2006 within the MOJAVE program. The observations were performed at 8.1, 8.4, 12.1, and 15.4 GHz. We implemented a method of determining the core shift vector based on (i) image registration by two-dimensional normalized cross-correlation and (ii) model-fitting the source brightness distribution to take into account a non-zero core component offset from the phase center. The 15.4-8.1, 15.4-8.4, and 15.4-12.1 GHz core shift vectors are derived for 163 sources, and have median values of 0.128, 0.125, and 0.088 mas, respectively, compared to the typical measured errors of 0.050, 0.051, 0.035 mas. The effect occurs predominantly along the jet direction, with departures smaller than 45 deg from the median jet position angle in over 80% of the cases. Despite the moderate ratio of the observed frequencies (<2), core shifts significantly different from zero (>2sigma) are detected for about 55% of the sources. These shifts are even better aligned with the jet direction, deviating from the latter by less than 30 deg in over 90% of the cases. There is an indication that the core shift decreases with increasing redshift. Magnetic fields in the jet at a distance of 1 parsec from the central black hole, calculated from the obtained core shifts, are found to be systematically stronger in quasars (median B1~0.9 G) than those in BL Lacs (median B1~0.4 G). We also constrained the absolute distance of the core from the apex of the jet at 15 GHz as well as the magnetic field strength in the 15 GHz core region.

astro-ph.HE↗

arXiv2012

Apparent parsec-scale jet opening angles and γ-ray brightness of active galactic nuclei

We have investigated the differences in apparent opening angles between the parsec-scale jets of the active galactic nuclei (AGN) detected by the Fermi Large Area Telescope (LAT) during its first 24 months of operations and those of non-LAT-detected AGN. We used 15.4 GHz VLBA observations of 215 sources from the 2 cm VLBA MOJAVE program. The apparent opening angles were determined by analyzing transverse jet profiles from the data in the image plane by using stacked images constructed from all available MOJAVE epochs for a given source. We confirm our earlier result based on the first three months of scientific operations of the LAT. The apparent opening angles of γ-ray bright AGN are preferentially larger than those of γ-ray weak sources, suggesting smaller viewing angles to the γ-ray bright AGN. Intrinsic opening angles for BL Lacs are wider than those in quasars.

astro-ph.HE↗

arXiv2012

Faraday rotation in the MOJAVE blazars: Connection with gamma-ray studies

We have conducted a survey of Faraday rotation in a sample of 191 compact radio-loud AGNs as part of the MOJAVE (Monitoring of Jets in Active galactic nuclei with VLBA Experiments) project. The observations were carried out with the VLBA at 8.1, 8.4, 12.1 and 15.3 GHz over 12 epochs in 2006. We detect sufficiently strong linear polarization in 159 out of 211 observations to calculate the rotation measure values, resulting in a large enough sample for statistical analysis of the Faraday rotation in blazars. These Faraday rotation measures can be used to study the intrinsic magnetic field order and orientation in parsec-scale blazar jets. Our sample includes 119 sources listed in the 1FGL or 2FGL catalogs and we detect rotation measure values in 111 out of 131 maps. Of the 72 sources that are not in the gamma-ray catalogs we detect RM in 48 out of 80 maps. The median RM values of the LAT-detected sources do not differ significantly from the non-LAT-detected sources. Nine of the sources in our sample have resolved enough jets to study the transverse Faraday rotation structure, and we detect significant transverse rotation measure gradients in four sources. In two of these (3C~273 and 3C~454.3) there is additional evidence to support helical magnetic field in the parsec-scale jets. The two others (0923+392 and 2230+114) require further observations to identify the nature of the gradient. It is interesting that three of the four sources with significant rotation measure gradients are sources that have shown large gamma-ray flares.

astro-ph.HE↗