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H. D. Aller

Publications and source records attributed to H. D. Aller.

At least 73 records · Page 4Linked to original sources
arXiv2008

AGILE detection of intense gamma-ray emission from the blazar PKS 1510-089

We report the detection by the AGILE (Astro-rivelatore Gamma a Immagini LEggero) satellite of an intense gamma-ray flare from the source AGL J1511-0909, associated with the powerful quasar PKS 1510-089, during ten days of observations from 23 August to 1 September 2007. During the observation period, the source was in optical decrease following a flaring event monitored by the GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope (WEBT). The simultaneous gamma-ray, optical, and radio coverage allows us to study the spectral energy distribution and the theoretical models based on the synchrotron and inverse Compton (IC) emission mechanisms. AGILE observed the source with its two co-aligned imagers, the Gamma-Ray Imaging Detector and the hard X-ray imager Super-AGILE sensitive in the 30 MeV - 50 GeV and 18 - 60 keV bands, respectively. Between 23 and 27 August 2007, AGILE detected gamma-ray emission from PKS 1510-089 when this source was located about 50 degrees off-axis, with an average flux of (270 +/- 65) x 10^{-8} photons cm^{-2} s^{-1} for photon energy above 100 MeV. In the following period, 28 August - 1 September, after a satellite re-pointing, AGILE detected the source at about 35 degrees off-axis, with an average flux (E > 100 MeV) of (195 +/- 30) x 10^{-8} photons cm^{-2} s^{-1}. No emission was detected by Super-AGILE, with a 3-sigma upper limit of 45 mCrab in 200 ksec. The spectral energy distribution is modelled with a homogeneous one-zone synchrotron self Compton (SSC) emission plus contributions by external photons: the SSC emission contributes primarily to the X-ray band, whereas the contribution of the IC from the external disc and the broad line region match the hard gamma-ray spectrum observed.

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arXiv2008

Long term radio variability of AGN

A large number of AGN have been monitored for nearly 30 years at 22, 37 and 87 GHz in Metsähovi Radio Observatory. These data were combined with lower frequency 4.8, 8.0 and 14.5 GHz data from the University of Michigan Radio Astronomy Observatory, higher frequency data at 90 and 230 GHz from SEST, and supplementary higher frequency data from the literature to study the long-term variability of a large sample of AGN. Both the characteristics of individual flares from visual inspection and statistically-determined variability timescales as a function of frequency and optical class type were determined. Based on past behaviour, predictions of sources expected to exhibit large flares in 2008--2009 appropriate for study by GLAST and other instruments are made. The need for long-term data for properly understanding source behaviour is emphasised.

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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

The multiwavelength variability of 3C 273

We present an update of 3C 273's database hosted by the ISDC, completed with data from radio to gamma-ray observations over the last 10 years. We use this large data set to study the multiwavelength properties of this quasar,especially focussing on its variability behaviour. We study the amplitude of the variations and the maximum variability time scales across the broad-band spectrum and correlate the light curves in different bands, specifically with the X-rays, to search for possible connections between the emission at different energies. 3C 273 shows variability at all frequencies, with amplitudes and time scales strongly depending on the energy and being the signatures of the different emission mechanisms. The variability properties of the X-ray band imply the presence of either two separate components (possibly a Seyfert-like and a blazar-like) or at least two parameters with distinct timing properties to account for the X-ray emission below and above ~20 keV. The dominant hard X-ray emission is most probably not due to electrons accelerated by the shock waves in the jet as their variability does not correlate with the flaring millimeter emission, but seems to be associated to long-timescale variations in the optical. This optical component is consistent with being optically thin synchrotron radiation from the base of the jet and the hard X-rays would be produced through inverse Compton processes (SSC and/or EC) by the same electron population. We show evidence that this synchrotron component extends from the optical to the near-infrared domain, where it is blended by emission of heated dust that we find to be located within about 1 light-year from the ultraviolet source.

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arXiv2008

Long-term radio variability of AGN: flare characteristics

We have studied the flare characteristics of 55 AGN at 8 different frequency bands between 4.8 and 230 GHz. Our extensive database enables us to study the various observational properties of flares in these sources and compare our results with theoretical models. We visually extracted 159 individual flares from the flux density curves and calculated different parameters, such as the peak flux density and duration, in all the frequency bands. The selection of flares is based on the 22 and 37 GHz data from Metsähovi Radio Observatory and 90 and 230 GHz data from the SEST telescope. Additional lower frequency 4.8, 8, and 14.5 GHz data are from the University of Michigan Radio Observatory. We also calculated variability indices and compared them with earlier studies. The observations seem to adhere well to the shock model, but there is still large scatter in the data. Especially the time delays between different frequency bands are difficult to study due to the incomplete sampling of the higher frequencies. The average duration of the flares is 2.5 years at 22 and 37 GHz, which shows that long-term monitoring is essential for understanding the typical behaviour in these sources. It also seems that the energy release in a flare is independent of the duration of the flare.

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arXiv2008

The Trails of Superluminal Jet Components in 3C111

In 1996, a major radio flux-density outburst occured in the broad-line radio galaxy 3C111. It was followed by a particularly bright plasma ejection associated with a superluminal jet component, which has shaped the parsec-scale structure of 3C111 for almost a decade. Here, we present results from 18 epochs of Very Long Baseline Array (VLBA) observations conducted since 1995 as part of the VLBA 2 cm Survey and MOJAVE monitoring programs. This major event allows us to study a variety of processes associated with outbursts of radio-loud AGN in much greater detail than has been possible in other cases: the primary perturbation gives rise to the formation of a leading and a following component, which are interpreted as a forward and a backward-shock. Both components evolve in characteristically different ways and allow us to draw conclusions about the work flow of jet-production events; the expansion, acceleration and recollimation of the ejected jet plasma in an environment with steep pressure and density gradients are revealed; trailing components are formed in the wake of the primary perturbation possibly as a result of coupling to Kelvin-Helmholtz instability pinching modes from the interaction of the jet with the external medium. The interaction of the jet with its ambient medium is further described by the linear-polarization signature of jet components traveling along the jet and passing a region of steep pressure/density gradients.

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arXiv2007

A Possible Periodicity in the Radio Lightcurves of 3C454.3

During the period 1966.5 - 2006.2 the 15GHz and 8GHz lightcurves of 3C454.3 (z=0.859) show a qsasi-periodicity of ~12.8 yr (~6.9 yr in the rest frame of the source) with a double-bump structure. This periodic behaviour is interpreted in terms of a rotating double-jet model in which the two jets are created from the black holes in a binary system and rotate with the period of the orbital motion. The periodic variations in the radio fluxes of 3C454.3 are suggested to be mainly due to the lighthouse effects (or the variation in Doppler boosting) of the precessing jets which are caused by the orbital motion. In addition, variations in the mass-flow rates accreting onto the black holes may be also involved.

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arXiv2007

Doppler Boosting, Superluminal Motion, and the Kinematics of AGN Jets

We discuss results from a decade long program to study the fine-scale structure and the kinematics of relativistic AGN jets with the aim of better understanding the acceleration and collimation of the relativistic plasma forming AGN jets. From the observed distribution of brightness temperature, apparent velocity, flux density, time variability, and apparent luminosity, the intrinsic properties of the jets including Lorentz factor, luminosity, orientation, and brightness temperature are discussed. Special attention is given to the jet in M87, which has been studied over a wide range of wavelengths and which, due to its proximity, is observed with excellent spatial resolution. Most radio jets appear quite linear, but we also observe curved non-linear jets and non-radial motions. Sometimes, different features in a given jet appear to follow the same curved path but there is evidence for ballistic trajectories as well. The data are best fit with a distribution of Lorentz factors extending up to gamma ~30 and intrinsic luminosity up to ~10^26 W/Hz. In general, gamma-ray quasars may have somewhat larger Lorentz factors than non gamma-ray quasars. Initially the observed brightness temperature near the base of the jet extend up to ~5x10^13 K which is well in excess of the inverse Compton limit and corresponds to a large excess of particle energy over magnetic energy. However, more typically, the observed brightness temperatures are ~2x10^11 K, i.e., closer to equipartition.

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arXiv2007

Statistical analyses of long-term variability of AGN at high radio frequencies

We present a study of variability time scales in a large sample of Active Galactic Nuclei at several frequencies between 4.8 and 230 GHz. We investigate the differences of various AGN types and frequencies and correlate the measured time scales with physical parameters such as the luminosity and the Lorentz factor. Our sample consists of both high and low polarization quasars, BL Lacertae objects and radio galaxies. The basis of this work is the 22 GHz, 37 GHz and 87 GHz monitoring data from the Metsahovi Radio Observatory spanning over 25 years. In addition,we used higher 90 GHz and 230 GHz frequency data obtained with the SEST-telescope between 1987 and 2003. Further lower frequency data at 4.8 GHz, 8 GHz and 14.5 GHz from the University of Michigan monitoring programme have been used. We have applied three different statistical methods to study the time scales: The structure function, the discrete correlation function and the Lomb-Scargle periodogram. We discuss also the differences and relative merits of these three methods. Our study reveals that smaller flux density variations occur in these sources on short time scales of 1-2 years, but larger outbursts happen quite rarely, on the average only once in every 6 years. We do not find any significant differences in the time scales between the source classes. The time scales are also only weakly related to the luminosity suggesting that the shock formation is caused by jet instabilities rather than the central black hole.

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arXiv2006

The Kinematics of S5 1803+784

We present the results of a multi-frequency analysis of the structural variability in the parsec-scale jet of the blazar S5 1803+784. More than 90 epochs of observations at 6 frequencies from 1.6 GHz up to 22 GHz have been combined and analyzed. We discuss an alternative jet model for the source. In contrast to previously discussed motion scenarios for S5 1803+784, we find that the jet structure within 12 mas of the core can most easily be described by seven "oscillating" jet features. We find that the parameters of jet features, such as core separation, position angle and flux density, change in a periodic way with a timescale of about 4 years. We also find evidence for a correlation between these parameters and the total flux density variability. We suggest a scenario incorporating a periodic form of motion (e.g. rotation, precession), with a non-negligible geometrical contribution to explain the observational results.

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arXiv2006

"Oscillating" components in the BL Lac object 0716+714?

15 VLBA observations of the BL Lac object 0716+714, performed between 1994.67 and 2006.40 at 15 GHz, have been analyzed. Part of the data (12 epochs) were obtained within the Mojave-survey project and reanalyzed by us. We present a new motion scenario for jet model component motion in this source which suggests that no longterm-outward motion but rather an oscillation of components around an average core separation is taking place. Although no significant outward motion in the core separation can be found, motion with regard to the position angle is observed. We give lower limits for the derived apparent motions and Doppler factors and compare them with values published in the literature. We find a relation between the total flux-density evolution at 14.5 GHz (UMRAO data) and the position angle changes observed for the jet component closest to the core. We suggest a significant geometric contribution to the longterm flux-density variability in 0716+714.

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arXiv2006

Frequency-dependent time delays for strong outbursts in four blazars from the Metsahovi and UMRAO monitoring databases

The combined data of the University of Michigan Radio Astronomy Observatory and Metsahovi Radio Observatory provide us with radio light curves for Active Galactic Nuclei monitored by both observatories from 4.8 to 37 GHz covering time intervals up to ~25 years. We consider here such composite light curves for four gamma-ray blazars that have been nearly continuously monitored at both observatories: 0458-020, 0528+134, 1730-130 and 2230+114. We have decomposed the most prominent outbursts in the light curves of these four blazars into individual components using Gaussian model fitting, and estimated the epochs, amplitudes, and half-widths of these components as functions of frequency. We attempt to distinguish "core outbursts", which show frequency-dependent time delays and are associated with brightening of the core, from "jet outbursts", which appear nearly synchronously at all frequencies and are accompanied by the emergence of new jet components and their subsequent evolution. Available 43 GHz VLBA images allow us to identify only one pure core outburst (in 2230+114) and one pure jet outburst (0458-020). Most of the outbursts analyzed are mixed, in the sense that they display frequency-dependent time delays (i.e., they are optically thick) and are associated with the eventual emergence of new jet components. The maxima of the jet and mixed outbursts probably correspond to epochs when newly ejected components become fully optically thin. These epochs are also marked by a significant increase in the angular velocities of the ejected components. There is evidence that the outbursts in 2230+114 repeat every 8.0+-0.3 years, with the positions of individual sub-outbursts being preserved from one quasi-periodic eight-year cycle to another, even though their amplitudes vary by more than a factor of two.

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arXiv2006

A Quasi-Periodic Modulation of the Radio Light Curve of the Blazar PKS B0048-097

In this letter we present the results of a wavelet analysis of the radio light curve of the BL Lac Object PKS B0048-097 from the University of Michigan Radio Astronomy Observatory monitoring program at 8GHz during twenty-five years, from 1979 to 2004. The results show a remarkable periodicity of 450-470 days in the early 1980s that changed to a ~585 day periodicity in the late 1980s to early 1990s. A less pronounced ~400 day periodicity is found after \~1995. Very-long-baseline interferometry imaging at 15GHz shows dramatic structural changes in the usually unresolved source between two epochs, 1995.57 and 2002.38. The pronounced northward directed jet seen in the 2002 image differs by more than 90 degrees in direction from the source structure found in earlier epochs. These findings make PKS B0048-097 a primary target for multi-wavelength observations and intensive radio monitoring to decipher the blazar-variability phenomenon.

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arXiv2006

Structure and flux variability in the VLBI jet of BL Lacertae during the WEBT campaigns (1995--2004)

BL Lacertae has been the target of several observing campaigns by the Whole Earth Blazar Telescope (WEBT) collaboration and is one of the best studied blazars at all accessible wavelengths. A recent analysis of the optical and radio variability indicates that part of the radio variability is correlated with the optical light curve. Here we present an analysis of a huge VLBI data set including 108 images at 15, 22, and 43 GHz obtained between 1995 and 2004. The aim of this study is to identify the different components contributing to the single-dish radio light curves. We obtain separate radio light curves for the VLBI core and jet and show that the radio spectral index of single-dish observations can be used to trace the core variability. Cross-correlation of the radio spectral index with the optical light curve indicates that the optical variations lead the radio by about 100 days at 15 GHz. By fitting the radio time lags vs. frequency, we find that the power law is steeper than expected for a freely expanding conical jet in equipartition with energy density decreasing as the square of the distance down the jet as in the Königl model. There is a section of the compact radio jet where the emission is weak such that flares propagating down the jet are bright first in the core region with a secondary increase in flux about 1.0 mas from the core. This illustrates the importance of direct imaging to the interpretation of multi-wavelength light curves that can be affected by several distinct components at any given time. We discuss how the complex behaviour of the light curves and correlations can be understood within the framework of a precessing helical jet model.

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arXiv2006

A historic jet-emission minimum reveals hidden spectral features in 3C 273

Aims. The aim of this work is to identify and study spectral features in the quasar 3C 273 usually blended by its strong jet emission. Method. A historic minimum in the sub-millimetre emission of 3C 273 triggered coordinated multi-wavelength observations in June 2004. X-ray observations from the INTEGRAL, XMM-Newton and RXTE satellites are complemented by ground-based optical, infrared, millimetre and radio observations. The overall spectrum is used to model the infrared and X-ray spectral components. Results. Three thermal dust emission components are identified in the infrared. The dust emission on scales from 1 pc to several kpc is comparable to that of other quasars, as expected by AGN unification schemes. The observed weakness of the X-ray emission supports the hypothesis of a synchrotron self-Compton origin for the jet component. There is a clear soft-excess and we find evidence for a very broad iron line which could be emitted in a disk around a Kerr black hole. Other signatures of a Seyfert-like X-ray component are not detected.

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arXiv2005

VSOP monitoring of the compact BL Lac object AO 0235+164

In 1999, the highly compact and variable BL Lac object AO 0235+164 was identified as the highest brightness temperature active galactic nucleus observed with the VLBI Space Observatory Programme (VSOP), with T_B > 5.8 x 10^{13} K. The sub-milliarcsecond radio structure of this source has been studied with dual-frequency (1.6 and 5 GHz), polarization-sensitive VSOP observations during 2001 and 2002. Here we present the results of this monitoring campaign. At the time of these observations, the source was weakly polarized and characterized by a radio core that is clearly resolved on space-ground baselines.

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arXiv2005

Sub-milliarcsecond Imaging of Quasars and Active Galactic Nuclei. IV. Fine Scale Structure

We have used VLBA fringe visibility data obtained at 15 GHz to examine the compact structure in 250 extragalactic radio sources. For 171 sources in our sample, more than half of the total flux density seen by the VLBA remains unresolved on the longest baselines. There are 163 sources in our list with a median correlated flux density at 15 GHz in excess of 0.5 Jy on the longest baselines. For about 60% of the sources, we have at least one observation in which the core component appears unresolved (generally smaller than 0.05 mas) in one direction, usually transverse to the direction into which the jet extends. BL Lacs are on average more compact than quasars, while active galaxies are on average less compact. Also, in an active galaxy the sub-milliarcsecond core component tends to be less dominant. IDV sources typically have a more compact, more core-dominated structure on sub-milliarcsecond scales than non-IDV sources, and sources with a greater amplitude of intra-day variations tend to have a greater unresolved VLBA flux density. The objects known to be GeV gamma-ray loud appear to have a more compact VLBA structure than the other sources in our sample. This suggests that the mechanisms for the production of gamma-ray emission and for the generation of compact radio synchrotron emitting features are related. The brightness temperature estimates and lower limits for the cores in our sample typically range between 10^11 and 10^13 K, but they extend up to 5x10^13 K, apparently in excess of the equipartition brightness temperature, or the inverse Compton limit for stationary synchrotron sources. The largest component speeds are observed in radio sources with high observed brightness temperatures, as would be expected from relativistic beaming (abridged).

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