arXiv subjects

Hugh D. Aller

Publications and source records attributed to Hugh D. Aller.

29 records · Page 2Linked to original sources

arXiv2010

Probing the Physics of Gamma-Ray Blazars with Single-Dish Monitoring Data

In the 1990s a comparison of sparse EGRET measurements with single-dish flux density monitoring from the Metsahovi and UMRAO programs established a temporal connection between the onset of flaring at radio band and the occurrence of gamma-ray activity. Correlations between the emergence of new VLBI components from the core, flares in linearly polarized radio flux, and gamma-ray activity in bright EGRET-detected blazars supported a picture in which the gamma-ray and the radio band emission arises in the same shocked region of the jet, with the high energy emission produced via inverse Compton scattering by the synchrotron-emitting electrons in the jet. Quantitative tests of this scenario, however, were hampered by insufficient temporal sampling of the data and the simple nature of the models adopted. The extensive data from Fermi coupled with the wealth of well-sampled radio band data from old as well as new programs such as the F-GAMMA project now permit statistical studies for large numbers of sources, including weak HBLs, and detailed analyses of individual highly-active class members. I summarize progress in understanding the origin of the gamma-ray emission using these new measurements. I focus on three areas: attempts to isolate the physical site of the high energy emission using time delay information; investigation of the emission process using the characteristics of the variability; and quantitative tests of the shock model picture using high-time-sampled multifrequency linear polarization data, VLBP imaging, and new models of propagating oblique relativistic shocks incorporating detailed radiative transfer calculations.

astro-ph.HE↗

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.

astro-ph.CO↗

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.

astro-ph.HE↗

arXiv2006

An Extremely Curved Relativistic Jet in PKS 2136+141

We report the discovery of an extremely curved jet in the radio-loud quasar PKS2136+141. Multi-frequency Very Long Baseline Array (VLBA) images show a bending jet making a turn-around of 210 degrees in the plane of the sky, which is, to our knowledge, the largest ever observed change in the position angle of an astrophysical jet. Images taken at six different frequencies, from 2.3 to 43 GHz, reveal a spiral-like trajectory, which is likely a sign of an intrinsic helical geometry. A space-VLBI image, taken with the HALCA satellite at 5 GHz and having comparable resolution to our ground-based 15 GHz data, confirms that the bend is a frequency-independent structure. VLBA monitoring data at 15 GHz, covering eight years of observations, show knots in the jet clearly deviating from ballistic motion, which suggests that the bending may be caused by a growing helical Kelvin-Helmholtz normal mode. The jet appearance suggests a helical wave at a frequency well below the "resonant" frequency of the jet, which indicates that the wave is driven by a periodic perturbation at the base of the jet. We fit the observed structure in the source with a helical twist, and we find that a simple isothermal model with a constant wave speed and wavelength gives a good fit. The measured apparent velocities indicate some degree of acceleration along the jet, which together with an observed change in the apparent half-opening angle of the jet allow us to estimate the changes in the angle between the local jet direction and our line of sight. We suggest that the jet in PKS2136+141 is distorted by a helical Kelvin-Helmholtz normal mode externally driven into the jet (e.g. by precession), and that our line of sight falls within the opening angle of the helix cone.

astro-ph↗

arXiv2004

Radio-optical flux behavior and spectral energy distribution of the intermediate blazar GC 0109+224

About twenty years of radio observations in five bands (from 4.8 to 37 GHz) of the BL Lac object GC 0109+224 (S2 0109+22, RGB J0112+227), are presented and analysed together with the optical data. Over the past ten years this blazar has exhibited enhanced activity. There is only weak correlation between radio and optical flares delays, usually protracted on longer timescales in the radio with respect to the optical. In some cases no radio flare counterpart was observed for the optical outbursts. The radio variability, characterised by peaks superposition, shows hints of some characteristic timescales (around the 3-4 years), and a fluctuation mode between the flickering and the shot noise. The reconstructed spectral energy distribution, poorly monitored at high energies, is preliminarily parameterised with a synchrotron-self Compton description. The smooth synchrotron continuum, peaked in the near-IR-optical bands, strengthens the hypothesis that this source could be an intermediate blazar. Moreover the intense flux in millimetre bands, and the optical and X-ray brightness, might suggest a possible detectable gamma-ray emission.

astro-ph↗

arXiv2003

The Cross-Wavelet Transform and Analysis of Quasiperiodic Behavior in the Pearson-Readhead VLBI Survey Sources

We introduce an algorithm for applying a cross-wavelet transform to analysis of quasiperiodic variations in a time-series, and introduce significance tests for the technique. We apply a continuous wavelet transform and the cross-wavelet algorithm to the Pearson-Readhead VLBI survey sources using data obtained from the University of Michigan 26-m parabloid at observing frequencies of 14.5, 8.0, and 4.8 GHz. Thirty of the sixty-two sources were chosen to have sufficient data for analysis, having at least 100 data points for a given time-series. Of these thirty sources, a little more than half exhibited evidence for quasiperiodic behavior in at least one observing frequency, with a mean characteristic period of 2.4 yr and standard deviation of 1.3 yr. We find that out of the thirty sources, there were about four time scales for every ten time series, and about half of those sources showing quasiperiodic behavior repeated the behavior in at least one other observing frequency.

astro-ph↗

arXiv2001

Parsec-Scale Blazar Monitoring: Flux and Polarization Variability

We present analysis of the flux and polarization variability of parsec scale radio jets from a dual-frequency, six-epoch, VLBA polarization experiment monitoring 12 blazars. The observations were made at 15 and 22 GHz at bimonthly intervals over 1996. Here we analyze the flux, fractional polarization, and polarization position angle behavior of core regions and jet features, considering both the linear trends of these quantities with time and more rapid fluctuations about the linear trends. The dual frequency nature of the observations allows us to examine spectral evolution, to separate Faraday effects from changes in magnetic field order, and also to deduce empirical estimates for the uncertainties in measuring properties of VLBI jet features (abridged).

astro-ph↗

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.

astro-ph↗

arXiv1999

A major radio outburst in III Zw 2 with an extremely inverted, millimeter-peaked spectrum

III Zw 2 is a spiral galaxy with an optical spectrum and faint extended radio structure typical of a Seyfert galaxy, but also with an extremely variable, blazar-like radio core. We have now discovered a new radio flare where the source has brightened more than twenty-fold within less than two years. A broad-band radio spectrum between 1.4 and 666 GHz shows a textbook-like synchrotron spectrum peaking at 43 GHz, with a self-absorbed synchrotron spectral index +2.5 at frequencies below 43 GHz and an optically thin spectral index -0.75 at frequencies above 43 GHz. The outburst spectrum can be well fitted by two homogenous, spherical components with equipartition sizes of 0.1 and 0.2 pc at 43 and 15 GHz, and with magnetic fields of 0.4 and 1 Gauss. VLBA observations at 43 GHz confirm this double structure and these sizes. Time scale arguments suggest that the emitting regions are shocks which are continuously accelerating particles. This could be explained by a frustrated jet scenario with very compact hotspots. Similar millimeter-peaked spectrum (MPS) sources could have escaped our attention because of their low flux density at typical survey frequencies and their strong variability.

astro-ph↗

arXiv1998

Cm-Wavelength Total Flux and Linear Polarization Properties of Radio-Loud BL Lacertae Objects

Results from a long-term program to quantify the range of behavior of the cm-wavelength total flux and linear polarization variability properties of a sample of 41 radio-loud BL Lac objects using weekly to tri-monthly observations with the University of Michigan 26-m telescope operating at 14.5, 8.0, and 4.8 GHz are presented; these observations are used to identify class-dependent differences between these BL Lacs and QSOs in the Pearson-Readhead sample. The BL Lacs are found to be more highly variable in total flux density than the QSOs, exhibiting changes that are often nearly-simultaneous and of comparable amplitude at 14.5 and 4.8 GHz in contrast to the behavior in the QSOs and supporting the existence of class-dependent differences in opacity within the parsec-scale jet flows. Structure function analyses of the flux observations quantify that a characteristic timescale is identifiable in only 1/3 of the BL Lacs. The time-averaged fractional linear polarizations are only on the order of a few percent and are consistent with the presence of tangled magnetic fields within the emitting regions. In many sources a preferred long-term orientation of the EVPA is present; when compared with the VLBI structural axis, no preferred position angle difference is identified. The polarized flux typically exhibits variability with timescales of months to a few years and shows the signature of a propagating shock during several resolved outbursts. The observations indicate that the source emission is predominately due to evolving source components and support the occurrence of more frequent shock formation in BL Lac parsec-scale flows than in QSO jets. The differences in variability behavior and polarization between BL Lacs and QSOs can be explained by differences in jet stability.

astro-ph↗

arXiv1998

Extraordinary Activity in the BL Lac Object OJ~287

We use a continuous wavelet transform to analyze more than two decades of data for the BL~Lac object OJ~287 acquired as part of the UMRAO variability program. We find clear evidence for a persistent modulation of the total flux and polarization with period $\sim 1.66$ years, and for another signal that dominates activity in the 1980s with period $\sim 1.12$ years. The relationship between these two variations can be understood in terms of a `shock-in-jet' model, in which the longer time scale periodicity is associated with an otherwise quiescent jet, and the shorter time scale activity is associated with the passage of a shock; the different periodicities of these two components may reflect different internal conditions of the two flow domains, leading to different wave speeds, or different contractions of a single underlying periodicity, due to the different Doppler factors of the two flow components. We suggest that the modulation arises from a wave driven by some asymmetric disturbance close to the central engine. The periodic behavior in polarization exhibits excursions in $U$ which correspond to a direction $\sim 45^{\circ}$ from the VLBI jet axis. This behavior is not explained by the random walk in the $Q$-$U$ plane which is expected from models in which a pattern of randomly aligned magnetic field elements propagate across the visible portion of the flow, and suggests a small amplitude, cyclic variation in the flow direction in that part of the flow that dominates cm-wavelength emission.

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