Esko Valtaoja

Multiwavelength Observations of a Dramatic High-Energy Flare in the Blazar 3C 279

The blazar 3C 279, one of the brightest identified extragalactic objects in the γ-ray sky, underwent a large (factor of ~10 in amplitude) flare in γ-rays toward the end of a 3 week pointing by Compton Gamma Ray Observatory (CGRO), in 1996 January-February. The flare peak represents the highest γ-ray intensity ever recorded for this object. During the high state, extremely rapid γ-ray variability was seen, including an increase of a factor of 2.6 in ~8 hr, which strengthens the case for relativistic beaming. Coordinated multifrequency observations were carried out with Rossi X-Ray Timing Explorer (RXTE), Advanced Satellite for Cosmology and Astrophysics (ASCA; or, Astro-D), Roentgen Satellite (ROSAT), and International Ultraviolet Explorer (IUE) and from many ground-based observatories, covering most accessible wavelengths. The well-sampled, simultaneous RXTE light curve shows an outburst of lower amplitude (factor of 3) well correlated with the γ-ray flare without any lag larger than the temporal resolution of ~1 day. The optical-UV light curves, which are not well sampled during the high-energy flare, exhibit more modest variations (factor of ~2) and a lower degree of correlation. The flux at millimetric wavelengths was near a historical maximum during the γ-ray flare peak, and there is a suggestion of a correlated decay. We present simultaneous spectral energy distributions of 3C 279 prior to and near to the flare peak. The γ-rays vary by more than the square of the observed IR-optical flux change, which poses some problems for specific blazar emission models. The synchrotron self-Compton (SSC) model would require that the largest synchrotron variability occurred in the mostly unobserved submillimeter/far-infrared region. Alternatively, a large variation in the external photon field could occur over a timescale of a few days. This occurs naturally in the mirror model, wherein the flaring region in the jet photoionizes nearby broad emission line clouds, which, in turn, provide soft external photons that are Comptonized to γ-ray energies.

Kinematics of the Parsec-Scale Relativistic Jet in Quasar 3C 279: 1991-1997

We present results of long-term high-frequency VLBI monitoring of the relativistic jet in 3C 279, consisting of 18 epochs at 22 GHz from 1991 to 1997 and 10 epochs at 43 GHz from 1995 to 1997. Three major results of this study are apparent speeds measured for six superluminal components range from 4.8c to 7.5c (H0 = 70 km s-1 Mpc-1, q0 = 0.1), variations in the total radio flux are due primarily to changes in the VLBI core flux, and the uniform-sphere brightness temperature of the VLBI core is ~1 × 1013 K at 22 GHz after 1995, one of the highest direct estimates of a brightness temperature. If the variability brightness temperature measured for 3C 279 by Lahteenmaki & Valtaoja is an actual value and not a lower limit, then the rest-frame brightness temperature of 3C 279 is quite high and limited by inverse Compton effects rather than equipartition. The parsec-scale morphology of 3C 279 consists of a bright, compact VLBI core, a jet component (C4) that moved from ~2 to ~3.5 mas from the core during the course of our monitoring, and an inner jet that extends from the core to a stationary component, C5, at ~1 mas from the core. Component C4 followed a curved path, and we reconstruct its three-dimensional trajectory using polynomial fits to its position versus time. Component C5 faded with time, possibly due to a previous interaction with C4 similar to interactions seen in simulations by Gomez et al. Components in the inner jet are relatively short lived and fade by the time they reach ~1 mas from the core. The components have different speeds and position angles from each other, but these differences do not match the differences predicted by the precession model of Abraham & Carrara. Although VLBI components were born about six months prior to each of the two observed γ-ray high states, the sparseness of the γ-ray data prevents a statistical analysis of possible correlations.

Multifrequency observations of blazars. III - The spectral shape of the radio to X-ray continuum

Multifrequency, quasi-simultaneous spectra for a sample of 11 blazars are presented. The spectral shape of the violently variable millimeter to ultraviolet flux is consistent with emission from a very compact single component which becomes self-absorbed at wavelengths longer than about 3 mm. The centimeter emission can be attributed to a separate, more slowly varying component. Three out of four optically violent variable quasars also exhibit evidence of a UV excess component. Values for the size of the flaring regions of 0.001-0.1 pc and magnetic fields of order 1 G are deduced. It is found that photon energy densities may dominate over magnetic field energy densities, in which case inverse Compton scattering may be the dominant energy loss mechanism in flaring components.

A complete sample of Northern hemisphere active galactic nuclei : differences in flux and spectra at high radio frequencies

We present two-epoch 22 and 31 GHz flux measurements of a complete 1 Jy sample of flat-spectrum extragalactic radio sources. The spectral and variability properties of quasars and blazars in our sample are similar. Radio galaxies, which have steeper cm-to-mm spectra, are the only sources which can be identified solely on the basis of their radio continuum properties. Surprisingly, among the flat-spectrum sources the BL Lacs and low polarization quasars with the steepest spectra seem to be the most variable ones. The implications of these results for orientation-dependent unified models for active galaxies are discussed

THE MULTIFREQUENCY SPECTRAL EVOLUTION OF BLAZAR 3C 345 DURING THE 1991 OUTBURST

The blazar 3C 345 underwent a 2.5 mag optical outburst between 1990 November and 1991 May. We have obtained 10 nearly simultaneous multifrequency spectra during the course of the outburst in order to study the multifrequency spectral variations of 3C 345 as a function of time. Although our observations were not sampled frequently enough to completely resolve the variations in every frequency band, the general rise and decline of the outburst were seen in the UV through radio with differing rise times. Simulations of an electron distribution injected into a tangled magnetic field show a relationship between frequency and characteristic timescale that was also observed in the radio variations of 3C 345. The two X-ray observations made during the monitoring period showed no evidence of variability. The multifrequency spectrum was modeled with two major components: a relativistic jet and a relativistic thermal accretion disk. Models calculated for each spectrum indicate that the outburst can be explained in terms of these models by varying the high-energy cutoff of the injected electron distribution in the jet model, while also varying the mass accretion rate in the disk model. There is marginal evidence that the inferred accretion rate varies with the jet luminosity.

Radio to γ-ray observations of 3C 454.3:1993–1995

Results from a multiwaveband study of the γ-ray-bright compact radio source 3C 454.3 are presented.

Long term radio variability of AGN

Talvikki Hovatta∗,1 Merja Tornikoski,1 Harry J. Lehto,2,3 Elina Nieppola,1 Esko Valtaoja,2,3 Markku Lainela,2 Ilona Torniainen,1 Anne Lahteenmaki1 Margo F. Aller4 and Hugh D. Aller4 1Metsahovi Radio Observatory, TKK, Helsinki University of Technology Metsahovintie 114, 02540 Kylmala, Finland 2Tuorla Observatory, University of Turku Vaisalantie 20, 21500 Piikkio, Finland 3Department of Physics, University of Turku 20140 University of Turku, Finland 4Department of Astronomy, University of Michigan Ann Arbor, MI 48109, USA E-mail: tho@kurp.hut.fi

The radio/gamma-ray connection in AGNs

We have continued the comparison of the Metsahovi total flux density radio variations and EGRET gamma-ray observations from Cycles 1–4. This work compliments the arguments presented at the 4th Compton Symposium. We have also attempted to remove the effect that different radio and gamma-ray spectral indices have on the observed properties. We discuss the relationship between the two frequency regions and the consequences it has on the source modeling. It seems that the radio emission and the gamma-rays originate within the same shocked area of the relativistic jet and that the gamma-rays are most likely produced by the synchrotron self-Compton (SSC) mechanism.

Multiwavelength observations of the February 1996 high-energy flare in the blazar 3C 279

We report CGRO, RXTE, ASCA, ROSAT, IUE, HST and ground-based observations of a large flare in 3C 279 in February 1996. X-rays and γ-rays peaked simultaneously (within one day). We show simultaneous spectral energy distributions prior to and near the flare peak. The γ-ray flare was the brightest ever observed in this source.