Showing papers by "L. Maraschi published in 1998"

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

Abstract: 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 simeq3) 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.

The hidden X-ray Seyfert nucleus in 3C 273: BeppoSAX results

Abstract: We present the results of 5 BeppoSAX AO1 Core Program observations of 3C 273 performed in Jan. 1997 and compare them in detail with data obtained during the satellite Science Verification Phase (SVP), in Jul. 1996 (Grandi et al. 1997). 3C 273 was about 15% brighter in the first 1997 observation than in the last one, and, on average, a factor 2 brighter than the SVP observation. A count rate variation in the 2-10 keV band of ' 12 %i n 0:5 day was clearly detected during the last of the four pointings. Power-law fits with Galactic absorption to all observations

BeppoSAX spectral survey of soft X-ray selected BL Lacertae objects

Abstract: We present X-ray spectra obtained with BeppoSAX (Satellite per Astronomia X) of 10 BL Lac objects, selected from the Einstein Medium Sensitivity and Slew Surveys. We find that in about half of the objects a fit in the 0.1-10 keV range with a single power law and free absorption yields values of NH larger than the Galactic ones. In most of these cases, however, broken power law fits with NH fixed at the Galactic values yield an alternative, better description of the data and indicate a steepening of the spectrum with increasing energy. One object (1ES1101-232) is detected up to ∼ 100 keV. Its spectral energy distribution (SED) peaks in the medium energy X-ray band. For each object we compute the peak frequency of the SED from multifrequency data. The spectral indices αx in the 2-10 keV band (Fν ∝ ν−αx) are smaller (i.e. flatter spectrum) for objects with higher peak frequencies. We therefore confirm and extend to higher energies the behavior already known for X-ray selected BL Lac objects in the ROSAT band. We do not find spectral indices smaller than 1; however, the flat distribution of αx and the correlation between αx and peak frequency found from our data suggest that a number of objects may exist, which in the quiescent status have flatter spectrum and peak frequency in the hard X-ray range.

Reprocessing and Variable Cold Absorption in the Broad-Line Radio Galaxy 3C390.3

Abstract: A BeppoSAX observation of the BLRG 3C390.3 is reported. For the first time, both the 6.4 keV iron line and a strong reflection hump, produced by the illumination of the primary X-ray emission on cold matter, are detected in this source. The 0.1-100 keV continuum is modeled by an absorbed power law (\Gamma~ 1.8) reflected at high energies by material with a fairly large covering factor (\Omega/2\pi~1). The iron line is intrinsically narrow ($\sigma=73(+207,-73) eV) and has an equivalent width of ~140$ eV. We discuss the results in the context of current models for AGNs and suggest that the primary X-ray power law continuum is probably produced by a hot inner flow, while the reprocessed radiation comes from an outer cold thin disk, and/or from a thick torus at even larger radii. Further observations with BeppoSAX could distinguish between the latter two cases. Beamed radiation associated to the radio jet is unlikely to contribute significantly to the X-ray emission. Finally, an historical study of the column density NH, also reported here, shows that the absorption along the line of sight changes in time. The NH time variability, which is not correlated with that of the primary continuum, seems to imply variations of the geometry of the absorber rather than variations in the ionization state of the gas.

The complex 0.1-100 keV X-ray spectrum of PKS 2155-304

Abstract: A long (> 100;000 seconds) observation of the bright BL Lac object PKS 2155-304 has been carried out with the Narrow Field Instruments of the BeppoSAXsatellite as part of the Science Verification Phase. The source was detected be- tween 0.1 and about 100 keV at an intermediate intensity level compared to previous observations. The unique spectral cover- age of BeppoSAXhas allowed us to detect a number of spectral features. Between 0.1 and 10 keV the spectrum can be well de- scribed by a convex spectrum with (energy) slope gradually steepening from 1.1 to 1.6. At higher energies evidence for a sharp spectral hardening is found, while in the soft X-rays (0.1- 1.0 keV) some evidence for an absorption feature was found. Indication for an emission line at 6.4 keV in the source rest frame is present. Repeated variability of 20-30% around the mean flux is clearly detected on time scales of a few hours. From the symmetry and timescale of the observed variations we de- rive limits on the magnetic field and on the maximum energy of the emitting particles, implying that PKS 2155-304 should not be bright at TeV energies.

BeppoSax Spectral Survey of soft X-ray selected BL Lacs

Abstract: We present X-ray spectra obtained with BeppoSAX (Satellite per Astronomia X) of 10 BL Lac objects, selected from the Einstein Medium Sensitivity and Slew Surveys. We find that in about half of the objects a fit in the 0.1-10 keV range with a single power law and free absorption yields values of NH larger than the Galactic ones. In most of these cases, however, broken power law fits with NH fixed at the Galactic values yield an alternative, better description of the data and indicate a steepening of the spectrum with increasing energy. One object (1ES1101-232) is detected up to ~100 keV. Its spectral energy distribution (SED) peaks in the medium energy X-ray band. For each object we compute the peak frequency of the SED from multifrequency data. The spectral indices alpha_x in the 2-10 keV band (Fnu prop to nu^{-alpha_x}) are smaller (i.e. flatter spectrum) for objects with higher peak frequencies. We therefore confirm and extend to higher energies the behavior already known for X-ray selected BL Lac objects in the ROSAT band. We do not find spectral indices smaller than 1; however, the flat distribution of alpha_x and the correlation between alpha_x and peak frequency found from our data suggest that a number of objects may exist, which in the quiescent status have flatter spectrum and peak frequency in the hard X-ray range.