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Astron. Astrophys. 362, 105-112 (2000)

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1. Introduction

An understanding of the physical processes leading to the generation of the enormous luminosities observed from active galactic nuclei (AGN) and the explanation of the physical and morphological differences of the nuclear regions around the black hole giving rise to the various manifestations of the AGN phenomenon is one of the great challenges of modern astrophysics.

BL Lac objects represent a relatively small but highly exciting subgroup of the AGN population characterized by large luminosities, rapid variability, and high and variable polarization of the emitted radiation. As their spectra are smooth and nearly featureless at all wavelengths it is rather challenging to determine any physical parameters of these systems.

BL Lacs are thought to be dominated by relativistic jets seen at small angles to the line of sight (Urry & Padovani 1995, Königl 1989), and their radio-through-X-ray spectra are well fitted by inhomogeneous jet models (Bregman et al. 1987). However, the high energy emission of X-ray bright (or blue) BL Lacs (e.g. Maraschi 1998) may well be dominated by a single emission region, as suggested by the success of spectral models based on synchrotron and inverse Compton emission from a homogenous region (Tavecchio et al. 1998). On the basis of this model Chiappetti et al. (1999) determined recently the physical parameters of the emission region in PKS 2155-304 using simultaneous X-ray and gamma-ray observations obtained with BeppoSAX and EGRET. At the same time the source was detected for the first time at TeV energies and the measured flux turned out to be in agreement with that predicted by the model. Kataoka et al. (1999) arrived at similar values of the physical parameters from an ASCA observation of PKS 2155-304 , using a time-dependent one-zone homogeneous Synchrotron-Self-Compton model.

Combining spectral and temporal information greatly constrains the jet physics, since different models predict different variability time scales. The measured lags between the light curves at different energies as well as spectral changes during intensity variations allow to probe the micro-physics of particle acceleration and radiation in the jet. Elucidating the structure of blazar jets, possible through multi-wavelength monitoring, is an essential precursor to understanding their formation and thus the extraction of energy from the central engine. The critical elements for effective multi-wavelength monitoring are a sufficiently rapid sampling to resolve variability, temporal coverage long enough to view several flares, and a wide wavelength coverage of the optically thin synchrotron component and, if possible, the Compton-scattered component.

The BL Lac object PKS 2155-304 is one of the best candidates for blazar monitoring because it is both rapidly variable and one of the brightest extragalactic objects in the ultraviolet and X-ray sky, i.e., bright enough that its variability can be resolved at UV and X-ray wavelengths. Like most BL Lac objects, PKS 2155-304 has no strong emission features; the reported redshift of [FORMULA] (Falomo et al. 1993) was obtained from spectroscopy of the nebulosity surrounding the BL Lac object. PKS 2155-304 has previously been observed to be highly variable at both ultraviolet (Maraschi et al. 1986, Urry et al. 1988, Edelson et al. 1991) and X-ray wavelengths (Snyder et al. 1980, Treves et al. 1989, Sembay et al. 1993, Kataoka et al. 1999).

Multi-wavelength monitoring of PKS 2155-304 in November 1991 and in May 1994 has led to the best available data for any blazar at the time (Urry et al. 1993, Brinkmann et al. 1994, Courvoisier et al. 1995, Edelson et al. 1995, Urry et al. 1997, Pian et al. 1997, Pesce et al. 1997). This X-ray/EUV/UV/optical monitoring of PKS 2155-304 established for the first time that: (1) the X-ray through optical emission in blazars are closely related; (2) variability occurs on time scales less than 1 day; and (3) the X-ray flux leads the UV by a few hours to a few days. The tight X-ray/UV correlation and the overall UV to X-ray spectral shape confirmed the supposition that synchrotron emission is responsible for the optical-through-X-ray continuum in blazars and ruled out conclusively any significant optical/UV continuum in an accretion disk.

Nevertheless, the light curves of PKS 2155-304 from the two campaigns showed significant differences (Urry et al. 1997). In 1991 the X-ray/UV/optical variations were of similar low amplitude, the measured lag was quite small and the variations were nearly achromatic. In 1994, the X-ray flare was much larger than the nearest UV flare and the lag was at least two days. The larger-amplitude wavelength-dependent variability and larger lag in 1994 likely result from physical processes in the relativistic jet itself.

Neither campaign on PKS 2155-304 (much less for any other blazar) was ideal for good cross-correlations. The first campaign had only 3.5 days of overlap between X-ray and UV/optical, enough to show a clear correlation and to measure a tentative lag, but the X-rays and UV flux appear to diverge at the end. The second campaign was excellent in UV coverage but had less than two days of ASCA data. The X-ray flare appears to precede the EUV and UV flares by 1 and 2 days, respectively, but because only 2 of 12 days were covered in X-rays, this depends on the uncertain association of the X-ray flare with the UV (e.g., it could be related to another flare at the beginning of the UV light curve).

Therefore we designed a monitoring program that would produce high-quality light curves in several bands, including unprecedented spectral coverage in the ultraviolet, extreme ultraviolet, and soft-X-ray from the combination of IUE, EUVE, the ROSAT Wide Field Camera (WFC), and the ROSAT High Resolution Imager (HRI). At infrared wavelengths a 12-day light curve from ISO was obtained, at the highest X-ray energies a detailed monitoring and spectroscopy of PKS 2155-304 was performed with the Rossi XTE. The observations took place in 1996 May and November.

In this paper we will first present the results of the ROSAT HRI observations and then compare the results to previous measurements of PKS 2155-304 taken over the life time of ROSAT. A first comparison of the soft X-ray data with the Rossi XTE data at higher X-ray energies have been presented by Urry et al. (1998); Sambruna et al. (2000) present the results of an analysis of the temporal variations of the source in the RXTE X-ray band, and ISO results can be found in Bertone et al. (2000).

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© European Southern Observatory (ESO) 2000

Online publication: October 30, 19100