Search

ABSTRA C T We present details of a large X-ray flare in the blazar 3C 279 detected during a 3-week period of daily observations by the satellite RXTE in early 1996. The flare lasted for a total of 7 d. The shape of the flare is well described by a symmetrical, exponential rise and fall with e-folding time-scales in each case of 1.1 d. The peak measured flux is three times the quiescent level. The flare is superimposed on a well-defined quiescent level and appears to represent a separate emission component. There is no statistically significant variability of the X-ray spectral index during the observations, but the errors are large and the data hint at a hardening of about 0.1 at the peak of the flare. Such a hardening is required to allow the X-ray flare spectrum to join on, in a single power law, to the g-ray flare that occurred at the same time. The exponential rise and decay most likely correspond to a variation in the acceleration of relativistic electrons or in the flux of seed photons, if the latter do not originate in the jet, as the energy loss time-scales are shorter than the rise and decay time-scales and the flare profile does not match that expected if light-travel delays determine the light curve.

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.

The high sensitivity and improved spectral resolution (ΔE/E = 0.4 at 1 keV) of ROSAT has enabled high quality spectra to be obtained in the soft X-ray (0.1−2 keV) band. When added to simultaneous Ginga data, spectra extending up to 20 keV are achieved. 3C273 was observed by ROSAT in a pointed observation in June 1990 and again in December 1990 during the survey, when it was also observed by Ginga . The December observation yielded a 3σ detection in the SI filter of the Wide Field Camera. E1821+643, benefits from being at a high galactic latitude and was observed for a total of 40 days during the ROSAT survey, with an effective exposure time of 9ks. Ginga observed the source during this period. It was also observed serendipitously during the pointed phase in February 1991. Both sources need a two component model to fit their combined 0.1−20 keV spectrum (see Tables 1 & 2). The spectrum of 3C273 has a significant soft excess which exists below 1.2 keV (source rest frame). This may be adequately parameterised by a 200eV Bremsstrahlung or a power law of α = 3.4, however, a single temperature Blackbody or Raymond-Smith model is not a good fit to the data, neither are elementary disc models. E1821+643 has a strong, steep soft excess which rises below 0.65 keV (source rest frame) in both ROSAT observations.