RXTE observations of 3C 279 during a high-energy flare

We present details of a large X-ray flare in the blazar 3C 279 detected dur ing a 3-week period of daily observations by the satellite RXTE in early 19 96. The flare lasted for a total of 7 d. The shape of the flare is well des cribed by a symmetrical, exponential rise and fall with e-folding time-scal es in each case of 1.1 d. The peak measured flux is three times the quiesce nt level. The flare is superimposed on a well-defined quiescent level and a ppears to represent a separate emission component. There is no statisticall y significant variability of the X-ray spectral index during the observatio ns, 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 f lare spectrum to join on, in a single power law, to the gamma-ray hare that occurred at the same time. The exponential rise and decay most likely corr espond to a variation in the acceleration of relativistic electrons or in t he 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 dete rmine the light curve.