X-Ray Spectral Variability of PKS 2005-489 During the Spectacular November 1998 Flare

We report on monitoring of the BL Lac object PKS 2005 - 489 by the Rossi X-ray Timing Explorer (RXTE) covering the period October-December 1998. During these three months, the source underwent a spectacular flare; at its peak on November 10, its 2-10 keV flux was $\sim 3.3 \times 10^{-10} erg cm^{-2} s^{-1}$, over 30 times brighter than its average in X-ray observations prior to 1995. The structure of this flare was remarkably simple and symmetrical, lasting about a month at half-maximum and nearly three months from end to end. Thanks to the fortuitous timing of RXTE observations in the weeks before maximum and a flare alert by the all-sky monitor team, we were able to study the X-ray spectral behavior of PKS 2005 - 489 during this event. During the flare's rising phase, the X-ray spectrum of PKS 2005 - 489 hardened considerably, its 2 - 10 keV spectral index reaching $\alpha = 1.32 (F_\nu \propto \nu^{-\alpha})$ near maximum. During the declining phase, the X-ray spectrum first steepened rapidly, reaching $\alpha = 1.82$, then became somewhat harder towards the end of December ($\alpha \sim 1.6$). While behavior of this type has been seen before in flare observations of BL Lac objects, the simplicity, magnitude and long duration of this flare allowed us to study it in great detail. We argue that this flare was caused by the injection of particles into the jet, and that the spectral steepening which followed the flare maximum was the result of synchrotron cooling.