X-ray spectral variability of PKS 2005-489 during the spectacular 1998 November flare

We report on monitoring of the BL Lac object PKS 2005-489 by the Rossi X-Ra y Timing Explorer (RXTE) in 1998 October-December. During these months, the source umderwent a spectacular flare; at its peak on November 10, its 2-10 keV flux was 3.35 x 10(-10) ergs cm(-2) s(-1), over 30 times brighter than in quiescence. During the rising phase, the X-ray spectrum of PKS 2005-489 hardened considerably, reaching alpha = 1.32 (F-nu proportional to nu(-alp ha)) near maximum. During the declining phase, the X-ray spectrum steepened rapidly, reaching alpha = 1.82, then became somewhat harder toward the end of December (alpha similar to 1.6). While such behavior has been seen befo re, the simplicity, magnitude, and duration of this flare allowed us to stu dy it in great detail. We argue that this flare was caused by either the in jection of particles into the jet or in situ particle acceleration and that the spectral steepening which followed the flare maximum was the result of synchrotron cooling. Contrary to other recently observed blazar flares (e. g., Mrk 501, 3C 279, PKS 2155-304), our results do not imply a major shift in the location of the synchrotron peak during this flare.