We report on monitoring of the BL Lac object PKS 2005 - 489 by the Rossi X–ray Timing Explorer ( RXTE ) in October-December 1998 .
During these months , the source underwent a spectacular flare ; at its peak on November 10 , its 2 - 10 keV flux was 3.33 \times 10 ^ { -10 } { ~ { } erg~ { } 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 } \propto \nu ^ { - \alpha } ) near maximum .
During the declining phase , the X–ray spectrum steepened rapidly , reaching \alpha = 1.82 , then became somewhat harder towards the end of December ( \alpha \sim 1.6 ) .
While such behavior has been seen before , the simplicity , magnitude and duration of this flare allowed us to study it in great detail .
We argue that this flare was caused by either the injection 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. , Mkn 501 , 3C 279 , PKS 2155-304 ) , our results do not imply a major shift in the location of the synchrotron peak during this flare .