A Rapid X-ray Flare from Markarian 501.

We present X-ray observations of the BL Lacertae object Mrk 501 that were taken with the Rossi X-Ray Timing Explorer in 1998 May as part of a multiwavelength campaign. The X-ray light curve shows a very rapid flare in which the 2-10 keV flux increased by approximately 60% in less than 200 s. This rapid rise is followed by a drop-off in the 2-10 keV flux of approximately 40% in less than 600 s. The 10-15 keV variation in this flare is roughly a factor of 2 on similar timescales. During the rise of the flare, the 3-15 keV spectral index hardened from 2.02+/-0.03 to 1.87+/-0.04, where it remained during the decay of the flare. This is the fastest variation ever seen in X-rays from Mrk 501 and among the fastest seen at any wavelength for this object. The shift in the energy at which the spectral power peaks (from less, similar3 keV before the flare to greater, similar30 keV during the flare) is also among the most rapid shifts seen from this object. This flare occurs during an emission state (2-10 keV flux approximately 1.2x10-10 ergs cm-2 s-1) that is approximately 25% of the peak flux observed in 1997 April from this object but that is still high compared to its historical average X-ray flux. The variations in the hardness ratio are consistent with the low-energy variations leading those at high energies during the development and decay of the flare. This pattern is rare among high-frequency peaked BL Lac objects like Mrk 501, but has been seen recently in two other TeV-emitting BL Lac objects, Mrk 421 and PKS 2155-304. The hard lag is consistent with a flare dominated by the acceleration timescale for a simple relativistic shock model of flaring.