2000ApJ...534L..39C

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 ∼60% in less than 200 s. This rapid rise is followed by a drop-off in the 2-10 keV flux of ∼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 ≲3 keV before the flare to ≳30 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 ~ 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.