X-ray flaring activity of Markarian 421

We report results from a systematic search for X-ray flares from Mrk 421, using archival data from the Rossi X-Ray Timing Explorer (RXTE) satellite. The flares are clearly seen over a wide range of timescales. The quasicontinuous coverage of the source with the All-SkyMonitor (ASM) reveals frequent occurrence of major flares that last for months. On a few occasions, the source was intensively monitored with the more sensitive pointing instruments aboard RXTE. The data from these observations show the presence of X-ray flares of much shorter duration, ranging from weeks down to less than an hour. For the first time, we clearly resolved the most rapid flares from Mrk 421. Moreover, Fourier analyses reveal variability on even shorter timescales, up to about 0.01 Hz. The source appears to behave differently in its spectral properties during different flares, large or small, which is intriguing. While significant hysteresis is observed to be associated with spectral evolution in some cases, little is seen in other cases. Sometimes, the shape of the X-ray spectrum hardly varies across a flare. Therefore, the phenomenology is complex. The observed hierarchical structure of the X-ray flares seems to imply the scaleinvariant nature of the phenomenon, perhaps similar in this regard to solar flares or the rapid X-ray flares observed from stellar-mass black holes. Combined with other results, the observed flaring timescales seriously constrain the physical properties of X-ray–emitting regions in the jets of Mrk 421. Subject headings: BL Lacertae objects: individual (Markarian 421) — galaxies: active — radiation mechanisms: nonthermal — X-rays: galaxies