Variable X‐Ray Absorption in the Seyfert 2 Galaxy Markarian 348

Abstract

We present Rossi X-Ray Timing Explorer monitoring observations of the Seyfert 2 galaxy Mrk 348 spanning a 6 month period. The time-averaged spectrum in the 3-20 keV band shows many features characteristic of a Compton-thin Seyfert 2 galaxy, namely, a hard underlying power-law continuum (Γ ≈ 1.8) with heavy soft X-ray absorption (NH ~ 1023 cm-2) plus measurable iron Kα emission (equivalent width ~100 eV) and, at high energy, evidence for a reflection component (R 1). During the first half of the monitoring period, the X-ray continuum flux from Mrk 348 remained relatively steady. However, this was followed by a significant brightening of the source (by roughly a factor of 4) with the fastest change corresponding to a doubling of its X-ray flux on a timescale of about 20 days. The flux increase was accompanied by a marked softening of the X-ray spectrum most likely attributable to a factor of ~3 decline in the intrinsic line-of-sight column density. In contrast, the iron Kα line and the reflection components showed no evidence of variability. These observations suggest a scenario in which the central X-ray source is surrounded by a patchy distribution of absorbing material located within about a light-week of the nucleus of Mrk 348. The random movement of individual clouds within the absorbing screen, across our line of sight, produces substantial temporal variations in the measured column density on timescales of weeks to months and gives rise to the observed X-ray spectral variability. However, as viewed from the nucleus, the global coverage and typical thickness of the cloud layer remains relatively constant.