The 2001 April Burst Activation of SGR 1900+14: X-Ray Afterglow Emission

After nearly two years of quiescence, the soft gamma-ray repeater SGR 1900+14again became burst-active on April 18 2001, when it emitted a large flare,preceded by few weak and soft short bursts. After having detected the X andgamma prompt emission of the flare, BeppoSAX pointed its narrow field X-raytelescopes to the source in less than 8 hours. In this paper we present ananalysis of the data from this and from a subsequent BeppoSAX observation, aswell as from a set of RossiXTE observations. Our data show the detection of anX-ray afterglow from the source, most likely related to the large hard X-rayflare. In fact, the persistent flux from the source, in 2-10 keV, was initiallyfound at a level $\sim$5 times higher than the usual value. Assuming anunderlying persistent (constant) emission, the decay of the excess flux can bereasonably well described by a t$^{-0.9}$ law. A temporal feature - a$\sim$half a day long bump - is observed in the decay light curve approximatelyone day after the burst onset. This feature is unprecedented in SGR afterglows.We discuss our results in the context of previous observations of this sourceand derive implications for the physics of these objects.Comment: 18 pages - 2 Figures (one in color) - Astrophysical Journal (Part I), in pres