X-Ray Flashes and X-Ray Counterparts of Gamm-Ray Bursts

摘要

The brightest transients in the sky, Supernovae and Gamma-Ray Bursts, are associated with the collapse of cores of massive stars. They shine in the optical and in the gamma-ray sky. On theoretical grounds one would expect to see similar events in the x-ray and ultra-violet sky. Here we summarize recent observational evidence demonstrating the existence of X-ray bursts, termed X-ray Flashes (XRFs). We argue that they are most likely very large explosions on a cosmological distant scale, similar to Gamma-Ray Bursts (GRBs). They may either be highly redshifted GRBs, GRBs viewed at a large angle or another geometrical effect. Or they may be a new type of cosmic explosion expanding mildly relativistically, much larger than the initial expansion of supernova remnants, but less than the extreme relativistic cosmic fireballs of Gamma-Ray Bursts. The ratio of the energy contained in the X-ray part (2-10 keV) of Gamma-Ray Bursts to the y-range (50-350 keV) varies widely. GRBs for which this fraction is typically more than half, are referred to as X-ray Rich GRBs. The new class of x-ray transients, the X-ray Flashes, show a further extension of this ratio to above 1, where most of the energy is contained in the x-ray range. They have properties similar to the X-ray counterpart of GRBs, but do not trigger Gamma-Ray Burst experiments since they are not detected in the gamma-ray range above typically 100 keV. X-ray Flashes nevertheless appear to be related to Gamma-Ray Bursts. Observationally X-ray Flashes are found as a subset of Fast X-ray Transients, which have been seen by almost all x-ray satellites. Their nature has remained unclear in most cases. It was generally assumed that the origin of FXTs is a mixture of detector artifacts and several types of astronomical events, including coronal emission from late type stars. Using the Wide Field camera on board BeppoSAX we have made a systematic study of FXTs and identify at least two types on the basis of their duration. The subset with a typical duration of order an hour contain identifications with active coronal sources. The other subset which last typically of the order of minutes, are the X-ray Flashes. A recent X-ray Flash, XRF011030 or SAX J2043.6+7717 is shown to be associated with an afterglow in the radio and in the X-ray range and thus demonstrates the relation with a cosmic explosive event, which may very well be a new type of cosmic explosion associated with stellar collapse.