X-ray flashes and X-ray rich Gamma Ray Bursts

X-ray flashes are detected in the Wide Field Cameras on BeppoSAX in the energy range 2-25 keV as bright X-ray sources lasting of the order of minutes, but remaining undetected in the Gamma Ray Bursts Monitor on BeppoSAX. They have properties very similar to the x-ray counterparts of GRBs and account for some of the Fast X-ray Transient events seen in almost every x-ray satellite. We review their X-ray properties and show that x-ray flashes are in fact very soft, x-ray rich, untriggered gamma ray bursts, in which the peak energy in 2-10 keV x-rays could be up to a factor of 100 larger than the peak energy in the 50-300 keV gamma ray range. The frequency is ∼ 100 yr. 1 Fast X-ray Transients/High-latitude X-ray Transients Fast X-ray Transients have been observed with many x-ray satellites. In particular they are seen with x-ray instruments that scan the entire sky on a regular basis. Such events are detected in one sky scan and disappeared in the next, typically limiting the duration to be longer than a minute and shorter than a few hours. For this reason they are called Fast Transients. The first transients of this type were seen with UHURU (Forman et al. [6]). We review some of the other observations. Ariel V. Ariel V scanned the sky for 5.5 years with a time resolution of one satellite orbit (∼ 100 min) and in 1983 Pye and McHardy [11] reported 27 events. In contrast to LMXBs, Fast Transients are also seen at high galactic latitude and are, therefore, also called High Latitude Transient X-ray Sources. About 20% of the sources seen with Ariel V are identified with RS CVn systems and have a duration of order hours. The authors conclude that all Ariel V observations are consistent with as yet unknown coronal sources, but remark that two of the transients are time coincident with gamma ray burst sources. One of them also was spatially coincident with a GRB to within ∼ 1. The frequency is estimated as one Fast Transient every ∼ 3 days above 4× 10 erg/s/cm (2-10 keV). HEAO-1 also had complete sky coverage. Ambruster et al. [1] report the analysis of the first 6 month of HEAO-1 scanning data. They observe 10 Fast Xray Transients with the A1 instrument (0.5-20 keV) above∼ 7×10 erg/s/cm, of which 4 are identified with flare stars. The duration is > 10 s and < 1.5 hr. They estimate an all sky rate of ∼ 1500 per year. In the A2 instrument (2-60 keV), 5 more Fast Transients have been detected [2] with peak fluxes between ∼ 10 and ∼ 10 erg scm (2-10 keV). They suggest that most of the