Abstract
Given the paucity of empirical constraints, the nature of the newly recognized phenomena called X-ray flashes (XRFs) has been an open question. However, with the recent detections of radio and X-ray afterglow it is finally possible to study the large- and small-scale environments of XRFs. We present Chandra, Hubble Space Telescope (HST), and Keck observations of the fields of XRFs 011030 and 020427. Astrometric comparisons of the X-ray transient positions and the HST images reveal the XRFs to be associated with faint blue galaxies. Photometric evidence of these putative hosts suggests that these two XRFs originated from redshifts less than z ~ 3.5, and thus cannot be due to GRBs at very high redshifts. In both host-burst offsets and host properties, these XRFs could have been drawn from distributions similar to those measured of gamma-ray bursts (GRBs). We conclude with a discussion of the implications of this XRF-GRB host connection for the possible progenitors of XRFs.
Highlights
A new class of high-energy transients called X-ray flashes (XRFs) has been identified
In both host-burst offsets and host properties, these XRFs could have been drawn from distributions similar to those measured of gamma-ray bursts (GRBs)
Host magnitudes, and the formulation in Bloom et al (2002), we estimate this chance to be PchðXRF 011030Þ 1⁄4 0:00797 and PchðXRF 020427Þ 1⁄4 0:00595 : we believe that, as with most other GRBs localized to date, these XRFs are likely to be physically associated with galaxies
Summary
A new class of high-energy transients called X-ray flashes (XRFs) has been identified Kulkarni 2003, in preparation, and Yamazaki et al 2003 for recent reviews) These events have an annual allsky rate that is between one-third and one-half of the gamma-ray burst (GRB) rate and make a substantial contribution to the cosmic explosion rate. XRFs could arise from a new physical class of explosions, GRBs that originate from very large redshifts (ze6; Heise et al 2001), or lower redshift variants of GRBs (e.g., GRBs beamed away from Earth; Yamazaki et al 2002, GRBs with dense ambient gas, or transition GRBs with lower Lorentz factor outflows; Dermer, Chiang, & Bottcher 1999). The only reason to possibly associate XRFs with the death of massive stars is that XRFs appear to have a duration distribution similar to those of the long-duration GRBs. A basic discriminator of the various XRF progenitor models is a measurement of the distance to the explosions.
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