The First Two Host Galaxies of X‐Ray Flashes: XRF 011030 and XRF 020427

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.

On the origin of X-ray flashes

“…Its AG was discovered by Fox et al (2003b). Later measurements were reported by Dullighan et al (2003a,b), Smith et al (2003), Bond et al (2003) and Fynbo et al (2003) who reported a "rebrightening" in the optical AG 14 days after the XRF, which may be due to the contribution of a SN.…”

“…3 In the CB model GRBs (and, more so, their individual pulses) are much better standard candles than in the FB models Berger et al 2003;Bloom et al 2003b). …”

“…The direct comparison of optical AGs is not an issue because that of XRF 020903 is due to the CBs, while the very large observation angle of GRB 980425 implied . The CB-model fit to the R-band AG of XRF 030723 (Prigozhin et al 2003;Fox et al 2003b;Dullighan et al 2003a,b;Smith et al 2003;Bond et al 2003;Fynbo et al 2003). The first two data points have been deduced from the unfiltered measurements of Smith et al (2003), assuming an early F ν ∼ ν −0.5 (Dado et al 2003a).…”

“…Finally, in the AGs of relatively nearby XRFs it should be possible to observe the contribution of a SN akin to SN1998bw, displaced to the XRF's position and peaking about one month after the SN exploded and the XRF was emitted (Dar & De Rújula 2003). Such a shining-gun signature may already have been observed in the AG of XRF 030723 (Fynbo et al 2003), see 2 Subsequent to critiques (Dar 1998(Dar , 1999(Dar , 2003Dar & Plaga 1999;Dado et al 2002a;De Rújula 2002 of the on-axis viewingangle anzatz, θ = 0, of the "collimated-fireball" or conical-jet models (e.g., Rhoads 1997Rhoads , 1999Sari et al 1999;Piran 2000;Panaitescu & Kumar 2000;Frail et al 2001;Berger et al 2003;Bloom et al 2003b), this assumption has recently been replaced by a θ 0 of uniform or structured conical jets (e.g., Rossi et al 2002;Granot et al 2002;Zhang & Mészáros 2002;Panaitescu & Kumar 2003;Jin & Wei 2003;Salmonson 2003). The jets' opening angles are θ v > 1/γ, but getting close to inverting the inequality (Waxman 2003;Lazzatti et al 2003) to spouse the CB-model's geometry.…”

The FirstSwiftX‐Ray Flash: The Faint Afterglow of XRF 050215B

“…Follow-up observations have in some cases successfully found afterglow emission, most commonly at X-ray wavelengths, and occasionally in the optical and NIR. Accurate positions have indicated that, like GRBs, XRFs are found in star-forming galaxies at cosmological distances (e.g., Bloom et al 2003b) but, based on statistics of a few, at a rather lower median redshift than that for GRBs. Recently, the very low redshift XRF 060218, 145 Mpc distant, was shown to be associated with SN 2006aj Pian et al 2006;Modjaz et al 2006), clearly demonstrating that some XRFs, like long-duration GRBs, originate in the core collapse of a massive star in a Type Ic supernova (SN Ic; Hjorth et al 2003;Stanek et al 2003).…”