Ray Flashes

X-ray flashes are brief transient sources of X-rays which are distributed isotrop-ically on the sky - in fact, they are very similar to GRBs except for their softer spectra [38]. It has been shown that X-ray flashes can indeed be interpreted as a continuation of the GRB phenomenon towards lower peak energy (typically below 100 keV) [4]. Though optical/radio afterglows have been identified for only a fraction of the X-ray flashes, the afterglow properties are also consistent with those of GRBs. Interestingly, the light curve of one X-ray flash (XRF 030723) also exhibits the excess emission, suggestive of a supernova Ic at z = 0.6, and XRF 020903 has spectroscopic evidence for a related supernova [86].

Initially it had been speculated that XRFs are the high-redshift tail of the GRB distribution. However, the first two out of three redshift estimates of XRFs are below one, suggesting that the cause of the softer spectra is rather a different emission process and/or geometry. Different explanations have been proposed to account for the properties of XRFs: (i) they are classical GRBs but seen off-axis [74, 102],

Fig. 24.9 X-ray afterglow spectra of GRB 011211 (left) and GRB 030227 (right). The detector response has not been unfolded. The Ka lines of Mg, Si, S, Ar, and Ca, redshifted by z = 1-88 (left; optical z = 2-14) and z = 1-39 (right) are marked. No Fe, Co, or Ni emission is detected in these two GRBs (from [73,97])

Fig. 24.9 X-ray afterglow spectra of GRB 011211 (left) and GRB 030227 (right). The detector response has not been unfolded. The Ka lines of Mg, Si, S, Ar, and Ca, redshifted by z = 1-88 (left; optical z = 2-14) and z = 1-39 (right) are marked. No Fe, Co, or Ni emission is detected in these two GRBs (from [73,97])

(ii) they are "dirty" fireballs, e.g., explosions with a larger baryon load and consequently lower r [20]; (iii) fireballs with large Lorentz-factors and/or low baryon loading that in the case of internal shocks lead to the emission of less energetic photons [103]; (iv) low contrast between the bulk Lorentz factors of the colliding relativistic shells [5].

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