X-ray flashes or soft gamma-ray bursts?

Abstract

Context. The origin of X-ray Flashes (XRFs) is still a mystery and several models have been proposed. To disentangle among these models, an important observational tool is the measure of the XRF distance scale, so far available only for a few of them. Aims. In this work, we present a multi-wavelength study of XRF 040912, aimed at measuring its distance scale and the intrinsic burst properties. Methods. We performed a detailed spectral and temporal analysis of both the prompt and the afterglow emission and we estimated the distance scale of the likely host galaxy. We then used the currently available sample of XRFs with known distance to discuss the connection between XRFs and classical Gamma-ray Bursts (GRBs). Results. We found that the prompt emission properties unambiguously identify this burst as an XRF, with an observed peak energy of E p = 17 ± 13 keV and a burst fluence ratio S 2−30 keV /S 30−400 keV > 1. A non-fading optical source with R ∼ 24 mag and with an apparently extended morphology is spatially consistent with the X-ray afterglow, likely the host galaxy. XRF 040912 is a very dark burst since no afterglow optical counterpart is detected down to R > 25 mag (3σ limiting magnitude) at 13.6 h after the burst. The host galaxy spectrum detected from 3800 Å to 10 000 Å, shows a single emission line at 9552 Å. The lack of any other strong emission lines blue-ward of the detected one and the absence of the Lyα cut-off down to 3800 Å are consistent with the hypothesis of the [OII] line at redshift z = 1.563 ± 0.001. The intrinsic spectral properties rank this XRF among the soft GRBs in the E peak −E iso diagram. Similar results were obtained for most XRFs at known redshift. Only XRF 060218 and XRF 020903 represent a good example of instrinsic XRF (i-XRF) and are possibly associated with a different progenitor population. This scenario may call for a new definition of XRFs.

Intrinsic properties of a complete sample ofHETE-2 gamma-ray bursts

“…However, half of the sub-sample of the observed XRFs are part of the cluster of intrinsic classical GRBs when considered in their source frame. This fact was also noted by Stratta et al (2007).…”

“…Moreover, two other GRBs detected in 2003 and 2004 by HETE-2 had a pseudo-redshift ) but no redshift. Some later observations of the host galaxies resulted in the determination of their spectroscopic redshifts, in agreement with their pseudo-redshifts: Rau et al (2005) measured the redshift of the host galaxy of GRB 030531 to be z = 0.782 ± 0.001 (ẑ = 0.64 ± 0.15, , and Stratta et al (2007) obtained the redshift of the X-ray flash 040912: z = 1.563 ± 0.001 from the [OII] line of the host-galaxy spectrum, also consistent with its previously determined pseudoredshift (ẑ = 2.90 ± 1.60, .…”

“…Indeed, according to the HETE-2 sample, several XRFs are situated in the gap between the group of intrinsic classical GRBs and the two intrinsic XRFs 020903 iso,52 (with E intr peak in keV and E iso,52 in units of 10 52 erg) found by Amati (2006) and also the vertical logarithmic deviation of 0.4 (dotted lines) displayed in Amati (2006). In the top right panel the dashed curve marks the redshift evolution of the E peak , assuming a burst with E (z=0) peak = 40 keV (see Stratta et al 2007). 060218 (Amati et al 2007), as is XRF 050416, a soft burst detected by Swift (Sakamoto et al 2006a,b). However, half of the sub-sample of the observed XRFs are part of the cluster of intrinsic classical GRBs when considered in their source frame.…”

“…We consider in this paragraph the question posed, e.g., in Stratta et al (2007): do the XRFs form a continuum extending the "classical GRBs" to soft energies, or do they form a distinct population, the intrinsic XRFs (i-XRFs)? Figure 13 (panel 1) shows the Amati relation for the complete sample of 82 HETE-2 bursts and Fig.…”

Intrinsic properties of a complete sample of HETE-2 Gamma-Ray Bursts

“…In the top left panel, by way of comparison, we show (solid line) the best-fit power-law E intr peak =77× 0.57 iso,52 (with E intr peak in keV and E iso,52 in units of 10 52 erg) found by Amati (2006) and also the vertical logarithmic deviation of 0.4 (dotted lines) displayed in Amati (2006). In the top right panel the dashed curve marks the redshift evolution of the E peak , assuming a burst with E (z=0) peak = 40 keV (see Stratta et al 2007).…”