X-ray Flashes or soft Gamma-ray Bursts? The case of the likely distant XRF 040912

TL;DR

This study analyzes XRF 040912 across multiple wavelengths to determine its distance and properties, revealing it as a soft GRB with a likely host galaxy at redshift 1.563, and discusses the connection between XRFs and classical GRBs.

Contribution

It provides a detailed spectral and temporal analysis of XRF 040912, estimating its redshift and intrinsic properties, and compares it with other XRFs to explore their relation to GRBs.

Findings

XRF 040912 has an observed peak energy of 17 keV.

The host galaxy is likely at redshift 1.563.

XRF 040912 is a very dark burst with no optical afterglow detected.

Abstract

In this work, we present a multi-wavelength study of XRF 040912, aimed at measuring its distance scale and the intrinsic burst properties. 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). 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(2-30keV)/S(30-400keV)>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 (3 sigma limiting magnitude) at 13.6 hours after the burst. The host galaxy spectrum detected from 3800A to 10000A, shows a single emission line at 9552A. The lack of any other strong emission lines blue-ward of the detected one and the absence of the Ly alpha cut-off down to 3800A 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 calls for a new definition of XRFs.