Abstract
We present Swift and XMM observations of GRB 050326, detected by Swift-BAT. The fluence was 7.7x10^-6 erg cm^-2 (20-150 keV), and its spectrum was hard, with a power law photon index 1.25. The afterglow light curve did not show any break nor flares between ~1 hr and ~6 d after the burst, and decayed with a slope 1.70. The afterglow spectrum is well fitted by a power-law model, suffering absorption both in the Milky Way and in the host galaxy. The rest-frame Hydrogen column density is significant, N_H_z > 4x10^21 cm^-2, and the redshift of the absorber is z > 1.5. There was good agreement between the Swift-XRT and XMM results. By comparing the prompt and afterglow fluxes, we found that an early break occurred before the XRT observation. The properties of the GRB 050326 afterglow are well described by a spherical fireball expanding in a uniform external medium, so a further steepening is expected at later times. The lack of such a break constrains the jet angle to be >7 deg. Using the redshift constraints provided by the X-ray analysis, we also estimated that the beaming-corrected gamma-ray energy was >3x10^51 erg, at the high end of GRB energies. Despite the brightness in X rays, only deep limits could be placed by Swift-UVOT at optical/UV wavelengths. Thus, this GRB was "truly dark", with the optical-to-X-ray spectrum violating the synchrotron limit. The optical and X-ray observations are consistent either with an absorbed event or with a high-redshift one. To obey the Ghirlanda relation, a moderate/large redshift z>4.5 is required. (abridged)
Highlights
The Swift satellite (Gehrels et al 2004) is a mission dedicated to the study of gamma-ray bursts (GRBs) and their afterglows
We present Swift and XMM-Newton observations of the bright gamma-ray burst GRB 050326, detected by the Swift Burst Alert Telescope
In order to extract the light curve, we considered all photon counting (PC) data, but discarded the windowed timing (WT) data taken after the second orbit of the X-Ray Telescope (XRT) observation (t > 20 ks)
Summary
The Swift satellite (Gehrels et al 2004) is a mission dedicated to the study of gamma-ray bursts (GRBs) and their afterglows. In the remaining four cases, the beginning of the observation was delayed by approximately 50 min due to the Earth occultation constraints This is the case for the bright GRB 050326, which was discovered by BAT on 2005 Mar. 26 at 9:53:55 UT (Markwardt et al 2005). The decay of the light curve prevented any further detection of the afterglow This object was observed for 45.8 ks by XMM-Newton (Ehle & Perez Martinez 2005; De Luca et al 2005a), starting 8.5 h after the trigger. Limited ground-based follow-up was reported for this burst This was likely due to its unfavorable location in the sky (very few telescopes can point at such low declination), as well as to the brightness of the Moon (which was 99% full at the time of the GRB explosion).
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