Swift and XMM-Newton observations of the dark GRB 050326

¹ INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy e-mail: moretti@merate.mi.astro.it
² INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano, via E. Bassini 15, 20133 Milano Italy
³ International School for Advanced Studies (SISSA/ISAS), via Beirut 2-4, 34014 Trieste, Italy
⁴ Department of Astronomy and Astrophysics, Pennsylvania State University, 525 Davey Lab, University Park, PA 16802, USA
⁵ NASA/Goddard Space Flight Center, Greenbelt Road, Greenbelt, MD20771, USA
⁶ Università degli studi di Milano-Bicocca, Dipartimento di Fisica, piazza delle Scienze 3, 20126 Milano, Italy
⁷ ASI Science Data Center, via G. Galilei, 00044 Frascati (Roma), Italy
⁸ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica di Palermo, via U. La Malfa 153, 90146 Palermo, Italy
⁹ X-Ray Observational Astronomy Group, Department of Physics and Astronomy, University of Leicester, LE1 7RH, UK
¹⁰ Universities Space Research Association, 10211 Wincopin Circle, Suite 500, Columbia, MD, 21044-3432, USA
¹¹ Department of Physics and Astronomy, The Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
¹² Department Physics, University of Nevada, Las Vegas, Nevada, 89154-4002, USA
¹³ Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
¹⁴ Department of Physics and Astronomy, Sonoma State University, Rohnert Park, California 94928, USA

Received: 26 July 2005
Accepted: 14 December 2005

Abstract

We present Swift and XMM-Newton observations of the bright gamma-ray burst GRB 050326, detected by the Swift Burst Alert Telescope. The Swift X-Ray Telescope (XRT) and XMM-Newton discovered the X-ray afterglow beginning 54 min and 8.5 h after the burst, respectively. The prompt GRB 050326 fluence was  erg cm^-2 (20–150 keV), and its spectrum was hard, with a power law photon index . The X-ray afterglow was quite bright, with a flux of  erg cm^-2 s^-1 (0.3–8 keV), 1 h after the burst. Its light curve did not show any break nor flares between  h and  d after the burst, and decayed with a slope . 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,  cm^-2, and the redshift of the absorber was constrained to be . There was good agreement between the spatial, temporal, and spectral parameters as derived by Swift-XRT and XMM-Newton. By comparing the prompt and afterglow fluxes, we found that an early break probably occurred before the beginning of the XRT observation, similarly to many other cases observed by Swift. However, 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 allowed us to constrain the jet half-opening angle . Using the redshift constraints provided by the X-ray analysis, we also estimated that the beaming-corrected gamma-ray energy was larger than  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 and ultraviolet wavelengths. Thus, this GRB was a “truly dark” event, with the optical-to-X-ray spectrum violating the synchrotron limit. The optical and X-ray observations are therefore consistent either with an absorbed event or with a high-redshift one. To obey the Ghirlanda relation, a moderate/large redshift is required.