GCN Circular 2698
Subject
GRB040827: analysis of the XMM-Newton observation
Date
2004-09-08T18:09:54Z (20 years ago)
From
Sergio Campana at INAF-OAB <campana@merate.mi.astro.it>
A. DeLuca & D. Gotz (IASF-Mi), S. Campana (OA Brera) on behalf of a larger
collaboration report:
"We have analysed the data XMM-Newton observation of GRB 040827 (Gotz et
al.
GCN 2670). A preliminary account of these data has been given by
Rodriguez-Pascual & Gonzalez-Riestra (GCN 2688).
The observation started on 2004-08-27 at 18:07:56.UT, lasting 54.3 ks.
Thanks to quick-look data it was soon realised that an afterglow was
present
but it was more than 1arcmin off the camera boresight. Therefore, the
telescope
was re-pointed during the observation (causing problems in the generation
of the ancillary file for spectral analysis).
The source intensity profile is consistent within errors with the
instrumental PSF (possible earlier reports of an extended source might be
due to the re-pointing of the satellite).
Astrometry of the XMM-Newton/EPIC images was done by matching X-ray
sources in the field to stars in the USNO-B1 catalogue. The refined
position (J2000) for the X-ray afterglow (source XMMUJ151701.3-160828,
GCN 2678) is: RA(2000) 15h 17m 01.39s, Dec(2000):-16d 08' 28.77''.
The positional accuracy is at a level of 1.5'' (68% c.l.).
This position is consistent with earlier reports (Rodriguez-Pascual &
Gonzalez-Riestra GCN 2688, XMM-2) and with the position of the likely
afterglow established based on optical-NIR data (Berger et al. GCN 2680;
Gladders & Berger GCN 2681; Kaplan & Berger GCN 2683; Tanvir et al. GCN
2684;
Malesani et al. GCN 2685).
We extracted spectra from the three EPIC cameras. We took the arf file
from the same region of extraction of a different observation.
A single absorbed power law model provides a good description of the data
(chi2_red=1.2 with 187 degrees of freedom), however the column density
(N_H=3.7_{-1.2}^{+4.2}x10^21 cm-2, errors at 90% c.l.) is much larger
than the column density in this direction of the Galaxy (N_H=8x10^20 cm-2,
Dickey & Lockman 1990, or N_H=5x10^20 cm-2 based on Schlegel et al. 1998
maps).
Fixing the column density to N_H=8x10^20 cm-2 results in a unacceptable fit
with chi2_red=3.0.
We then include an absorption component at the redshift of the afterglow,
keeping fixed the Galactic column density to N_H=8x10^20 cm-2.
The new fit is good (chi2_red=1.0) providing a heavily absorbed
afterglow with
N_H=1.3_{-0.3}^{+0.6}x10^22 cm-2. The redshift is relatively well
constrained to
z=0.9_{-0.2}^{+0.9}. The power law photon index is Gamma=2.3+/-0.1 and
the unabsorbed 0.5-10 keV flux is 3.4x10^{-13} erg cm-2 s-1.
The 0.2-10 keV pn light curve shows a clear decay in time. We excluded from
this analysis the first 4 ks when the XMM-Newton pointing was not
definitive.
A power law decay with t^{-1.6+/-0.2} can account for the main decay."
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