GCN Circular 4274
Subject
GRB050925: analysis of the XMM-Newton observation
Date
2005-11-15T18:44:45Z (19 years ago)
From
Andrea De Luca at IASF-CNR,Milano <deluca@iasf-milano.inaf.it>
A.De Luca, P.Caraveo, P.Esposito, S.Mereghetti, A.Tiengo (INAF/IASF Mi),
report:
we have analyzed the data from the XMM-Newton observation of
the soft short burst GRB050925, observed by Swift to have
characteristics similar to that of Soft Gamma Repeaters (SGRs)
(Holland et al., GCN4034). An independent analysis of the XMM
data was reported by Rea et al.(GCN4264).
We give here the detailed results of a source detection on the
EPIC dataset. Such results will become an important reference
in case the putative SGR will enter a phase of enhanced activity.
We used independently the pn camera data and the combined
MOS1+MOS2 data. We selected the energy ranges 0.3-2 keV,
2-8 keV and 0.3-8 keV. The MOS and pn positions are found to
be consistent within ~1.5 arcsec, in agreement with the known
astrometric accuracy of the EPIC cameras. Thus, we assume 1.5
arcsec as the 1sigma uncertainty on the X-ray sources' positions.
In the source list below we quote the J2000 coordinates
as derived from the pn camera (unless otherwise specified).
Since the field of GRB050925 is very crowded we focus on the
region closer to the 1.5 arcmin BAT error circle (Markwardt
et al., GCN4037). 6 X-ray sources (named XMM1...XMM6) are
detected within 2 arcmin of the BAT coordinates.
Finding charts from pn and MOS data may be found at
http://www.iasf-milano.inaf.it/~deluca/grb050925
Please note that the circles around XMM sources are meant
to help locate them. In the scale used for the figures the
XMM error boxes are smaller than 1 pixel.
Sources inside the BAT circle:
XMM1 - RA 20:13:47.81, Dec +34:19:51.9 - it is the brightest
source located within the BAT error circle. It is the
steady source discovered in Swift/XRT data by Beardmore
et al. (GCN4043) and interpreted as a field star.
Its coordinates, within errors, are consistent with a bright
optical source (B~12.4, R~11.9, source id.1243-0400741 in
USNO-B1). Its spectrum is well described by a mekal plasma
model with NH<5x10^20 cm^-2, kT~0.6 keV, abundance ~0.3 solar
values and observed flux of 2.7x10^-14 erg cm^-2 s^-1
in 0.3-8 keV. The resulting Fx/Fopt is of ~3x10^-4.
Thus, in agreement with Beardmore et al.(GCN4043), we
conclude that source XMM1 is most likely a nearby star.
Rea et al.(GCN4264), assuming a smaller X-ray error circle,
questioned the association with the bright optical source
XMM2 - RA 20:13:57.85, Dec +34:19:56.5 - detected with S/N~3,
in the 0.3-2 keV range only.
Assuming a Crab spectrum with Galactic absorption
(1.1x10^22 cm^-2 according to Dickey & Lockman, 1990), its
observed flux is (2-3)x10^-15 erg cm^-2 s^-1 (0.3-2 keV).
No obvious optical counterparts are found in the DSS plates.
Sources outside the BAT circle:
XMM3 - RA 20:13:54.97, Dec +34:18:22.4 - located at 1.55 arcmin
from the BAT position, it is detected with S/N~3
in MOS data only (in the pn detector it lies on a bad column),
with a flux similar to source XMM2.
An optical source with R~17 is found within 3 arcsec.
XMM4 - RA 20:13:56.06, Dec +34:21:28.9 - located at 1.61 arcmin
from the BAT position, it is detected with S/N~3
in MOS data only (in the pn detector it lies on a bad column),
with a flux similar to source XMM2.
An optical source with R~16.5 is found within 2.2 arcsec.
XMM5 - RA 20:14:01.59, Dec +34:21:03.1 - located at 1.89 arcmin
from the BAT position, it lies within 1.5 arcsec of a
bright optical source (B~13.1, R~11.4, source id.1243-0401091
in USNO-B1 catalog), its spectrum is consistent with a mekal
plasma model with absorption smaller than the Galactic value,
kT~0.67 keV and abundance ~0.4 solar values; the observed flux
is ~1x10^-14 erg cm^-2 s^-1 (0.2-5 keV). Fx/Fopt is ~10^-4.
It is most likely a star.
XMM6 - RA 20:13:45.17, Dec +34:20:23.8 - located at 1.93 arcmin
from the BAT position, its spectrum is consistent
with a power law (photon index ~1.8) absorbed by the Galactic
column. Its observed flux is ~2x10^-14 erg cm^-2 s^-1
(0.2-8 keV). No obvious optical counterparts.
No other sources within 2 arcmin of the BAT position are seen.
Using the present XMM observation we can estimate the upper limit
flux as follows: assuming a Crab-like spectrum with Galactic
absorption, a S/N=3 detection corresponds to a flux of ~3x10^-15 erg
cm^-2 s^-1 in the 0.3-2 keV band and ~8x10^-15 erg cm^-2 s^-1 in
the 0.3-8 keV band.
We thank the XMM Project Scientist, N.Schartel, for approval of
this TOO, and the XMM people at Vilspa for performing it.