[vsnet-grb-info 2427] GRB/SGR 050925: XMM-Newton ToO observation

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  4264
SUBJECT: GRB/SGR 050925: XMM-Newton ToO observation
DATE:    05/11/11 19:22:00 GMT
FROM:    Nanda Rea at SRON  <N.Rea at sron.nl>

Nanda Rea (SRON), Elisa Costantini (SRON) and Gianluca Israel (INAF-OAR)

report:

We report on the preliminary analysis of a 27ks XMM-Newton ToO
observation of GRB 050925 (GCN 4034 Holland et al.; GCN 4037 Markwardt
et al.) performed on 2005 October 12. The very short (128 ms) duration
of this GRB and the lack of any bright UV/Opt/IR afterglow (GCN 4038
Rosen et al.; GCN 4035 Guidorzi et al.; GCN 4036 Qiu et al.; GCN 4042
Bloom et al.) suggested a probable Soft Gamma-Ray Repeater (SGR)
nature of this burst. A bright, seemingly not variable, radio source
was reported at 4.9 GHz, just outside the refined Swift-BAT error
circle (GCN 4039 and GCN 4057 van der Horst et al.).

In the Swift-BAT 1.5 arcminutes error circle of GRB 050925 (GCN 4037
Markwardt et al.) the XMM-PN detects only one source at RA: 20 13
47.84, Dec: 34 19 52.0 (1.2 arcsec associated error circle; errors given 
at 90%). The Epic-PN source count rate is 0.022+/-0.001 counts/s.
This source is consistent with the one detected by the Swift-XRT (GCN
4043 Beardmore et al.). The USNO object, previously consistent with
the XRT position, lies now outside the refined XMM error circle.

A preliminary timing analysis does not show any significant periodic
signal. The source spectrum is soft and not well fitted by simple
models as absorbed power-law, blackbody or bremsstrahlung
(chi^2/dof=4.0, 2.8 and 3.1, respectively). The fit improves using two
components, power-law plus blackbody or two blackbodies (chi^2/dof=0.9). 
The correspondent absorbed flux (4x10^-14 ergs*cm^-2*s^-1 in the
0.5-10 keV band) is consistent with the XRT value.
A detailed spectral analysis is on-going. As already noted in
GCN 4043 by Page et al., the absorption value of this source
(~10^21 cm^-2) is very low compared with the Galactic absorption in
that direction (~10^22 cm^-2), suggesting a probable nearby object.
Note that there is not any X-ray source (flux upper limit of ~10^-15
ergs*cm^-2*s^-1) coincident with the position of the bright radio
source reported in GCN 4039 and GCN 4057 (van der Horst et al.).

At this stage, this source cannot be unambiguously identified neither
as a Soft Gamma-Ray Repeater in quiescence, nor as the responsible
for the GRB 050925.

We thank the XMM-Newton team for having performed this observation.