[vsnet-grb-info 7218] GRB 090107B: Swift XRT afterglow confirmation

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  8791
SUBJECT: GRB 090107B: Swift XRT afterglow confirmation
DATE:    09/01/09 16:08:56 GMT
FROM:    Matteo Perri at ISAC/ASDC  <perri at asdc.asi.it>

G. Stratta (ASDC), M. Perri (ASDC) and J. R. Cummings (GSFC/UMBC)
report on behalf of the Swift-XRT team:

The Swift ToO observations of GRB 090107B started on 2009 Jan 7th
at 17:30:50 UT. XRT data set consists of 5.7 ks exposure in PC mode.
The single, uncatalogued X-ray source observed in the field
of INTEGRAL, previously reported by Perri et al. (GCN Circ. 8789),
shows fading flux, confirming its afterglow nature.

The 0.3-10 keV light curve from T + 4.2 ks to T + 96.4 ks
possibly indicates evidence for a plateau phase
followed by a steeper decay, but the available data prevented
us from constraining the parameters (i.e. the temporal break and
the two decay indexes). Assuming a simple power law model,
the best fit decay index is alpha = 1.3 +/- 0.1.

The average spectrum accumulated in the same time interval
was fitted with an absorbed power law. The best fit photon
index is 1.8 +/- 0.4. The spectrum shows evidence of absorption
at the level of (9 +/- 1)e20 cm-2, in excess of the
estimated Galactic value (5e20 cm-2, Kalberla et al. 2005).
The average observed[unabsorbed] 0.3-10 keV average flux is
about  2.8[3.6]e-12 ergs cm-2 s-1.

The predicted count rate 48 hours after the trigger is 0.002 cts/s
The count-rate-to-flux conversion factor is 5.2e-11 ergs cm-2 cts-1

All quoted errors are at 90% confidence level.

Burst Advocate for this burst is J. R. Cummings.

This circular is an official product of the Swift-XRT team.