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GCN Circular 15286

NuSTAR observations of GRB 130925A
2013-10-02T15:16:42Z (11 years ago)
Eric Bellm at Caltech <>
E. C. Bellm, F. A. Harrison, K. Forster, K. K. Madsen, V. Rana
(Caltech), S. E. Boggs, J. Tomsick (U.C. Berkeley), J. M. Miller
(Michigan), and S. B. Cenko (NASA/GSFC) report on behalf of the NuSTAR

NuSTAR observed the unusual ultra-long GRB/TDE candidate GRB 130925A
(GCN 15246) beginning 44.3 hours after the Swift trigger for a total
of 39.2 ksec exposure time.

NuSTAR detects the source to >20 keV; the average 3-20 keV flux over
the interval was 1.3E-12 erg/cm^-2/s^-1.  The data may be fit with an
absorbed power law with spectral index Gamma ~ 3.

NuSTAR observes a broad absorption feature centered near 6 keV.
Simultaneous Swift-XRT spectra support the existence of this feature
at lower SNR.  An F-test of a Gaussian absorption feature fit to the
combined data sets yields a detection significance of 5.5 sigma.
Preliminary searches for features in the early-time XRT data have not
identified any clear signatures; additional tests are ongoing.

Further observations are in progress; we encourage continued
monitoring of this interesting event at all wavelengths.

We thank the NuSTAR Mission Operations Team for the rapid turnaround
that enabled this observation.
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