GCN Circular 15286

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
collaboration:

NuSTAR observed the unusual ultra-long GRB/TDE candidate GRB 130925A
(GCN 15246
Not found
 
 
) 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.