GRB 121025A

M. Asada, H. Negoro (Nihon U.), M. Serino (RIKEN), M. Morii  (Tokyo Tech), 
Nakahira (JAXA), K. Yamaoka (Waseda U.), S. Ueno, H. Tomida, M. Ishikawa (JAXA),
T. Mihara, M. Sugizaki, T. Yamamoto, J. Sugimoto, M. Matsuoka (RIKEN), 
N. Kawai, R. Usui, K. Ishikawa, T. Yoshii  (Tokyo Tech),
A. Yoshida (AGU), H. Tsunemi, M. Kimura (Osaka U.), 
M. Nakajima (Nihon U.), 
Y. Ueda, K. Hiroi, M. Shidatsu, R. Sato (Kyoto U.),
Y. Tsuboi, M. Higa (Chuo U.)
M. Yamauchi, Y. Nishimura, T. Hanayama, K. Yoshidome (Miyazaki U.)
report on behalf of the MAXI team:

At 2012-10-25T07:46:30 UT, the MAXI/GSC nova alert system triggered
on a bright uncatalogued X-ray transient source.
The transient emission lasted at least 20 seconds
within the 44 second long triangular transit response of
MAXI/GSC. We identify this event as GRB 121025A.

Assuming that the source flux was constant over the transit,
we obtain the source position at

 (R.A., Dec) = (248.75 deg, +27.73 deg) = (16 35 00, +27 44 05)(J2000)

with a 90% C.L. statistical error of 0.27 deg and an additional systematic uncertainty
of 0.1 deg (90% containment radius). The 4-10 keV flux was  129 (+26/-22) mCrab.

Within the scan transit, the variability of the source was marginally detected with a chance 
probability of 0.084 for 4 - 10 keV band.

There was no significant excess flux in the previous transit at 06:13 UT and 
in the next transit at 09:18 UT with an upper limit of 20 mCrab for each.

Follow-up observations are encouraged.

K.L. Page, R.L.C. Starling and P.A. Evans (U. Leicester) report on behalf
of the Swift-XRT team:

Swift performed a Target of Opportunity observation of the MAXI-detected
GRB 121025A (Asada et al., GCN Circ. 13895
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 ), starting 26.1 ks after the
trigger. A series of four tiled pointings was performed, in order to cover
the MAXI error circle. Within these fields, one > 3 sigma uncatalogued
source was detected, at RA, Dec = 248.38182, 27.67189, which is equivalent
to:

RA(J2000)  = 16h 33m 31.64s
Dec(J2000) = 27d 40' 18.8"

with an uncertainty of 3.8 arcsec (radius, 90% containment). This source
has a mean count rate of 0.022 +/- 0.004 count s^-1. A spectrum formed
from the 1.8 ks of data can be fitted with an absorbed power-law with a
photon spectral index of 2.36 (+0.57,-0.56). The best-fitting absorption
column is consistent with the Galactic value of 3.8 x 10^20 cm^-2
(Kalberla et al. 2005). The counts to observed (unabsorbed) 0.3-10 keV
flux conversion factor deduced from this spectrum is 3.7 x 10^-11 (4.5 x
10^-11) erg cm^-2 count^-1.

It is not possible to determine whether this source is fading at this
time and, therefore, whether it is the X-ray afterglow.

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

K.L. Page (U. Leicester) reports on behalf of the Swift-XRT team:

Swift performed a follow-up observation of the possible X-ray afterglow of
MAXI-detected GRB 121025A (Asada et al., GCN Circ. 13895
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 ) announced in GCN
Circ. 13909
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  (Page, Starling & Evans). This second observation took place
on 2012-10-31, 5.8 days after the burst. The source has now faded by at
least an order of magnitude, with an upper limit on the count rate of
0.002 count s^-1 (corresponding to an observed flux of 8.1 x 10^-14 erg
cm^-2 s^-1). This source is therefore confirmed as the X-ray afterglow of
GRB 121025A.

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