GRB 110426A

S. Nakahira (RIKEN), H. Negoro (Nihon U.), M. Matsuoka ,
T. Mihara, M. Sugizaki, M. Serino, T. Yamamoto, T. Sootome (RIKEN), 
S. Ueno, H. Tomida, M. Kohama, M. Ishikawa (JAXA),
N. Kawai, M. Morii, K. Sugimori, R. Usui, T. Toizumi (Tokyo Tech),
A. Yoshida, K. Yamaoka (AGU),
H. Tsunemi, M. Kimura, H. Kitayama (Osaka U.), 
M. Nakajima, F. Suwa (Nihon U.), 
Y. Ueda, K. Hiroi, M. Shidatsu (Kyoto U.),
Y. Tsuboi, T. Matsumura, K. Yamazaki (Chuo U.)
report on behalf of the MAXI team:

At the scan transit centered at UT 2011-04-26T15:08:36,
MAXI/GSC detected a short X-ray transient lasting about 18 seconds 
within the 46 seconds triangular transit response of MAXI/GSC.
Judging from its location and spectral hardness ratio, this transient is
probably a gamma-ray burst.
  
Since the emission was detected only for a fraction of the triangular 
transit response, the uncertainty of its position along the scan direction 
is large.  Without assuming the source constancy, we obtain a rectangular 
error box (90%C.L.) with the following corners: 
  
(R.A., Dec) = (+219.64 deg,-9.51 deg) = (14 38 34, -09 30 36)(J2000)
(R.A., Dec) = (+222.36 deg,-12.97 deg) = (14 49 26, -12 58 12)(J2000)
(R.A., Dec) = (+222.57 deg,-12.77 deg) = (14 50 17, -12 46 12)(J2000)
(R.A., Dec) = (+219.84 deg,-9.31 deg) = (14 39 21, -09 18 36)(J2000)

There is additional systematic uncertainty of 0.2 deg (90% containment radius).

The preliminary flux (2-20 keV) of the source was at least 200 mCrab.
There was no significant detection at the transit location in the
previous and following orbits (92 min before or after the detection) 
with an upper limit of 20 mCrab.  There is no known bright X-ray source 
at the detected position. 
  
  Follow-up observations are encouraged.

O. Gres, K.Ivanov, V.A.Poleshchuk, S.Yazev, N.M.Budnev, O.Chuvalaev
Irkutsk State University

E. Gorbovskoy, V. Lipunov, V.Kornilov, D.Kuvshinov, A.Belinski, N.Tyurina, 
N.Shatskiy, P.Balanutsa, V.V.Chazov, P.V.Kortunov, A.Kuznetsov, D.Zimnukhov, M. 
Kornilov, A.Kuznetsov, D.Zimnukhov, M. Kornilov, A.Sankovich
Sternberg Astronomical Institute, Moscow State University

A. Tlatov, A.V. Parhomenko, D. Dormidontov
Kislovodsk Solar Station of the Pulkovo Observatory

V.Krushinski, I.Zalozhnich, T.Kopytova, A. Popov
Ural State University, Kourovka

V.Yurkov, Yu.Sergienko, D.Varda, I.Kudelina
Blagoveschensk Educational State University, Blagoveschensk


First we pointed to FERMI trigger N325523188 (ra=217.133 dec=-12.333
err=32.42) at 15:07:28 (62 sec after trigger time). We observed this area 
to 15:08:33, until the next more exact FERMI  alert  (ra=205.160 
dec=-11.570 err=10.9) comes.
Unfortunately, after that replacement, the part of MAXI error-box has left 
a FOV of ours widefield cameras. We observ this new place to 15:13:33.

We coadd all wide field (500 square degrees) camera unfiltered images. 
There is no OT from automatical identification. 
So we have following results:


T_start     T_s-T_fermi     T_s-T_maxi     exptime    limit    Cover_MAXI
15:07:28       62 sec        -68 sec        55 sec     13.0      100 %
15:08:33      126 sec         -3 sec       250 sec     13.8       85 %

First  epoch image 
http://observ.pereplet.ru/images/evgeny/grb110426a1.jpeg
Second epoch image 
http://observ.pereplet.ru/images/evgeny/grb110426a2.jpeg
Black lines is MAXI error_box.

  The main  MASTER  telescopes (40 sm) also made observations in 
2 polarizations, in the center of each of FERMI error-boxes, but them FOV 
(4 sq. degree) it has appeared not enough for observ at least parts MAXI
error-box.

H. A. Krimm (GSFC/USRA),V. D'Elia (ASDC) and G. Stratta (ASDC), report on
behalf of the Swift team:

Swift XRT and UVOT instruments observed the center of the error rectangle
for the MAXI-detected GRB 110426A (Nakahira et al, GCN Circ. 11980
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 ) on
two occasions.  The first observation was for 5 ks starting 7 ks after
the MAXI trigger.  In this observation four point sources were found.
One is consistent with a source in the Digitized Sky Survey and the other
three were  of <3-sigma significance.  The second observation, also 5 ks,
began 250 ks after then trigger.  In the second observation, the DSS source
was detected without having faded and two other weak sources were seen at
different locations.  Since there is no evidence for a fading source in
the XRT field of view, we conclude that we have not detected the afterglow
of GRB 110426A.
     However, we note that the reported 90% error box for GRB 110426A
(Nakahira et al, GCN Circ. 11980
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 ) is approximately eight times the size
of the XRT field of view. Therefore, it is likely that the true GRB
location is outside the observed field.

A.J. van der Horst and A. Camero-Arranz (USRA) report on behalf of
the Fermi GBM Team:

"At 15:06:26.61 UT on 26 April 2011, the Fermi Gamma-Ray Burst Monitor
triggered on GRB 110426A (trigger 325523188 / 110426629). The on-ground
calculated location, using the GBM trigger data, is RA = 219.9, Dec = -8.7
(J2000 degrees, equivalent to 14h40m, -8d43'), with an uncertainty of
2.1 degrees (radius, 1-sigma containment, statistical only; there is
additionally a systematic error which is currently estimated to be 2
to 3 degrees). The angle from the Fermi LAT boresight is 88 degrees.

This is most likely the same source as the one that MAXI/GSC reported
in GCN 11980
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 . Although MAXI/GSC triggered 130 seconds later than GBM on
GRB 110426A, the locations obtained by both instruments are consistent.
The GBM light curve consists of multiple peaks with a duration (T90) of
328 +/- 3 s (50-300 keV), indicating that the source was still active
when MAXI/GSC detected it.

The time-averaged spectrum from T0-4.1 s to T0+327.7 s is adequately fit
by a simple power law function with index -2.28 +/- 0.01. The event fluence
(10-1000 keV) in this time interval is (4.54 +/- 0.06)E-5 erg/cm^2.
The 1-sec peak photon flux measured starting from T0+216.1 s in the
10-1000 keV band is 6.7 +/- 0.4 ph/s/cm^2.

We note that there is bursting emission detected in GBM starting ~40
minutes before the trigger. This emission is significantly softer than
the ~6 minute emission episode after the GBM trigger time. The start of
the spectrally soft bursts is consistent with Sco X-1 coming out of
Earth occultation. The GRB 110426A location, however, is not consistent
with Sco X-1 and thus a separate source.

The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog."