GRB 150428C

F. Honda, M. Fujita, H. Negoro (Nihon U.), M. Serino (RIKEN), S. Nakahira, 
S. Ueno, H. Tomida,  M. Kimura, M. Ishikawa, Y. E. Nakagawa (JAXA)
T. Mihara, M. Sugizaki, M. Shidatsu, J. Sugimoto, T. Takagi, M. Matsuoka (RIKEN),
N. Kawai, T. Yoshii, Y. Tachibana, Y. Ono (Tokyo Tech),
A. Yoshida, T. Sakamoto, Y. Kawakubo, H. Ohtsuki (AGU),
H. Tsunemi, R. Imatani (Osaka U.),
M. Nakajima, T. Namba, T. Masumitsu, K. Tanaka (Nihon U.),
Y. Ueda, T. Kawamuro, T. Hori (Kyoto U.),
Y. Tsuboi, S. Kanetou (Chuo U.),
M. Yamauchi, D. Itoh (Miyazaki U.),
K. Yamaoka (Nagoya U.),
M. Morii (ISM)
report on behalf of the MAXI team:

The MAXI/GSC nova alert system triggered a bright uncatalogued X-ray transient source at 08:24:15 UT.
Assuming that the source flux was constant over the transit,
we obtain the source position at
(R.A., Dec) = (60.247 deg, 67.818 deg) = (04 00 59, +67 49 03) (J2000)
with a 90% C.L. statistical error of 0.29 deg and an additional systematic uncertainty of 0.1 deg (90% containment radius).
The X-ray flux averaged over the scan was 161 +- 27 mCrab
(4-10keV, 1 sigma error).

Without assumptions on the source constancy, we obtain a rectangular error
box for the transient source with the following corners:
(59.061 deg, 67.515 deg) = (03 56 14, +67 30 54) (J2000)
(58.970 deg, 68.084 deg) = (03 55 52, +68 05 00) (J2000)
(61.826 deg, 67.549 deg) = (04 07 18, +67 32 57) (J2000)
(61.794 deg, 68.119 deg) = (04 07 10, +68 07 09) (J2000)

There was no significant excess flux in the previous transit at 06:51 UT
with an upper limit of 20 mCrab.

We note that no significant X-ray flux above 10 keV was detected,
and the spectrum was represented by a blackbody model with
the temperature of approximately 1 keV. Thus, the burst might be an X-ray burst
from an unknown source. We tentatively name the source MAXI J0401+678.

P. A. Evans (U. Leicester) reports on behalf of the Swift team:

Swift has initiated a series of observations, tiled on the sky, of the
MAXI GRB 150428C. Automated analysis of the XRT data will
be presented online at http://www.swift.ac.uk/xrt_products/TILED_GRB00043

Any uncatalogued X-ray sources detected in this analysis will be
reported on this website and via GCN COUNTERPART notices. The probability of finding
serendipitous sources, unrelated to the MAXI event is high: any X-ray source
considered to be a probable afterglow candidate will be reported via a GCN Circular
after manual consideration.

Details of the XRT automated analysis methods are detailed in Evans et
al. (2007, A&A, 469, 379; 2009, MNRAS, 397, 1177 and 2014, ApJS, 210, 8).

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

V.Yurkov, Yu.Sergienko, D.Varda, E.Sinyakov
Blagoveschensk Educational State University, Blagoveschensk

E. Gorbovskoy, V. Lipunov,  N.Tyurina, V.Kornilov, P.Balanutsa, 
A.Kuznetsov,D.Kuvshinov,
Lomonosov Moscow State University, Sternberg Astronomical Institute

D.Buckley, S. Potter, A.Kniazev, M.Kotze
South African Astronomical Observatory

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

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

V.Krushinski, I.Zalozhnih,  A. Popov
Ural Federal University, Kourovka

Hugo Levato and Carlos Saffe
Instituto de Ciencias Astronomicas, de la Tierra y del Espacio (ICATE)

Carlos Lopez, Claudio Mallamaci and Federico Podest
Observatorio Astronomico Felix Aguilar (OAFA)

MASTER II  robotic telescope (MASTER-Net: http://observ.pereplet.ru) 
located in Blagoveschensk was pointed to the  GRB150428C 12184 sec after 
notice time (after sun set) and 15390 sec after trigger time at 
2015-04-28 12:40:45 UT.

We did not find OT brighter then 17.4 mag .

The message may be cited.

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

Swift-XRT has performed follow-up observations of the MAXI-detected
burst GRB 150428C (Honda et al. GCN Circ. 17772
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 ) in a series of
observations tiled on the sky. The total exposure time is 2.1 ks,
distributed over 7 tiles; the maximum exposure at a single sky location
was 819 s. The data were collected between T0+16.2 ks and T0+28.9 ks,
and are entirely in Photon Counting (PC) mode.

No real uncatalogued sources have been detected. Although a
low-significance source was reported by the automatic software, we believe
this is an artifact caused by high background emission. In addition, a
previously-catalogued X-ray source has been detected, however because it
is a catalogued object it is unlikely to be the afterglow.

The 3-sigma upper limit in the field ranges from ~0.01 to ~0.05 ct s^-1,
corresponding to a 0.3-10 keV observed flux of 4e-13 to 2e-12 erg cm^-2
s^-1 (assuming a typical GRB spectrum).

The results of the XRT-team automatic analysis of the XRT observations,
including a position-specific upper limit calculator, are available at
http://www.swift.ac.uk/xrt_products/TILED_GRB00043/