GRB 150424B

Y. Ono (Tokyo Tech), S. Nakahira (JAXA), T. Masumitsu, H. Negoro  (Nihon U.), 
S. Ueno, H. Tomida,  M. Kimura, M. Ishikawa, Y. E. Nakagawa (JAXA),
T. Mihara, M. Sugizaki, M. Serino, M. Shidatsu, J. Sugimoto, T. Takagi, M. Matsuoka (RIKEN),
N. Kawai, T. Yoshii, Y. Tachibana (Tokyo Tech),
A. Yoshida, T. Sakamoto, Y. Kawakubo, H. Ohtsuki (AGU),
H. Tsunemi, R. Imatani (Osaka U.),
M. Nakajima,  T. Namba, M. Fujita, F. Honda, 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 
09:40:14 UT.

Assuming that the source flux was constant over the transit,
we obtain the source position at
(R.A., Dec) = (182.759 deg, -16.212 deg) = (12 11 02, -16 12 43) (J2000) 
with a statistical 90% C.L. elliptical error region 
with long and short radii of 0.13 deg and 0.11 deg, respectively. 
The roll angle of long axis from the north direction is 23.0 deg counterclockwise. 
There is an additional systematic uncertainty of 0.1 deg (90% containment radius).

Without assumptions on the source constancy, we obtain a rectangular error
box for the transient source with the following corners:
(182.383 deg, -15.974 deg) = (12 09 31, -15 58 27) (J2000)
(182.575 deg, -15.799 deg) = (12 10 18, -15 47 55) (J2000)
(182.974 deg, -16.261 deg) = (12 11 53, -16 15 40) (J2000)
(182.781 deg, -16.436 deg) = (12 11 07, -16 26 10) (J2000)

The X-ray flux averaged over the scan was 544 +- 53 mCrab
(4-10keV, 1 sigma error).
There was no significant excess flux in the previous transit at 08:07 UT
with an upper limit of 20 mCrab.

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 150424B. Automated analysis of the XRT data will
be presented online at http://www.swift.ac.uk/xrt_products/TILED_GRB00042

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.

Bin-Bin Zhang (UAH) and A. von Kienlin (MPE) reports on behalf of the
Fermi GBM Team:

"At 09:39:59.68 UT on 24 April 2015, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 150424B (trigger 451561202 / 150424403),
which was also detected by the MAXI/GSC (Ono et al. 2015, GCN 17746
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 ).
The GBM on-ground location is consistent with the MAXI/GSC position.
The angle from the Fermi LAT boresight is about 107 degrees.

The GBM light curve shows a single pulse with a duration (T90)
of about 36.1 s (50-300 keV). The time-averaged spectrum from
T0-8 s to T0+28 s is best fit by a Band function with
Epeak = 56.6 +/- 8.1 keV, alpha = -0.8 +/- 0.3, and beta = -2.4 +/- 0.2.

The event fluence (10-1000 keV) in this time interval is
(3.85 +/- 0.31)E-06 erg/cm^2. The 1.024-sec peak photon flux
measured starting from T0-0.4 s in the 10-1000 keV band
is 3.8 +/- 0.3 ph/s/cm^2.

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