GRB 150418A

GRB 150418A: MAXI/GSC detection
M. Serino (RIKEN), H. Negoro (Nihon U.), S. Nakahira (JAXA),
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 (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, T. Masumitsu (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 00:52:08 UT.
Without assumptions on the source constancy, we obtain a rectangular error
box for the transient source with the following corners:
(159.791 deg, -4.326 deg) = (10 39 09, -04 19 34) (J2000)
(159.898 deg, -4.194 deg) = (10 39 35, -04 11 39) (J2000)
(161.225 deg, -5.494 deg) = (10 44 53, -05 29 39) (J2000)
(161.114 deg, -5.624 deg) = (10 44 27, -05 37 25) (J2000)
There is an additional systematic uncertainty of 0.1 deg (90% containment radius).
The X-ray flux averaged over the scan was 597 +- 45 mCrab
(4-10keV, 1 sigma error).
There was no significant excess flux in the previous transit at UT 23:19 on
April 17 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 150418A. Automated analysis of the XRT data will
be presented online at http://www.swift.ac.uk/xrt_products/TILED_GRB00041

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.

B. Sbarufatti (INAF-OAB/PSU), A. Amaral-Rogers (U. Leicester), J.P.
Osborne (U. Leicester), K.L. Page (U. Leicester), P. D'Avanzo
(INAF-OAB), V. D'Elia (ASDC), M. de Pasquale (INAF-IASFPA), D.N.
Burrows (PSU), J.A. Kennea (PSU) 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 150418A (Serino et al. GCN Circ. 17719
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 ) in a series of
observations tiled on the sky. The total exposure time is 3.7 ks,
distributed over 7 tiles; the maximum exposure at a single sky location
was 1.5 ks. The data were collected between T0+12.9 ks and T0+32.6 ks,
and are entirely in Photon Counting (PC) mode. 

Two uncatalogued X-ray sources have been detected, however none of them
is above the RASS limit or shows definitive signs of fading. Therefore,
at the present time we cannot identify which, if any, is the afterglow.
Details of these sources are given below:

Source 1:
  RA (J2000.0):  160.2608  =  10:41:2.60
  Dec (J2000.0): -4.7593  =  -04:45:33.5
  Error: 6.4 arcsec (radius, 90% conf.)
  Count-rate: 0.0124 [+0.0060, -0.0047] ct s^-1
  Flux: (5.0 [+2.4, -1.9])e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Source 2:
  RA (J2000.0):  160.6825  =  10:42:43.80
  Dec (J2000.0): -5.3357  =  -05:20:08.4
  Error: 5.8 arcsec (radius, 90% conf.)
  Count-rate: 0.0215 [+0.0089, -0.0074] ct s^-1
  Flux: (8.6 [+3.6, -3.0])e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)

The results of the XRT-team automatic analysis of the tiled XRT
observations, including a position-specific upper limit calculator, are
available at http://www.swift.ac.uk/xrt_products/TILED_GRB00041.

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

B. Sbarufatti (INAF-OAB/PSU), L. Hagen (PSU) report on behalf of the Swift team

Swift performed a follow up observation of GRB 150418A, with a pointing  in the direction of Source #1 reported by Sbarufatti et al. (GCN Circ. 17722
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 ).
In 3.7 ks of exposure we don���t detect any X-ray source down to a 3-sigma upper limit of 9.8 E-3 counts/s in the position of Source 1.
We note that this new observation and the original one are both affected by a higher than usual background, which may have led to spurious detections of background fluctuations as Sources #1 and #2.

No further Swift observations of this field are planned.

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

N.P.M. Kuin (UCL/MSSL) reports on behalf of the Swift UVOT team:

Swift UVOT observations were made concurrently with the XRT observations
reported by  Sbarufatti et al. (GCN Circ. 17722
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  and 17725
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 ) of
GRB 150418A which was discovered by MAXI (Serino et al., GCN Circ.
No. 17719). At the location of the possible X-ray source detections
named source 1 and source 2 no new source was detected in the
u-band, with the following limits:

Source 1: u > 22.9 at 72.2 ks since the MAXI trigger at
    2015-04-18T00:52:08UT for an exposure time of 3.9ks
Source 2: u > 21.17 at t = 19ks since the MAXI trigger,
    for an exposure time of 322s.

However, we note that source 1 is at a distance of 4.6 arcsec of
a known nearby source in GSC2.3 (designated S5FU004949). That
source was detected with a magnitude u=20.38 +/- 0.15.

No correction was made for the galactic reddening
of E(B-V) = 0.043 and 0.373 respectively (Schlafly & Finkbeiner
2011 (ApJ 737, 103) ) at the locations of the sources 1 and 2.

S. Golenetskii, R.Aptekar, D. Frederiks, V. Pal'shin, P. Oleynik,
M. Ulanov, D. Svinkin, A. Tsvetkova, A.Lysenko, and T. Cline
on behalf of the Konus-Wind team, report:

The long GRB 150418A localized by MAXI/GSC (Serino et al., GCN 17719
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 )
was detected by Konus-Wind in the waiting mode.

The burst light curve shows a double-peaked structure
started at ~T0(MAXI)-16 s with a total  duration of ~29 s.

As observed by Konus-Wind, the burst had a fluence of
2.94(-0.37,+0.47)x10^-6 erg/cm2 and a 2.944-s peak flux,
measured from ~T0(MAXI)+5 s, of 2.71(-0.37,+0.49)x10^-7 erg/cm2/s
(both in the 20 - 1200 keV energy range).

Fitting the K-W 3-channel time-integrated spectrum (from ~T0(MAXI)-16 s
to ~T0(MAXI)+14 s) by a simple power-law model yields a power law index
of -2.24(-0.17,+0.17), chi2=0.04/1 dof.

All the quoted errors are estimated at the 1 sigma confidence level.
All the presented results are preliminary.

The K-W light curve of this burst is available at
http://www.ioffe.rssi.ru/LEA/GRBs/GRB150418A/