GRB 150907A

J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU), J.P. Osborne (U.
Leicester), K.L. Page (U. Leicester), A. D'ai (INAF-IASFPA), A. Maselli
 (INAF-IASFPA), A. Melandri (INAF-OAB), D.N. Burrows (PSU) and P.A.
Evans (U. Leicester) report on behalf of the Swift-XRT team:

Swift-XRT has conducted further observations of the field of the
MAXI-detected burst GRB 150907A (Nakagawa et al. GCN Circ. 18246
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 ). The
observations now extend from T0+8.8 ks to T0+489.4 ks. The source
previously reported by Kennea et al. (GCN Circ. 18250
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 ), "Source 1", is
fading with 3-sigma significance, and is therefore likely the GRB
afterglow.  The position of this source is RA, Dec=225.0528, -47.7080
which is equivalent to:

RA (J2000): 15:00:12.68
Dec(J2000): -47:42:28.8

with an uncertainty of 6.8 arcsec (radius, 90% confidence).  This
position is 10.9 arcmin from the MAXI position.  The source is fading
with alpha >0.5.

The results of the XRT-team automatic analysis of the likely afterglow
are at http://www.swift.ac.uk/xrt_products/TILED_GRB00048/index_1.php.
The results of the full analysis of the tiled XRT observations are
available at http://www.swift.ac.uk/xrt_products/TILED_GRB00048.

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

M. De Pasquale (UCL-MSSL) and D. Malesani (DARK/NBI) report on behalf of
the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 150907A
8821 s after the MAXI trigger (Suzuki et al., GCN Circ. 18246
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 ).

No optical source consistent with the XRT Source 1 position given in
Kennea et al. (GCN Circ. 18250
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 ) is detected in the summed u band exposures.

We note however, that there is source in the DSS catalogue consistent
with the XRT Source 1 position.


Preliminary 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the exposure
for source 1 is:

Filter         T_start(s)   T_stop(s)      Exp(s)         Mag

u                8821        18835         336           >19.5


The magnitudes in the table are not corrected for the Galactic 
extinction due
to the reddening of E(B-V) = 0.08 in the direction of the burst 
(Schlegel et al. 1998).

-- 

Dr. Massimiliano De Pasquale
Research associate - Swift UVOT scientist
Mullard Space Science Laboratory, University College London

J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU), T.G.R. Roegiers (PSU),
J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A. D'ai
(INAF-IASFPA), A. Maselli  (INAF-IASFPA), A. Melandri (INAF-OAB), D.N.
Burrows (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 150907A (Nakagawa et al. GCN Circ. 18246
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 ) in a series of
observations tiled on the sky. The total exposure time is 1.7 ks,
distributed over 4 tiles; the maximum exposure at a single sky location
was 1.1 ks. The data were collected between T0+8.8 ks and T0+18.8 ks,
and are entirely in Photon Counting (PC) mode. 

One uncatalogued X-ray source has been detected, it is below the RASS
limit and shows no definitive signs of fading. Therefore, at the
present time we cannot confirm this as the afterglow. Details of this
source are given below:

Source 1:
  RA (J2000.0):  225.0526  =  15:00:12.62
  Dec (J2000.0): -47.7084  =  -47:42:30.3
  Error: 6.0 arcsec (radius, 90% conf.)
  Count-rate: 0.037 +/- 0.010 ct s^-1
  Flux: (2.50 +/- 0.70)e-12 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_GRB00048.

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

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 150907A. Automated analysis of the XRT data will
be presented online at http://www.swift.ac.uk/xrt_products/TILED_GRB00048

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.

Y. E. Nakagawa (JAXA), H. Negoro, (Nihon U.), M. Serino (RIKEN),
M. Ishikawa, S. Nakahira, S. Ueno, H. Tomida,  M. Kimura(JAXA),
T. Mihara, M. Sugizaki,  M. Shidatsu, J. Sugimoto, T. Takagi, M. Matsuoka (RIKEN), 
N. Kawai, Y. Tachibana, T. Yoshii (Tokyo Tech),
A. Yoshida, T. Sakamoto, Y. Kawakubo, H. Ohtsuki (AGU), 
H. Tsunemi, R. Imatani (Osaka U.), M. Nakajima, 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
16:00:50 UT on 7 September 2015. Assuming that the source flux was constant over the transit,
we obtain the source position at
(R.A., Dec) = (224.845 deg, -47.592 deg) = (14 59 22, -47 35 31) (J2000)
with a 90% C.L. statistical error of 0.06 deg. 
There is an additional systematic uncertainty of 0.1 deg (90% containment radius).
The X-ray flux averaged over the scan was 1464 +- 75 mCrab (4-10keV, 1 sigma error).
There was no significant excess flux in the previous transit at 14:28 UT on 7 September 2015
with an upper limit of 20 mCrab.