[vsnet-grb-info 16235] GRB 150301C : (Swift trigger 633105) Swift observations

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  17516
SUBJECT: GRB 150301C : (Swift trigger 633105) Swift observations
DATE:    15/03/01 20:31:02 GMT
FROM:    Jay R. Cummings at NASA/GSFC/Swift  <james.r.cummings at nasa.gov>

M. Stamatikos (OSU), S. D. Barthelmy (GSFC),  W. H. Baumgartner (GSFC/UMBC),
J. R. Cummings (CPI), N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA),
A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC), D. M. Palmer (LANL),
T. Sakamoto (AGU), J. Tueller (GSFC), T. N. Ukwatta (LANL),
F. Marshall (NASA/GSFC), D. Malesani (DARK/NBI), L. Hagen (PSU),
M. de Pasquale (INAF-IASFPA), A. Maselli (INAF-IASFPA), D.N. Burrows (PSU),
K. Page (UCL-MSSL), N. P. M. Kuin (UCL-MSSL)

  
Using the data set from T-60 to T+243 sec from the recent telemetry downlink,
we report further analysis of possible GRB 150301C (trigger #633105)
(Lien, et al., GCN Circ. 17512).  The BAT ground-calculated position is
RA, Dec = 11.319, 41.861 deg which is
    RA(J2000)  =  00h 45m 16.6s
    Dec(J2000) = +41d 51' 40.0"
with an uncertainty of 2.2 arcmin, (radius, sys+stat, 90% containment).
The significance in the BAT image was 10.1, so we are sure that this was
a real astrophysical source.  The partial coding was 69%.

The location is well within the visible light of M31.  A more distant galaxy,
[JSD2012] 170, which has a size of 0.11 arcmin, is within the BAT error
circle.  In 1.9 ks of data, the Swift XRT found only a single very weak source
(2.8 sigma), consistent with the position of a known source within the BAT
error circle, [PFH2005] 622, with a count rate of (5.3+/-1.9) x 10^-03 c/s.  We
cannot state whether there is any variability of this source at this time.  The
XRT position of this source is:

RA (J2000)  00h 45m 14.4s
Dec (J2000) +41d 50' 37.7"

The BAT mask-weighted light curve shows a single weak FRED peak.  T90 (15-350 keV)
is 14 +- 3 sec (estimated error including systematics).
  
The time-averaged BAT spectrum from T-0.6 to T+15.0 sec is best fit by a power-
law model with an exponential cutoff.  The power law index of the time-averaged
spectrum is poorly defined but is nominally -0.8 +- 2.  The peak energy is
38 +- 7 keV.  The fluence in the 15-150 keV band is 2.1 +- 0.4 x 10^-07 erg/cm2.
The 1-sec peak photon flux measured from T+1.32 sec in the 15-150 keV band is
0.6 +- 0.2 ph/cm2/sec.  A simple power-law fit has a photon index of 1.96 +- 0.25.
All the quoted errors are at the 90% confidence level.

If this source was in M31, its isotropic energy output was 1.5 x 10^43 ergs
in 15.6 seconds in the range 15-150 keV.

It is very possible, given the softness of the source, that this is a non-GRB
in M31.  The color:duration is within the distribution of long GRBs.  No XRT
source was seen in the initial observations, whereas we might expect most
sources in M31 bright enough to be seen in BAT would be very bright in XRT.
Therefore the source is likely to be highly absorbed.  The results of the
batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/633105/BA/

The Swift/UVOT began settled observations of the field 102 s after
the BAT trigger (Lien et al., GCN Circ. 17512). No new source within
the BAT error circle is detected in the initial UVOT exposures.
Preliminary 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first
exposures are:

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

white              102         4151          369        >20.39
v                  592         4562          216        >18.55
b                  518         5241           96        >19.08
u                  261         5177          461        >19.69
uvw1               641         4973          216        >19.27
uvm2               617         4768          216        >19.20
uvw2               568         4357          216        >19.41

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.06 in the direction of the burst
(Schlegel et al. 1998).

Further Swift observations are planned.