GRB 150720A

A. Y. Lien (GSFC/UMBC), S. D. Barthelmy (GSFC), J. R. Cummings (GSFC/UMBC),
N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA), C. B. Markwardt (GSFC),
D. M. Palmer (LANL), T. Sakamoto (AGU), E. Sonbas (NASA/GSFC/Adiyaman Univ.),
M. Stamatikos (OSU), T. N. Ukwatta (LANL)
(i.e. the Swift-BAT team):
 
Using the data set from T-30 to T+273 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 150720A (trigger #649706)
(Sonbas, et al., GCN Circ. 18037
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 ).  The BAT ground-calculated position is
RA, Dec = 119.577, -28.281 deg, which is 
   RA(J2000)  =   07h 58m  18.5s
   Dec(J2000) =  -28d 16'  50"
with an uncertainty of 1.5  arcmin, (radius, sys+stat, 90% containment).
The partial coding was 100%.
 
The mask-weighted light curve shows 3 peaks starting at T-30 sec
and the emission ending at ~T+130 sec.
T90 (15-350 keV) is 151 +- 11 sec (estimated error including systematics).
 
The time-averaged spectrum from T-35.1 to T+126.4 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
2.61 +- 0.33.  The fluence in the 15-150 keV band is 5.6 +- 1.0 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T+116.68 sec in the 15-150 keV band
is 0.3 +- 0.1 ph/cm2/sec.  All the quoted errors are at the 90% confidence
level. 
 
The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/649706/BA/

We note that this has a soft spectrum (in the bottom ~10-percentile
of the BAT hardness range) and that the Galactic lattitude is 0.5deg,
so we can not rule out a non-GRB Galactic source origin for this event.

V. D'Elia (ASDC), L. Izzo (URoma/ICRA), K. Page (U Leicester) and E. Sonbas (NASA/GSFC/Adiyaman Univ.) report on behalf of the Swift-XRT team:

We have analysed 11 ks of XRT data for the possible GRB 150720A (Sonbas et al.  GCN Circ. 18037
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 ), from 138 s to 40 ks after the BAT trigger. The data comprise 96 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The refined XRT position is RA, Dec = 119.58144, -28.26249 which is equivalent to:

RA (J2000): 07 58 19.55
Dec(J2000): -28 15 45.0

with an uncertainty of 1.8 arcsec (radius, 90% confidence).

The late time light curve can be modelled with a power-law decay with a decay index of alpha ~ 1.5.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 3.16 (+0.10, -0.10). The best-fitting absorption column is 3.1 (+0.6, -0.6) x 10^21 cm^-2, in excess of the Galactic value of 5.72 x 10^21 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 2.7 x 10^-11 (1.6 x 10^-10) erg cm^-2 count^-1.

A summary of the WT-mode spectrum is thus:
Total column:	     8.8 (+0.6, -0.6) x 10^21 cm^-2
Galactic foreground: 5.72 x 10^21 cm^-2
Excess significance: 5.1 sigma
Photon index:	     3.16 (+0.10, -0.10)

If the light curve continues to decay with a power-law decay index of 1.5, the count rate at T+24 hours will be 1.8 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.9 x 10^-14 (2.9 x 10^-13) erg cm^-2 s^-1.

Given the light curve and spectrum behaviour, we believe that this is a real GRB and not a galactic transient.

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

A.P. Beardmore, P.A. Evans, M.R. Goad and J.P. Osborne (U. Leicester) 
report on behalf of the Swift-XRT team.

Using 653 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 150720A, we find an astrometrically corrected X-ray
position (using the XRT-UVOT alignment and matching UVOT field sources
to the USNO-B1 catalogue): RA, Dec = 119.58146, -28.26251 which is equivalent
to:

RA (J2000): 07h 58m 19.55s
Dec (J2000): -28d 15' 45.0"

with an uncertainty of 1.7 arcsec (radius, 90% confidence).

This position may be improved as more data are received. The latest
position can be viewed at http://www.swift.ac.uk/xrt_positions. Position
enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans
et al. (2009, MNRAS, 397, 1177).

This circular was automatically generated, and is an official product of the
Swift-XRT team.

M. H. Siegel (PSU) and E. Sonbas (NASA/GSFC/Adiyaman Univ.)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 150720A
142 s after the BAT trigger (Sonbas et al., GCN Circ. 18037
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 ).
No optical afterglow consistent with the XRT position (Beardmore et al.,
GCN Circ. 18040
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 )  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
finding chart (FC) exposure and subsequent exposures are:

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

white (fc)         142          292          147         >20.5
u (fc)             301          551          246         >20.4
white              142         1551          392         >21.2
v                  631         1601          117         >19.3
b                  557         1527           97         >19.6
u                  301         1502          324         >20.2
w1                 680         1478           97         >19.3
m2                 655         1623          114         >19.8
w2                 606         1577          117         >19.6

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.68 in the direction of the burst
(Schlegel et al. 1998).

[GCN OPS NOTE(22jul15): Added the "A" suffix to the GRB name.]