GRB 150724A

J. L. Racusin (NASA/GSFC), A. P. Beardmore (U Leicester),
J. R. Cummings (NASA/UMBC), K. L. Page (U Leicester),
D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the
Swift Team:

At 05:46:35 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 150724A (trigger=650141).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 97.540, -19.154 which is 
   RA(J2000) = 06h 30m 09s
   Dec(J2000) = -19d 09' 12"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed multiple peaks
structure with a duration of about 40 sec.  The peak count rate
was ~1412 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 05:48:18.6 UT, 102.6 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 97.56060,
-19.16395 which is equivalent to:
   RA(J2000)  = 06h 30m 14.54s
   Dec(J2000) = -19d 09' 50.2"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 78 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at http://www.swift.ac.uk/sper. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 2.94
x 10^21 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 1.27e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 250 seconds with the U filter starting
108 seconds after the BAT trigger. No credible afterglow candidate has been
found in the initial data products. The 2.7'x2.7' sub-image covers 100% of the
XRT error circle. The typical 3-sigma upper limit has been about 19.2 mag. The
8'x8' region for the list of sources generated on-board covers 100% of the XRT
error circle. The list of sources is typically complete to about 18.0 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.27. 

Burst Advocate for this burst is J. L. Racusin (judith.racusin AT nasa.gov). 
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/too.html.)

P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:

Using  promptly downlinked XRT event data for GRB 150724A, we find an
enhanced XRT position of the afterglow: RA, Dec: 97.5600, -19.1646
which is equivalent to:
   RA (J2000)  = 06 30 14.41
   Dec (J2000) = -19 09 52.5
with an uncertainty of 2.0 arcseconds (radius, 90% confidence).
Analysis of the promptly available data is online at
http://www.swift.ac.uk/sper/650141.

Position enhancement is is described by Goad et al. (2007, A&A, 476,
1401) and Evans et al. (2009, MNRAS, 397, 1177).

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

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

Using 2472 s of XRT Photon Counting mode data and 4 UVOT
images for GRB 150724A, 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 = 97.56044, -19.16479 which is equivalent
to:

RA (J2000): 06h 30m 14.51s
Dec (J2000): -19d 09' 53.3"

with an uncertainty of 1.9 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.

D. M. Palmer (LANL), S. D. Barthelmy (GSFC), J. R. Cummings (GSFC/UMBC),
N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA), A. Y. Lien (GSFC/UMBC),
C. B. Markwardt (GSFC), J. L. Racusin (NASA/GSFC), T. Sakamoto (AGU),
M. Stamatikos (OSU), T. N. Ukwatta (LANL)
(i.e. the Swift-BAT team):
 
Using the data set from T-240 to T+902 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 150724A (trigger #650141)
(Racusin, et al., GCN Circ. 18050
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 ).  The BAT ground-calculated position is
RA, Dec = 97.549, -19.147 deg, which is 
   RA(J2000)  =  06h 30m 11.7s 
   Dec(J2000) = -19d 08' 48.1" 
with an uncertainty of 1.9 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 48%.
 
The mask-weighted light curve has at least 4 peaks starting at ~T-10 sec,
with peaks at ~T+1 sec, ~T+24 sec, ~T+160 sec, & ~T+220 sec, and the
emission ending at ~T+300 sec.  T90 (15-350 keV) is 280 +- 44 sec
(estimated error including systematics).
 
The time-averaged spectrum from T-1.07 to T+325.42 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
2.06 +- 0.15.  The fluence in the 15-150 keV band is 2.2 +- 0.2 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T+24.08 sec in the 15-150 keV band
is 0.9 +- 0.2 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/650141/BA/

M. De Pasquale (UCL-MSSL) and J. L. Racusin (NASA/GSFC)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 150724A
108 s after the BAT trigger (Racusin et al., GCN Circ. 18050
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 ).
No optical afterglow consistent with the XRT position (Goad et al.
GCN Circ. 18054
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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 summed exposures are:

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

u_FC               108          358          246         >19.7

v                  415         6930          510         >19.2
b                  364        12898         1165         >20.6
u                  108        12238         1769         >20.7
w1                 464        11324         1387         >20.5
m2                 439         7135          502         >19.9
w2                 390         6726          529         >20.2

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.27 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

T.G.R. Roegiers (PSU), B.P. Gompertz (U. Leicester), J.P. Osborne (U.
Leicester), K.L. Page (U. Leicester), P. D'Avanzo (INAF-OAB), V. D'Elia
(ASDC), A. D'ai (INAF-IASFPA), L.M. McCauley (PSU), J.A. Kennea (PSU)
and J.L. Racusin report on behalf of the Swift-XRT team:

We have analysed 11 ks of XRT data for GRB 150724A (Racusin et al. GCN
Circ. 18050
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 ), from 91 s to 24.4 ks after the  BAT trigger. The data
comprise 349 s in Windowed Timing (WT) mode (the first 9 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. The enhanced XRT position for this burst was given by Goad et al.
(GCN Circ. 18052
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 ). The late-time light curve (from T0+4.7 ks) is
consistent with a constant source of mean count rate 2.2e+00 ct/sec. A
power-law fit gives an index of -0.02 (+0.19, -0.20).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.70 (+/-0.04). The
best-fitting absorption column is  3.70 (+0.21, -0.20) x 10^21 cm^-2,
in excess of the Galactic value of 2.9 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.13 (+0.18, -0.17)
and a best-fitting absorption column of 3.6 (+0.8, -0.7) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.6 x 10^-11 (5.8 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.6 (+0.8, -0.7) x 10^21 cm^-2
Galactic foreground: 2.9 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     2.13 (+0.18, -0.17)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00650141.

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