GRB 150616A

A. Y. Lien (GSFC/UMBC), K. L. Page (U Leicester), D. M. Palmer (LANL)
and M. H. Siegel (PSU) report on behalf of the Swift Team:

At 22:49:19 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 150616A (trigger=644259).  The Swift slewed to the burst
after a delay due to the Earth limb constraint. 
The BAT on-board calculated location is 
RA, Dec 314.705, -53.394 which is 
   RA(J2000) = 20h 58m 49s
   Dec(J2000) = -53d 23' 36"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of at least 200 sec.  The peak count rate
was ~1500 counts/sec (15-350 keV), at ~3 sec after the trigger. 

The XRT began observing the field at 22:58:50.1 UT, 570.9 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 314.7156, -53.3923 which is equivalent to:
   RA(J2000)  = +20h 58m 51.74s
   Dec(J2000) = -53d 23' 32.3"
with an uncertainty of 4.8 arcseconds (radius, 90% containment). This
location is 23 arcseconds from the BAT onboard position, within the BAT
error circle. No event data are yet available to determine the column
density using X-ray spectroscopy. 

The initial flux in the 2.5 s image was 4.54e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 579 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 19.6 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 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.04. 

Burst Advocate for this burst is A. Y. Lien (amy.y.lien 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, M.R. Goad, J.P. Osborne and A.P. Beardmore (U. Leicester) 
report on behalf of the Swift-XRT team.

Using 2598 s of XRT Photon Counting mode data and 6 UVOT
images for GRB 150616A, 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 = 314.71689, -53.39383 which is equivalent
to:

RA (J2000): 20h 58m 52.05s
Dec (J2000): -53d 23' 37.8"

with an uncertainty of 1.4 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.

K.L. Page (U. Leicester), C. Pagani (U. Leicester), P. D'Avanzo
(INAF-OAB), V. D'Elia (ASDC), A. D'ai (INAF-IASFPA), J.A. Kennea (PSU),
B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), A.P. Beardmore (U.
Leicester) and A.Y. Lien report on behalf of the Swift-XRT team:

We have analysed 10 ks of XRT data for GRB 150616A (Lien et al. GCN
Circ. 17936
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 ), from 561 s to 31.6 ks after the  BAT trigger. The data
comprise 979 s in Windowed Timing (WT) mode (the first 8 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 Evans et
al. (GCN Circ. 17937
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 ).

The late-time light curve (from T0+6.3 ks) can be modelled with an
initial power-law decay with an index of alpha=3.785 (+0.019, -0.768),
followed by a break at T+7888 s to an alpha of 0.94 (+0.14, -0.16).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.75 (+/-0.04). The
best-fitting absorption column is  5.06 (+0.31, -0.30) x 10^21 cm^-2,
in excess of the Galactic value of 3.4 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.15 (+0.10, -0.09)
and a best-fitting absorption column of 4.6 (+/-0.5) x 10^21 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.7 x 10^-11 (6.5 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     4.6 (+/-0.5) x 10^21 cm^-2
Galactic foreground: 3.4 x 10^20 cm^-2
Excess significance: 15.3 sigma
Photon index:	     2.15 (+0.10, -0.09)

If the light curve continues to decay with a power-law decay index of
0.94, the count rate at T+24 hours will be 0.050 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.9 x
10^-12 (3.3 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00644259.

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

M. Stamatikos (OSU), S. D. Barthelmy (GSFC),  W. H. Baumgartner (GSFC/UMBC),
J. R. Cummings (GSFC/UMBC), N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA),
A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC), D. M. Palmer (LANL),
T. Sakamoto (AGU), T. N. Ukwatta (LANL) (i.e. the Swift-BAT team):

Using the data set from T-240 to T+962 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 150616A (trigger #644259)
(Lien, et al., GCN Circ. 17936
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 ).  The BAT ground-calculated position is
RA, Dec = 314.713, -53.403 deg which is
  RA(J2000)  =  20h 58m 51.1s
  Dec(J2000) = -53d 24' 11.3"
with an uncertainty of 1.2 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 28%.

The mask-weighted light curve shows a long multi-peaked structure that starts
at ~ T-50 s and ends at ~ T+710 s. The major pulse starts at ~ T+60 s, peaks
at ~ T+93 s, and ends at ~ T+140 s. There is a weak emission from the source
from T-240 s to T-200 s. There may be additional low-level emission before
this time that we cannot confirm nor rule out. T90 (15-350 keV) is 599.5 +- 99.3 sec
(estimated error including systematics).

The time-averaged spectrum from T-52.3 to T+705.9 sec is best fit by a power law
with an exponential cutoff.  This fit gives a photon index 1.37 +- 0.25,
and Epeak of 92.5 +- 45.9 keV (chi squared 55.79 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 1.9 +- 0.1 x 10^-5 erg/cm2
and the 1-sec peak flux measured from T+93.58 sec in the 15-150 keV band is
2.9 +- 0.4 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 1.73 +- 0.06 (chi squared 62.56 for 57 d.o.f.).  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/644259/BA/

S. J. LaPorte (PSU) and A. Y. Lien (GSFC/UMBC) report on behalf of the
Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 150616A
579 s after the BAT trigger (Lien et al., GCN Circ. 17936
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 ).
No optical afterglow consistent with the XRT position
(Evans et al. GCN Circ. 17937
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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           579          729          147         >21.0
white              579         7913          616         >21.8
v                  736         2624          214         >19.6
b                  836         2878          365         >20.5
u                  810         2700          214         >20.5
w1                 786         2674          214         >20.2
m2                 760         2649          136         >19.9
w2                1016         2250          156         >20.1

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.04 in the direction of the burst
(Schlegel et al. 1998).

D. Murphy (UCD), A. Martin-Carrillo (UCD), M. Topinka (CTU), L. Hanlon (UCD), H. J. van Heerden (UFS), B. van Soelen (UFS) and P. J. Meintjes (UFS):

We observed the field of GRB 150616A (Lien et al., GCN Circ. #17936
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 ) using the 40cm Watcher telescope at Boyden Observatory in South Africa. 

Observations started at 22:52:09 UT, 170 seconds after the burst trigger. A series of 5 sec images were taken in the SDSS r' filter using an Andor CCD camera. 
No credible afterglow is found in any of the individual images at the enhanced XRT location (Evans et al., GCN Circ. #17937
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 ). A 3sigma upper limit for a combined 50 second exposure is given below.

Start             | End              | Exposure | Filter | Magnitude
-------------------------------------------------------------------------------------------------
22:52:09 UT | 22:53:15 UT | 10x5 sec |  r'       |  >18.9 (3sigma upper limit)

The upper limit was calculated using nearby stars from the APASS catalogue.

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