GRB 170516A

A. Y. Lien (GSFC/UMBC), S. D. Barthelmy (GSFC), A. D'Ai (INAF-IASFPA),
P.A. Evans (U Leicester), C. Gronwall (PSU), J. A. Kennea (PSU),
H. A. Krimm (NSF/USRA), N. P. M. Kuin (UCL-MSSL),
F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester) and
R. L. C. Starling (U Leicester) report on behalf of the Swift Team:

At 12:49:20 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 170516A (trigger=753110).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 41.598, -55.899, which is 
   RA(J2000)  =  02h 46m 24s
   Dec(J2000) = -55d 53' 54"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a couple peaks
with a total duration of about 35 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 12:50:41.1 UT, 80.5 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 41.5663, -55.9092 which is equivalent to:
   RA(J2000)  = +02h 46m 15.91s
   Dec(J2000) = -55d 54' 33.1"
with an uncertainty of 6.4 arcseconds (radius, 90% containment). This
location is 73 arcseconds from the BAT onboard position, within the BAT
error circle. 

The initial flux in the 2.5 s image was 4.96e-10 erg cm^-2 s^-1 (0.2-10
keV). 

Automated UVOT data processing is currently not working, but manual
inspection of the UVOT image shows no GRB afterglow counterpart. 

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.)

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

Using 2335 s of XRT Photon Counting mode data and 6 UVOT
images for GRB 170516A, 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 = 41.56858, -55.90968 which is equivalent
to:

RA (J2000): 02h 46m 16.46s
Dec (J2000): -55d 54' 34.8"

with an uncertainty of 1.8 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.

N. P. M. Kuin (UCL-MSSL) 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 170516A 87 s after the BAT trigger (Lien et al., GCN Circ.
21097
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 ). No optical afterglow consistent with the XRT position
(Lien et al. GCN Circ. 21097
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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            87          237          147         >20.1
white               87         5703          541         >21.1
v                 4478         6113          393         >19.8
b                 3863         5498          393         >20.6
u                  299         5293          379         >19.4
w1                4888         5088          197         >19.4
m2                4683         6222          299         >20.5
w2                4274         5909          393         >20.5

The magnitudes in the table are not corrected for the
Galactic extinction due to the reddening of E(B-V) = 0.03
in the direction of the burst (Schlegel et al. 1998).

D.N. Burrows (PSU), A. Cholden-Brown (PSU), S. J. LaPorte (PSU), S.L.
Gibson (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) and A.Y. Lien report on behalf of the Swift-XRT team:

We have analysed 7.2 ks of XRT data for GRB 170516A (Lien et al. GCN
Circ. 21097
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 ), from 69 s to 34.9 ks after the  BAT trigger. The data
comprise 7 s in Windowed Timing (WT) mode (taken while Swift was
slewing), with the remainder in Photon Counting (PC) mode. The enhanced
XRT position for this burst was given by Osborne et al. (GCN Circ.
21098
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 ).

The light curve can be modelled with an initial power-law decay with an
index of alpha=3.78 (+4.22, -0.29), followed by a break at T+232 s to
an alpha of 0.55 (+/-0.06).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.07 (+0.25, -0.23). The
best-fitting absorption column is  9.1 (+6.2, -5.2) x 10^20 cm^-2, in
excess of the Galactic value of 3.1 x 10^20 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.2 x 10^-11 (3.9 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     9.1 (+6.2, -5.2) x 10^20 cm^-2
Galactic foreground: 3.1 x 10^20 cm^-2
Excess significance: 1.9 sigma
Photon index:	     2.07 (+0.25, -0.23)

If the light curve continues to decay with a power-law decay index of
0.55, the count rate at T+24 hours will be 0.015 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.9 x
10^-13 (6.0 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00753110.

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

J. R. Cummings (CPI), S. D. Barthelmy (GSFC),  H. A. Krimm (NSF/USRA),
A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC), D. M. Palmer (LANL),
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+962 sec from the recent telemetry 
downlink,
we report further analysis of BAT GRB 170516A (trigger #753110)
(Lien, et al., GCN Circ. 21097
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 ). The BAT ground-calculated position is
RA, Dec = 41.543, -55.929 deg which is
    RA(J2000)  =  02h 46m 10.4s
    Dec(J2000) = -55d 55' 44.2"
with an uncertainty of 1.6 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 67%.

The mask-weighted light curve shows two fairly symmetrical triangular 
peaks.  The first
starts at T-5 s, peaks at T+2 s and decays by T+10 s.  The second peak 
starts
at T+10 s, has a broad peak from T+15 to T+20 and decays by T+28 s.   The
spacecraft slewed away from the burst position at about T+550 sec.
T90 (15-350 keV) is 36.78 +- 11.61 sec (estimated error including 
systematics).

The time-averaged spectrum from T-19.86 to T+31.22 sec is best fit by a 
power law
with an exponential cutoff.  This fit gives a photon index 1.13 +- 0.87,
and Epeak of 19.1 +- 12.4 keV (chi squared 37.46 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 8.5 +- 0.9 x 10^-7 erg/cm2
and the 1-sec peak flux measured from T+1.27 sec in the 15-150 keV band is
1.3 +- 0.2 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 2.67 +- 0.17 (chi squared 47.73 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/753110/BA/