GRB 160625A

T. Schweyer, T.-W. Chen, and J. Greiner (all MPE Garching) report on behalf of the GROND team:

We observed the field of GRB 160625A (Swift trigger 701503; Maselli et al., GCN #19577
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 ) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m MPG telescope at ESO La Silla Observatory (Chile).

Observations started at 22:55 UT on 2016-06-26, 24.24 hrs after the GRB trigger.
They were performed through cirrus at an average seeing of 1.4" and at an average airmass of 1.2.

We do not detect any source within the Swift-XRT error circle reported by Burrows et al. (GCN #19578
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 ) down to

g > 22.6 mag,
r > 23.9 mag,
i > 22.5 mag,
z > 22.6 mag,
J > 19.9 mag,
H > 19.4 mag,
K > 19.5 mag.

Given magnitudes are calibrated against USNO as well as 2MASS field stars and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V)=1.2 mag in the direction of the burst (Schlafly and Finkbeiner 2011).

M. De Pasquale (UCL-MSSL) and A. Maselli (INAF-IASFPA)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 160625A
7183 s after the BAT trigger (Maselli et al., GCN Circ. 19577
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 ).
No optical afterglow consistent with the XRT position
(Burrows et al. GCN Circ. 19578
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 ) is detected in the UVOT exposures.
We note the presence of a source approximately 3'' from the
XRT position. This source is present in the USNO catalogue.
Preliminary 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the
exposures are:

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

white             7183         7952          344         >20.8
v                 8163         8363          197         >19.0
b                 7548         7747          197         >19.9
u                 7342         7542          197         >19.4
m2                8368         8483          113         >18.9
w2                7958         8158          197         >19.6

The magnitudes in the table are not corrected for the substantial Galactic
extinction due to the reddening of E(B-V) = 1.51 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

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),
A. Maselli (INAF-IASFPA), 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 downlinks,
we report further analysis of BAT GRB 160625A (trigger #701503)
(Maselli, et al., GCN Circ. 19577
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 ).  The BAT ground-calculated position is
RA, Dec = 176.939, -65.142 deg, which is 
   RA(J2000)  =  11h 47m 45.3s 
   Dec(J2000) = -65d 08' 30.9" 
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 57%.
 
The mask-weighted light curve shows at least 7 peaks, the first starting
at ~T-35 sec, the brightest peak at ~T+36 sec, and the last peak ending at ~T+70 sec.
The peak mentioned in GCN Circ 19577
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  at T+170 sec is due to GRB 160625B (as suggested
by Kann, Circ 19579) and confirmed by our analysis.
T90 (15-350 keV) is 53.9 +- 3.1 sec (estimated error including systematics).
 
The time-averaged spectrum from T-15.04 to T+47.41 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
2.08 +- 0.07.  The fluence in the 15-150 keV band is 3.3 +- 0.1 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T+35.07 sec in the 15-150 keV band
is 3.0 +- 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/701503/BA/

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

We have analysed 6.4 ks of XRT data for GRB 160625A (Maselli et al. GCN
Circ. 19577
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 ), from 1.3 ks to 48.7 ks after the	BAT trigger. The data
comprise 1.7 ks in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Evans et al. (GCN Circ. 19583
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 ).

The late-time light curve (from T0+7.4 ks) can be modelled with a
power-law decay with a decay index of alpha=0.71 (+0.31, -0.28).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.2 (+/-0.5). The
best-fitting absorption column is  2.2 (+0.9, -0.7) x 10^22 cm^-2, in
excess of the Galactic value of 1.0 x 10^22 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 5.8 x 10^-11 (1.6 x 10^-10) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.2 (+0.9, -0.7) x 10^22 cm^-2
Galactic foreground: 1.0 x 10^22 cm^-2
Excess significance: 2.8 sigma
Photon index:	     2.2 (+/-0.5)

If the light curve continues to decay with a power-law decay index of
0.71, the count rate at T+24 hours will be 9.8 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.7 x
10^-13 (1.6 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/00701503.

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

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

Using 830 s of XRT Photon Counting mode data and 3 UVOT
images for GRB 160625A, 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 = 176.91548, -65.14460 which is equivalent
to:

RA (J2000): 11h 47m 39.72s
Dec (J2000): -65d 08' 40.6"

with an uncertainty of 2.5 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. A. Kann (TLS Tautenburg) speculates:

Swift just triggered on GRB 160625A (Swift Trigger 701503, Maselli et al.,
GCN #19577
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 ). They reported several rather small peaks followed by very
strong emission ca. three minutes later.

This burst is seen to be extremely powerful in INTEGRAL SPI ACS and CALET
GBM. It seems to have begun with a short spike at about the same time as
the Swift BAT trigger (Fermi GBM trigger 488587220).

Three minutes later, Fermi LAT sent out a flight position at:

RA = 308.267
Dec. = +6.900

with an error of 0.5 degrees.

This position differs very strongly from the Swift BAT position. (Dec. of
-65!)

Additionally, Swift BAT has recently slewed to the BAT position and found
a faint X-ray afterglow (Burrows et al., GCN 19578
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 ), which is in contrast
to the very bright one one would expect from the extreme burst seen by
INTEGRAL and CALET.

I suggest that these are two different events, with the bright GRB seen
through side of the Swift spacecraft.

The Fermi LAT position is affected by a relatively low amount of
extinction (E_(B-V)=0.1), in contrast to the Swift position (E_(B-V)=1.3).

I strongly urge observations of the LAT position by Swift to determine an
XRT position, and by optical observatories as well. Judging from the
INTEGRAL SPI ACS light curve, this GRB is of similar intensity to GRB
160623A.

D.N. Burrows (PSU), J.A. Kennea (PSU), S. Campana (INAF-OAB), G.
Cusumano (INAF-IASF PA), P.A. Evans (U. Leicester) and J.P. Osborne (U.
Leicester) report on behalf of the Swift-XRT team:

The XRT began observing the field of GRB 160625A at 23:02:36.8 UT,
1325.3 seconds after the BAT trigger. Using promptly downlinked data we
find an uncatalogued X-ray source located at RA, Dec 176.91418,
-65.14362 which is equivalent to:
   RA(J2000)  = 11h 47m 39.40s
   Dec(J2000) = -65d 08' 37.0"
with an uncertainty of 5.8 arcseconds (radius, 90% containment). This
location is 112 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.  We cannot determine whether the source is
fading at the present time. No spectrum from the promptly downlinked
event data is yet available to determine the column density.

A. Maselli (INAF-IASFPA), S. D. Barthelmy (GSFC),
A. D'Ai (INAF-IASFPA), N. Gehrels (NASA/GSFC), C. Gronwall (PSU),
A. Y. Lien (GSFC/UMBC), D. M. Palmer (LANL), J. L. Racusin (NASA/GSFC)
and T. N. Ukwatta (LANL) report on behalf of the Swift Team:

At 22:40:31 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 160625A (trigger=701503).  Swift did not slew to the burst due to
an observing constraint.  The BAT on-board calculated location is 
RA, Dec 176.937, -65.114, which is 
   RA(J2000)  =  11h 47m 45s
   Dec(J2000) = -65d 06' 49"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows several peaks in the first 
40 sec and then a large peak at T+170 sec
structure with a duration of at least 180 sec.  The peak count rate
was ~14000 counts/sec (15-350 keV), at ~175 sec after the trigger. 

Due to an observing constraint, Swift will not slew until T0+20.3
minutes. There will be no XRT or UVOT data until this time. 

Burst Advocate for this burst is A. Maselli (maselli AT ifc.inaf.it). 
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.)