GRB 161004B

A. D'Ai (INAF-IASFPA), V. D'Elia (ASDC), F. E. Marshall (NASA/GSFC),
D. M. Palmer (LANL) and B. Sbarufatti (INAF-OAB/PSU) report on behalf
of the Swift Team:

At 23:07:59 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 161004B (trigger=715246).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 112.157, -39.895 which is 
   RA(J2000) = 07h 28m 38s
   Dec(J2000) = -39d 53' 40"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single FRED peak
with substructure and  a duration of about 20 sec.  The peak count rate
was ~12000 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 23:09:12.7 UT, 73.4 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 112.15290, -39.89858 which
is equivalent to:
   RA(J2000)  = 07h 28m 36.70s
   Dec(J2000) = -39d 53' 54.9"
with an uncertainty of 3.6 arcseconds (radius, 90% containment). This
location is 17 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. 

The initial flux in the 2.5 s image was 5.23e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 82 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
0.54. 

Burst Advocate for this burst is A. D'Ai (antonino.dai 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.)

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

Using  promptly downlinked XRT event data for GRB 161004B, we find an
enhanced XRT position of the afterglow: RA, Dec: 112.1514, -39.8976
which is equivalent to:
   RA (J2000)  = 07 28 36.34
   Dec (J2000) = -39 53 51.5
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/715246.

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.

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

Using 3535 s of XRT Photon Counting mode data and 5 UVOT
images for GRB 161004B, 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 = 112.15126, -39.89791 which is equivalent
to:

RA (J2000): 07h 28m 36.30s
Dec (J2000): -39d 53' 52.5"

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.

V. D'Elia (ASDC), D.N. Burrows (PSU), T.G.R. Roegiers (PSU), L.M.
McCauley (PSU), P.A. Evans (U. Leicester), S.L. Gibson (U. Leicester),
J.P. Osborne (U. Leicester), A. Melandri (INAF-OAB) and A. D'Ai report
on behalf of the Swift-XRT team:

We have analysed 7.1 ks of XRT data for GRB 161004B (D'Ai et al. GCN
Circ. 19987
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 ), from 63 s to 28.2 ks after the  BAT trigger. The data
comprise 77 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 Evans et
al. (GCN Circ. 19988
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 ).

The light curve can be modelled with an initial power-law decay with an
index of alpha=0.89 (+/-0.05), followed by a break at T+2716 s to an
alpha of 1.44 (+0.30, -0.22).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.99 (+0.19, -0.18). The
best-fitting absorption column is  1.14 (+0.21, -0.19) x 10^22 cm^-2,
in excess of the Galactic value of 3.5 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 5.1 x 10^-11 (9.7 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.14 (+0.21, -0.19) x 10^22 cm^-2
Galactic foreground: 3.5 x 10^21 cm^-2
Excess significance: 7.1 sigma
Photon index:	     1.99 (+0.19, -0.18)

If the light curve continues to decay with a power-law decay index of
1.44, the count rate at T+24 hours will be 2.1 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x
10^-13 (2.0 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/00715246.

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

T. Sakamoto (AGU), S. D. Barthelmy (GSFC), J. R. Cummings (GSFC/UMBC),
A. D'Ai (INAF-IASFPA), N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA),
A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC), D. M. Palmer (LANL),
M. Stamatikos (OSU), T. N. Ukwatta (LANL)
(i.e. the Swift-BAT team):

Using the data set from T-60 to T+243 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 161004B (trigger #715246)
(D'Ai, et al., GCN Circ. 19987
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 ).  The BAT ground-calculated position is
RA, Dec = 112.156, -39.896 deg which is
    RA(J2000)  =  07h 28m 37.4s
    Dec(J2000) = -39d 53' 47.3"
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 75%.

The mask-weighted light curve shows two overlapping peaks starting at T-10 sec,
peaking at T+1 sec and ending at T+40 sec.  T90 (15-350 keV) is
15.9 +- 0.7 sec (estimated error including systematics).

The time-averaged spectrum from T-5.16 to T+27.31 sec is best fit by a power law
with an exponential cutoff.  This fit gives a photon index 1.21 +- 0.13,
and Epeak of 177 +- 73 keV (chi squared 35.21 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 8.8 +- 0.2 x 10^-6 erg/cm2
and the 1-sec peak flux measured from T+0.36 sec in the 15-150 keV band is
10.1 +- 0.4 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 1.47 +- 0.03 (chi squared 47.75 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/715246/BA/

V. Kumar, V. Sharma, D. Bhattacharya and V. Bhalerao (IUCAA), A. R. Rao (TIFR) and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration:

Analysis of AstroSat CZTI data showed clear detection of GRB161004B (Swift BAT detection: D.Palmer et al., GCN Circ. 19987
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 ) in the 40-200 keV energy range. The light curve shows a single peak at 23:07:59.0 UT, coincident with the Swift Trigger. The measured peak count rate is 187.3 counts/sec above the background in combined data of four quadrants, with a total of 1393.0 counts. The local mean background count rate was 313.7 counts/sec. Using cumulative rates, we measure a T90 of 13.1 secs.

CZTI GRB detections are reported regularly on the payload site at http://astrosat.iucaa.in/czti/?q=grb.  CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, ISAC, IUCAA, SAC and PRL. The Indian Space Research Organisation funded, managed and facilitated the project.

F. E. Marshall (NASA/GSFC) and A. D'Ai (INAF-IASFPA)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 161004B
82 s after the BAT trigger (D'Ai et al., GCN Circ. 19987
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 ).
No optical afterglow consistent with the XRT position
(Evans et al., GCN Circ. 19991
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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            82          232          147         >20.8
u_FC               294          544          246         >19.6
white               82         1886          431         >21.2
v                  624         1937          156         >19.5
b                  550         1862          136         >19.6
u                  294         1836          363         >19.6
w1                 674         1812          136         >18.9
m2                5622         5747          123         >18.8
w2                 600         1736          136         >18.7

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.54 in the direction of the burst
(Schlegel et al. 1998).

R. Hamburg (UAH) and C. Meegan (UAH)
report on behalf of the Fermi GBM Team:

"At 23:07:54.79 UT on 04 October 2016, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 161004B (trigger 497315278 / 161004964)
which was also detected by the Swift BAT
(D'Ai et al. 2016, GCN 19987
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 )
The GBM on-ground location is consistent with the Swift position.

The trigger resulted in an Autonomous Repoint Request (ARR)
by the GBM Flight Software owing to the high peak flux
of the GRB. This ARR was accepted and the spacecraft slewed to the GBM
in-flight
location. The initial angle from the Fermi LAT boresight to
the Swift location is 69 degrees.

The GBM light curve consists of a single peak
with a duration (T90) of about 14 s (50-300 keV).
The time-averaged spectrum from T0-0.0 s to T0+18.4 s is
best fit by a Band function with Epeak = 132 +/- 6 keV,
alpha = -0.75 +/- 0.04, and beta = -2.4 +/- 0.1.

The event fluence (10-1000 keV) in this time interval is
(1.877 +/- 0.039)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+4.7 s in the 10-1000 keV band
is 15.4 +/- 0.4 ph/s/cm^2.

The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog."

Y. Yamada, A. Yoshida, T. Sakamoto, Y. Kawakubo, M. Moriyama (AGU),
K. Yamaoka (Nagoya U), S. Nakahira (JAXA), I. Takahashi (IPMU), Y. Asaoka,
S. Ozawa, S. Torii (Waseda U), Y. Shimizu, T. Tamura (Kanagawa U), W. Ishizaki (ICRR),
M. L. Cherry (LSU), S. Ricciarini (U of Florence), P. S. Marrocchesi (U of Siena)
and the CALET collaboration:

The long-duration GRB 161004B (D'Ai et al., GCN Circ. 19987
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 ; Kumar et al.,
GCN Circ. 19996
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 ;  Hamburg et al., GCN Circ. 19998
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 ; INTEGRAL-SPI/ACS trigger #7590)
triggered the CALET Gamma-ray Burst Monitor (CGBM) at 23:07:52.64 on 4 October 2016.
The burst signal was seen by the SGM instrument.

The light curve of the SGM shows a single peak starting at T0, peaking at
T+7 sec and ending at T+15 sec.  The T90 duration measured by the SGM data
is 11.1 +- 0.9 sec (40-450 keV).

The light curve is available at

http://cgbm.calet.jp/cgbm_trigger/flight/1159657616/

The CALET data used in this analysis are provided by the Waseda
CALET Operation Center located at the Waseda University.

A. Tsvetkova, S. Golenetskii, R.Aptekar, D. Frederiks, P. Oleynik,
M. Ulanov, D. Svinkin, A. Lysenko, A. Kozlova and T. Cline
on behalf of the Konus-Wind team, report:

The long-duration GRB 161004B (Swift-BAT trigger #715246:
D'Ai et al., GCN 19987
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 ; Sakamoto et al., GCN 19995
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 ;
Fermi GBM detection: Hamburg & Meegan, GCN 19998
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 ;
CALET Gamma-Ray Burst Monitor detection: Yamada et al., GCN 20003
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 )
triggered Konus-Wind at T0=83274.317 s UT (23:07:54.317).

The burst light curve shows a single pulse with a total duration of ~18 s.
The emission is seen up to ~2 MeV.

As observed by Konus-Wind, the burst
had a fluence of 1.71(-0.07,+0.08)x10^-5 erg/cm2,
and a 64-ms peak energy flux, measured from T0+4.144 s,
of 3.43(-0.80,+0.80)x10^-6 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum of the burst
(measured from T0 to T0+15.616 s)
is best fit in the 20 keV - 2 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with  alpha = -1.05(-0.09,+0.09),
and Ep = 175(-12,+14) keV (chi2 = 56/58 dof).
Fitting by the GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index:
beta < -3.14 (chi2 = 56/57 dof).

The spectrum near the peak count rate
(measured from T0 to T0+7.424 s)
is best fit in the 20 keV - 2 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with  alpha = -0.71(-0.10,+0.11),
and Ep = 210(-13,+15) keV (chi2 = 67/58 dof).
Fitting by the GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index:
beta < -3.12 (chi2 = 67/57 dof).

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB161004_T83274/

All the quoted errors are at the 90% confidence level.
All the quoted values are preliminary.