GRB 191004A

S. B. Cenko (GSFC), S. D. Barthelmy (GSFC),
A. P. Beardmore (U Leicester), A. D'Ai (INAF-IASFPA),
V. D'Elia (SSDC), J.D. Gropp (PSU), N. J. Klingler (PSU),
S Laha (CRESST/GSFC/UMD), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), M. J. Moss (GWU),
K. L. Page (U Leicester), B. Sbarufatti (PSU) and
A. Tohuvavohu (Toronto) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 18:07:02 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 191004A (trigger=927825).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 31.721, -36.932 which is 
   RA(J2000)  =  02h 06m 53s
   Dec(J2000) = -36d 55' 53"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a double-peaked
structure with a duration of about 5 sec.  The peak count rate
was ~8000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 18:08:24.2 UT, 81.5 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 31.66950, -36.93368 which is equivalent
to:
   RA(J2000)  = 02h 06m 40.68s
   Dec(J2000) = -36d 56' 01.2"
with an uncertainty of 3.6 arcseconds (radius, 90% containment). This
location is 148 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. No spectrum from the promptly downlinked
event data is yet available to determine the column density. 

UVOT took a finding chart exposure of 150 seconds with the White
filter  starting 84 seconds after the BAT trigger. Results from the
list of sources generated on-board shows an optical source consistent
with the XRT position.  Because there was not automatic aspect
solution for the image,  the precise position and magnitude will be
reported later.  The 2.7'x2.7' sub-image covers none of the XRT error
circle.  No correction has been made for the expected  extinction
corresponding to E(B-V) of 0.01. 

Burst Advocate for this burst is S. B. Cenko (brad.cenko 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/)

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

Using 2401 s of XRT Photon Counting mode data and 5 UVOT
images for GRB 191004A, 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 = 31.66842, -36.93302 which is equivalent
to:

RA (J2000): 02h 06m 40.42s
Dec (J2000): -36d 55' 58.9"

with an uncertainty of 1.7 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. E. Heintz (Univ. of Iceland), D. B. Malesani (DTU space), and S. Moran (NOT), report on behalf of a larger collaboration:

We observed the field of the Swift-detected GRB 191004A (Cenko et al., GCN 25945
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 ) with the Nordic Optical Telescope (NOT) equipped with StanCAM. We obtained 3x300s exposures with the Bessel R filter starting at 02:10:34 UT on October 5 (i.e. 8.05 hr post-burst). We do not detect the previously reported optical afterglow (Cenko et al., GCN 25945
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 ; Lipunov et al., GCN 25944
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 ), down to a limit of R > 23.5 mag, calibrated against the photometry of nearby field stars from the USNO-B1 catalog.

T. Sakamoto (AGU), S. D. Barthelmy (GSFC), J. R. Cummings (CPI),
S. B. Cenko (GSFC), H. A. Krimm (NSF), S. Laha (GSFC/UMBC),
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-61 to T+242 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 191004A (trigger #927825)
(Cenko, et al., GCN Circ. 25945
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 ).  The BAT ground-calculated position is
RA, Dec = 31.716, -36.932 deg, which is
   RA(J2000)  =  02h 06m 51.8s
   Dec(J2000) = -36d 55' 54.0"
with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 26%.

The mask-weighted light curve has two overlapping peaks starting at
~T-0.2 sec, peaking at ~T_zero, and ending at ~3.1 sec.
T90 (15-350 keV) is 2.44 +- 0.08 sec (estimated error including systematics).

The time-averaged spectrum from T-0.09 to T+2.58 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.35 +- 0.08.  The fluence in the 15-150 keV band is 1.3 +- 0.1 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T-0.09 sec in the 15-150 keV band
is 7.8 +- 0.5 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/927825/BA/

J. D. Gropp (PSU), A. Tohuvavohu (U. Toronto), A.P. Beardmore (U.
Leicester), P.A. Evans (U. Leicester), J.P. Osborne (U. Leicester),
M.G. Bernardini (INAF-OAB), E. Ambrosi	(INAF-IASFPA) , M. Capalbi
(INAF-IASFPA), J.A. Kennea (PSU) and V. Lipunov report on behalf of the
Swift-XRT team: report on behalf of the Swift-XRT team:

We have analysed 4.8 ks of XRT data for GRB 191004A (Lipunov et al. GCN
Circ. 25944
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 ), from 86 s to 12.4 ks after the  BAT trigger. The data
comprise 88 s in Windowed Timing (WT) mode with the remainder in Photon
Counting (PC) mode. The enhanced XRT position for this burst was given
by Osborne et al. (GCN Circ. 25950
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 ).

The light curve can be modelled with an initial power-law decay with an
index of alpha=0.74 (+0.07, -0.08), followed by a break at T+883 s to
an alpha of 1.36 (+0.34, -0.10).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.04 (+0.21, -0.13). The
best-fitting absorption column is  2.0 (+3.8, -0.5) x 10^20 cm^-2,
consistent with the Galactic value of 1.5 x 10^20 cm^-2 (Willingale et
al. 2013). The PC mode spectrum has a photon index of 1.81 (+0.14,
-0.13) and a best-fitting absorption column of 6.0 (+3.4, -3.1) x 10^20
cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.6 x 10^-11 (4.0 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     6.0 (+3.4, -3.1) x 10^20 cm^-2
Galactic foreground: 1.5 x 10^20 cm^-2
Excess significance: 2.4 sigma
Photon index:	     1.81 (+0.14, -0.13)

If the light curve continues to decay with a power-law decay index of
1.36, the count rate at T+24 hours will be 2.9 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x
10^-13 (1.2 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/00927825.

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

P. Schady (Univ. of Bath), J. Balmer (MPE Garching), and A. Nicuesa Guelbenzu (TLS Tautenburg) report:


We observed the field of GRB 191004A (Swift trigger 927825; Cenko et al., GCN #25945
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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 01:56 UT on 5th October, 7.8 hours after the GRB trigger. They were performed at an average seeing of 1.4" and at an average airmass of 1.5.


We do not detect a source at the position of the Swift-XRT and MASTER afterglow (Osborne et al., GCN #25950
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 ; Lipunov et al., GCN #25944
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 ) down to the following 3-sigma limits (all AB magnitude):


g' > 23.7 mag
r' > 24.2 mag
i' > 23.5 mag
z' > 23.3 mag
J > 21.4 mag
H > 20.4 mag
K > 19.9 mag


The given limits are derived based on calibrating the images against Sky-Mapper and 2MASS field stars and are not corrected for the Galactic foreground extinction corresponding to a reddening of E_(B-V)= 0.01 in the direction of the burst (Schlafly & Finkbeiner 2011).

We acknowledge the excellent support from Regis Lachaume in La Silla in acquiring these observations.

R. Strausbaugh (U. of the Virgin Islands), A. Cucchiara (U. of the Virgin Islands/College of Marin) report on behalf of a larger collaboration:

We observed Swift GRB 191004A (Barthelmy et al., GCN 25945
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 ) with
the LCO 1-m Sinistro instrument on September 4, from 20:12 to 20:47 UT
(corresponding to 1.70 to 2.28 hours from the GRB trigger time)
with the Bessel R and I filters.

We performed a series of 10x180s exposures in each band.  We do not detect a significant source in individual frames, but we do  detect a source in co-added frames that is not present in either USNO-B1.0 or 2MASS surveys and is consistent with the optical afterglow location (Evans et al., GCN 25950
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 ) with the following magnitudes:

R = 22.89 +/- 0.10
I = 21.00 +/- 0.09


This flux is calibrated against several USNO-B1.0 objects near the GRB location but is not corrected for Galactic Extinction.

These observations were possible thanks to the USVI NASA-EPSCoR
Research Infrastructure Development (RID) grant NNX16AL44A.

GRB 191004A: Swift/UVOT Detection

S. J. LaPorte (PSU) and S. B. Cenko (GSFC)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 191004A
84 s after the BAT trigger (Cenko et al., GCN Circ. 25945
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 ).
A fading source consistent with the XRT position
(Osborne et al. GCN Circ. 25950
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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

b                       582             775             38         >18.83
uvm2                680             873             38         >18.00
u                       327             750           265         >19.60
v                       656            1104            58         >18.06
uvw1                705              898            38         >17.89
uvw2                632            1080            58         >18.28
white                115            1054          333           19.78+-0.16

white              5567          12438          956         >21.24

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.01 in the direction of the burst
(Schlegel et al. 1998).

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

The long-duration GRB 191004A
(Swift-BAT detection: Cenko et al., GCN circ. 25945
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 ;
Sakamoto et al., GCN circ. 25953
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 )
triggered Konus-Wind at T0=65226.378 s UT (18:07:06.378).

The burst light curve shows a multi-peaked structure
which starts at ~T0-0.2 s and has a total duration of ~5 s.
The emission is seen up to ~1 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB191004_T65226/

As observed by Konus-Wind, the burst
had a fluence of 2.00(-0.41,+0.62)x10^-6 erg/cm2,
and a 64-ms peak flux, measured from T0+1.872 s,
of 2.38(-0.83,+1.02)x10^-6 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the burst
(measured from T0 to T0+8.448 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 = -0.69(-0.61,+0.83)
and Ep = 157(-37,+95) keV (chi2 = 49/57 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.0
(chi2 = 48/56 dof).

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

K. Yamaoka (Nagoya U),
A. Yoshida, T. Sakamoto, V. Pal'shin, S. Sugita (AGU),
Y. Kawakubo (LSU), S. Nakahira (RIKEN),
Y. Asaoka, S. Torii (Waseda U), Y. Shimizu, T. Tamura (Kanagawa U),
N. Cannady (GSFC/UMBC), M. L. Cherry (LSU),
S. Ricciarini (U of Florence), P. S. Marrocchesi (U of Siena),
and the CALET collaboration:

The soft GRB 191004A (Swift-BAT detection: Cenko et al., GCN Circ. 25945
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 ,
Sakamoto et al., GCN Circ. 25953
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 ; Konus-Wind observation: Svinkin et al.,
GCN Circ. 25973
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 ) triggered the CALET Gamma-ray Burst Monitor (CGBM) at
18:07:05.528 UTC on 4 October 2019. The burst signal was seen by all CGBM
detectors.
No real-time CGBM GCN notice was distributed about this trigger because
the real-time communication from the ISS was off (loss of signal).

The burst light curve shows two pulses which start at T-2.7 sec and end
at T+0.0 sec. The T90 and T50 durations measured by the SGM data are
2.6 +- 0.3 sec and 1.9 +- 1.4 sec (40-1000 keV), respectively.

The ground processed light curve is available at

http://cgbm.calet.jp/cgbm_trigger/ground/1254247608/

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

P. Schady (Univ. of Bath) reports:

I would like to correct a mistake in the GROND g'r'i'z' upper limits reported in Schady et al. (GCN #25959
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 ). The reported afterglow position (Osborne et al., GCN #25950
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 ; Lipunov et al., GCN #25944
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 ) lay at the edge of the g'r'i'z' field of view, where the exposure coverage is not complete. Taking into account the exposure map reduces the optical band 3 sigma upper limits reported in GCN #25959
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  by around 0.5 mag. The correct 3 sigma upper limits (AB system) are as follows:

g� > 23.3
r� > 23.7
i� > 23.1
z� > 22.7

I would like to thank Jochen Greiner for bringing this mistake to my attention.