GRB 191218A

E. Troja (NASA/GSFC/UMCP), N. J. Klingler (PSU),
A. Y. Lien (GSFC/UMBC), M. J. Moss (GWU), K. L. Page (U Leicester),
D. M. Palmer (LANL), M. H. Siegel (PSU) and A. Tohuvavohu (U Toronto)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 02:42:43 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 191218A (trigger=944916).  Swift did not slew to this burst due
to a Sun constraint. 
The BAT on-board calculated location is 
RA, Dec 301.078, -40.274 which is 
   RA(J2000) = 20h 04m 19s
   Dec(J2000) = -40d 16' 25"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a double-peaked
structure with a duration of about 60 sec.  The peak count rate
was ~4000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

Due to a Sun observing constraint, Swift cannot slew to the BAT
position until 12:12 UT on 2020 February 27. There will thus be no XRT
or UVOT data for this trigger before this time. 

Burst Advocate for this burst is E. Troja (eleonora.troja 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/)

R. Gaikwad, S.Gupta, V. Sharma, A. Vibhute and D. Bhattacharya (IUCAA), V. Bhalerao (IIT-B), A. R. Rao (TIFR) and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration:

Analysis of AstroSat CZTI data showed the detection of a GRB 191218A, which was also detected by Swift (Troja E. et al., GCN #26480
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 ).

The source was clearly detected in the 40-200 keV energy range. The light curve shows multiple peaks of emission with the strongest peak at 2019-12-18 02:42:43.0 UT. The measured peak count rate associated with the burst is 348 cts/s above the background in the combined data of four quadrants, with a total of 3669 cts. The local mean background count rate was 507 cts/s. Using cumulative rates, we measure a T90 of 51.03 s.

It was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range.

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.

C. B. Markwardt (GSFC), S. D. Barthelmy (GSFC),
J. R. Cummings (CPI), H. A. Krimm (NSF),
S. Laha (GSFC/UMBC), A. Y. Lien (GSFC/UMBC),
D. M. Palmer (LANL), T. Sakamoto (AGU),
M. Stamatikos (OSU), E. Troja (GSFC/UMD),
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 191218A (trigger #944916)
(Troja et al., GCN Circ. 26480
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 ).  The BAT ground-calculated position is
RA, Dec = 301.070, -40.277 deg which is
   RA(J2000)  =  20h 04m 16.7s
   Dec(J2000) = -40d 16' 38.8"
with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 9%.

The mask weighted light curve shows two peaks at ~T+4 s and ~T+45 s. The
overall structure starts at ~T-2 s and lasts till ~T+58 s.
T90 (15-350 keV) is 44.67 +- 0.73 sec (estimated error including systematics).

The time-averaged spectrum from T-0.23 to T+46.03 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.12 +- 0.10.  The fluence in the 15-150 keV band is 7.8 +- 0.4 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T+0.66 sec in the 15-150 keV band
is 6.3 +- 0.9 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/944916/BA/

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

The long-duration GRB 191218A (Swift-BAT trigger #944916:
Troja et al., GCN 26480
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 ; Markwardt et al., GCN 26495
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 ;
AstroSat CZTI detection: Gaikwad et al., GCN 26482
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 )
triggered Konus-Wind at T0=9758.735 s UT (02:42:38.735).

The burst light curve shows a double-peaked structure
which starts at ~T0-0.5 s and has a total duration of ~78.8 s.
The emission is seen up to ~4 MeV.

As observed by Konus-Wind, the burst
had a fluence of 3.92(-0.37,+0.45)x10^-5 erg/cm2,
and a 64-ms peak flux, measured from T0+1.440 s,
of 4.08(-1.07,+1.07)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+81.664 s)
is best fit in the 20 keV - 4 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with  alpha = -0.76(-0.18,+0.20)
and Ep = 330(-44,+61) keV (chi2 = 54/69 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.2
(chi2 = 53/68 dof).

The spectrum near the maximum count rate
(measured from T0+0.256 to T0+7.936 s)
is best fit in the 20 keV - 4 MeV range
by a power law with exponential cutoff model
with  alpha = -0.27(-0.15,+0.17)
and Ep = 322(-24,+28) keV (chi2 = 74/69 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.7
(chi2 = 73/68 dof).

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB191218_T09758/

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