- GCN Circular #29663
E. Troja (NASA/GSFC, UMCP), K. L. Page (U. Leicester), A. Lien (GSFC/UMBC),
M. Siegel (PSU), and P. Evans (U. Leicester) report on behalf of the Neil
Gehrels Swift Observatory Team:
At 03:08:33 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210318B (trigger=1037335). Swift slewed immediately to the
burst.
The BAT on-board calculated location is
RA, Dec 264.227, -35.819 which is
RA(J2000) = 17h 36m 55s
Dec(J2000) = -35d 49' 08"
with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty). The BAT light curve showed several overlapping
pulses with a duration of about 20 sec. The peak count rate
was ~22000 counts/sec (15-350 keV), at ~3 sec after the trigger.
We have analysed 1.4 ks of XRT data for GRB 210318B, from 52 s to 1.5
ks after the BAT trigger. The data comprise 584 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 refined XRT position is RA,
Dec = 264.2328, -35.8220 which is equivalent to:
RA (J2000): 17 36 55.88
Dec(J2000): -35 49 19.2
with an uncertainty of 3.5 arcsec (radius, 90% confidence).
The light curve can be modelled with an initial power-law decay with an
index of alpha=0.71 (+0.03, -0.04), followed by a break at T+564 s to
an alpha of 1.23 (+0.18, -0.24).
A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.78 (+/-0.09). The
best-fitting absorption column is 1.43 (+/-0.13) x 10^22 cm^-2, in
excess of the Galactic value of 7.1 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.69 (+0.27, -0.25)
and a best-fitting absorption column of 1.6 (+/-0.4) x 10^22 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum is 6.5 x 10^-11 (1.1 x 10^-10) erg cm^-2
count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 1.6 (+/-0.4) x 10^22 cm^-2
Galactic foreground: 7.1 x 10^21 cm^-2
Excess significance: 4.0 sigma
Photon index: 1.69 (+0.27, -0.25)
If the light curve continues to decay with a power-law decay index of
1.23, the count rate at T+24 hours will be 0.011 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 6.9 x
10^-13 (1.1 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/01037335.
UVOT took a finding chart exposure of 150 seconds with the White filter
starting 57 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 with a 3-sigma upper limit of 20.1 mag.
No correction has been made for the large, but uncertain, extinction
expected.
Due to a temporary TDRSS outage, no automatic GCN notices or alerts were
promptly distributed.
Burst Advocate for this burst is E. Troja (eleonora@umd.edu).
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/)
- GCN Circular #29664
D. Svinkin, S. Golenetskii, D. Frederiks, M. Ulanov,
A. Tsvetkova, A. Lysenko, A. Ridnaia, and T. Cline
on behalf of the Konus-Wind team, report:
The long-duration GRB 210318B
(Swift-BAT detection: Troja et al., GCN Circ. 29663)
triggered Konus-Wind at T0=11309.57 s UT (03:08:29.570).
The burst light curve shows a multipeaked structure
which starts at ~T0-0.5 s and has a total duration of ~14 s.
The emission is seen up to ~3 MeV.
The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB210318_T11309/
As observed by Konus-Wind, the burst
had a fluence of 3.83(-0.44,+0.50)x10^-5 erg/cm2,
and a 64-ms peak flux, measured from T0+1.872 s,
of 1.57(-0.28,+0.29)x10^-5 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).
The time-averaged spectrum of the burst
(measured from T0 to T0+13.824 s)
is best fit in the 20 keV - 10 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -0.81(-0.12,+0.15),
the high energy photon index beta = -2.51(-0.39,+0.23),
the peak energy Ep = 264(-32,+33) keV
(chi2 = 70/82 dof).
The spectrum near the maximum count rate
(measured from T0+0.256 to T0+5.632 s)
is best fit in the 20 keV - 10 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -0.64(-0.13,+0.14),
the high energy photon index beta = -2.59(-0.45,+0.23),
the peak energy Ep = 297(-32,+36) keV
(chi2 = 67/81 dof).
All the quoted errors are at the 90% confidence level.
All the quoted values are preliminary.
- GCN Circular #29666
M. Stamatikos (OSU), S. D. Barthelmy (GSFC), H. A. Krimm (NSF),
S. Laha (GSFC/UMBC), A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC),
D. M. Palmer (LANL), T. Sakamoto (AGU), E. Troja (NASA/GSFC, UMCP),
T. N. Ukwatta (LANL) (i.e. the Swift-BAT team):
Using the data set from T-61 to T+242 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 210318B (trigger #1037335)
(Troja, et al., GCN Circ. 29663). The BAT ground-calculated position is
RA, Dec = 264.228, -35.823 deg which is
RA(J2000) = 17h 36m 54.7s
Dec(J2000) = -35d 49' 21.4"
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 95%.
The mask-weighted light curve shows a multi-peak structure running from ~T-4 sec
to T+15 sec. T90 (15-350 keV) is 14.95 +- 0.50 sec (estimated error including systematics).
The time-averaged spectrum from T-11.22 to T+19.42 sec is best fit by a power law
with an exponential cutoff. This fit gives a photon index 1.10 +- 0.13,
and Epeak of 252.1 +- 191.6 keV (chi squared 41.33 for 0 d.o.f.). For this
model the total fluence in the 15-150 keV band is 1.0 +- 0.02 x 10^-5 erg/cm2
and the 1-sec peak flux measured from T+2.70 sec in the 15-150 keV band is
14.5 +- 0.4 ph/cm2/sec. A fit to a simple power law gives a photon index
of 1.31 +- 0.03 (chi squared 48.69 for 0 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/1037335/BA/
- GCN Circular #29668
C. Malacaria (NASA-MSFC/USRA) and C. Meegan (UAH)
report on behalf of the Fermi GBM Team:
At 03:08:46.50 UT on 18 March 2021, the Fermi Gamma-Ray Burst Monitor (GBM)
triggered and located GRB 210318B (trigger 637729731 / 210318131),
which was also detected by the Swift/BAT (Troja et al. 2019, GCN 29663)
and by Konus-Wind (Svinkin et al. 2021, GCN 29664).
The GBM on-ground location is consistent with the Swift position.
The angle from the Fermi LAT boresight at the GBM trigger time is 91 degrees.
The GBM light curve shows a peak
with a duration (T90) of about 24.3 s (50-300 keV).
The time-averaged spectrum from T0-4.1 s to T0+0.0 s is
best fit by a power law function with an exponential
high-energy cutoff. The power law index is 2.72 +/- 1.53 and
the cutoff energy, parameterized as Epeak, is 84.5 +/- 8.7 keV.
The event fluence (10-1000 keV) in this time interval is
(2.537 +/- 0.393)E-07 erg/cm^2. The 1-sec peak photon flux measured
starting from T0-1.9 s in the 10-1000 keV band
is 1.4 +/- 0.2 ph/s/cm^2.
The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog:
https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html
For Fermi GBM data and info, please visit the official Fermi GBM Support Page:
https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/
- GCN Circular #29675
R. Hosokawa, K. L. Murata, R. Adachi, M. Niwano, F. Ogawa, N.Nakamura,
N. Ito, S. Ogata, H. Takamatsu, H. Hara, Y. Yatsu, and N.Kawai
(TokyoTech) report on behalf of the MITSuME collaboration:
We observed the field of GRB 210318B (Troja et al. GCN Circular
#29663, Svinkin et al. GCN Circular #29664, Stamatikos et al. GCN
Circular #29666, Lipunov et al. GCN Circular #29667, Malacaria et al.
GCN Circular #29668) with the optical three color (g', Rc, and Ic) CCD
cameras attached to the MITSuME 50 cm telescope of Okayama
Astrophysical Observatory, Okayama, Japan. The observation with a
series of 60 sec exposures started at 2021-03-19 19:11:03 UT.(40.4
hours after Swift BAT trigger) We stacked the images with good
conditions. We did not find any new point sources within the Swift XRT
circle (Troja et al. GCN Circular #29663) in all three bands.
We obtained the 5-sigma limits of the stacked images as follows.
T0+[hour] MID-UT T-EXP[sec] 5-sigma limits
------------------------------------------------------------------------------------------------
40.4 19:50:32 2040 g'>14.3, Rc>14.2, Ic>13.4
------------------------------------------------------------------------------------------------
T0+ : Elapsed time after the burst
T-EXP: Total Exposure time
We used the UCAC4 catalog for flux calibration.
The magnitudes are expressed in the AB system.
The images were processed in real-time through the MITSuME GPU
reduction pipeline (Niwano et al. 2021, PASJ, Vol.73, Issue 1, Pages
4-24; https://github.com/MNiwano/Eclaire).
- GCN Circular #29695
GRB 210318B: Swift/UVOT Upper Limits
M. H. Siegel (PSU) and E. Troja (NASA/GSFC, UMCP)
report on behalf of the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 210318B
70 s after the BAT trigger (Troja et al., GCN Circ. 29663).
No optical afterglow consistent with the XRT position
(Troja et al. GCN Circular 29663)
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 70 219 147 >20.0
u_FC 282 532 246 >20.1
white 70 1357 373 >20.9
v 612 1406 97 >18.2
b 538 1500 92 >19.1
u 282 1479 324 >20.1
w1 661 1455 97 >18.8
m2 1066 1430 58 >19.3
w2 588 1382 97 >19.0
The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 1.718 in the direction of the burst
(Schlegel et al. 1998).
Previous IAU Circulars 