GRB 240918B
GCN Circular 37558
Subject
GRB 240918B: Fermi GBM Detection
Date
2024-09-19T15:47:35Z (8 months ago)
From
rachel.hamburg@ijclab.in2p3.fr
Via
Web form
The Fermi GBM team reports the detection of a likely LONG GRB.
At approximately 20:56:58 UT on 18 Sep 2024, the Fermi Gamma-ray Burst Monitor (GBM)
detected GRB 240918B, which was observed about 107 s after GBM triggered on
GRB 240918A (trigger 748385716/240918872) (Fermi GBM Team 2024; GCN 37549).
Due to the GBM trigger buffer of 10 min, the GBM did not trigger on this GRB.
The on-ground calculated location, using the Fermi GBM trigger data from 107.52 s
to 115.71 s after trigger time, is RA = 142.54, Dec = -17.36 (J2000 degrees,
equivalent to J2000 9h 30m, -17d 36'), with a statistical uncertainty of 2.03 degrees.
The angle from the Fermi LAT boresight is 64 degrees.
The GBM Targeted Search [1], the most sensitive, coherent search for GRB-like signals,
was also used to identify GRB-like emission around the time of GRB 240918B. This search
finds a multi-peaked emission episode on timescales up to 4.096 s and showing a
localization consistent with the on-ground location. The brightest peak (at 20:57:01 UT)
was found to be the most significant (FAR = 5.4e-06 Hz) on the 0.512 s timescale and
best fit with a "normal" GRB spectrum as described in [1]."
[1] Goldstein et al. 2019 arXiv:1903.12597
GCN Circular 37588
Subject
GRB 240918B: AstroSat CZTI detection
Date
2024-09-23T00:21:43Z (8 months ago)
From
Gaurav Waratkar at IIT Bombay <gauravwaratkar@iitb.ac.in>
Via
Web form
J. Joshi (IUCAA), G. Waratkar (IITB), A. Vibhute (IUCAA), V. Bhalerao (IITB), D. Bhattacharya (Ashoka University/IUCAA), A. R. Rao (IUCAA/TIFR), and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration:
Analysis of AstroSat CZTI data with the CIFT framework (Sharma et al., 2021, JApA, 42, 73) showed the detection of a long-duration GRB 240918B which was also detected by Fermi/GBM (Fermi-GBM Team, GCN Circ. 37558) and INTEGRAL SPI-ACS (Pawar et al., GCN Circ. 37560).
The source was detected in the CZT detectors in the 20-200 keV energy range. The light curve peaks at 2024-09-18 20:57:00.50 UTC. The measured peak count rate associated with the burst is 123 (+35, -30) counts/s above the background in the combined data of all quadrants, with a total of 339 (+116, -102) counts. The local mean background count rate was 316 (+3, -5) counts/s. Using cumulative rates, we measure a T90 of 4 (+2, -1) s.
The source was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve peaks at 2024-09-18 20:57:01.32 UTC. The measured peak count rate associated with the burst is 295 (+60, -56) counts/s above the background in the combined data of three quadrants (out of four), with a total of 1092 (+181, -204) counts. The local mean background count rate was 1049 (+6, -6) counts/s. We measure a T90 of 6 (+1, -2) s from the cumulative Veto light curve.
CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, URSC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project.
CZTI GRB detections are reported regularly on the payload site at:
http://astrosat.iucaa.in/czti/?q=grb
GCN Circular 37602
Subject
IPN triangulation of GRB 240918B
Date
2024-09-24T19:25:27Z (8 months ago)
From
Anna Ridnaia at Ioffe Institute <ridnaia@mail.ioffe.ru>
Via
Web form
A.S. Kozyrev, D.V. Golovin, M.L. Litvak, I.G. Mitrofanov, and A.B. Sanin
on behalf of the HEND/Mars Odyssey team,
D. Svinkin, D. Frederiks, A. Ridnaia, A. Lysenko,
and T. Cline on behalf of the Konus-Wind team,
A. Goldstein, M. S. Briggs, C. Wilson-Hodge,
and E. Burns on behalf of the Fermi GBM team,
E. Bozzo and C. Ferrigno, on behalf of the INTEGRAL SPI-ACS GRB team,
and
W. Boynton, C. Fellows, K. Harshman, H. Enos, R. Starr,
and A.S. Gardner on on behalf of the GRS-Odyssey GRB team,
report:
The long-duration GRB 240918B
(Fermi-GBM detection: The Fermi GBM team, GCN 37558;
INTEGRAL SPI-ACS detection: Pawar, GCN 37560;
AstroSat CZTI detection: Joshi et al., GCN 37588)
was detected by Fermi (GBM), Konus-Wind, INTEGRAL (SPI-ACS),
AstroSat (CZTI), and Mars-Odyssey (HEND)
at about 75416 s UT (20:56:56).
We have triangulated it to a preliminary, 3 sigma error box
whose coordinates are:
---------------------------------------------
RA(2000), deg Dec(2000), deg
---------------------------------------------
Center:
143.280 (09h 33m 07s) -17.435 (-17d 26' 07")
Corners:
141.159 (09h 24m 38s) -19.395 (-19d 23' 40")
141.379 (09h 25m 31s) -19.587 (-19d 35' 15")
145.293 (09h 41m 10s) -15.401 (-15d 24' 02")
145.075 (09h 40m 18s) -15.211 (-15d 12' 41")
---------------------------------------------
The error box area is 1.6 sq. deg, and its maximum
dimension is 5.6 deg (the minimum one is 17 arcmin).
The Sun distance was 35 deg.
The IPN localization is consistent with, but reduces the area of,
the Fermi-GBM localization (GCN 37558).
This localization may be improved.
A triangulation map and HEALPix FITS file are posted at
http://www.ioffe.ru/LEA/GRBs/GRB240918_T75415/IPN/
The HEALPix triangulation map is the multi-order HEALPix in units of
probability density.
The Konus-Wind time history and spectrum will be given
in a forthcoming GCN Circular.
GCN Circular 37603
Subject
Konus-Wind detection of GRB 240918B
Date
2024-09-24T19:27:38Z (8 months ago)
From
Anna Ridnaia at Ioffe Institute <ridnaia@mail.ioffe.ru>
Via
Web form
A. Ridnaia, D. Frederiks, A. Lysenko, D. Svinkin,
A. Tsvetkova, M. Ulanov, and T. Cline,
on behalf of the Konus-Wind team, report:
The long-duration GRB 240918B
(Fermi-GBM detection: The Fermi GBM team, GCN 37558;
INTEGRAL SPI-ACS detection: Pawar, GCN 37560;
AstroSat CZTI detection: Joshi et al., GCN 37588;
IPN triangulation: Kozyrev et al., GCN 37602)
triggered Konus-Wind at T0=75415.787 s UT (20:56:55.787).
The burst light curve shows a multipeaked structure
which starts at ~T0-1.9 s and has a total duration of ~4.4 s.
The emission is seen up to ~2 MeV.
The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB240918_T75415/
As observed by Konus-Wind, the burst
had a fluence of 4.19(-0.60,+0.71)x10^-6 erg/cm2,
and a 64-ms peak flux, measured from T0+1.264 s,
of 2.56(-0.89,+0.93)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 - 10 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with alpha = -0.49(-0.44,+0.56)
and Ep = 173(-28,+45) keV (chi2 = 112/99 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 = 111/98 dof).
All the quoted errors are at the 90% confidence level.
All the quoted values are preliminary.