GRB 250121A
GCN Circular 39023
Samuele Ronchini (PSU), James DeLaunay (PSU), Aaron Tohuvavohu (Caltech), Gayathri Raman (PSU), Jamie A. Kennea (PSU), Tyler Parsotan (NASA GSFC) report:
Swift/BAT did not localize GRB 250121A onboard (T0: 2025-01-21T23:36:05.78 UTC, Fermi GBM GCN 39006 and 39018, Fermi LAT GCN 39010, INTEGRAL GCN 39011, CALET GCN 39021)
The Fermi notice, distributed in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; Tohuvavohu et al. 2020, ApJ, 900, 1).
Upon trigger by this notice, GUANO sent a command to the Swift Burst Alert Telescope (BAT) to save 200 seconds of BAT event-mode data from [-50,+150] seconds around the time of the burst. All the requested event mode data was delivered to the ground.
The BAT likelihood search, NITRATES (DeLaunay + Tohuvavohu 2022, ApJ, 941, 169), performed on the temporal window [T0-20 s, T0+20 s], detects the burst with a sqrt(TS) of 38.1 in a 2.048 s analysis time bin, starting at T0 - 0.0001 s.
Using the NITRATES analysis, parameter estimation was performed to obtain the localization of this burst in the form of a HEALPIX Multi-Order Coverage (MOC) skymap. This localization accounts for both statistical and systematic errors. More details in the creation and calibration of these maps will soon be published (DeLaunay et al. 2025. in prep)
The 90% credible area is 153 deg2 and the 50% credible area is 46 deg2. The integrated probability inside the coded field of view is <1%.
The NITRATES skymap is consistent with the Fermi-GBM and Fermi-LAT localizations. The joint NITRATES+GBM localization has a 90% credible area of 68 deg2 and a 50% credible area of 17 deg2
A plot of the probability skymap can be viewed here:
The probability skymap file can be downloaded from the link here
Instructions on how to read and manipulate this map can be found here:
https://guano.swift.psu.edu/documentation
More details about this burst can be found on the trigger report page here:
https://guano.swift.psu.edu/trigger_report?id=759195400
GUANO is a fully autonomous, extremely low latency, spacecraft commanding pipeline designed for targeted recovery of BAT event mode data around the times of compelling astrophysical events to enable more sensitive GRB searches.
A live reporting of Swift/BAT event data recovered by GUANO can be found at: https://www.swift.psu.edu/guano/
GCN Circular 39021
T. Sakamoto, A. Yoshida, S. Sugita, Y. Kawakubo (AGU),
K. Yamaoka (Nagoya U), S. Nakahira (JAXA), Y. Asaoka (ICRR), S. Torii,
Y. Akaike, K. Kobayashi (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 long GRB 250121A (Fermi GBM Final Real-time Localization: Fermi GBM team, GCN Circ. 39006;
Fermi-LAT detection: Holzmann Airasca et al., GCN Circ. 39010;
INTEGRAL/SPI-ACS detection: Barria et al., GCN Circ. 39011;
Fermi GBM Observation: Godwin et al., GCN Circ. 39018;
) was detected in the ground analysis of the CALET Gamma-ray
Burst Monitor (CGBM) data around 23:36:05.78 on 21 January 2025 (referenced
to the Fermi-GBM Observation: GCN Circ. 39018).
(https://cgbm.calet.jp/cgbm_trigger/flight/1421537655/index.html).
The burst signal was seen by only the SGM detector.
The burst light curve shows a double-peaked structure that starts
at T+0.1 sec, peaks at T+0.7 sec, and ends at T+5.4 sec.
The T90 and T50 durations measured by the SGM data are 4.7 +/- 0.2 sec
and 2.6 +/- 0.4 sec (40-1000 keV), respectively.
The ground-processed light curve is available at
https://cgbm.calet.jp/cgbm_trigger/ground/1421537655/
The CALET data used in this analysis are provided by
the Waseda CALET Operation Center located at Waseda University.
GCN Circular 39018
Matt Godwin (UAH) and C.Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team:
"At 23:36:05.78 UT on 21 January 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250121A (trigger 759195370/250121983).
which was also detected by Fermi-LAT (Holzmann Airasca et al. 2025, GCN 39010).
The Fermi GBM on-ground location is consistent with the Fermi-LAT position. (Fermi GBM Team et al. 2025, GCN 39006)
The angle from the Fermi LAT boresight is 15 degrees.
This burst was also independently detected by INTEGRAL SPI-ACS.
The GBM light curve consists of one peak with a duration (T90)
of about 5.2 s (50-300 keV). The time-averaged spectrum
from T0-0.4 to T0+5.8 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -0.65 +/- 0.05 and the cutoff energy,
parameterized as Epeak, is 230 +/- 10 keV.
The event fluence (10-1000 keV) in this time interval is
(5.8 +/- 0.2)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+0.32 s in the 10-1000 keV band is 14.0 +/- 0.3 ph/s/cm^2.
A Band function fits the spectrum equally well
with Epeak= 210 +/- 20 keV, alpha = -0.59 +/- 0.07 and beta = -2.5 +/- 0.3.
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 39011
Patrizia Barria(a,b), Giulia Gianfagna(a), James Craig Rodi(a), Aishwarya Linesh Thakur(a), Lorenzo Natalucci(a,b), Luigi Piro(a) report:
GRB 250121A was discovered by Fermi/GBM (GCN 39006, Fermi/LAT GCN 39010) on 2025-01-21T23:36:05 (UTC). We searched for a corresponding counterpart in the INTEGRAL/SPI-ACS data.
In an SPI-ACS light curve above 80 keV, we find a signal temporally coincident with the GBM detection, having an approximate duration of ~ 4 sec. The signal consists of a single pulse over this duration.
The approximate peak count rate in SPI-ACS is 74,000 cts/s for E>80 keV, over a median background rate of 62,800 cts/s.
This work is based on observations with INTEGRAL, an ESA project with instruments and a science data centre funded by ESA member states (especially the PI countries: Denmark, France, Germany, Italy, Switzerland, and Spain), and with the participation of Russia and the USA. The SPI-ACS detector system has been provided by MPE Garching/Germany.
-----
(a) INAF/IAPS-Rome
(b) ICSC National Research Centre for High-Performance Computing
GCN Circular 39010
A. Holzmann Airasca (UniTrento and INFN Bari), D. Depalo (Politecnico and INFN Bari), E. Bissaldi (Politecnico and INFN Bari), R. Gupta (NASA GSFC), C. Bartolini (UniTrento and INFN Bari), S. Lopez (CNRS / IN2P3) on behalf of the Fermi-LAT Collaboration:
On January 21, 2025, Fermi-LAT detected high-energy emission from GRB 250121A, which was also detected by Fermi-GBM (trigger 759195370 / 250121983, GCN 39006).
The best LAT on-ground location is found to be:
RA, Dec = 225.14, -34.37 (J2000)
with an error radius of 0.24 deg (90 % containment, statistical error only). This was 15 deg from the LAT boresight at the time of the GBM trigger (T0 = 23:36:05.78 UT). The data from the Fermi-LAT shows a significant increase in the event rate that is spatially and temporally correlated with the GBM emission with high significance. The photon flux above 100 MeV in the time interval 0 - 500 s after the GBM trigger is (2.5 ± 1.2) E-6 ph/cm2/s. The estimated photon index above 100 MeV is 1.7 ± 0.3. The highest-energy photon is a 2.6 GeV event which is observed ~ 15 seconds after the GBM trigger.
The Fermi-LAT point of contact for this burst is Aldana Holzmann Airasca (aldana.holzmannairasca@ba.infn.it).
The Fermi-LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.
GCN Circular 39006
The Fermi GBM team reports the detection of a likely LONG GRB
At 23:36:05 UT on 21 Jan 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250121A (trigger 759195370.779232 / 250121983).
The on-ground calculated location, using the Fermi GBM trigger data, is RA = 225.1, Dec = -36.9 (J2000 degrees, equivalent to J2000 15h 00m, -36d 53'), with a statistical uncertainty of 1.1 degrees.
The angle from the Fermi LAT boresight is 16.0 degrees.
The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250121983/quicklook/glg_skymap_all_bn250121983.png
The HEALPix FITS file, including the estimated localization systematic, can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250121983/quicklook/glg_healpix_all_bn250121983.fit
The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250121983/quicklook/glg_lc_medres34_bn250121983.gif