GRB 230426A

The Fermi GBM team reports the detection of a likely LONG GRB

At 20:28:59 UT on 26 Apr 2023, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 230426A (trigger 704233744.96626 / 230426853).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 38.5, Dec = -82.4 (J2000 degrees, equivalent to J2000 02h 34m, -82d 24'), with a statistical uncertainty of 1.0 degrees.

The angle from the Fermi LAT boresight is 49.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2023/bn230426853/quicklook/glg_skymap_all_bn230426853.png

The HEALPix FITS file, including the estimated localization systematic, can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2023/bn230426853/quicklook/glg_healpix_all_bn230426853.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2023/bn230426853/quicklook/glg_lc_medres34_bn230426853.gif

P K. Navaneeth (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 ML pipeline (Abraham et al., 2021, MNRAS, 504, 3084) and the CIFT framework (Sharma et al., 2021, JApA, 42, 73) showed the detection of a bright long GRB 230426A which was also detected by Fermi (Fermi GBM Team, GCN Circ. 33684
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 ).

The source was clearly detected in the 20-200 keV energy range. The light curve peaks at 2023-04-26 20:29:23.50 UTC. The measured peak count rate associated with the burst is 1773 (+77, -81) counts/s above the background in the combined data of all quadrants, with a total of 14876 (+653, -615) counts. The local mean background count rate was 454 (+2, -3) counts/s. Using cumulative rates, we measure a T90 of 23 (+3, -2) s. In the preliminary analysis, we find 1069 Compton events associated with this event.

The source was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve peaks at 2023-04-26 20:29:23.50 UTC. The measured peak count rate associated with the burst is 1179 (+82, -77) counts/s above the background in the combined data of all quadrants, with a total of 8609 (+655, -665) counts. The local mean background count rate was 1491 (+4, -5) counts/s. We measure a T90 of 18 (+5, -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

J. Wood (NASA/MSFC) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 20:28:59.97 UT on 26 April 2023, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 230426A (trigger 704233744/230426853)
which was also detected by AstroSat CZTI (P K. Navaneeth et al. 2023, GCN 33686
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 ).

The on-ground calculated location, using the Fermi GBM trigger data,
is RA = 38.53, Dec = -82.40 (J2000 degrees, equivalent to
J2000 2h 34m, -82d 24'), with a statistical uncertainty of 1.00 degrees.
(radius, 1-sigma containment, statistical only; there is additionally a
systematic error which we have characterized as a mixture of two Gaussians,
one with a radius of 1.8 degrees (52% contribution) and one with a radius
of 4.1 degrees (47% contribution) [A. Goldstein et al. 2020, ApJ, 895, 1]).

The angle from the Fermi LAT boresight is 49 degrees.

The GBM light curve consists of multiple pulses with a duration (T90)
of about 25.9 s (50-300 keV). The time-averaged spectrum
from T0+8.2 to T0+35.8 s is best fit by
a Band function with Epeak = 77 +/- 2 keV,
alpha = -1.34 +/- 0.02, and beta = -2.38 +/- 0.03.

The event fluence (10-1000 keV) in this time interval is
(5.21 +/- 0.04)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+23 s in the 10-1000 keV band is 66.6 +/- 0.7 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/"

V.Lipunov (Lomonosov MSU), D.A.H.Buckley (SAAO), Ya.Kechin, A.Chasovnikov,
A.Kuznetsov, N.Tiurina, P.Balanutsa, O.Gress, E.Gorbovskoy, G.Antipov, K.Zhirkov, D.Vlasenko,
V.Senik, V.Topolev, Yu.Tselik, Siyu Wu (Lomonosov Moscow State University, SAI, Physics Department),
C.Francile, F. Podesta, C.Lopez, R. Podesta (Observatorio Astronomico Felix Aguilar OAFA),
R. Rebolo, M. Serra (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.Corella,
L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory),
N.M.Budnev, O.Ershova (ISU,API),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A. Gabovich, V.Yurkov (Blagoveschensk Educational State University),
K.Vetrov, V.Shumkov, T.Pogrosheva

MASTER-SAAO robotic telescope (Global MASTER Net  http://observ.pereplet.ru,Lipunov et al., 2010, Advances in Astronomy, v.2010, 30L)
located in South African Astronomical Observatory
started (Lipunov et al. GCN 33685
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 ) inspect of the FERMI GRB230426.85 (trigger 704233744, Wood et al. GCN 33689
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 , Ttrigger 20:28:59.97UT) errorbox
4359 sec after notice time (4391 sec after trigger time) at 2023-04-26 21:42:11UT with upper limit up to 19.5m,
see our cover map https://master.sai.msu.ru/site/master2/observ.php?id=2212931

MASTER OT J023135.16-792104.8 - optical counterpart of GRB 230426A discovery
2023gwu

MASTER-SAAO auto-detection system found optical counterpart of GRB 230426.85
at R.A.,Dec(2000) = 02h 31m 35.16s , -79d 21m 04s.8 with unfiltered m_OT=17.4 at
2023-04-26 21:56:50UT and with optical decay at 10 images
 https://www.wis-tns.org/object/2023gwu .

There is no any sources in VIZIER database, no any objects in MPC at this time
and no OT in MASTER database since Dec.2014. We used reference image on
2018-08-18.99148 UT with  unfiltered mlim=20.6m.

Analysis will be continued.

Deep photometry and spectral observations are required.

The message may be cited.

M. Dafcikova, J. Ripa (Masaryk U.), A. Pal (Konkoly Observatory),  N. Werner  (Masaryk U.), M. Ohno (Hiroshima U.),  L. Meszaros, B. Csak (Konkoly Observatory), H. Takahashi (Hiroshima U.), F. Munz , M. Topinka, F. Hroch, N. Husarikova, J.-P. Breuer (Masaryk U.), J. Hudec, J. Kapus, M. Frajt, M. Rezenov (Spacemanic s.r.o), R. Laszlo (Needronix), G. Galgoczi (Wigner Research Center/Eotvos U.), N. Uchida (ISAS/JAXA), T. Enoto (Kyoto U.), Zs. Frei (Eotvos U.), Y. Fukazawa, K. Hirose, H. Matake (Hiroshima U.), S. Hisadomi (Nagoya U.), Y. Ichinohe (Rikkyo U.), L. L. Kiss (Konkoly Observatory),  T. Mizuno (Hiroshima U.), K. Nakazawa (Nagoya U.), H. Odaka (Univ of Tokyo), K. Torigoe (Hiroshima U.), P. Svoboda, V. Daniel, J. Dudas, M. Junas, J. Gromes (VZLU), I. Vertat (FEL ZCU)  -- the VZLUSAT-2/GRB payload collaboration.

The long duration GRB 230426A (Fermi/GBM detection: GCN 33684
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 ; Astrosat/CZTI detection: GCN 33686
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 ; INTGERAL/SPI-ACS detection at 2023-04-26 20:29:23 UT) was detected by the GRB detectors on board of the VZLUSAT-2 3U CubeSat (https://www.vzlusat2.cz/en/).

The data acquisition was performed by the GRB detector units no. 0 and no. 1. The detection was confirmed at the peak time 2023-04-26 20:29:21 UTC. The T90 duration measured by VZLUSAT-2 is 35 s (25 s) and the significance during T90 reaches 18 sigma (68 sigma) for detector unit no. 0 (no. 1).

The light curve obtained by VZLUSAT-2 is available here:
https://vzlusat2.konkoly.hu/static/share/GRB230426A_GCN_VZLUSAT2.pdf

All VZLUSAT-2 detections are listed at: https://monoceros.physics.muni.cz/hea/VZLUSAT-2/
The GRB detectors on VZLUSAT-2 are a demonstration payload for a future CubeSat constellation (Werner et al. Proc. SPIE 2018). Two GRB modules of VZLUSAT-2 are placed in a perpendicular manner and each consists of a 75 x 75 x 5 mm3 CsI scintillator read out by a SiPM array, covering the energy range from ~30 keV to ~1000 keV. VZLUSAT-2 was launched on 2022 January 13 from Cape Canaveral.