GRB 231230A

Chenyu Wang, Zirui Yang and Longhao Li report on behalf of the GRID Collaboration:

GRID-04 reports the detection of the long-duration GRB 231230A, which was also detected by Swift/BAT and Fermi/GBM(GCN Circular 35434
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  and 35436
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 ).

The event was triggered with GRID on 2023-12-30 at 01:29:08​ UTC. The GRID light curve consists of multiple spikes with a duration (T90) of about 17 +/- 1 s (30-2000 keV).
The time-averaged spectrum from T0+0.4 to T0+17.3 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -1.03 +/- 0.2 and the cutoff energy, parameterized as Epeak, is 132 +/- 45 keV. The event fluence (10-1000 keV) in this time interval is about 6.0E-06 erg/cm^2.​

The GRID light curve of this event can be found at https://mirrors.tuna.tsinghua.edu.cn/GRID/data/GRID-GCN/GRB231230A/GRID_231230A_ltcv.pdf. The GRID spectrum of this event can be found at https://mirrors.tuna.tsinghua.edu.cn/GRID/data/GRID-GCN/GRB231230A/GRID_231230A_spec_cpl.pdf.

GRID is a student-led project to monitor the transient gamma-ray sky with multiple detectors onboard different nanosatellites in the era of multi-messenger astronomy. For more information about GRID, please refer to the following references: https://doi.org/10.1007/s10686-019-09636-w
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  and https://doi.org/10.1007/s10686-021-09819-4.
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 ​​

S. Laha (GSFC/UMBC), S. D. Barthelmy (GSFC),  H. A. Krimm (NSF),
A. Y. Lien (U Tampa), C. B. Markwardt (GSFC), M. J. Moss (GSFC),
D. M. Palmer (LANL), T. Parsotan (GSFC/UMBC), T. Sakamoto (AGU),
M. Stamatikos (OSU) (i.e. the Swift-BAT team):

Using the data set from T-239 to T+963 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 231230A (trigger #1205319)
(Moss, et al., GCN Circ. 35436
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 ).  The BAT ground-calculated position is
RA, Dec = 245.223, 58.132 deg which is
   RA(J2000)  =  16h 20m 53.5s
   Dec(J2000) = +58d 07' 54.2"
with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 35%.

The mask-weighted light curve shows a complex structure extending from
around T+0 to T+20.  T90 (15-350 keV) is 15.2 +- 0.9 sec (estimated error including systematics).

The time-averaged spectrum from T+0.26 to T+17.26 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.28 +- 0.10.  The fluence in the 15-150 keV band is 2.1 +- 0.1 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T+7.64 sec in the 15-150 keV band
is 2.2 +- 0.4 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/1205319/BA/

C. Fletcher (USRA) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 01:29:10.05 UT on 30 December 2023, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB231230A (trigger 725592555/231230062).
which was also detected by Swift-BAT (Moss et al. 2023, GCN 35436
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 ).
The Fermi GBM Real-time Localization (GCN 35434
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 ) is consistent with the Swift-BAT position.

The angle from the Fermi LAT boresight is 157 degrees.

The GBM light curve consists of multiple spikes with a duration (T90)
of about 17 s (50-300 keV). The time-averaged spectrum
from T0-2.6 to T0+17.9 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -1.2 +/- 0.2 and the cutoff energy,
parameterized as Epeak, is 220 +/- 40 keV.

The event fluence (10-1000 keV) in this time interval is
(6.1 +/- 0.5)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+9.3 s in the 10-1000 keV band is 5 +/- 1 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/"

A. Tohuvavohu (U. Toronto), D.N. Burrows (PSU), J. D. Gropp (PSU), K.L.
Page (U. Leicester), A.P. Beardmore (U. Leicester), R. Brivio
(INAF-OAB), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAR) and P.A.
Evans report on behalf of the Swift-XRT team:

We have analysed 9.1 ks of XRT data for GRB 231230A, from 127 s to 41.2
ks after the  BAT trigger. The data comprise 68 s in Windowed Timing
(WT) mode (the first 8 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. 

The light curve can be modelled with an initial power-law decay with an
index of alpha=0.835 (+0.027, -0.028), followed by a break at T+17.4 ks
to an alpha of 2.0 (+/-0.4).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.93 (+/-0.13). The
best-fitting absorption column is  4.0 (+0.7, -0.6) x 10^21 cm^-2, in
excess of the Galactic value of 1.3 x 10^20 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 4.1 x 10^-11 (6.1 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     4.0 (+0.7, -0.6) x 10^21 cm^-2
Galactic foreground: 1.3 x 10^20 cm^-2
Excess significance: 9.9 sigma
Photon index:	     1.93 (+/-0.13)

If the light curve continues to decay with a power-law decay index of
2.0, the count rate at T+24 hours will be 5.1 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.1 x
10^-13 (3.1 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01205319.

This circular is an official product of the Swift-XRT team.

A. Kumar; B. P. Gompertz; G. Ramsay; R. Starling; K. Ulaczyk; K. Ackley; M. J. Dyer; J. Lyman; F. Jimenez-Ibarra; D. O'Neill; D. Steeghs; D. K. Galloway; V. Dhillon; P. O'Brien; K. Noysena; R. Kotak; R. P. Breton; L. K. Nuttall; E. Pall'e and D. Pollacco report on behalf of the GOTO collaboration:

The Gravitational-wave Optical Transient Observer (GOTO, Steeghs et al. 2022) performed a targeted observation in response to Fermi and Swift detected GRB 231230A (Fermi GBM Team, GCN 35434
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 ; Moss et al., GCN 35436
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 ) at 05:32:28; 05:47:36 and 06:40:25 UT on 2023-12-30 (at respectively ~4.1, 4.3 and 5.2 hours after the trigger). Each set of observations consisted of 4x90s exposures in the GOTO L-band (400-700 nm).

Images were processed immediately after acquisition using the GOTO pipeline. No optical counterpart is detected within the enhanced Swift-XRT position (Evans et al., GCN 35437
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 ; Osborne et al., GCN 35441
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 ) in any of the three sets of observations up to the 5-sigma limits of L > 18.7, 19.0 and 19.5 magnitudes (AB), respectively. The upper limits are consistent with those reported by Zhu et al., GCN 35439
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  and Kuin et al., GCN 35440
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 .

Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and are not corrected for Galactic extinction.

GOTO (https://goto-observatory.org) is a network of telescopes that is principally funded by the STFC and operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester and the Instituto de Astrofisica de Canarias (IAC).

J.P. Osborne, A.P. Beardmore, P.A. Evans and M.R. Goad (U. Leicester) 
report on behalf of the Swift-XRT team.

Using 2051 s of XRT Photon Counting mode data and 3 UVOT
images for GRB 231230A, we find an astrometrically corrected X-ray
position (using the XRT-UVOT alignment and matching UVOT field sources
to the USNO-B1 catalogue): RA, Dec = 245.21436, +58.12396 which is equivalent
to:

RA (J2000): 16h 20m 51.45s
Dec (J2000): +58d 07' 26.2"

with an uncertainty of 2.1 arcsec (radius, 90% confidence).

This position may be improved as more data are received. The latest
position can be viewed at http://www.swift.ac.uk/xrt_positions. Position
enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans
et al. (2009, MNRAS, 397, 1177).

This circular was automatically generated, and is an official product of the
Swift-XRT team.

N. P. M. Kuin (UCL-MSSL) and M. J. Moss (GSFC)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 231230A
1511 s after the BAT trigger (Moss et al., GCN Circ. 35436
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 ).
No optical afterglow consistent with the XRT position
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 initial exposures are:

Filter         T_start(s)   T_stop(s)      Exp(s)         Mag

white             1511         1705           39         >18.9
b                 1661         1681           19         >18.5
w1                1612         1632           19         >18.0

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.014 in the direction of the burst
(Schlegel et al. 1998).

Z. Zhu, D. Xu (NAOC) report on behalf a larger collaboration:

We observed the position of GRB 231230A (Moss et al., GCN 35436
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 ) using the Nordic Optical Telescope equipped with the ALFOSC images. Observations were carried out with the SDSS r and z filters, with an exposure time of 3x300 s and 5x200 s respectively. 

No new objects were dectected in our stacked image within or around the enhanced XRT position (Evans et al., GCN 35437
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 ), down to magnitudes r > 23.4 @ T-mid = 4.02 hr and z > 22.7 @ T-mid = 4.32 hr, calibrated with the nearby Pan-STARRS field.

P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:

Using  promptly downlinked XRT event data for GRB 231230A, we find an
enhanced XRT position of the afterglow: RA, Dec: 245.21382, 58.12618
which is equivalent to:
   RA (J2000)  = 16 20 51.32
   Dec (J2000) = +58 07 34.2
with an uncertainty of 1.9 arcseconds (radius, 90% confidence).
Analysis of the promptly available data is online at
http://www.swift.ac.uk/sper/1205319.

Position enhancement is is described by Goad et al. (2007, A&A, 476,
1401) and Evans et al. (2009, MNRAS, 397, 1177).

This circular is an official product of the Swift-XRT team.

M. J. Moss (GSFC), C. Gronwall (PSU), J.D. Gropp (PSU),
H. A. Krimm (NSF), A. Y. Lien (U Tampa), K. L. Page (U Leicester) and
M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 01:29:08 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 231230A (trigger=1205319).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 245.197, +58.120 which is 
   RA(J2000) = 16h 20m 47s
   Dec(J2000) = +58d 07' 11"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi-peaked
structure with a duration of about 30 sec.  The peak count rate
was ~1500 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 01:32:45.0 UT, 216.3 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 245.21706, 58.12324 which is equivalent
to:
   RA(J2000)  = 16h 20m 52.09s
   Dec(J2000) = +58d 07' 23.7"
with an uncertainty of 3.5 arcseconds (radius, 90% containment). This
location is 39 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at https://www.swift.ac.uk/sper.  We
cannot determine whether the source is fading at the present time. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (1.31 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 4.3
(+4.82/-3.83) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 145 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. The typical 3-sigma upper limit has been about 19.6 mag. 
The 8'x8' region for the list of sources generated on-board covers 100% of the
XRT error circle. The list of sources is typically complete to about 18 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.015. 

Burst Advocate for this burst is M. J. Moss (mikejmoss3 AT gmail.com). 
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/)

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

At 01:29:10 UT on 30 Dec 2023, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 231230A (trigger 725592555.052228 / 231230062).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 240.7, Dec = 53.6 (J2000 degrees, equivalent to J2000 16h 02m, 53d 36'), with a statistical uncertainty of 5.2 degrees.

The angle from the Fermi LAT boresight is 157.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2023/bn231230062/quicklook/glg_skymap_all_bn231230062.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/bn231230062/quicklook/glg_healpix_all_bn231230062.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2023/bn231230062/quicklook/glg_lc_medres34_bn231230062.gif