GRB 210928A

M. Axelsson (KTH & Stockholm Univ.), N. Di Lalla (Stanford Univ.), E. Bissaldi (Politecnico & INFN Bari), N. Omodei (Stanford Univ.), C. C. Cheung (NRL) and G. La Mura (LIP) report on behalf of the Fermi-LAT Collaboration:

On September 28th, 2021, Fermi-LAT detected high-energy emission from GRB 210928A, which was also detected by Fermi GBM (trigger 654487236 / 210928084). This source has previously been reported as the Fermi-LAT transient J1623-1752 (Rani et al., ATel #14939) and Swift X-ray transient J162315.0-175233 (Page et al.; ATel #14945). Further analysis shows that the LAT source is consistent with being a GRB, making the Swift transient the afterglow candidate.

The best LAT on-ground location is found to be

RA, Dec = 245.88, -17.91 (degrees, J2000)

with an error radius of 0.15 deg (90% containment, statistical error only). This location is consistent with the Swift X-ray transient.

The position was 16 deg from the LAT boresight at the time of the GBM trigger:

T0 = 02:00:31 UT.

The data from the Fermi-LAT show a significant increase in the event rate after the trigger that is spatially correlated with the GBM emission with high significance. The photon flux above 100 MeV in the time interval 0-1000 s after the trigger is 8.4 (-/+ 1.5) e-06 ph/cm2/s.

The estimated photon index above 100 MeV is -2.2 (-/+ 0.2).

The highest-energy photon is a 6 GeV event which is observed ~50 s after the trigger.

The Fermi-LAT point of contact for this burst is Magnus Axelsson (magaxe@kth.se).

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.

C. Fletcher (USRA) and C. Meegan (UAH) report on behalf of the Fermi GBM Team:

"At 02:00:31.01 UT on 28 September 2021, the Fermi Gamma-Ray Burst Monitor (GBM)
triggered and located GRB 210928A (trigger 654487236 / 210928084)
which was also detected by the Fermi LAT (M. Axelsson et al. 2021, GCN 30912
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 ).
The GBM on-ground location is consistent with the LAT position.

The angle from the Fermi LAT boresight at the GBM trigger time is 16 degrees.

The GBM light curve consists of a multiple peaks
with a duration (T90) of about 24 s (50-300 keV).
The time-averaged spectrum from T0+2.1 s to T0+26.6 s is
best fit by a power law function with an exponential
high-energy cutoff.  The power law index is -0.45 +/- 0.02 and
the cutoff energy, parameterized as Epeak, is 465 +/- 10 keV.

The event fluence (10-1000 keV) in this time interval is
(4.925 +/- 0.052)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+18.04 s in the 10-1000 keV band
is 10.6 +/- 0.3 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/"
FSSC: Data � Data Access � GBM<https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/>
GBM Data Products. This page lists the science data products created by the GIOC (GBM Instrument Operations Center) and provided to the FSSC. GBM Daily, Trigger, and Burst Data
fermi.gsfc.nasa.gov

V. D'Elia (SSDC & INAF-OAR), A. D'Ai (INAF-IASFPA), J.A. Kennea (PSU),
A. Tohuvavohu (U. Toronto), B. Sbarufatti (PSU), K.L. Page (U.
Leicester), A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester),
A. Melandri (INAF-OAB) and  report on behalf of the Swift-XRT team:
report on behalf of the Swift-XRT team:

We have analysed 5.3 ks of XRT data for the -detected burst GRB
210928A, from 67.4 ks to 262.9 ks after the   trigger. The data are
entirely in Photon Counting (PC) mode. Using 1636 s of PC mode data and
2 UVOT images, we find an enhanced XRT position (using the XRT-UVOT
alignment and matching UVOT field sources to the USNO-B1 catalogue):
RA, Dec = 245.51109, -17.94788 which is equivalent to:

RA (J2000): 16h 22m 02.66s
Dec(J2000): -17d 56' 52.4"

with an uncertainty of 4.4 arcsec (radius, 90% confidence). This
position is 6793.5 arcmin from the  position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.3 (+1.8, -1.3).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.4 (+0.6, -0.3). The
best-fitting absorption column is  2.342 (+3.358, -0.021) x 10^21
cm^-2, consistent with the Galactic value of 2.3 x 10^21 cm^-2
(Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10
keV flux conversion factor deduced from this spectrum  is 4.8 x 10^-11
(5.5 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.342 (+3.358, -0.021) x 10^21 cm^-2
Galactic foreground: 2.3 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.4 (+0.6, -0.3)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00014850.

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

P. A. Evans and K.L. Page (U. Leicester) report on behalf of the Swift-XRT team:

GCN Circ (30914) was sent in error, prematurely. The corrected information is below,
and we apologise for the confusion.

We have analysed 5.3 ks of XRT data for the Fermi-detected burst GRB 210928A
(GCN Circs. 30912
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 , 30913
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 ). The data were originally collected as a ToO to
observe U Sco, the possible counterpart to the Fermi-LAT source Fermi J1623-1752
(ATEL #14939). While U Sco was not detected, a new transient, Swift
J162315.0-175233 was discovered (ATEL #14945). Further Swift observations were
requested and are ongoing. The discovery of an unannounced Fermi-GBM trigger has
identified this object as GRB 210928A (ATEL #14948), and the manual insertion 
of the already-observed data into the GRB analysis system resulted in the premature
dispatch of this circular.

The Swift-XRT observations at present extend from 67.4 ks to 262.9 ks after the
GBM trigger time. The data are entirely in Photon Counting (PC) mode. Using 1636
s of PC mode data and 2 UVOT images, we find an enhanced XRT position (using the
XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue):
RA, Dec = 245.51109, -17.94788 which is equivalent to:

RA (J2000): 16h 22m 02.66s
Dec(J2000): -17d 56' 52.4"

with an uncertainty of 4.4 arcsec (radius, 90% confidence). This
position is 29 arcsec from the Fermi-LAT  position. 

The light curve can be modelled with a power-law decay with a decay index of
alpha=1.3 (+1.8, -1.3); however comprising only 3 data points and an upper
limit, this should be viewed with the caution the large error-bars imply.

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.4 (+0.6, -0.3). The
best-fitting absorption column is  2.342 (+3.358, -0.021) x 10^21
cm^-2, consistent with the Galactic value of 2.3 x 10^21 cm^-2
(Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10
keV flux conversion factor deduced from this spectrum  is 4.8 x 10^-11
(5.5 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.342 (+3.358, -0.021) x 10^21 cm^-2
Galactic foreground: 2.3 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.4 (+0.6, -0.3)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00014850.

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

T.-W. Chen, S. Yang (Stockholm), P. Evans (U. Leicester), W.-J. Hou, C.-S. Lin, Y.-C. Pan, C.-C. Ngeow, H.-Y. Hsiao, and J.-K. Guo (IANCU) report:

We observed the field of the GRB 210928A (Axelsson et al., GCN Circ. 30912
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 ), as part of the Kinder survey (Chen et al., AstroNote 2021-92). We used the SLT-40cm at Lulin Observatory, Taiwan, to obtain r-band images.

Observations started at 10:18 UT on 01 of October 2021 (MJD = 59488.430), 3.3 days after the GBM trigger time (Fletcher et al., GCN Circ. 30913
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 ), and were centred on the Swift X-ray source J162315.0-175233 (Page et al., ATel #14945), which had been identified as the likely counterpart to the Fermi source (first reported in Rani et al., ATel #14939). The images were combined from 12 frames with 300 sec exposure time for each (two additional frames removed due to poor image quality), taken under a various seeing conditions 2".7 and at an average airmass of 2.4.

We subtracted the template of this field with our combined image, and derived the following preliminary 2.5-sigma limit (in the AB system):
r > 21.22 mag.

Given magnitudes are calibrated against PanSTARRS1 field stars and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V) = 0.29 mag in the direction of the counterpart (Schlafly & Finkbeiner 2011).

K.L. Page (U. Leicester) reports on behalf of the Swift-XRT team:

Incorrect coordinates for the X-ray afterglow of GRB 210928A were
accidentally reported in GCN 30915
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 . The correct position (as earlier
stated in ATel 14945) is:

RA, Dec(J2000) = 245.81275, -17.87598, which is equivalent to

RA(J2000) = 16h 23m 15.06s
Dec(J2000) = -17d 52m 33.5s
with an estimated 90% uncertainty radius of 2.9 arcsec.

The information provided regarding the light-curve and spectrum were for
the afterglow; only the position was incorrect.

We apologise for any confusion.

A. Ridnaia, D. Frederiks, S. Golenetskii, A. Lysenko,
D. Svinkin, A. Tsvetkova,  M. Ulanov, and T. Cline
on behalf of the Konus-Wind team, report:

The long-duration GRB 210928A
(Fermi-LAT detection: Axelsson et al., GCN 30912
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 ;
Fermi-GBM observation: Fletcher & Meegan, GCN 30913
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 )
triggered Konus-Wind at T0=7234.398 s UT (02:00:34.398).

The burst light curve shows a single multi-peaked pulse
started at ~T0-4.3 s with a total duration of ~27.3 s.
The emission is seen up to ~6 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB210928_T07234/

As observed by Konus-Wind, the burst
had a fluence of 6.79(-0.66,+0.71)x10^-5 erg/cm2,
and a 64-ms peak flux, measured from T0+14.416 s,
of 6.45(-1.72,+1.77)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+24.320 s)
is best fit in the 20 keV - 6 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -0.34(-0.15,+0.16),
the high energy photon index beta = -2.55(-0.35,+0.21),
the peak energy Ep = 361(-33,+38) keV
(chi2 = 61/78 dof).

The spectrum near the maximum count rate
(measured from T0+8.448 to T0+16.128 s)
is best fit in the 20 keV - 6 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -0.07(-0.21,+0.23),
the high energy photon index beta = -2.73(-0.55,+0.28),
the peak energy Ep = 328(-30,+36) keV
(chi2 = 75/78 dof).

All the quoted errors are at the 90% confidence level.
All the quoted values are preliminary.

L. Rhodes (Oxford/MPIfR), A. van der Horst, S. Chastain (GWU), R. Fender (Oxford/UCT) and E. Tremou (CEA-Saclay) on behalf of the ThunderKAT collaboration.

We observed Fermi J1623-1752 (Atel #14948) as part of the ThunderKAT X-ray binary monitoring program prior to it being reclassified as GRB 210928A. The observation was made as 1.3GHz on 30th September 2021 at 17:07:48UT. The 15-minute observation revealed a point source at the XRT position of GRB 210928A with a peak flux density of 215 +/- 6uJy. The noise in the field is 16uJy.

We thank the staff at the South African Radio Astronomy Observatory for scheduling these observations.