Gamma-ray Burst 210928A

(All information courtesy of the instrument teams.)

Previous IAU Circulars

none

Results of Observations

ATEL #14939
```
Title:  Fermi-LAT detection of Fermi J1623-1752 and its possible association
                with recurrent nova U Scorpii
Author: B. Rani (Korea Astronomy and Space Science Institute), G.
                La Mura (LIP), C. C. Cheung (NRL), on behalf of the Fermi Large Area
                Telescope Collaboration
Queries:        Teddy.Cheung@nrl.navy.mil
Posted: 28 Sep 2021; 19:31 UT
Subjects:Gamma Ray, >GeV, Nova, Transient
```
The Large Area Telescope (LAT), one of two instruments on the Fermi Gamma-ray Space Telescope, has detected a new transient gamma-ray source, Fermi J1623-1752. >From data obtained on 2021 Sep 28 (00:00-06:00 UTC), preliminary analysis indicates a detection of greater than 5-sigma significance with a (E >100 MeV) flux of (7.2 +/- 5.2) x 10^-7 photons cm^-2 s^-1 and a relatively hard spectrum (single power-law photon index = 1.8 +/- 0.4); statistical uncertainties only. The best-fit position is RA, Dec (J2000) = 245.82 deg, -17.88 deg, with a 95% (99%) confidence error radius of 0.16 deg (0.20 deg).
A possible counterpart to the LAT transient is the recurrent nova U Scorpii with position offset by 0.18 deg from the LAT position, thus just outside the 95% confidence error circle, but within the 99% confidence error circle. U Sco has a known recurrence interval of 10.3 years (Schaefer 2010, ApJS, 187, 275) prior to its outburst in January 2010, thus a new outburst from U Sco could be anticipated. The last observation of U Sco in the AAVSO lightcurve database was on 2021 Sep 19.
Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. In consideration of the ongoing activity of this source, we encourage multi-wavelength observations, particularly optical follow-up of U Sco. For this source the Fermi LAT contacts are B. Rani (binduphysics@gmail.com) and C.C. Cheung (Teddy.Cheung@nrl.navy.mil).
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.
ATEL #14945
```
Title:  New X-ray transient Swift J162315.0-175233: a possible counterpart
                to Fermi J1623-1752
Author: K. L. Page, P. A. Evans and J. P. Osborne (U. Leicester)
Queries:        kpa@star.le.ac.uk
Posted: 29 Sep 2021; 15:15 UT
Subjects:Ultra-Violet, X-ray, Transient
```
Following the announcement by Fermi-LAT of a new gamma-ray source Fermi J1623-1752 (Rani et al., ATel #14939), with an error region encompassing the recurrent nova U Sco, Swift performed a ~5 ks Target of Opportunity observation of the field of U Sco between 2021 September 28 20:40 UT and September 29 03:17 UT. No X-ray source is detected at the location of U Sco by the XRT, to a 3-sigma upper limit of 2.2x10^-3 count s^-1. The UVOT measures magnitudes of u = 18.95 +/- 0.25, uvw1 = 19.60 +/- 0.32, uvm2 = 19.81 +/- 0.18 and uvw2 = 19.36 +/- 0.30. Given that U Sco in outburst reaches a visual magnitude of ~7.5, these observations show that the nova is not currently erupting, a conclusion also reached by Sokolovsky et al. in ATel #14941.
There is, however, a previously uncatalogued X-ray source, at a position of RA, Dec(J2000) = 245.81275 deg, -17.87598 deg, 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.
This source has a 0.3-10 keV XRT count rate of 0.020 +/- 0.003 count s^-1, with a spectrum which can be approximated by a power-law of Gamma = 1.5 +0.7/-0.6, with an absorbing column consistent with the Galactic value of 2.32x10²¹ cm^-2 (Willingale et al. 2013). The 0.3-10 keV flux is 9.3 x 10^-13 erg cm^-2 s^-1 (observed; 1.1 x 10^-12 erg cm^-2 s^-1 unabsorbed).
Combining all the previous Swift-XRT observations (a total of > 300 ks), the earlier upper limit at this position is 1.85 x 10^-4 count s^-1.
This new source is not detected by the UVOT, to the following limits: u > 19.61, uvw1 > 20.02, m2 > 20.83 and uvw2 > 19.78.
This source was automatically detected as a likely transient by the forthcoming Live Swift-XRT Point Source Catalogue (LSXPS), which is currently undergoing pre-release testing, and is named Swift J162315.0-175233.
The position of Swift J162315.0-175233 is only 29 arcsec from the Fermi-LAT position given in ATel #14939, and therefore fully consistent with that localisation. This new X-ray source may be the counterpart of the new Fermi-LAT detection.
GCN Circular #30912
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.
GCN Circular #30913
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). 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 ť Data Access ť 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
GCN Circular #30914
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:
Text replaced by GCN Circular #30915 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.
GCN Circular #30915
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, 30913). 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.
GCN Circular #30918
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), 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), 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).
GCN Circular #30919
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. 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.
GCN Circular #30921
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; Fermi-GBM observation: Fletcher & Meegan, GCN 30913) 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.
GCN Circular #30924
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.

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Jochen Greiner, last update: 06-Oct-2021 [Disclaimer]