Gamma-ray Burst 210506A

(All information courtesy of the instrument teams.)

Previous IAU Circulars

none

Results of Observations

GCN Circular #29947
Aaron Tohuvavohu (U Toronto), James DeLaunay (PSU), Jamie A. Kennea (PSU) report:
Swift/BAT did not trigger on GRB 210506A (T0: 2021-05-06 00:39:48.54 UTC, INTEGRAL/SPI-ACS trigger 9191).
The INTEGRAL/SPI-ACS 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 90 seconds of BAT event-mode data from [-45,+45] seconds around the time of the burst. All the requested event mode data was delivered to the ground.
The burst is strongly detected in BAT with a duration of ~0.2 seconds.
The position of the burst was found with the typical BAT imaging procedure with an SNR of 6.8. We note that this position would imply an extremely hard spectrum for this burst, beyond what is typically expected. This could imply that the position is incorrect, or that this is a truly exceptional event. We encourage other GRB instruments to provide spectral measurements that can help clarify the situation.
The BAT position is RA, Dec = 132.853, 4.582 deg which is
```
   RA(J2000)  = 08h 51m 24.72s
   Dec(J2000) =  04d 34’ 55.2”
```
with an estimated uncertainty of 4 arcmin.
XRT and UVOT follow-up has been requested. Results of follow-up observations will be reported in future circulars.
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 #29949
P. A. Evans (U. Leicester) reports on behalf of the Swift team:
Swift has initiated a ToO observation of the Swift/BAT GRB 210506A. Automated analysis of the XRT data will be presented online at https://www.swift.ac.uk/ToO_GRBs/00021430
Any uncatalogued X-ray sources detected in this analysis will be reported on this website and via GCN COUNTERPART notices. These are not necessarily related to the Swift/BAT event. Any X-ray source considered to be a probable afterglow candidate will be reported via a GCN Circular after manual consideration.
Details of the XRT automated analysis methods are detailed in Evans et al. (2007, A&A, 469, 379; 2009, MNRAS, 397, 1177 and 2014, ApJS, 210, 8).
This circular is an official product of the Swift-XRT team.
GCN Circular #29955
P. D'Avanzo (INAF-OAB), V. D'Elia (ASI-SSDC), D. Malesani (DTU Space), E. Palazzi, A. Rossi (INAF-OAS), G. Andreuzzi, D. Carosati (INAF-TNG) on behalf of the CIBO collaboration report:
We observed the field of the short GRB 210506A (Tohuvavohu & DeLaunay, GCN Circ. 29947) with the Italian 3.6m TNG telescope equipped with NICS in imaging mode. A series of images were obtained with the J filter on 2021-05-06 from 21:15:04 UT to 21:55:59 UT (i.e. at a mid time of about 20.9 hours after the burst).
Our observations (with a field of view of 4.2' x 4.2') were centered at the position of the XRT source #3 (see: https://www.swift.ac.uk/ToO_GRBs/00021430/ and Evans, GCN Circ. 29949).
No obvious source is found within the position of source XRT #3 down to a 3-sigma limiting magnitude of J > 20.6 (Vega, calibrated against the 2MASS catalogue). Unfortunately an image artefact, possibly due to bad sky subtraction, prevented us to visually investigate a portion (~ 5%) of the XRT error circle for source #3.
GCN Circular #29956
S.Y. Fu (NAOC), Z.P. Zhu (NAOC, HUST), X. Liu, D. Xu (NAOC), X. Gao (Urumqi No.1 Senior High School), J.Z. Liu (XAO) report:
We observed the field of GRB 210506A (Tohuvavohu, DeLaunay & Kennea, GCN 29947) using the NEXT-0.6m telescope located at Nanshan, Xinjiang, China. We obtained 9x300 s frames in the Sloan r-band, starting at 15:17:27 UT on 2021-05-06, i.e., 14.63 hr after the BAT trigger.
No credible optical source is detected in our stacked image at the position of the XRT source #3 that lies in the BAT error circle (ref. https://www.swift.ac.uk/ToO_GRBs/00021430/ and Evans, GCN 29949), down to a limiting magnitude of r~21.7, calibrated with the nearby PanSTARRS field.
GCN Circular #29960
J. D. Gropp (PSU), J.A. Kennea (PSU), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB), A. Tohuvavohu (U. Toronto) and P.A. Evans (U. Leicester) report on behalf of the Swift-XRT team:
Swift-XRT has performed follow-up observations of the Swift/BAT-detected burst GRB 210506A (Aaron Tohuvavohu et al. GCN Circ. 29947), collecting 4.8 ks of Photon Counting (PC) mode data between T0+29.0 ks and T0+45.6 ks.
Three uncatalogued X-ray sources have been detected consistent with being within 296 arcsec of the Swift/BAT position, however none of them is above the RASS limit or shows definitive signs of fading. Therefore, at the present time we cannot identify which, if any, is the afterglow. Details of these sources are given below:
Source 3:
```
  RA (J2000.0):  132.9074  =  08:51:37.78
  Dec (J2000.0): +4.5999  =  +04:35:59.6
  Error: 6.4 arcsec (radius, 90% conf.)
  Count-rate: (1.46 [+0.89, -0.69])e-3 ct s^-1	 
  Distance: 206 arcsec from Swift/BAT position.
```
Source 8:
```
  RA (J2000.0):  132.8470  =  08:51:23.29
  Dec (J2000.0): +4.6632  =  +04:39:47.4
  Error: 7.4 arcsec (radius, 90% conf.)
  Count-rate: (1.72 [+1.01, -0.78])e-3 ct s^-1	 
  Distance: 291 arcsec from Swift/BAT position.
  Flux: (1.05 [+0.62, -0.48])e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)
```
Source 9:
```
  RA (J2000.0):  132.7917  =  08:51:10.02
  Dec (J2000.0): +4.6042  =  +04:36:15.2
  Error: 7.9 arcsec (radius, 90% conf.)
  Count-rate: (1.76 [+0.96, -0.76])e-3 ct s^-1	 
  Distance: 232 arcsec from Swift/BAT position.
  Flux: (2.8 [+1.5, -1.2])e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)
```
Six uncatalogued sources were also detected too far from the GRB position to be likely afterglow candidates.
The results of the XRT-team automatic analysis of the XRT observations, including a position-specific upper limit calculator, are available at https://www.swift.ac.uk/ToO_GRBs/00021430.
This circular is an official product of the Swift-XRT team.
GCN Circular #29961
V. Lipunov, E. Gorbovskoy, V.Kornilov, N.Tyurina, P.Balanutsa, A.Kuznetsov, F.Balakin, V.Vladimirov, D. Vlasenko, I.Gorbunov, D.Zimnukhov, V.Senik, T.Pogrosheva, D.Kuvshinov, D. Cheryasov (Lomonosov Moscow State University, SAI, Physics Department),
R. Podesta, C.Lopez, F. Podesta, C.Francile (Observatorio Astronomico Felix Aguilar OAFA),
R. Rebolo, M. Serra (The Instituto de Astrofisica de Canarias),
D. Buckley (South African Astronomical Observatory),
O.A. Gres, N.M. Budnev, O.Ershova (Irkutsk State University, API),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
V. Yurkov, A. Gabovich, Yu. Sergienko (Blagoveschensk Educational State University)
MASTER-Tavrida robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Russia (Lomonosov MSU, SAI Crimea astronomical station) was pointed to the Integral GRB210506.78 (trigger No 9194,08h 27m 08.00s , +19d 55m 21.0s, R=0.001) errorbox 48590 sec after notice time and 84186 sec after trigger time at 2021-05-07 18:02:19 UT, with upper limit up to 16.4 mag. Observations started at twilight. The observations began at zenith distance = 43 deg. The sun altitude is -11.5 deg.
The galactic latitude b = 30 deg., longitude l = 205 deg.
Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=1608498
We obtain a following upper limits.
```
Tmid-T0  |          Site       |Filt.| Expt. | Limit| Comment
_________|_____________________|_____|_______|______|________

   84276 |      MASTER-Tavrida |   C |   180 | 15.4 |        
   84675 |      MASTER-Tavrida |   C |   180 | 16.4 |        
```
Filter C is a clear (unfiltred) band.
The observation and reduction will continue. The message may be cited.
GCN Circular #29968
D. Svinkin, S. Golenetskii, D. Frederiks, A. Ridnaia, A. Lysenko, and T. Cline on behalf of the Konus-Wind team,
K. Hurley, on behalf of the IPN,
S. Barthelmy, J. Cummings, H. Krimm, D. Palmer, and A. Tohuvavohu on behalf of the Swift-BAT team,
A. von Kienlin, X. Zhang, A. Rau, V. Savchenko, E. Bozzo, and C. Ferrigno, on behalf of the INTEGRAL SPI-ACS GRB team,
and
S. Xiao, S. L. Xiong, X. B. Li, Y. Huang, and S. N. Zhang, on behalf of the Insight-HXMT team, report:
The short-duration GRB 210506A (Swift/BAT-GUANO detection: Tohuvavohu et al., GCN Circ. 29947) was detected by Swift (BAT), INTEGRAL (SPI-ACS), Insight-HXMT (HE), and Konus-Wind at about 2388 s UT (00:39:48).
We have triangulated it to a preliminary, error box whose coordinates are:
```
  ------------------------------------
   RA(2000), deg       Dec(2000), deg
  ------------------------------------
  Corners:
     224.231           -32.736
     175.778           -26.218
     177.243           -30.663
     225.586           -36.393
  ------------------------------------
```
The error box area is about 160 sq. deg, and its maximum dimension is 43 deg (the minimum one is 3.7 deg). The Sun distance was 93 deg.
This box may be improved.
The IPN box is consistent with the Fermi-GBM nondetection, inconsistent with the BAT-GUANO localization, and is outside the BAT coded FoV.
A triangulation map is posted at http://www.ioffe.ru/LEA/GRBs/GRB210506_T02388/IPN/
GCN Circular #29969
Aaron Tohuvavohu (U Toronto), James DeLaunay (PSU), Jamie A. Kennea (PSU) report:
As stated in the detection circular (GCN 29947) the candidate position reported was uncertain, with a near-threshold significance and an implied spectral index outside of typical astrophysical priors for GRB prompt emission.
Without an independent spectral measurement of this burst, the BAT candidate image-domain arcmin localization cannot be ruled out via BAT analysis alone. However, the IPN localization (GCN 29968; which utilizes the BAT-GUANO rates-domain detection) strongly suggests that the GRB originated from outside the BAT FoV. An origin for the GRB counts from along this line of sight implies a much more reasonable (though still hard) emission spectrum. The BAT-GUANO rates-domain detection of this GRB remains robust and we are confident that the counts in the detector originate from GRB 210506A.
The BAT maximum likelihood analysis (DeLaunay et al. 2021, in prep) independently shows a slight preference for an out of FoV origin. We did not report the test statistic results for this source from the maximum likelihood analysis in the detection circular due to delayed run times on the cluster.
We do not identify any detector glitches, noise sources, or cosmic ray induced showers that can account for the image source.
We conclude that the SNR 6.8 image source reported is likely to be rare high-significance spurious image noise caused by an unfortunate spatial correlation of true GRB counts in the detector plane.
The False Alarm Probability for a BAT source at SNR 6.8 in a background subtracted image from ground analysis is low (~1-5%) and we stress that other GRBs with lower image significance GUANO localizations have been confirmed by XRT afterglow discovery.

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Jochen Greiner, last update: 09-May-2021 [Disclaimer]