GRB 210506A

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/

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.

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
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 ) 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
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 ). 

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.

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
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 ) 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
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 ), down 
to a limiting magnitude of r~21.7, calibrated with the nearby PanSTARRS 
field.

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
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 ), 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.

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
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 )
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/

Aaron Tohuvavohu (U Toronto), James DeLaunay (PSU), Jamie A. Kennea
(PSU) report:

As stated in the detection circular (GCN 29947
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 ) 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
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 ; 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.