GRB 210605B

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

At 14:55:58 UT on 5 Jun 2021, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 210605B (trigger 644597763.075646 / 210605622).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 15.1, Dec = -6.5 (J2000 degrees, equivalent to J2000 01h 00m, -6d 30'), with a statistical uncertainty of 3.8 degrees.

The angle from the Fermi LAT boresight is 18.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2021/bn210605622/quicklook/glg_skymap_all_bn210605622.png

The HEALPix FITS file, including the estimated localization systematic, can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2021/bn210605622/quicklook/glg_healpix_all_bn210605622.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2021/bn210605622/quicklook/glg_lc_medres34_bn210605622.gif

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

Swift/BAT did not trigger on GRB 210605B (T0: 2021-06-06 14:55:58 UTC,
Fermi/GBM GCN 31029
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 ).

The Fermi/GBM 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 200 seconds of BAT event-mode data from
[-50,+150] seconds around the time of the burst. All the requested
event mode data was delivered to the ground.

The burst is detected in BAT with a duration of at least 20 seconds.
The burst occurred during a Swift slew.
The position of the burst was found with SNR of 8.4 using a novel slew
image mosaicing procedure (DeLaunay et al. 2021, in prep.), and so the
positional uncertainty is not yet well characterized.

The BAT position is
RA, Dec = 15.732, -6.467 deg which is
   RA(J2000)  = 01h 02m 55.68s
   Dec(J2000) =  -6d 28��� 1.2���
with an estimated uncertainty of 5 arcmin.

This position is consistent with the Ferm/GBM localization (GCN 31029
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 ).

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 210605B. 
Automated analysis of the XRT data will be presented online at
https://www.swift.ac.uk/ToO_GRBs/00021450

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.

A. D'Ai (INAF-IASFPA), A. Tohuvavohu (U. Toronto), B. Sbarufatti (PSU),
D.N. Burrows (PSU), J.P. Osborne (U. Leicester), K.L. Page (U.
Leicester), A.P. Beardmore (U. Leicester), A. Melandri (INAF-OAB), T.
Sbarrato (INAF-OAB) 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 210605B (James DeLaunay et al. GCN Circ.
30130
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 ), collecting 5.0 ks of Photon Counting (PC) mode data between
T0+74.2 ks and T0+96.6 ks. 

One uncatalogued X-ray sources has been detected, however it is not
above the RASS limit or shows definitive signs of fading. Therefore, at
the present time we cannot confirm if this source is the afterglow.
Details of the source are given below:

Source 1:
  RA (J2000.0):  15.7456  =  01:02:58.95
  Dec (J2000.0): -6.4290  =  -06:25:44.3
  Error: 5.7 arcsec (radius, 90% conf.)
  Count-rate: (2.94 [+1.05, -0.86])e-3 ct s^-1	 
  Distance: 145 arcsec from Swift/BAT position.
  Flux: (9.3 [+3.3, -2.7])e-14 erg cm^-2 s^-1 (observed, 0.3-10 keV)

The field has been previously observed by XRT for the Swift
Gravitational Wave Galaxy Survey (https://www.swift.ac.uk/SGWGS/). No
source was detected at this position on a 5.3 ks integrated exposure,
down to a 3-�� upper limit of 2.7 e-3 ct s^-1.

A catalogued source and a spurious source caused by Earth limb
contamination were also detected.

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/00021450.

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

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

"At 14:55:58.08 UT on 5 June 2021, the Fermi Gamma-Ray Burst Monitor (GBM)
triggered and located GRB 210605B (trigger 644597763 / 210605622)
which was also detected by the Swift/BAT-GUANO (DeLaunay et al. 2021, GCN
30130
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 )
The Fermi GBM Final Real-time Localization (GCN 30129
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 ) is consistent with
the Swift position.

The angle from the Fermi LAT boresight at the GBM trigger time is 19
degrees.

The GBM light curve consists of multiple peaks followed by some
extended emission with a duration (T90) of about 110 s (50-300 keV).
The time-averaged spectrum from T0-3.07 s to T0+113.66 s
is best fit by a power law function with an exponential
high-energy cutoff. The power law index is -0.78 +/- 0.08  and
the cutoff energy, parameterized as Epeak, is 150 +/- 11 keV.

The event fluence (10-1000 keV) in this time interval is
(10.12 +/- 0.44)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+26.36 s in the 10-1000 keV band
is 3.3 +/- 0.2 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. D'Ai (INAF-IASFPA), A. Tohuvavohu (U. Toronto), B. Sbarufatti (PSU),
D.N. Burrows (PSU), J.P. Osborne (U. Leicester), K.L. Page (U.
Leicester), A.P. Beardmore (U. Leicester), A. Melandri (INAF-OAB), T.
Sbarrato (INAF-OAB) and P.A. Evans (U. Leicester) report on behalf of
the Swift-XRT team:

Swift-XRT has conducted further observations of the field of the
Swift/BAT-GUANO detected burst GRB 210605B (James DeLaunay et al. GCN
Circ. 30130
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 ). The observations now extend from T0+74.3 ks to T0+734.1
ks. 

Of the sources reported by D'Ai et al. (GCN Circ. 30146
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 ), "Source 1" is
fading with 2.5 sigma significance and thus is believed to be the GRB
afterglow.  The position of this source is RA, Dec=15.7456, -6.4290
which is equivalent to:

RA (J2000): 01:02:58.95
Dec(J2000): -06:25:44.3

with an uncertainty of 5.7 arcsec (radius, 90% confidence).  This
position is 2.4 arcmin from the Swift/BAT position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=0.8 (+0.7, -0.4).

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00021450.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/00021450.

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