GRB 210618A

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

At 01:43:36 UT on 18 Jun 2021, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 210618A (trigger 645673421.140258 / 210618072).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 227.8, Dec = 42.5 (J2000 degrees, equivalent to J2000 15h 11m, 42d 30'), with a statistical uncertainty of 7.6 degrees.

The angle from the Fermi LAT boresight is 106.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2021/bn210618072/quicklook/glg_skymap_all_bn210618072.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/bn210618072/quicklook/glg_healpix_all_bn210618072.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2021/bn210618072/quicklook/glg_lc_medres34_bn210618072.gif

M. G. Bernardini (INAF-OAB), P. D'Avanzo (INAF-OAB),
H. A. Krimm (NSF), A. Y. Lien (GSFC/UMBC), A. Melandri (INAF-OAB),
K. L. Page (U Leicester), D. M. Palmer (LANL), T. Sbarrato (INAF-OAB)
and M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 01:43:37 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210618A (trigger=1056426).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 235.820, +46.043 which is 
   RA(J2000) = 15h 43m 17s
   Dec(J2000) = +46d 02' 36"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of about 3 sec.  The peak count rate
was ~3600 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 01:44:42.2 UT, 65.3 seconds after
the BAT trigger. No source was detected in the 2.5-s promptly available
image. Due to a short visibility interval, no further data were immediately 
collected. We are waiting for the full dataset to detect and localise the
XRT counterpart. 

Due to the short visibility window, UVOT data are not available at this time. 

Burst Advocate for this burst is M. G. Bernardini (grazia.bernardini AT brera.inaf.it). 
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/)

Oc��lotl Lopez (UNAM), Margarita Pereyra (UNAM), Rosa L. Becerra (UNAM),
Nat Butler (ASU), Alan M. Watson (UNAM), Simone Dichiara (GSFC/UMD),
Alexander Kutyrev (GSFC/UMD), William H. Lee (UNAM), Diego Gonzalez (UNAM),
Srihari Ravi (ASU), and Eleonora Troja (GSFC/UMD) report:

We observed the field of the GRB 210618A (Fermi GBM Team, GCN circ. 30248
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and Bernardini et al., GCN circ. 30250
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 ) with the DDOTI wide-field imager at
the Observatorio Astron��mico Nacional on Sierra San Pedro Martir (
http://ddoti.astroscu.unam.mx) from 2021-06-18 03:52:24 UTC to 2021-03-18
06:39:28 UTC (from 2.14 to 4.93 hours after the trigger).

We observed a field of approximately 3.4 degrees in RA and 3.4 degrees in
declination (about 11.55 square degrees), centered on the Swift BAT ground
position of RA 235.74010768 , Dec 46.14225361(J2000), covering  the BAT
error region. We obtained 7604.6 seconds of exposure in the w filter. We
calibrated our images against the APASS catalog.

We find no new sources within the BAT error region. To our 10-sigma limit
we obtain an upper limit of w = 20.47 mag for the optical afterglow.

We thank the staff of the Observatorio Astron��mico Nacional in San Pedro
Martir.

J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore
(U. Leicester), E. Ambrosi  (INAF-IASFPA) , M. Capalbi (INAF-IASFPA),
J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto), B. Sbarufatti (PSU) and
M. G. Bernardini (INAF-OAB) report on behalf of the Swift-XRT team:

We have analysed 5.2 ks of Photon Counting (PC) mode XRT data for the
Swift-BAT-detected burst GRB 210618A (Bernardini et al. GCN Circ.
30250
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 ), collected between T0+65 s and T0+22.6 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. Two
sources (Source 1 and Source 4) are consistent within the XRT error
circle with two spectroscopically identified quasars, and the third one
(Source 2) is consistent with a SDSS galaxy with photo_z=0.59 and
marginally consistent with a candidate quasar. Therefore, for the
moment the most plausible afterglow candidate is Source 2. Swift will
perform follow-up observations to confirm this candidate.

Details of these sources are given below:

Source 1:
  RA (J2000.0):  235.8870  =  15:43:32.87
  Dec (J2000.0): +46.0253  =  +46:01:31.0
  Error: 8.9 arcsec (radius, 90% conf.)
  Count-rate: (3.82 [+1.19, -1.00])e-3 ct s^-1	 
  Distance: 179 arcsec from Swift-BAT position.
  Flux: (1.17 [+0.36, -0.30])e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)
This source is consistent with SDSS J154332.15+460133.2,
spectroscopically classified as a quasar at z = 0.657. 

Source 2:
  RA (J2000.0):  235.7698  =  15:43:4.75
  Dec (J2000.0): +46.0691  =  +46:04:08.6
  Error: 9.3 arcsec (radius, 90% conf.)
  Count-rate: (4.0 +/- 1.0)e-3 ct s^-1	 
  Distance: 156 arcsec from Swift-BAT position.
  Flux: (1.35 +/- 0.35)e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV) 
This source is positionally consistent with SDSS J154305.56+460405.0, a
galaxy with photo_z = 0.590 +/- 0.153 and 12.2 arcsec away from SDSS
J154305.40+460418.7, a candidate quasar (Richards et al., 2015, ApJS,
219, 39).

Source 4:
  RA (J2000.0):  235.7183  =  15:42:52.39
  Dec (J2000.0): +46.0336  =  +46:02:01.0
  Error: 8.7 arcsec (radius, 90% conf.)
  Count-rate: (1.48 [+0.78, -0.60])e-3 ct s^-1	 
  Distance: 256 arcsec from Swift-BAT position.
  Flux: (7.2 [+3.8, -2.9])e-14 erg cm^-2 s^-1 (observed, 0.3-10 keV)
This source is positionally consistent with SDSS J154251.96+460206.1,
spectroscopically classified as a quasar at z = 2.130.

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/01056426.

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

J. Wood (NASA/MSFC) and C. Meegan (UAH)
report on behalf of the Fermi GBM Team:

"At 01:43:36.14 UT on 18 June 2021, the Fermi Gamma-Ray Burst Monitor (GBM)
triggered and located GRB 210618A (trigger 645673421 / 210618072)
which was also detected by the Swift/BAT (M. G. Bernardini et al. 2019, GCN 30250
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 ).

The Fermi GBM Final Real-time Localization (GCN 30248
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 ) is consistent with the Swift position.

The angle from the Fermi LAT boresight at the GBM trigger time is 113 degrees.

The GBM light curve consists of multiple peaks
with a duration (T90) of about 3.6 s (50-300 keV).
The time-averaged spectrum from T0-1.6 s to T0+2.2 s is
well fit by a power law function with an exponential
high-energy cutoff.  The power law index is -0.62 +/- 0.26 and
the cutoff energy, parameterized as Epeak, is 440 +/- 140 keV

The event fluence (10-1000 keV) in this time interval is
(1.2 +/- 0.2)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+0.3 s in the 10-1000 keV band
is 2.1 +/- 0.3 ph/s/cm^2.

A Band function fits the spectrum equally well
with Epeak= 400 +/- 190 keV, alpha = -0.60 +/- 0.32 and beta = -2.3 +/- 0.9.

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/"

S. Laha (GSFC/UMBC), S. D. Barthelmy (GSFC),  M. G. Bernardini (INAF-OAB),
H. A. Krimm (NSF), A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC),
D. M. Palmer (LANL), T. Sakamoto (AGU), M. Stamatikos (OSU),
T. N. Ukwatta (LANL) (i.e. the Swift-BAT team):

Using the data set from T-70 to T+960 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 210618A (trigger #1056426)
(Bernardini, et al., GCN Circ. 30250
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 ).  The BAT ground-calculated position is
RA, Dec = 235.763, 46.072 deg which is 
  RA(J2000)  =  15h 43m 03.2s 
  Dec(J2000) = +46d 04' 17.5" 
with an uncertainty of 2.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 100%.

The mask-weighted light curve shows a fairly complex structure between roughly
T-1.5 and T+0.5 seconds.  The spacecraft slewed away from the burst location
starting at around T+320 seconds. T90 (15-350 keV) is 2.13 +- 0.44 sec (estimated 
error including systematics).

The time-averaged spectrum from T-1.42 to T+0.96 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.18 +- 0.28.  The fluence in the 15-150 keV band is 1.8 +- 0.3 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T-0.50 sec in the 15-150 keV band
is 1.1 +- 0.2 ph/cm2/sec.  All the quoted errors are at the 90% confidence
level. 

The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/1056426/BA/

Y.-D. Hu, T.-R. Sun, E. Fernandez-Garcia, A. J. Castro-Tirado, M.D. Caballero-Garcia (IAA-CSIC), C. Perez del Pulgar, A. Castellon, I. Carrasco (Univ. de Malaga), R. Fernandez-Munoz (IHSM/UMA-CSIC) and M. Jelinek (ASU-CAS), on behalf of a larger collaboration, report:

The 60cm BOOTES-2/TELMA robotic telescope at IHSM La Mayora (UMA-CSIC) in Algarrobo Costa (Malaga, Spain) automatically responded to the Swift and Fermi trigger of GRB 210618A (Fermi GBM team GCNC 30248, Bernardini et al. GCNC 30250). Images were taken starting at 01:54 UT, but due to poor weather conditions, only images taken after 02:17 UT (~34 min after trigger) could be used. In the co-added 10 x 60 s exposure image (clear filter), no optical afterglow is neither detected within the BAT/Swift error box (Bernardini et al. GCNC 30250) nor associated with the reported XRT/Swift sources (Osborne et al. GCNC 30252) down to 19.6 mag. This non-detection is consistent with the report from Lipunov et al. (GCNC 30249) and Lopez et al. (GCNC 30251).

We thank the staff at La Mayora for its excellent support.

N. P. M. Kuin (UCL-MSSL) and M. G. Bernardini (INAF-OAB) report
on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 210618A
3728 s after the BAT trigger (Bernardini et al., GCN Circ. 30250
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 ).
No optical afterglow consistent with the XRT candidates source 1, 2
or 4 (Osborne et al. , GCN Circ. 30252
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 ) is detected in the initial
UVOT exposures.
Preliminary 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first
finding chart (FC) exposure and subsequent exposures are:

Filter         T_start(s)   T_stop(s)      Exp(s)         Mag

white_FC          3728         3878          147         >20.4
white             3728         4496          344         >21.1
v                 4707         4843          133         >19.2
b                 4092         4291          197         >20.5
u                 3886         4086          197         >20.0
w2                4502         4702          197         >20.1

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.015 in the direction of the burst
(Schlegel et al. 1998).

Since this is a short GRB, further Swift search for late time optical
emission is being planned.

J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore
(U. Leicester), M.G. Bernardini (INAF-OAB), E. Ambrosi	(INAF-IASFPA),
M. Capalbi (INAF-IASFPA), J.A. Kennea (PSU), A. Tohuvavohu (U.
Toronto), B. Sbarufatti (PSU) 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-detected burst GRB 210618A (Bernardini et al. GCN Circ.
30250
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 ). The observations now extend from T0+3.7 ks to T0+424.9 ks. 

Of the sources reported by Osborne et al. (GCN Circ. 30252
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 ), "Source 2"
is believed to be the afterglow.  The position of this source is RA,
Dec=235.7698, +46.0691 which is equivalent to:

RA (J2000): 15:43:04.75
Dec(J2000): +46:04:08.6

with an uncertainty of 9.3 arcsec (radius, 90% confidence).  This
position is 2.6 arcmin from the Swift-BAT position.  The results of the
XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01056426.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/01056426.

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

Y.-D. Hu, F. J. Aceituno, T.-R. Sun, A. J. Castro-Tirado, M. D. Caballero-Garcia, M. A. Castro Tirado and E. Fernandez-Garcia (IAA-CSIC) on behalf of a larger collaboration, report:

Following the detection of GRB210618A by Swift (Bernardini et al. GCNC 30250) and Fermi (Fermi GBM team. GCNC 30248), we triggered the 1.5m OSN telescope in Granada (Spain) for the possible X-ray afterglow detection, "Source 2" reported in Osborne et al. GCNC 30252. Optical images in the I-band were obtained starting on Jun 24, 21:11 UT (~6.8 d after trigger). No-optical afterglow is detected in the co-added image (300s x 3) within the Swift/XRT error box (Osborne et al. GCNC 30312) which down to I=20.9.

We thank the staff at OSN for their excellent support.