GRB 210826A

P. D'Avanzo, M. G. Bernardini, A. Melandri, T. Sbarrato (INAF-OAB), P. A. Evans (U. Leicester) report on behalf of the Swift team:

Swift-XRT has performed further follow-up observations of GRB 210826A (Ohno et al. GCN Circ. 30721
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 ), collecting a total of 14.6 ks 
of Photon Counting (PC) mode data between T0+36.5 ks ks and T0+1.26e6 s. 

Of the 8 X-ray uncatalogued sources detected in the first observing epoch (D'Ai et al., GCN Circ. 30725
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 ), Source 1 and Source 5 are 
still detected with a consistent count-rate. The other sources are not detected down to a typical 3sigma upper limit of ~ 3e-3 ct/s. 
Therefore, we found no clear evidence for an X-ray afterglow candidate of GRB 210826A. 

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/xrt_products/TILED_GRB00101.

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

V. Prasad (IUCAA), P. Sawant (IUCAA), G. Waratkar (IITB), A. Vibhute
(IUCAA), V. Bhalerao (IITB), D. Bhattacharya (IUCAA), A. R. Rao
(IUCAA/TIFR), and S. Vadawale (PRL) report on behalf of the AstroSat
CZTI collaboration:

Analysis of AstroSat CZTI data with the CIFT framework (Sharma et al,
2020, arxiv:2011.07067) showed detection of a long GRB 210826A, which
was also detected by Fermi-GBM (trigger no. 210826293, GCN 30716
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 ) and
Fermi-LAT (Ohno et al., GCN 30721
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 ).

The source was clearly detected in the 20-200 keV energy range. The
light curve showed multiple peaks of emission with the strongest peak at
2021-08-26 07:01:53.50 UT. The measured peak count rate associated with
the burst is 183 (+45, -24) cts/s above the background in the combined
data of four quadrants, with a total of 2161 (+327, -549) cts. The local
mean background count rate was 560 (+4, -2) cts/s. Using cumulative
rates, we measure a T90 of 24 (+17, -10) s.

It was also detected in the CsI anticoincidence (Veto) detector in the
100-500 keV energy range. The light curve showed multiple peaks of
emission with the strongest peak at 2021-08-26 07:01:47.03 UT. The
measured peak count rate is 200 (+95, -5) cts/s above the background in
the combined Veto data of four quadrants, with a total of 1953 (+648,
-720) cts. The local mean background count rate was 1907 (+6, -6) cts/s.
We measure a T90 of 14 (+8, -5) s from the cumulative Veto light curve.

CZTI GRB detections are reported regularly on the payload site at
http://astrosat.iucaa.in/czti/?q=grb [1]. CZTI is built by a TIFR-led
consortium of institutes across India, including VSSC, ISAC, IUCAA, SAC
and PRL. The Indian Space Research Organisation funded, managed and
facilitated the project. 

Links:
------
[1] http://astrosat.iucaa.in/czti/?q=grb

A. A. Breeveld (UCL-MSSL) and T. Sbarrato (INAF-IASFPA) report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 210826A 36747 s after the LAT trigger 
(Ohno et al., GCN Circ. 30721
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 ). No new optical source consistent with the any of the 8 candidate XRT 
positions (D'Ai et al., GCN Circ. 30725
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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 initial exposures are:

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

v                36747        49716         4869         >20.9

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of 
E(B-V) = 0.032 in the direction of the burst (Schlegel et al. 1998).

A. D'Ai (INAF-IASFPA), B. Sbarufatti (PSU), D.N. Burrows (PSU), J. D.
Gropp (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
Fermi/LAT-detected burst GRB 210826A (Ohno et al. GCN Circ. 30721
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 ), for
a total exposure time of 4.9 ks. The data were collected between
T0+36.5 ks and T0+49.7 ks, and are entirely in Photon Counting (PC)
mode.

Eight uncatalogued X-ray sources have been detected, 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. We notice that one of the sources (Source 8) is consistent
with a catalogued galaxy at redshift z = 0.05. Details of these sources
are given below:

Source 1:
  RA (J2000.0):  45.3361  =  03:01:20.66
  Dec (J2000.0): -71.7072  =  -71:42:25.8
  Error: 8.7 arcsec (radius, 90% conf.)
  Count-rate: (1.46 [+0.80, -0.61])e-3 ct s^-1	 
  Distance: 48 arcsec from Fermi/LAT position.

Source 2:
  RA (J2000.0):  45.0955  =  03:00:22.93
  Dec (J2000.0): -71.6531  =  -71:39:11.1
  Error: 5.0 arcsec (radius, 90% conf. [Enhanced position])
  Count-rate: (3.06 [+1.16, -0.94])e-3 ct s^-1	 
  Distance: 286 arcsec from Fermi/LAT position.
  Flux: (8.2 [+3.1, -2.5])e-14 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Source 4:
  RA (J2000.0):  45.1820  =  03:00:43.67
  Dec (J2000.0): -71.5711  =  -71:34:15.8
  Error: 9.7 arcsec (radius, 90% conf.)
  Count-rate: (1.36 [+0.80, -0.59])e-3 ct s^-1	 
  Distance: 483 arcsec from Fermi/LAT position.
  Flux: (8.2 [+4.8, -3.6])e-14 erg cm^-2 s^-1 (observed, 0.3-10 keV)
This source is 10" away from  the AGN candidate WISEA
J030045.23-713422.8 (Assef et al., 2018, ApJS, 234, 23)

Source 5:
  RA (J2000.0):  45.4777  =  03:01:54.65
  Dec (J2000.0): -71.6539  =  -71:39:13.9
  Error: 8.4 arcsec (radius, 90% conf.)
  Count-rate: (3.7 [+1.3, -1.0])e-3 ct s^-1   
  Distance: 260 arcsec from Fermi/LAT position.
  Flux: (5.8 [+2.0, -1.6])e-14 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Source 6:
  RA (J2000.0):  45.1850  =  03:00:44.41
  Dec (J2000.0): -71.7836  =  -71:47:00.9
  Error: 10.0 arcsec (radius, 90% conf.)
  Count-rate: (1.60 [+0.86, -0.65])e-3 ct s^-1	 
  Distance: 327 arcsec from Fermi/LAT position.
  Flux: (4.2 [+2.2, -1.7])e-14 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Source 7:
  RA (J2000.0):  45.3683  =  03:01:28.39
  Dec (J2000.0): -71.6617  =  -71:39:42.1
  Error: 10.3 arcsec (radius, 90% conf.)
  Count-rate: (1.54 [+0.84, -0.63])e-3 ct s^-1	 
  Distance: 158 arcsec from Fermi/LAT position.
This source is 6" away from  the AGN candidate WISEA
J030129.33-713938.6 (Assef et al., 2018, ApJS, 234, 23)

Source 8:
  RA (J2000.0):  44.7903  =  02:59:9.68
  Dec (J2000.0): -71.6112  =  -71:36:40.2
  Error: 9.3 arcsec (radius, 90% conf.)
  Count-rate: (1.11 [+0.78, -0.56])e-3 ct s^-1	 
  Distance: 660 arcsec from Fermi/LAT position.
This source is 9" away from the catalogued galaxy 2MASX
J02591150-7136430 at redshift z = 0.05

Source 9:
  RA (J2000.0):  45.6834  =  03:02:44.02
  Dec (J2000.0): -71.6574  =  -71:39:26.7
  Error: 8.8 arcsec (radius, 90% conf.)
  Count-rate: (2.16 [+1.02, -0.79])e-3 ct s^-1	 
  Distance: 460 arcsec from Fermi/LAT position.
  Flux: (8.1 [+3.8, -3.0])e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)

The results of the XRT-team automatic analysis of the tiled XRT
observations, including a position-specific upper limit calculator, are
available at https://www.swift.ac.uk/xrt_products/TILED_GRB00101.

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

M. Ohno (Eotvos U./Hiroshima U.), M. Axelsson (KTH & Stockholm Univ.), F.
Longo (University and INFN, Trieste),
N. Omodei (Stanford Univ.) and F Dirirsa (LAPP, Annecy) report on behalf of
the Fermi-LAT Collaboration:

On August 26th, 2021, Fermi-LAT detected high-energy emission from
GRB 210826A, which was also detected by Fermi GBM (trigger 651654110.161386
/ 210826293; GCN 30716
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 ).

The best LAT on-ground location is found to be

RA, Dec 45.3, -71.7 (degrees, J2000)

with an error radius of 0.2 deg (90% containment, statistical error only).
This was 55 deg from the LAT boresight at the time of the GBM trigger:

T0 = 07:01:45 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 0-4000 s
after the trigger is 9.3 (-/+ 3.8) e-07  ph/cm2/s.

The estimated photon index above 100 MeV is -1.9 (-/+ 0.3).

The highest-energy photon is a 7.7 GeV event which is observed ~2.6 ks
after the trigger.

A Swift ToO has been approved for this burst.

The Fermi-LAT point of contact for this burst is
Feraol Fana Dirirsa (dirirsa@lapp.in2p3.fr).

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.

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-SAAO robotic telescope  (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L)  located in South Africa (South African Astronomical Observatory) started inspect of the Fermi GRB 210826A ( Fermi GBM team, GCN 30716
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 ) errorbox  36801 sec after notice time and 36879 sec after trigger time at 2021-08-26 17:16:24 UT, with upper limit up to  17.8 mag. Observations started at twilight.  The observations began at zenith distance = 71 deg. The sun  altitude  is -13.0 deg. 

The galactic latitude b = -35 deg., longitude l = 291 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=1700544

We obtain a following upper limits.  

Tmid-T0  |      Date Time      |          Site       |             Coord (J2000)          |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________

   36909 | 2021-08-26 17:16:24 |         MASTER-SAAO | (05h 11m 37.00s , -76d 38m 26.5s) |   C |    60 | 17.8 |        
Filter C is a clear (unfiltred) band. 


The observation and reduction will continue. 
The message may be cited.

P. A. Evans (U. Leicester) reports on behalf of the Swift team:

Swift has initiated a series of observations, tiled on the sky, of the
Fermi/LAT GRB 210826A. Automated analysis of the XRT data will
be presented online at http://www.swift.ac.uk/xrt_products/TILED_GRB00101

Any uncatalogued X-ray sources detected in this analysis will be
reported on this website and via GCN COUNTERPART notices. The probability of finding
serendipitous sources, unrelated to the Fermi/LAT event is high: 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.

B. Biltzinger, F. Kunzweiler, F. Berlato, J. Burgess & J. Greiner (all MPE Garching) report:

The public trigdat data of the Fermi Gamma-ray Burst Monitor (GBM) trigger
651654110 at 07:01:45 on 26 Aug. 2021 were automatically fitted for spectrum
and sky location with BALROG (Burgess et al. 2018, MNRAS 476, 1427;
Berlato et al. 2019, ApJ 873, 60).

The best-fit position (1 sigma statistical errors) is:
RA(2000.0) = 84.3+/-5.4 deg
Decl.(2000.0) = -75.6+/-0.7 deg
We estimate an additional systematic error of 2 deg.

Further details are available at:
https://grb.mpe.mpg.de/grb/GRB210826293/

The Healpix map can be downloaded from:
https://grb.mpe.mpg.de/grb/GRB210826293/healpix

The location parameters are available as JSON at:
https://grb.mpe.mpg.de/grb/GRB210826293/json

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

At 07:01:45 UT on 26 Aug 2021, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 210826A (trigger 651654110.161386 / 210826293).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 64.6, Dec = -77.1 (J2000 degrees, equivalent to J2000 04h 18m, -77d 05'), with a statistical uncertainty of 2.1 degrees.

The angle from the Fermi LAT boresight is 48.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2021/bn210826293/quicklook/glg_skymap_all_bn210826293.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/bn210826293/quicklook/glg_healpix_all_bn210826293.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2021/bn210826293/quicklook/glg_lc_medres34_bn210826293.gif