GRB 210606A

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

At 03:56:02 UT on 6 Jun 2021, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 210606A (trigger 644644567.122922 / 210606164).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 177.6, Dec = -0.5 (J2000 degrees, equivalent to J2000 11h 50m, 0d 30'), with a statistical uncertainty of 11.4 degrees.

The angle from the Fermi LAT boresight is 110.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2021/bn210606164/quicklook/glg_skymap_all_bn210606164.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/bn210606164/quicklook/glg_healpix_all_bn210606164.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2021/bn210606164/quicklook/glg_lc_medres34_bn210606164.gif

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-OAFA robotic telescope  (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L)  located in Argentina (OAFA observatory of San Juan National University) started inspect of the Fermi GRB 210606A ( Fermi GBM team, GCN 30131
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 ) errorbox  707 sec after notice time and 735 sec after trigger time at 2021-06-06 04:08:17 UT, with upper limit up to  17.4 mag. The observations began at zenith distance = 74 deg. The sun  altitude  is -79.9 deg. 

The galactic latitude b = 59 deg., longitude l = 274 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=1629491

We obtain a following upper limits.  

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

     765 | 2021-06-06 04:08:17 |         MASTER-OAFA | (11h 48m 10.39s , +00d 06m 01.9s) |   C |    60 | 16.3 |        
    1046 | 2021-06-06 04:12:58 |         MASTER-OAFA | (11h 56m 10.96s , +00d 05m 07.4s) |   C |    60 | 17.1 |        
    1206 | 2021-06-06 04:15:38 |         MASTER-OAFA | (11h 56m 17.42s , +02d 00m 54.3s) |   C |    60 | 15.7 |        
    1286 | 2021-06-06 04:16:57 |         MASTER-OAFA | (11h 48m 44.03s , -01d 47m 01.2s) |   C |    60 | 17.0 |        
    1365 | 2021-06-06 04:18:17 |         MASTER-OAFA | (11h 56m 52.70s , -01d 47m 56.7s) |   C |    60 | 17.4 |        
Filter C is a clear (unfiltred) band. 


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

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

Swift/BAT did not trigger on GRB 210606A (T0: 2021-06-06 03:56:02 UTC,
Fermi/GBM GCN 30131
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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 ~8 seconds.
With a maximum likelihood analysis (DeLaunay et al. 2021, in prep.) on
the event-mode data we detect a location for the burst with a square
root of the test statistic, sqrt(TS), of 15. The sqrt(TS) behaves
similarly to SNR.
Using the normal BAT imaging technique, we find the same location for
the GRB with an SNR of 5.2.

The BAT position is
RA, Dec = 170.904,  +0.718 deg which is
   RA(J2000)  = 11h 23m 36.96s
   Dec(J2000) =  00d 43��� 04.8���
with an estimated uncertainty of 4 arcmin.
The partial coding was 8%.

This position is consistent with the Ferm/GBM localization (GCN 30131
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 ).

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 OPS NOTE(06Jun21): Per author's request, the two places that cited
GCN 31031
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  have been changed to GCN 30131
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 .]

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

We observed the field of the likely Long Fermi GRB 210606A  (trigger
644644567, GCN #30131
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 , Fermi GBM Team) with the DDOTI/OAN wide-field imager
at the Observatorio Astron��mico Nacional on Sierra San Pedro M��rtir (
http://ddoti.astroscu.unam.mx) on 2021/06/06 from 04:12:31.000 to
06:46:24.000 UTC (0.27 to 2.83 hours after the event).

We observed a region of 20.4 degrees in RA by 20.4 degrees in declination,
with a 3 x 2 grid, centered on the Fermi GBM Final Position RA: 177.630000,
DEC: -0.470000 (J2000 degrees). This region contains 6 instrumental fields
or about 400 square degrees. We obtained 1075 to 1200 seconds of exposure
per instrumental field in the w filter. We obtained AB photometry by
calibration against the APASS catalog.

We detect no likely candidates for the afterglow to our 10-sigma upper
limits of w = 19.64 to 19.93 (inter-quartile).

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

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

Swift has initiated a ToO observation of the Swift/BAT GRB 210606A. 
Automated analysis of the XRT data will be presented online at
https://www.swift.ac.uk/ToO_GRBs/00021451

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.

T.-R. Sun, Y.-D. Hu, A. J. Castro-Tirado, V. Casanova, 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 GRB 210606A by both Fermi (Fermi/GBM GCNC 30131) and Swift (DeLaunay et al. GCNC 30134), we triggered the 1.5m OSN telescope in Granada (Spain) starting on June 6 at 21:08 UT (i.e. ~17.2 h post burst) in order to cover the Swift/BAT error region. Images were taken in BVRI bands. No optical afterglow is detected in the error box down to R=22.2 in the combined (3x600s) image, which is in agreement with the non-detections reported by Lipunov et al. (GCNC 30132) and Pereyra et al. (GCNC 30135).

We thank the staff at OSN for their excellent support.

B. Sbarufatti (PSU), D.N. Burrows (PSU), J. D. Gropp (PSU), K.L. Page
(U. Leicester), A.P. Beardmore (U. Leicester), A. Melandri (INAF-OAB),
T. Sbarrato (INAF-OAB), P. D'Avanzo (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-GUANO
detected burst GRB 210606A (James DeLaunay et al. GCN Circ. 30134
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 ),
collecting 7.3 ks of Photon Counting (PC) mode data between T0+62.3 ks
and T0+385.7 ks. 

Four uncatalogued X-ray sources are detected consistent with being
within 394 arcsec of the Swift/BAT position, of which one ("Source 1")
is fading with 4.6-sigma significance, and is therefore likely the GRB
afterglow. Using 4521 s of PC mode data and 5 UVOT images, we find an
enhanced XRT position (using the XRT-UVOT alignment and matching UVOT
field sources to the USNO-B1 catalogue): RA, Dec = 170.94110, +0.81293
which is equivalent to:

RA (J2000): 11h 23m 45.86s
Dec(J2000): +00d 48' 46.5"

with an uncertainty of 2.3 arcsec (radius, 90% confidence). This
position is 6.1 arcmin from the Swift/BAT position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.1 (+0.9, -0.6).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.8 (+0.5, -0.4). The
best-fitting absorption column is  1.4 (+1.6, -1.0) x 10^21 cm^-2,
consistent with the Galactic value of 3.8 x 10^20 cm^-2 (Willingale et
al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum  is 3.5 x 10^-11 (4.3 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.4 (+1.6, -1.0) x 10^21 cm^-2
Galactic foreground: 3.8 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.8 (+0.5, -0.4)

The results of the XRT-team automatic analysis of the likely afterglow
are at https://www.swift.ac.uk/ToO_GRBs/00021451/Source1.php.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/00021451.

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 03:56:02.12 UT on 6 June 2021, the Fermi Gamma-Ray Burst Monitor (GBM)
triggered and located GRB 210606A (trigger 644644567 / 210606164)
which was also detected by the Swift/BAT-GUANO (DeLaunay et al. 2021, GCN
30134
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 )
The Fermi GBM Final Real-time Localization (GCN 30131
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 ) is consistent with
the Swift position.

The angle from the Fermi LAT boresight at the GBM trigger time is 108
degrees.

The GBM light curve consists of a single peak followed by some
extended emission with a duration (T90) of about 11 s (50-300 keV).
The time-averaged spectrum from T0-4.09 s to T0-1.02 s
is best fit by a power law function with an exponential
high-energy cutoff. The power law index is -0.99 +/- 0.22  and
the cutoff energy, parameterized as Epeak, is 195 +/- 62 keV.

The event fluence (10-1000 keV) in this time interval is
(0.845 +/- 0.144)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0-2.05 s in the 10-1000 keV band
is 2.5 +/- 0.3 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/"