GRB 210406A

S.Mereghetti (INAF, IASF-Milano), D.Gotz (CEA, Saclay),  C.Ferrigno,
E.Bozzo, V.Savchenko (ISDC, Versoix), L.Ducci (IAAT, Germany and ISDC,
Versoix) and J.Borkowski (CAMK, Torun) report:

a gamma ray burst lasting about 60 s has been detected by IBAS in the
 IBIS/ISGRI data at 17:11:22  UT of April 6, 2021.

The refined coordinates (J2000) are:
R.A.=  132.4589 deg
DEC.=  +76.5322  deg
with an uncertainty of 1.5 arcmin (90% c.l.).

Due to telemetry saturation, we can derive only lower limits on the fluence
and peak flux (20-200 keV).
These are  >1.2e-6 erg/cm2 and >1.6  ph/cm2/s (1-s integration  time),
 respectively.

A plot of the light curve will  be posted at
http://ibas.iasf-milano.inaf.it/IBAS_Results.html

Y.-D. Hu, E. Fernandez-Garcia, A. J. Castro-Tirado, M.D. Caballero-Garcia, M. A. Castro Tirado (IAA-CSIC), C. Perez del Pulgar, A. Castellon, I. Carrasco (Univ. de Malaga), S. Guziy (Univ. of Nikolaev) and D. R. Xiong, Y. F. Fan, J. M. Bai, C. J. Wang, Y. X. Xin, X. H. Zhao (Yunnan Observatories of CAS) on behalf of a larger collaboration, report:

Following the detection of GRB 210406A by INTEGRAL (Mereghetti et al. GCNC 29767), the 0.6m BOOTES-4/MET robotic telescope at Lijiang Astronomical Observatory (China) automatically responded to the burst starting on Apr 6 at 17:13:27 UT  (~2 min after trigger). No new source is detected within the refined IBIS position (Mereghetti et al. GCNC 29767) down to 20.7 mag in the co-added image (878 s on the aggregate, clear filer). The non-detection is consistent with the limit reported by MASTER (Lipunov et al. GCNC 29766).

We thank the staff at Lijiang observatory for their excellent support.

J. Vinko (Konkoly/ELTE/U Szeged), R. Szakats, L. Kriskovics, A. Pal and K. Vida (Konkoly)
report on behalf of the "Transient Astrophysical Objects" project:

We observed the field of GRB 210406A (Mereghetti, GCN29767
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 ; Lipunov et al. GCN29766
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 ) with the 0.8m RC80 robotic telescope at Piszkesteto Mountain Station of Konkoly Observatory through SDSS r' and i' filters on 2021 Apr 06.86 UT
(MJD 59310.86, 3h 31.6m after the trigger). The total exposure time was 1800 s. The frames were tied
to PS1-photometry of local stellar sources. No optical counterpart for the GRB has been detected
above the 5-sigma upper limit of 21.9 AB-mag, which is consistent with other reports of non-detection
(Hu et al. GCN29768
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 ).

N. Pankov (HSE), S. Belkin (IKI), A. Pozanenko (IKI), E. Klunko (ISTP) 
report  on behalf of GRB IKI FuN:

We observed the field of GRB 210406A (Mereghetti  et al., GCN 29767
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 ) 
with AZT-33IK telescope of Sayan observatory (Mondy) starting on 
2021-04-06 (UT) 17:54:58. We obtained 15 images in R-filter. We do not 
find new sources in a stacked image in comparison with Pan-STARRS PS1 or 
Legacy Survey which is consistent with observations (Lipunov et al., GCN 
29766
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 ; Hu et al., GCN 29768
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 ; Vinko  et al., GCN 29769
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 ). So we have no 
immediate OT candidate. More observation is necessary to investigate 
variability of field sources.
Preliminary photometry of the field in the stacked image is following

Date       UT start   t-T0       Filter Exp.   OT   Err.  UL(3sigma)
                      (mid, days)        (s)

2021-04-06 17:54:58   0.04069    R     15*120  n/d   n/d  22.9

The photometry is based on the nearby USNO-B1.0 stars
USNO-B1.0_id RA DEC R2
1665-0048950 132.414156 +76.555625 16.49
1664-0050141 132.551831 +76.468300 18.01

C. Malacaria (NASA-MSFC/USRA) and C. Meegan (UAH)
report on behalf of the Fermi GBM Team:

"At 17:11:30.57 UT on 06 April 2021, the Fermi Gamma-Ray Burst Monitor (GBM)
triggered and located GRB 210406A (trigger 639421895 / 210406716),
which was also detected by the INTEGRAL/ISGRI (Mereghetti et al. 2021, GCN 29767
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 ).
The GBM on-ground location is consistent with the INTEGRAL position.

The angle from the Fermi LAT boresight at the GBM trigger time is 154 degrees.

The GBM light curve shows a long GRB with multiple spikes and
with a duration (T90) of about 17.4 s (50-300 keV).
The time-averaged spectrum from T0-2.0 s to T0+18.4 s is
best fit by a power law function with an exponential
high-energy cutoff. The power law index is -0.91 +/- 0.13 and
the cutoff energy, parameterized as Epeak, is 195.6 +/- 18.8 keV.

The event fluence (10-1000 keV) in this time interval is
(9.1 +/- 0.5)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+5.4 s in the 10-1000 keV band
is 4.6 +/- 0.6 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. Ridnaia, D. Frederiks, S. Golenetskii, A. Lysenko, D. Svinkin,
A. Tsvetkova,  M. Ulanov, and T. Cline, report:

The long GRB 210406A (INTEGRAL detection,
T0 = T0(IBIS/ISGRI)= 17:11:28 UT: Mereghetti, et al., GCN 29767
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 ;
Fermi/GBM detection: Malacaria & Meegan, GCN 29771
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 )
was detected by Konus-Wind (KW) in the waiting mode.

A Bayesian block analysis of the KW waiting mode data in the 20-400 keV band
reveals a >10 sigma count rate increase over background in the interval
from ~T0-0.470 s to ~T0+73.130 s.

As observed by Konus-Wind, the burst had a fluence of
9.13(-1.16,+1.40)x10^-6 erg/cm2 and a 2.944-s peak flux,
measured in the interval starting from ~T0+5.418 s,
of 5.74(-0.84,+0.97)x10^-7 erg/cm2/s
(both in the 10 keV - 10 MeV energy range).

Modeling the K-W 3-channel time-integrated spectrum
(from ~T0-0.470 s to ~T0+73.130 s)
by a power law with exponential cutoff (CPL) model
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
yields alpha = -1.04(-0.20,+0.35) and Ep = 176(-36,+54) keV.

The KW light curve of this burst is available at
http://www.ioffe.ru/LEA/GRBs/GRB210406A/

All the quoted errors are estimated at the 1 sigma confidence level.
All the presented results are preliminary.

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

Swift has initiated a ToO observation of the INTEGRAL GRB 210406A. 
Automated analysis of the XRT data will be presented online at
https://www.swift.ac.uk/ToO_GRBs/00021427

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 INTEGRAL 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.

S. Belkin (IKI), N. Pankov (HSE), A. Pozanenko (IKI), E. Klunko (ISTP) 
report  on behalf of GRB IKI FuN:

We observed the field of GRB 210406A (Mereghetti  et al., GCN 29767
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 ) 
with AZT-33IK telescope of Sayan observatory (Mondy) in second epoch 
starting on 2021-04-07 (UT) 16:54:14. The first epoch was obtained early 
(Pankov  et al., GCN 29770
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 ). We obtained 20 images in R-filter. In 
comparison of the sources we found one fading source in coordinates of 
(J2000) 08:49:49.09    76:33:20.2 with accuracy of 0.5 arcsec in each 
coordinate. The source is faded by 0.5mag between the two epochs. 
Preliminary photometry of the source in  stacked images is following

Date       UT start   t-T0       Filter Exp.   OT    Err.  UL(3sigma)
                     (mid, days)        (s)

2021-04-06 17:54:58   0.04069    R     15*120  21.90  0.14  22.9
2021-04-07 16:54:14   0.98811    R     20*120  22.41  0.14  23.2

The photometry is based on the nearby USNO-B1.0 stars
USNO-B1.0_id RA DEC R2
1665-0048950 132.414156 +76.555625 16.49
1664-0050141 132.551831 +76.468300 18.01

The source is not presented in Pan-STARRS PS1 but marginally  visible in 
Pan-STARRS (r-plate) and presented in Legacy Survey DR8.

We suggest the source as afterglow candidate of optical afterglow of GRB 
210406A.

V. D'Elia (SSDC & INAF-OAR), A. D'Ai (INAF-IASFPA), 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) and P.A. Evans
(U. Leicester) report on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the INTEGRAL-detected
burst GRB 210406A (Mereghetti et al. GCN Circ. 29767
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 ), collecting 5.1
ks of Photon Counting (PC) mode data between T0+169.5 and T0+244.5 ks. 


One uncatalogued X-ray source has been detected consistent with being
within 148 arcsec of the INTEGRAL position, it is below the RASS limit
and shows no definitive signs of fading. Therefore, at the present time
we cannot confirm this as the afterglow. Details of this source are
given below:

Source 3:
  RA (J2000.0):  132.4988  =  08:49:59.72
  Dec (J2000.0): +76.5289  =  +76:31:44.0
  Error: 6.4 arcsec (radius, 90% conf.)
  Count-rate: (1.28 [+0.75, -0.56])e-3 ct s^-1	 
  Distance: 35 arcsec from INTEGRAL position.
  Flux: (1.46 [+0.86, -0.64])e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)


An uncatalogued source was also detected, however this was too far from
the GRB position to be the afterglow.

No X-ray source is detected at the position of the optical afterglow
candidate reported by Belkin et al. (GCN Circ. 29775
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 ) down to a 3sigma
upper limit of 1.4e-3 ct s^-1.

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/00021427.

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

Y.-D. Hu, E. Fernandez-Garcia, A. J. Castro-Tirado, M.D. Caballero-Garcia, M. A. Castro Tirado (IAA-CSIC), V. Sokolov (SAO-RAS), I. Carrasco and A. Castellon (UMA) and  S. B. Pandey (ARIES), on behalf of a larger collaboration, report:

Following the detection of GRB 210406A by INTEGRAL (Mereghetti et al. GCNC 29767), we triggered the 10.4m GTC telescope equipped with OSIRIS in La Palma (Spain) starting on Apr 10 at 20:47 UT (~4.15 d after trigger). The 1.5 arcmin error region (Mereghetti et al. GCNC 29767) was covered by the two OSIRIS CCDs with an exception of a 9.4 arcsec gap in between.

For the optical candidate detected at Mondy Observatory (Belkin et al. GCNC 29775) a magnitude 22.45+-0.09 in r-band is measured. Since this value is consistent with the second epoch Mondy observation, a potential host galaxy cannot be excluded if this optical variability is finally confirmed although we notice that no X-ray afterglow has been reported by Swift/XRT at this position (D'Elia et al. GCNC 29785).

In addition to this, close to the source #3 position (6.4 arcsec error region) reported by Swift/XRT (D'Elia et al. GCNC 29785) we found two objects on the outskirts of it. The first one is located at RA: 8:50:00.18 DEC: +76:31:50.50 (J2000) for which we measure 22.96+-0.12 mag in r-band and seems to be recorded in Pan-STARRS DR2. A fainter object is found at RA: 8:50:01.42 DEC: +76:31:41.02 (J2000) for which we measure a magnitude of 24.00+-0.26 in r-band.

All magnitudes are calibrated with the Pan-STARRS catalog. Further observations are needed.

We thank the staff at Roque de Los Muchachos Observatory for their excellent support.