GRB 210413B

Swift/BAT did not successfully trigger on GRB 210413B (T0: 2021-04-13
23:46:52.45 UTC, Fermi/GBM TRIGGER 640050417).

The GUANO system is currently offline, thus no extended GUANO data is
available for this time, and so the sensitive GUANO MLL analysis
(DeLaunay et al. 2021, in prep) can not be run.

Swift/BAT triggered in real-time onboard in the rates domain (trigger
#1043009), but no strong source was found to be associated in the
onboard imaging. This 'failed trigger' resulted in 10s of BAT TTE data
being saved and sent to the ground.

The duration of the burst, as seen by BAT, is T90 = 1.088 s +/- 0.09 s.

In ground analysis of the available TTE data, we find a source using
typical imaging analysis, that is the likely location of the GRB, with
SNR 6 in the image domain.
The BAT position is
RA, Dec = 182.558 +55.965, which is:
    RA (J2000)     12h 10m 13.920s
    Dec (J2000)  +55d 57' 54.00"
with an estimated uncertainty of 3 arcminutes.

This location is consistent with the Fermi/GBM localization
distributed in the Final Position notice.

XRT and UVOT follow-up has been requested. Results of follow-up
observations will be reported in future circulars.

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

Swift has initiated a ToO observation of the Swift/BAT GRB 210413B. 
Automated analysis of the XRT data will be presented online at
https://www.swift.ac.uk/ToO_GRBs/00021428

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.

Gregory S.H. Paek (SNU ARC/SNU), Myungshin Im (SNU ARC/SNU), Chung-Uk Lee
(KASI), Seung-Lee Kim (KASI), Hyun-Il Sung (KASI) on behalf of a larger
collaboration

We searched for the optical counterpart of GRB 210413B (A. Tohuvavohu et
al. GCN #29819
Loading...
 
 , and P. A. Evans et al., GCN #29821
Loading...
 
 ) with the 1-m class
telescope in Lemonsan Optical Astronomy Observatory (LOAO), one of
facilities of the GW EM-Counterpart Korean Observatory (GECKO).

We observed the center of Swift/BAT localization (RA, Dec = 182.558 +55.965),
but could not find any transient within the field of view of LOAO
(28.1'x28.1'). We calibrated flux with the PS1 catalog and used an AB
magnitude system.

------
Filter Date-obs[UT] Exp.time[s] Depth(5sigma)
B 2021-04-16T03:17:51.667 180*3 20.780
V 2021-04-16T03:27:54.933 180*3 20.347
R 2021-04-16T03:37:58.867 180*3 19.594
I 2021-04-16T03:47:55.800 180*3 19.293

We thank the LOAO opertator for performing the observation.
Gravitational-wave EM Counterpart Korean Observatory (GECKO) is a network
of 10+ 0.5m to 1m class telescopes over the world.

J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto), J.P. Osborne (U.
Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), T.
Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), M.G. Bernardini
(INAF-OAB), B. Sbarufatti (PSU) 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 210413B (Swift/BAT did not successfully
trigger on GRB 210413B et al. GCN Circ. 29819
Loading...
 
 ), collecting 4.9 ks of
Photon Counting (PC) mode data between T0+185.0 ks and T0+206.7 ks. 

One uncatalogued X-ray source has been detected consistent with being
within 394 arcsec of the Swift/BAT 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 1:
  RA (J2000.0):  182.5333  =  12:10:7.98
  Dec (J2000.0): +56.0497  =  +56:02:59.0
  Error: 5.7 arcsec (radius, 90% conf. [Enhanced position])
  Count-rate: (4.5 +/- 1.2)e-3 ct s^-1	 
  Distance: 309 arcsec from Swift/BAT position.
  Flux: (1.42 +/- 0.39)e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Three 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/00021428.

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

N. Ito, K. L. Murata, R. Hosokawa, M. Niwano, H. Takamatsu, Y. Yatsu,
and N.Kawai (TokyoTech) report on behalf of the MITSuME collaboration:

We observed the field of GRB 210413B (Fermi/GBM TRIGGER 640050417, P.
A. Evans et al. GCN Circular #29821
Loading...
 
 , Gregory S.H. Paek et al. GCN
Circular #29825
Loading...
 
 , J.A. Kennea et al. GCN Circular #29827
Loading...
 
 ) with the
optical three color (g', Rc, and Ic) CCD cameras attached to the
MITSuME 50 cm telescope of Okayama Astronomical Observatory, Okayama,
Japan. The observation with a series of 60 sec exposures started at
2021-04-17 13:31:10 UT (85.8 hours after the Fermi/GBM trigger
640050417). We stacked the images with good conditions. We did not
find any transients within the BAT error region (A. Tohuvavohu et al.
GCN Circular #29819
Loading...
 
 ) in all three bands. We obtained the 5-sigma
limits of the stacked images as follows.

T0+[hour] MID-UT T-EXP[sec] 5-sigma limits
------------------------------------------------------------------------------------------------
86.8 15:06 2340 g'>17.4, Rc>17.6, Ic>17.3
------------------------------------------------------------------------------------------------
T0+ : Elapsed time after the burst
T-EXP: Total Exposure time
We used the PS1 catalog for flux calibration.
The magnitudes are expressed in the AB system.
The images were processed in real-time through the MITSuME GPU
reduction pipeline (Niwano et al. 2021, PASJ, Vol.73, Issue 1, Pages
4-24; https://github.com/MNiwano/Eclaire).

M. H. Siegel (PSU) and A. Tohuvavohu (U Toronto) report on behalf of the Swift/UVOT team:

Swift/UVOT performed follow-up observations of the Swift/BAT-detected burst GRB 210413B
(Tohuvavohu et al., GCN Circ. 29819
Loading...
 
 ) 185 ks after the BAT trigger. We detect a faint source
close to the XRT source reported by Kennea et al. (GCN Circ. 29827
Loading...
 
 ).

The preliminary UVOT position is:
   RA  (J2000) =  12:10:07.75 = 182.53230 (deg.)
   Dec (J2000) = +56:02:58.9  = 56.04970 (deg.)
with an estimated uncertainty of 0.4 arc sec. (radius, 90% confidence).

The preliminary detection using the UVOT photometric system (Breeveld et al. 2011, AIP Conf.
Proc. 1358, 373) is wh=19.68+-0.03 mag (Vega) or wh=20.48+-0.03 mag (AB).

We note, however, that the DECaLs survey shows faint source at this position at a g magnitude
of 21.09 mag  (AB).  It is thus not clear if this detection represents a brightening of that source,
which could indicate a host galaxy.

A. D���Ai (INAF-IASFPA), A. Tohuvavohu (U Toronto) 
and M. H. Siegel (PSU) report on behalf of the Swift team:

Swift-XRT has performed further follow-up observations of the
ground-detected burst GRB 200413B (Tohuvavohu et al. GCN 
Cir. 29819), collecting a total of 9.2 ks of Photon Counting (PC) 
mode data between T0+2.1 days and T0+37 days. 

The uncatalogued candidate X-ray source (Source 1) reported 
in Kennea et al. (GCN Circ. 29827
Loading...
 
 ) which had also a possible 
UV counterpart (Siegel, GCN Circ. 29912
Loading...
 
 ) is still detected in 
XRT at a count rate consistent with the first observations. 
We thus rule out a possible association with the burst.

The results of the XRT-team automatic analysis are at 
https://www.swift.ac.uk/ToO_GRBs/00021428/

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