[vsnet-grb-info 27159] GRB 210104A: early ATLAS detections of the bright afterglow

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  29262
SUBJECT: GRB 210104A: early ATLAS detections of the bright afterglow 
DATE:    21/01/05 17:47:51 GMT
FROM:    Stephen Smartt at Queen's U/Belfast  <s.smartt at qub.ac.uk>

S. J. Smartt, K. W. Smith (QUB), S. Srivastav,, D. R.  Young,
M. Fulton, (QUB) L. Denneau, A. Heinze, J.  Tonry, H. Weiland (IfA,
Univ. Hawaii), A. Rest (STScI), B. Stalder (LSST), C. Stubbs
(Harvard), O. McBrien, J. Gillanders, L. Shingles (QUB), 
T.-W. Chen (MPE)

We report serendipitous observations of GRB210104A (Troja et al. GCN
29233) by ATLAS, the twin telescope system on Haleakala and Mauna
Loa that surveys the whole northern sky every 2 days (see Tonry et
al. 2018, PASP, 130f4505).  Optical transients are automatically
detected on the difference images and discoveries (Smith et al. 2020,
PASP, 132h5002) are made public through the IAU Transient Name Server.

During the course of its survey, ATLAS happened to be observing the
field of GRB210104A starting 1min 47sec after the Swift BAT trigger at
2021-01-04 11:26:59 UT (Troja et al. GCN 29233). This was a fortunate
coincidence in normal survey mode and not a triggered pointing.

We detect the bright UVOT afterglow as initially reported in (Troja et
al. GCN 29233), and appear to have the caught it early and bright at
o = 13.9 along with rapid fading (see also Xin et al, GCN29235, Hu et
al. 29236, Hosokawa et al. GCN 29237, Kim et al. GCN 29238, Lipunov et
al. 29239),

It was detected at a position : 
RA = 06:55:05.37 (103.77240) DEC = +64:40:34.2 (+64.67618) 
              
The expoures were 30sec long in the o-band (an r+i composite),
with start time and magnitudes as follows : 

###MJD              UTC                                         m      dm  
59218.478319  2021-01-04 11:28:46.762 UTC 13.94  0.01
59218.481544  2021-01-04 11:33:25.402 UTC 15.56  0.01
59218.487080  2021-01-04 11:41:23.712 UTC 16.43  0.02
59218.497226  2021-01-04 11:56:00.326 UTC 16.96  0.03
59218.670448  2021-01-04 16:05:26.707 UTC >18.4  (3 sigma limit)