TITLE:   GCN CIRCULAR
NUMBER:  29716
SUBJECT: GRB 210312B: OSN Afterglow Observations
DATE:    21/03/24 19:45:39 GMT
FROM:    Alexander Kann at IAA-CSIC  <kann@iaa.es>

D. A. Kann (HETH/IAA-CSIC), M. Jelinek (ASU CAS Ondrejov), A. de Ugarte 
Postigo (HETH/IAA-CSIC, DARK/NBI), C. C. Thoene, M. Blazek, J. F. Agui 
Fernandez (all HETH/IAA-CSIC), A. Sota, and R. M. Eloy (both IAA-CSIC) 
report:

We observed the optical afterglow of GRB 210312B (INTEGRAL detection: 
Mereghetti et al., GCN #29650) with both the 1.5m T150 and the 0.9m T90 
telescopes of the Observatorio de Sierra Nevada (OSN),
Granada, Spain. We obtained 5 x 90 s exposures in B, V, Rc, and 6 x 90 s 
in Ic, followed by 21 x 180 s exposures in Rc with the T150 telescope, 
and 18 x 180 s in Rc with the T90 telescope in parallel, starting at 
2021-03-12 22:51:28.19 UT. The afterglow is detected in multiple 
combined images from T150, and in the complete combined T90 image. We 
furthermore obtained 10 x 500 s Rc images in the second night with the 
T150. The afterglow is detected in the combined image.

 From three stacked images observed from 0.09577 to 0.14106 days after 
the GRB, we detect a potential plateau phase, measuring Rc ~ 21.69 mag 
(AB mag, vs. a nearby Pan-STARRS star converted to Rc following the 
Lupton transformations, then transformed back to AB mag; errors range 
from 0.04 to 0.11 mag). Then at 1.0850 days, we find the afterglow has 
significantly decayed to Rc = 23.04 +/- 0.07 mag (AB).

Using photometry from our GTC observations (Kann et al., GCNs #29653, 
#29655) as well as earlier photometry from the Ondrejov D50, we find a 
broken power-law decay with a late slope of alpha ~ 2.2. The second-day 
OSN detection is in excess of the extrapolated decay, indicating that 
the host galaxy may contribute already, or that another rebrightening 
has taken place.

[GCN OPS NOTE(01apr21):  The typo in the SUBJECT line "230312B" was
changed to 210312B.]