[vsnet-grb-info 28235] GRB 210619B: Deep CAHA 2.2m detection and late-time light curve behavior

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  30338
SUBJECT: GRB 210619B: Deep CAHA 2.2m detection and late-time light curve behavior
DATE:    21/06/28 10:29:27 GMT
FROM:    Alexander Kann at IAA-CSIC  <kann at iaa.es>

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

We re-observed the afterglow of GRB 210619B (D'Avanzo et al., GCN 
#30261) with CAFOS mounted at the 2.2m Calar Alto telescope (Almeria, 
Spain). Image depth was influenced by moonlight but conditions were 
good. We obtained 20 x 180 s images in r'.

The afterglow is faintly detected in the stacked image. Against 
Pan-STARRS comparison stars, we derive r' = 22.56 +/- 0.16 mag at 
6.12346 d after the trigger.

Using selected data from GCN Circulars (Jelinek et al., GCN #30263, 
#30281; Pellegrin et al., GCN #30268; Perley, GCN #30271; Zheng & 
Filippenko, GCN #30273; Blazek et al., GCN #30274; Kann et al., GCN 
#30275; Xin et al., GCN #30277; D'Avanzo et al., GCN #30288; Moskvitin & 
Maslennikova, GCNs #303291, #30303, #30309; Romanov, GCN #30292; Hu et 
al., GCN #30293; Zhu et al., GCN #30294; Belkin et al., GCN #30299; 
Romanov & Lane, GCN #30305; Vinko et al., GCN #30320), we find the 
afterglow after 0.057 d can be fit by a smoothly broken power-law with a 
sharp break and parameters alpha_1 = 0.742 +/- 0.013, alpha_2 = 1.221 
+/- 0.045, and break time t_b = 0.465 +/- 0.048 d (40136 +/- 4108 s).

This fully confirms the steepening decay reported by Jelinek et al., GCN 
#30281; Kumar et al., GCN #30286.

Note that the best fit of the X-ray light curve at the time finds 
alpha_1 = 0.978 +0.012 -0.019, alpha_2 = 1.50 +/- 0.035, t_b = 0.152 
+0.015 -0.024 d (13100 +1300 -2100 s). However, the early decay (alpha_0 
= 0.773 +0.087 -0.141, up to 500 s) is very similar to the optical decay 
we find from ~5000 s onward to the (optical) break. We note that the 
post-break decay slope would be extremely shallow if this were actually 
a jet break, but no deviation from this decay is seen until ~6 d.