TITLE:   GCN CIRCULAR
NUMBER:  30791
SUBJECT: GRB 210619B: Maidanak and Assy optical observations, broken power law parametrization
DATE:    21/09/09 21:25:26 GMT
FROM:    Alexei Pozanenko at IKI, Moscow  <apozanen@iki.rssi.ru>

S. Belkin (IKI),  A. Pozanenko (IKI), O. Burhonov (UBAI), V. Kim (FAI, 
HSE), M. Krugov (FAI), N. Pankov (HSE), Sh. Ehgamberdiev (UBAI),  report 
on behalf of IKI-GRB-FuN:

We observed the field of GRB 210619B (D'Avanzo et al. GCN 30261) with 
AZT-22 telescope of Maidanak Observatory and AZT-20 telescope of 
Assy-Turgen observatory. The optical afterglow (D'Avanzo et al. GCN 
30261; Jelinek et al., GCN 30263; Kong, GCN 30265; Pellegrin et al., GCN 
30268; Perley, GCN 30271; Zheng and Filippenko, GCN 30273; Blazek et 
al., GCN 30274; Kann et al., GCN 30275; Xin et al., GCN 30277; Kiun et 
al., GCN 30278; Shrestha et al., GCN 30280; Jelkinek et al., GCN 30281; 
Kumar et al., GCN 30286; D'Avanzo et al., GCN 30288; Moskvitin and 
Maslennikova, GCN 303291; Romanov, GCN 30292; Hu et al., GCN 30293; Zhu 
et al., GCN 30294; Belkin et al., GCN 30299; Moskvitin and Maslennikova, 
GCN 30303; Romanov and Lane, GCN 30305; Moskvitin and Maslennikova, GCN 
30309; Vinko et al., GCN 30320; Kann et al., GCN 30338; Zaznobin et al., 
GCN 30343) is clearly detected on the most of stacked images. 
Preliminary photometry of the afterglow of some of observations is 
following.

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

2021-06-20 19:02:53 0.79963  R 8*300  19.47 0.06 23.0 AZT-22
2021-06-21 19:53:41 1.85019  R 12*300 20.43 0.05 23.2 AZT-22
2021-06-22 17:15:29 2.74206  r 75*60  21.82 0.26 22.6 AZT-20
2021-06-22 18:35:45 2.79606  R 12*300 21.21 0.09 23.1 AZT-22
2021-06-26 20:15:18 6.86520  R 20*180 22.55 0.24 23.2 AZT-22
2021-07-02 20:16:53 12.86630 R 12*300 23.5  0.3  23.5 AZT-22
2021-07-08 19:16:40 18.83003 r 76*60  n/d   n/d  23.2 AZT-20
2021-07-18 19:50:46 28.84816 R 12*300 n/d   n/d  23.9 AZT-22

Photometry is based on the USNO-B1.0 and PanSTARRS-PS1 nearby stars.
USNO-B1.0_id R2 I
1238-0493928 15.08 14.33
1238-0494511 15.67 15.03
1238-0493842 17.53 17.36
1238-0493787 16.81 16.12

PanSTARRS-PS1
RA DEC r
21:18:52.24182 +33:49:14.6348 15.2925
21:18:45.71408 +33:50:40.3319 18.1642

Using our data and those published in the GCN circulars cited above we 
plot a light curve which an be found at

http://grb.rssi.ru/GRB210619B/GRB210619B_LC_all.png

Using only Maidanak data after 0.7 days, it was found that the light 
curve can be fitted by a single power law with the index alpha = -1.48 
+/-0.04. Using early data published in GCNs we can describe the LC by a 
broken power law with parameters of alpha = -0.72+/-0.01, beta = -1.52 
+/-0.04, and break time of 0.57+/-0.12.

The fitted LC can be found at

http://grb.rssi.ru/GRB210619B/GRB210619B_AZT22_LC.png

It confirms the steeping decay suggested by Jelinek et al., GCN 30281, 
Kumar et al., GCN 30286. The power-law index before the break is 
approximately the same as that obtained by Kann et al., GCN 30338 and 
Oganesyan et al., arXiv:2109.00010, while the power law index after the 
break is steeper and coincides with XRT afterglow power law index.

We also not detected any influence of host galaxy on afterglow light 
curve, and after late time observations we can estimate brightness of 
the host galaxy R > 23.9.