TITLE:   GCN CIRCULAR
NUMBER:  24700
SUBJECT: GRB 190530A: Steep afterglow decay found by OSN
DATE:    19/05/31 23:13:05 GMT
FROM:    Alexander Kann at IAA-CSIC  <kann@iaa.es>

D. A. Kann, L. Izzo (both HETH/IAA-CSIC), A. de Ugarte Postigo 
(HETH/IAA-CSIC, DARK/NBI), M. Blazek, C. C. Thoene, K. Bensch (all 
HETH/IAA-CSIC), and V. Casanova (IAA-CSIC) report:

We observed the afterglow position (Lipunov et al., GCN 24680) of the 
extremely bright GRB 190530A (GBM detection and localization: Fermi GBM 
team, GCN 24676, Biltzinger et al., GCN 24677; Fermi-LAT localization: 
Longo et al., GCN 24679; Agile-MCAL/GRID detections: Lucarelli et al., 
GCN 24678, Verecchia et al., GCN 24683) a second time with the 1.5-m 
telescope of the Sierra Nevada Observatory (OSN), Spain. The afterglow 
is now only marginally detected in single frames, so we stack 7 x 120 s 
images.

We measure the following AB magnitude:

Rc = 20.60  +- 0.15 mag at 1.44509 days after the GRB.

The magnitude was derived against 1 nearby star from the SDSS catalog, 
using the transformation equations of Lupton (2005), and transformed 
back into AB mag.

This magnitude is significantly fainter than expected from an 
extrapolation of the decay from the first day, indicating a break in the 
light curve must have occurred. Using other observations (Lipunov et 
al., GCN 24680, GCN 24693; Kann et al., GCN 24684; Heintz et al., GCN 
24686; Izzo et al., GCN 24687; Xin et al., GCN 24688; Watson et al., GCN 
24690; Xin et al., GCN 24697) we find the afterglow is described by a 
broken power-law with pre-break decay slope alpha_1 = 1.21 +/- 0.07, 
post-break decay slope alpha_2 = 3.75 +/- 0.40, and break time t_b = 
0.78 +/- 0.06 days.

The steeper slope mentioned by Watson et al. (GCN 24690) may indicate 
the break was occurring during the COATLI observations. We note the 
magnitude given by Belkin et al. (GCN 2469 is significantly too bright, 
this may stem from their use of USNO stars as standard stars.

The post-break decay slope is extremely steep and generally not expected 
from the forward shock model and a typical electron energy distribution. 
Further observations are highly encouraged.