GCN Circular 24763

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

We checked the magnitude of our second-epoch OSN observation (Kann et 
al., GCN 24700
Not found
 
 
) of the Fermi GBM/LAT GRB 190530A (Fermi GBM team, GCN 
24676
Not found
 
 
; Longo et al., GCN 24679
Not found
 
 
) and found a calculation error which 
resulted in an incorrect zero-point. Remeasuring the magnitude against 
four SDSS stars (once again transformed to Rc via the equations of 
Lupton 2005) we now derive Rc(AB) = 19.51 +/- 0.04 mag. This is in good 
agreement with the value obtained by Moskvitin & Uklein (GCN 24708
Not found
 
 
). The 
magnitude of Belkin et al. (GCN 24698
Not found
 
 
) is still overly bright compared 
to our new result, and the revised value from Vinko et al. (GCN 24751
Not found
 
 
) 
is now significantly fainter.

We obtained 7 x 300 s images in SDSS r' with the 0.8m telescope of the 
Observatorio Astrofisico de Javalambre (Teruel, Spain). The first three 
images were taken too early in twilight and were discarded. The 
afterglow is clearly detected in the stack of the four last images, and 
we measure:

r'(AB) = 20.27 +/- 0.06 mag at 2.43735 days after the GRB.

This is in good agreement with an earlier value from Vinko et al. (GCN 
24751
Not found
 
 
) combined with a steep decay.

Using the further photometry published since Kann et al. (GCN 24700
Not found
 
 
) 
(Moskvitin et al., GCN 24708
Not found
 
 
; Belkin et al., GCN 24712
Not found
 
 
; Kumar et al., 
GCN 24729
Not found
 
 
; Nandi et al., GCN 24745
Not found
 
 
; Vinko et al., GCN 24751
Not found
 
 
) we find:

- The steep decay between the observation of Watson et al. (GCN 24690
Not found
 
 
) 
and Xin et al. (GCN 24697
Not found
 
 
) remains, and is not significantly affected by 
our revised OSN measurement.
- There may be a small flare at 1.4 days (this GCN [OSN]; Moskvitin et 
al., GCN 24708
Not found
 
 
).
- Starting at 2.2 days (Belkin et al., GCN 24712
Not found
 
 
; Vinko et al., GCN 
24751
Not found
 
 
; this GCN [OAJ]; Nandi et al., GCN 24745
Not found
 
 
; Kumar et al., GCN 
24729
Not found
 
 
), yet another steep decay sets in, for which we measure alpha = 
3.72 +/- 0.43. This value is perfectly in agreement with the one derived 
in Kann et al. (GCN 24700
Not found
 
 
) at an earlier time, but now based on 
significantly more measurements.

Further follow-up is warranted, if possible.