GCN Circular 17721

I. Bikmaev, E. Irtuganov, N. Sakhibullin (KFU/AST),
R. Burenin, M. Pavlinsky, R. Sunyaev (IKI),
I. Khamitov, H. Kirbiyik (TUG), U. Kiziloglu (METU), E. Gogus (Sabanci Uni.)

report:

The field of the optical afterglow of GRB 150413A (Markwardt et al.,
GCN 17688
Not found
 
 
, Ivanov et al., GCN 17689
Not found
 
 
) was observed with Russian-Turkish
1.5-m telescope (RTT150, Bakirlitepe, TUBITAK National Observatory,
Turkey) using the TFOSC instrument.

We obtained series of Rc exposures in April 13/14 (50 frames x 300
sec, UT = 20:27 - 02:30 ) and in April 14/15 (4 frames x 600 sec,
UT = 22:43 - 00:28, in partially cloudy conditions). The optical
transient is clearly detected in each images.

Using USNO-B1 1618-0098185 star at RA,DEC = 12:41:09.18 +71:50:32.4
(J2000) with R2 = 15.59 mag as a reference we estimated the
R-magnitudes of the afterglow:

T-T0, Mag. +/- Mag.err
(days)
0.2932 17.39 +/- 0.03
0.5200 18.70 +/- 0.05
1.4318 21.4  +/- 0.2

RTT-150 light curve in April 13/14, 2015, can be found at:

http://hea.iki.rssi.ru/grb/150413a/indexeng.html

The break in the light curve at (T-T0) = 0.33 days is evident (break
is seen also in V-band data of Bassano Bresciano Observatory, Quadri
et al, GCN 17704
Not found
 
 
).

Before the break at (T-T0) = 0.33 days, the power law decay index is
=~0.9, and it is close to 3.0 after that. Note, that decay index is
close to 2.0 after (T-T0) = 0.7 days. Therefore, there are multiple
breaks in the OT light curve.