[vsnet-grb-info 4370] GRB 070224: Optical afterglow confirmation

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  6154
SUBJECT: GRB 070224: Optical afterglow confirmation
DATE:    07/02/27 19:12:04 GMT
FROM:    Alexander Kann at TLS Tautenburg  <kann at tls-tautenburg.de>

Christina C. Thoene (Dark/NBI), D. Alexander Kann (TLS Tautenburg), Thomas                                                    
Augusteijn (NOT) and Celine Reyle-Laffont (Observatoire de Besancon) 
report:

We observed the OT candidate (Thoene et al., GCN 6142) of GRB 070224 
(Racusin et al., GCN 6137) again on Feb. 27 (2.27 days after the burst) 
with NOT and StanCam in R.

The proposed candidate (Thoene et al., GCN 6142, see also Chen et al., GCN
6145, Mirabal et al., GCN 6146, Rol et al., GCN 6147 and Chen et al., GCN 
6148) is clearly fading between our three epochs.                               

We get the following magnitudes:

t after burst   exptime   R mag:
0.20 d            900s        22.1 +- 0.2 
0.39 d            1800s       22.5 +- 0.2
2.27 d            2400s       24.1 +- 0.3

based on two nearby USNO-B1.0 stars.

We therefore conclude that this object is the afterglow of GRB 070224.
It thus may have had an early plateau phase (Rol et al., GCN 6147, Chen et 
al., GCN 6148).

Improved astrometry gives a refined position of the afterglow at (J2000):

RA = 11:56:06.65  
Dec = -13:19:48.8  
 
with an error of 0.5 arcsec. 

This is 1.1 arcsec from the center of the revised XRT error circle 
(Racusin et al., GCN Report 36.1) and 0.8 arcsec from the center of Nat 
Butler's error circle (v2.5) 
(http://astro.berkeley.edu/~nat/swift/xrt_pos.html).