TITLE:   GCN CIRCULAR
NUMBER:  11844
SUBJECT: GRB 110328A / Swift J164449.3+573451: No optical variability of the optical counterpart 
DATE:    11/03/30 20:49:44 GMT
FROM:    Giorgos Leloudas at Dark Cosmology Centre  <giorgos@dark-cosmology.dk>

G. Leloudas, D. Malesani (DARK/NBI), D. Xu (WIS), P. Jakobsson (U.  
Iceland), J. Telting (NOT), report on behalf of a larger collaboration.

We observed again the field of the peculiar source GRB 110328A / Swift  
J164449.3+573451 (Cummings et al., GCN 11823; Kennea et al., ATel  
3242) with the NOT equipped with ALFOSC, using the same instrument  
setup adopted in our first observation (Leloudas et al. GCN 11830).  
Observations were carried in the R and z filters with exposures of  
3x600 and 8x300 s, respectively. The observations were carried out  
starting on 04:10 UT, roughly 26 hr after our first observations and  
39 hr after the first Swift trigger.

The reported galaxy at z=0.35 (Cenko et al., GCN 11827; Levan et al.,  
GCN 11833; Thoene et al., GCN 11834), which is consistent with the  
position of the X-ray and radio emission (Osborne et al., GCN 11826;  
Zauderer et al., GCN 11836), does not show measurable variability over  
this time range.

Its R-flux has remained constant to within 0.02 +- 0.07 mag, relative  
to several stars in the field. Image subtraction using ISIS (Alard  
2000, A&AS 144, 363) also does not reveal any variable source on the  
potential host galaxy or nearby. Similarly, no variability is observed  
in the z-band up to 0.02 +- 0.15 mag. Given the strong variability of  
the X-ray counterpart over the same time span, this suggests that the  
optical emission is dominated by the host galaxy contribution.

Preliminary photometric calibration using the Landolt standard field  
SA 107 yields R = 19.35 for the USNO star 1475-0312998, close to its  
archival value (R=19.4) that was used in GCN 11830.