GCN Circular 17861

Owen Littlejohns (ASU), Nat Butler (ASU), Alan M. Watson (UNAM),
Alexander Kutyrev (GSFC), William H. Lee (UNAM), Michael G. Richer
(UNAM), Chris Klein (UCB), Ori Fox (UCB), J. Xavier Prochaska (UCSC),
Josh Bloom (UCB), Antonino Cucchiara (ORAU/GSFC), Eleonora Troja
(GSFC), Enrico Ramirez-Ruiz (UCSC), Jos� A. de Diego (UNAM), Leonid
Georgiev (UNAM), Jes�s Gonz�lez (UNAM), Carlos Rom�n-Z��iga (UNAM),
Neil Gehrels (GSFC), and Harvey Moseley (GSFC) report:

We continued to observe the field of GRB 150518A (Kawamuro, et al., GCN
17825
Not found
 
 
) with the Reionization and Transients Infrared Camera (RATIR;
www.ratir.org) on the 1.5m Harold Johnson Telescope at the Observatorio
Astron�mico Nacional on Sierra San Pedro M�rtir for two further epochs.

Our first full epoch now extends from 2015/05 20.15 to 2015/05 20.47 UTC
(29.94 to 37.69 hours after the BAT trigger), obtaining a total of 6.04
hours exposure in the r, i and z bands.

For the previously reported SDSS galaxy (Xu, et al., GCN 17829
Not found
 
 
; Mao,
et al., GCN 17830
Not found
 
 
; Littlejohns, et al., GCN 17831
Not found
 
 
), in comparison with
the SDSS DR9, we obtain the following detections:


  r     21.27 +/- 0.04
  i     21.02 +/- 0.04
  z     20.53 +/- 0.24

The second epoch of RATIR observations ranged from 2015/05 21.21 to
2015/05 21.45 UTC (55.33 to 61.20 hours after the BAT trigger),
obtaining a total of 3.59 hours exposure in the r and i bands, and
1.07 hours in the z band. For the same SDSS galaxy, again in
comparison with the SDSS DR9, we obtain the following detections
and upper limit (3-sigma):

  r     21.40 +/- 0.08
  i     21.08 +/- 0.08
  z     > 17.70

The third epoch of RATIR observations was from 2015/05 22.15 to
2015/05 22.47 UTC (77.99 to 85.61 hours after the BAT trigger),
obtaining a total of 6.04 hours exposure in the r, i and z bands.
In this epoch, we obtain the following detections of the SDSS galaxy:

  r     21.34 +/- 0.04
  i     21.08 +/- 0.04
  z     20.80 +/- 0.35

These magnitudes are in the AB system and are not corrected for
Galactic extinction in the direction of the GRB.

Combining the r and i band observations from the second and third
epochs, we find marginal evidence that the source has faded, changing
by 0.07 +/-0.04 magnitudes when compared to the first epoch of RATIR
observations (Littlejohns, et al., GCN 17831
Not found
 
 
). During the full first
epoch of observations, we also observed marginal (1-sigma) evidence
for fading by 0.05 magnitudes in the r and i bands. Finally, we note
that image subtraction between the third and first epochs, in both r
and i band, yield positive residuals in the Swift-XRT error circle at
the 1-sigma level.

We thank the staff of the Observatorio Astron�mico Nacional in San
Pedro M�rtir.