[vsnet-grb-info 13379] GRB 130521A: Skynet/PROMPT detection of an extremely red afterglow

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  14713
SUBJECT: GRB 130521A: Skynet/PROMPT detection of an extremely red afterglow
DATE:    13/05/28 19:52:35 GMT
FROM:    Adam S. Trotter at UNC-Chapel Hill/PROMPT/Skynet  <atrotter at physics.unc.edu>

D. James, C. Foster, P. Taylor, M. Carroll, A. Trotter, D. Reichart, A. 
LaCluyze, J. Haislip, T. Berger, H. T. Cromartie, R. Egger, A. Foster, 
N. Frank, K. Ivarsen, M. Maples, J. Moore, M. Nysewander, E. Speckhard, 
and J. A. Crain report

Skynet observed the Swift/BAT localization of GRB 130521A (Pagani et 
al., GCN 14691, Swift trigger #556344) with four 16" telescopes of the 
PROMPT array at CTIO, Chile. Starting at 2013-05-21, 22:49:16 UT and 
continuing until 23:01:04 UT (t=49.2s-11.8m post-trigger), it took 66 
exposures in the BVRI bands. The field was 28.5 degrees from the sun at 
this time, so the duration of the observation was limited by sky 
brightness and high airmass.

In James et al. (GCN 14697) we reported that we did not detect an 
afterglow. We conducted follow-up observations two nights later, and on 
reexamination discovered that an extremely red afterglow was in fact 
detected in our first night’s I and R band exposures.
The position of the afterglow is (J2000.0):
RA = 05h 50m 10.55s
Dec = 14d 27m 26.8s
A finder chart is at:
http://skynet.unc.edu/grb/grb130521a_finder.png

A preliminary light curve is at:
http://skynet.unc.edu/grb/grb130521a_2.png

Magnitudes are in the Vega system and are calibrated to 19 APASS stars 
in the field. We regard the V band detection to be questionable, but the 
R and I band detections are solid.

The galactic coordinates of the afterglow are l=193.3312, b=-5.9321. The 
dust emission maps of Schlegel et al. (1998) indicate E(B-V)=0.381 along 
this line of sight; the predicted extinction in each of the bands that 
we observed is:
Filter B V R I
A_lambda 1.65 1.24 1.00 0.75

The observed R band brightness is 1.5 magnitudes fainter than in the I 
band. Correcting for galactic dust extinction, the R band is still 1.25 
magnitudes fainter than in the I band. Such an extremely red afterglow 
could be due to an unusually large amount of dust extinction in the host 
galaxy or to the burst being at a redshift z-5. Follow-up observations 
are encouraged when the source is again visible in early August.

We encourage anyone who may have observed this field, particularly in 
the near infrared, to reexamine their images to see if this afterglow 
was detected in them.

No further Skynet observations are scheduled.