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 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.