[vsnet-grb-info 17135] GRB 151021A: Skynet PROMPT-CTIO/DSO17 observations of the optical afterglow

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  18472
SUBJECT: GRB 151021A: Skynet PROMPT-CTIO/DSO17 observations of the optical afterglow
DATE:    15/10/26 12:27:55 GMT
FROM:    Adam S. Trotter at UNC-Chapel Hill/PROMPT/Skynet  <atrotter at physics.unc.edu>

A. Trotter, D. Reichart, J. Haislip, A. Smith, D. Caton, L. Hawkins, J. Moore, N. Frank, M. Maples, E. Johnson, R. Joyner, J. Martin, C. Salemi, J. A. Crain, K. Ivarsen, A. LaCluyze, and M. Nysewander report:

Skynet observed the Swift BAT/XRT localization of GRB 151021A (Melandri et al., GCN 18425, Swift trigger=660671) with with one 16" telescope (PROMPT 5; I band) of the PROMPT array at CTIO, Chile, and with the 17" telescope at the Appalachian State University Dark Sky Observatory (DSO17; B band) in NC, USA. Starting at 2015-10-21 01:31:14 UT and continuing until 03:00 UT (t=2m-91m post-trigger), Skynet took a total of 38 exposures ranging from 10-160s. 

In the PROMPT I-band images, we clearly detect a fading optical afterglow at the position reported by de Ugarte Postigo et al. (GCN 18426), with I~14 at t=2m at I~19 at t=75m. A DSO17 B-band image taken at t=2.2m shows no source to a 3-sigma limit of B>15.7. 

A preliminary light curve is at:
http://www.skynet.unc.edu/grb/grb151021a.png

Limiting magnitudes are in the Vega System, calibrated to 4 APASS DR9 stars in the field.  Magnitudes have not been corrected for the small line-of-sight Milky Way dust extinction, with expected E(B-V)=0.009 (Schlafly & Finkbeiner 2011).

No further Skynet observations are scheduled.