[vsnet-grb-info 9140] GRB 100418A PAIRITEL NIR Observations

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  10627
SUBJECT: GRB 100418A PAIRITEL NIR Observations
DATE:    10/04/20 02:47:30 GMT
FROM:    Adam Morgan at U.C. Berkeley  <qmorgan at gmail.com>

C. R. Klein, A. N. Morgan, D. A. Perley, and J. S. Bloom (UC Berkeley) report:

We observed the location of the optical afterglow of GRB 100418A
(Marshall et al., GCN 10612) with the 1.3m PAIRITEL located at Mt.
Hopkins, Arizona. Observations began at 2010-04-19 07:51:27 UT, ~10.69
hours after the Swift Trigger, under non-ideal sky conditions, and
continued until clouds obscured the sky.  In  mosaics (effective
exposure time of 0.92 hours) taken simultaneously in the J, H, and Ks
filters, we detect the afterglow (Filgas et al., GCN 10617; Updike et
al., GCN 10619; Antonelli et al., GCN 10620; Pearson et al., GCN
10626).

The preliminary photometry yields:

post burst
t_mid (hr) exp.(hr) filt  mag   m_err
11.13      0.92     J     16.8  0.1
11.13      0.92     H     15.9  0.1
11.13      0.92     Ks    15.3  0.1

We note that our J band point is consistent with little to no fading
since the GROND detection ~3.6 hours prior (using J_AB = J_2mass +
0.90).  This is consistent with the shallow decay implied by the R
band magnitudes in this time period reported thus far (Filgas et al.,
GCN 10617; Updike et al., GCN 10619; Antonelli et al., GCN 10620;
Pearson et al., GCN 10626), and with the the non-fading behavior seen
in the UVOT data (Siegel et al., GCN 10625). Further, the XRT light
curve is roughly flat between ~1000s and ~1 day (see
http://astro.berkeley.edu/~nat/Swift/00419797/xrt/swx00419797.html),
indicating a long optical/X-ray plateau phase.  Continued observations
are encouraged.

Intermittent clouds led to highly variable transmission and sky
brightnesses during the PAIRITEL observations. All magnitudes are
given in the Vega system, calibrated to 2MASS. No correction for
Galactic extinction has been made to the above reported values.