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 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.