TITLE:   GCN CIRCULAR
NUMBER:  31264
SUBJECT: GRB 211211A: MMT/MMIRS Observations Indicate Fading of K-band Source
DATE:    21/12/18 21:26:07 GMT
FROM:    Wen-fai Fong at Northwestern U  <wfong@northwestern.edu>

J. Rastinejad, W. Fong (Northwestern), A. J. Levan, D. B. Malesani (Radboud), J. Jencson, D. Sand (U. Arizona), N. R. Tanvir (Leicester) report:

We observed the location of GRB 211211A (D’Ai et al., GCN 31202) with the MMT and Magellan Infrared Spectrograph (MMIRS) instrument mounted on the MMT 6.5-meter telescope on Mount Hopkins, Arizona. At a mid-time of 2021 December 18.52 UT (~6.9 days post-burst), we obtained 126 x 30 s of K-band imaging at a median airmass of 1.5 and seeing of 0.8 arcsec. A faint K-band source is still detected at the location of the optical afterglow (first reported by Zheng & Fillipenko; GCN 31203). Based on calibration to 2MASS, we estimate a preliminary magnitude for the source of K_AB ~ 24 mag, although we caution that the faintness of the source precludes a precise measure at this time. Visual inspection relative to our Gemini K-band imaging (Levan et al. GCN 31235) is in support of clear fading of the source.

Taken at face value, the K-band source has declined by ~1.5 mag between ~4 and 7 days post-burst, while AT2017gfo only faded by ~0.4 mag in the K band (Villar et al., 2017, ApJL, 851, L21) on these same timescales.

Further observations are planned. We thank Joannah Hinz and ShiAnne Kattner at MMT for the rapid planning and execution of these observations.