TITLE:   GCN CIRCULAR
NUMBER:  25583
SUBJECT: GRB 190829A: MMT detection of the optical afterglow
DATE:    19/08/30 23:41:32 GMT
FROM:    Wen-fai Fong at Northwestern U  <wfong@northwestern.edu>

W. Fong (Northwestern), T. Laskar (U. Bath), G. Schroeder, D. Coppejans and R. Margutti (Northwestern) report:

"We observed the location of the GRB 190829A (Fermi Collab. et al., GCN 25551; Dichiara et al., GCN 25552) with the MMTCam mounted on the MMT 6.5-meter telescope on Mount Hopkins, Arizona. We obtained 15x60-sec each in g- and r-bands at a mid-time of 2019 August 30.493 UT (15.89 hr post-burst). The observations have a mean airmass of 1.03, 1.0" seeing, and were taken in thin, variable clouds. We detect the optical afterglow (Xu et al., GCN 25555; Lipunov et al., GCN 25558; Kumar et al., GCN 25560; Heintz et al., GCN 25563; Valeev et al., GCN 25565; Chen et al., GCN 25569; Oates et al., GCN 25570; Zheng & Filippenko, GCN 25580) with a preliminary magnitude of r~20.0 +/- 0.1 AB mag, not corrected for Galactic extinction and calibrated to Pan-STARRS1. Our measurement is considerably fainter than the KAIT measurement at a similar epoch (Zheng & Filippenko, GCN 25580), likely attributable to the difference in filter, and substantial but uncertain host galaxy contribution.

Compared to earlier r-band observations of the afterglow at dt > 1.3 hr, we measure an optical flux decline rate of F~t^-1.3. This is consistent with the measured Swift/XRT afterglow decline rate on these timescales (https://www.swift.ac.uk/xrt_live_cat/00922968/).

Further observations are planned. We thank the MMT staff, and in particular ShiAnne Kattner and Nelson Caldwell, for their assistance with planning and executing these observations."