TITLE:   GCN CIRCULAR
NUMBER:  29720
SUBJECT: GRB 210323A: Gemini-North Optical Source Detection
DATE:    21/03/24 20:55:08 GMT
FROM:    Jillian Rastinejad at Northwestern Univ.  <jillianrastinejad2024@u.northwestern.edu>

J. Rastinejad, K. Paterson, W. Fong, C. D. Kilpatrick (Northwestern), T. Laskar (U. of Bath), A. Levan (Radboud U.), B. E. Cobb (GWU) report:

We observed the location of the Swift and Fermi short GRB 210323A (Gropp et al., GCN 29699, Hamburg et al., GCN 29709) with the Gemini Multi-object Spectrograph (GMOS) mounted on Gemini-North. We obtained 15x90-sec imaging in r-band at a mid-time of 2021 March 24.637 UT (0.72 days post-burst). Just to the East, outside of the updated XRT position (90% confidence; Evans et al., GCN 29704), we detect faint emission consistent with the putative optical afterglow (Malesani et al., GCN 29703, Pozanenko et al, GCN 29708, de Ugarte Postigo et al., GCN 29717). However, there is also a clear extension from this position to the North, which we propose to be a faint underlying host galaxy.

At present it is not possible to disentangle the contributions of faint afterglow and the underlying host. We measure a position for the source (or source complex) of RA = 21:11:47.34, Dec = +25:22:10.1 (J2000; 0.5'' uncertainty), ~0.7-0.9'' offset from the Mondy and NOT positions.

After calibrating our stacked image to isolated stars detected in the PS1 photometric catalog (Chambers et al., 2016), we perform aperture photometry of the entire extended source, and measure r = 24.6 +/- 0.2 AB mag (not corrected for Milky Way extinction). Compared to the NOT and GTC measurements Malesani et al., GCN 29703; de Ugarte Postigo et al., GCN 29717), this indicates fading of ~1.7 magnitudes over ~9.5 hours, indicating a rapid rate of decay of F~t^-1.86. This decline rate is likely contaminated by the host galaxy and the true decline rate is potentially steeper, consistent with the steep X-ray afterglow decline rate (Beardmore et al., GCN 29705).

Further observations are planned to assess any variability of the source. We thank Gemini staff Joan Font-Serra and Teo Mocnik for the rapid scheduling and execution of these observations.