Constraints on Millimeter-wave Extragalactic Transient Populations from 4 Years of SPT-3G data

Abstract

Wide-field cosmic microwave background (CMB) instruments have reached sufficient sensitivity and cadence to conduct untargeted millimeter-wave searches for astronomical transients. To date, such searches have primarily discovered stellar flares, while extragalactic detections remain limited to a few flaring blazars and active galactic nuclei. In contrast, recent simulations suggest that current and future CMB experiments should detect a few to dozens of extragalactic transients per year from sources including gamma-ray bursts (GRBs), tidal disruption events, and fast blue optical transients. At timescales under one week, predicted detections are dominated by flares from reverse shocks in long-duration GRBs, with some benchmark models predicting rates of ~10 detections per year for the SPT-3G camera on the South Pole Telescope. In this work, we conduct a transient search optimized for the bright and brief millimeter-wave emission from GRB reverse shocks. We update previous simulations by incorporating a more realistic model of the SPT-3G sensitivity, observing cadence, and detection algorithm, resulting in nearly a factor of two improvement in detection sensitivity compared to prior estimates. We subsequently carry out a search over four seasons of the 1500 deg2 SPT-3G Main field observations from 2019-22, finding no candidate extragalactic events. We use the lack of detections to constrain the parameters of the synthesized GRB populations, setting upper limits on the fraction of GRBs with reverse shock components, f_RS, and the all-sky rate of reverse shocks beamed at Earth, N_all-sky. For a standard 10^51 erg GRB with Lorentz factor Gamma = 100, we constrain f_RS <0.57 and N_all-sky <1226 bursts/year (at 90% confidence). The most readily detectable model we consider is a 10^52 erg "dirty" GRB with Gamma = 50, whose population we constrain to f_RS <0.09 and N_all-sky <120 bursts/year.