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.
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BibTeX
@misc{guns2026constraints,
title = {{Constraints on Millimeter-wave Extragalactic Transient Populations from 4 Years of SPT-3G data}},
author = {S. Guns and C. Tandoi and D. Anbajagane and E. Anderes and A. J. Anderson and B. Ansarinejad and M. Archipley and L. Balkenhol and D. R. Barron and K. Benabed and A. N. Bender and B. A. Benson and F. Bianchini and L. E. Bleem and S. Bocquet and F. R. Bouchet and E. Camphuis and M. G. Campitiello and J. E. Carlstrom and J. Carron and C. L. Chang and P. M. Chichura and A. Chokshi and T. -L. Chou and A. Coerver and T. M. Crawford and C. Daley and T. de Haan and K. R. Dibert and M. A. Dobbs and M. Doohan and D. Dutcher and T. Eftekhari and C. Feng and K. R. Ferguson and N. C. Ferree and K. Fichman and A. Foster and S. Galli and A. E. Gambrel and A. K. Gao and F. Ge and F. Guidi and N. W. Halverson and E. Hivon and A. Y. Q. Ho and G. P. Holder and W. L. Holzapfel and J. C. Hood and A. Hryciuk and N. Huang and T. Jhaveri and F. Kéruzoré and A. R. Khalife and L. Knox and K. Kornoelje and C. -L. Kuo and T. Laskar and K. Levy and Y. Li},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.28633},
eprint = {2610.28633},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.28633},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Constraints on Millimeter-wave Extragalactic Transient Populations from 4 Years of SPT-3G data
AU - S. Guns
AU - C. Tandoi
AU - D. Anbajagane
AU - E. Anderes
AU - A. J. Anderson
AU - B. Ansarinejad
AU - M. Archipley
AU - L. Balkenhol
AU - D. R. Barron
AU - K. Benabed
AU - A. N. Bender
AU - B. A. Benson
AU - F. Bianchini
AU - L. E. Bleem
AU - S. Bocquet
AU - F. R. Bouchet
AU - E. Camphuis
AU - M. G. Campitiello
AU - J. E. Carlstrom
AU - J. Carron
AU - C. L. Chang
AU - P. M. Chichura
AU - A. Chokshi
AU - T. -L. Chou
AU - A. Coerver
AU - T. M. Crawford
AU - C. Daley
AU - T. de Haan
AU - K. R. Dibert
AU - M. A. Dobbs
AU - M. Doohan
AU - D. Dutcher
AU - T. Eftekhari
AU - C. Feng
AU - K. R. Ferguson
AU - N. C. Ferree
AU - K. Fichman
AU - A. Foster
AU - S. Galli
AU - A. E. Gambrel
AU - A. K. Gao
AU - F. Ge
AU - F. Guidi
AU - N. W. Halverson
AU - E. Hivon
AU - A. Y. Q. Ho
AU - G. P. Holder
AU - W. L. Holzapfel
AU - J. C. Hood
AU - A. Hryciuk
AU - N. Huang
AU - T. Jhaveri
AU - F. Kéruzoré
AU - A. R. Khalife
AU - L. Knox
AU - K. Kornoelje
AU - C. -L. Kuo
AU - T. Laskar
AU - K. Levy
AU - Y. Li
PY - 2026
DA - 2026/10/20
PB - arXiv
DO - 10.48550/arxiv.2610.28633
UR - https://arxiv.org/abs/2610.28633
AB - 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.
ER -