Discovery of the Optical Afterglow and Host Galaxy of Short GRB 181123B at z = 1.754: Implications for Delay Time Distributions

K. Paterson, W. Fong, A. Nugent, A. Rouco Escorial, J. Leja, T. Laskar, R. Chornock, A.~A. Miller, J. Scharwächter, S.~B. Cenko, D. Perley, N.~R. Tanvir, A. Levan, A. Cucchiara, B.~E. Cobb, K. De, E. Berger, G. Terreran, K.~D. Alexander, M. NichollP.~K. Blanchard, D. Cornish

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5 Citations (Scopus)


We present the discovery of the optical afterglow and host galaxy of the Swift short-duration gamma-ray burst (SGRB) GRB 181123B. Observations with Gemini-North starting ≈9.1 hr after the burst reveal a faint optical afterglow with i ≈ 25.1 mag at an angular offset of 0.″59 0.″16 from its host galaxy. Using grizYJHK observations, we measure a photometric redshift of the host galaxy of z=1.77-0.17+0.30. From a combination of Gemini and Keck spectroscopy of the host galaxy spanning 4500-18000 Å, we detect a single emission line at 13390 Å, inferred as Hβ at z = 1.754 0.001 and corroborating the photometric redshift. The host galaxy properties of GRB 181123B are typical of those of other SGRB hosts, with an inferred stellar mass of ≈9.1 109 M o˙, a mass-weighted age of ≈0.9 Gyr, and an optical luminosity of ≈0.9L∗. At z = 1.754, GRB 181123B is the most distant secure SGRB with an optical afterglow detection and one of only three at z > 1.5. Motivated by a growing number of high-z SGRBs, we explore the effects of a missing z > 1.5 SGRB population among the current Swift sample on delay time distribution (DTD) models. We find that lognormal models with mean delay times of ≈4-6 Gyr are consistent with the observed distribution but can be ruled out to 95% confidence, with an additional ≈one to five Swift SGRBs recovered at z > 1.5. In contrast, power-law models with ∝t -1 are consistent with the redshift distribution and can accommodate up to ≈30 SGRBs at these redshifts. Under this model, we predict that ≈1/3 of the current Swift population of SGRBs is at z > 1. The future discovery or recovery of existing high-z SGRBs will provide significant discriminating power on their DTDs and thus their formation channels.

Original languageEnglish
Article numberL32
Pages (from-to)L32
JournalThe Astrophysical Journal Letters
Issue number2
Publication statusPublished - 1 Aug 2020

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  • Gamma-ray bursts
  • 629
  • Astrophysics - High Energy Astrophysical Phenomena


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