The luminous, slow-rising orphan afterglow AT2019pim as a candidate moderately relativistic outflow

ABSTRACT Classical gamma-ray bursts (GRBs) have two distinct emission episodes: prompt emission from ultrarelativistic ejecta and afterglow from shocked circumstellar material. While both components are extremely luminous in known GRBs, a variety of scenarios predict the existence of luminous afterglow emission with little or no associated high-energy prompt emission. We present AT 2019pim, the first spectroscopically confirmed afterglow with no observed high-energy emission to be identified. Serendipitously discovered during follow-up observations of a gravitational-wave trigger and located in a contemporaneous TESS sector, it is hallmarked by a fast-rising ($t \approx 2$ h), luminous ($M_{\rm UV,peak} \approx -24.4$ mag) optical transient with accompanying luminous X-ray and radio emission. No gamma-ray emission consistent with the time and location of the transient was detected by Fermi-GBM or by Konus, placing constraining limits on an accompanying GRB. We investigate several independent observational aspects of the afterglow in the context of constraints on relativistic motion and find all of them are consistent with an initial Lorentz factor of $\Gamma _0 \approx$ 10–30 for the on-axis material, significantly lower than in any well-observed GRB and consistent with the theoretically predicted ‘dirty fireball’ scenario in which the high-energy prompt emission is stifled by pair production. However, we cannot rule out a structured jet model in which only the line-of-sight material was ejected at low-$\Gamma$, off-axis from a classical high-$\Gamma$ jet core, and an on-axis GRB with below-average gamma-ray efficiency also remains a possibility. This event represents a milestone in orphan afterglow searches, demonstrating that luminous optical afterglows lacking detected GRB counterparts can be identified and spectroscopically confirmed in real time.

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Publication Details

Journal
Monthly Notices of the Royal Astronomical Society
Published
2025-01-22
DOI
https://doi.org/10.1093/mnras/staf125
Citations
13
Primary Topic
Gamma-ray bursts and supernovae
Type
article
Field-Weighted Citation Impact
15.01

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article

The luminous, slow-rising orphan afterglow AT2019pim as a candidate moderately relativistic outflow

Reed Riddle, Bryce Bolin, Shreya Anand, P. G. Jonker et al.
13 citations
Monthly Notices of the Royal Astronomical Society
Gamma-ray bursts and supernovae
15.01
article

The luminous, slow-rising orphan afterglow AT2019pim as a candidate moderately relativistic outflow

Reed Riddle, Bryce Bolin, Shreya Anand, P. G. Jonker, V. Bhalerao, A. J. Levan, A. Corsi, A. Ridnaia, Yuhan Yao, D. Frederiks, A. V. Filippenko, Harsh Kumar, Rahul Jayaraman, G. Waratkar, R. Hamburg, Michael Fausnaugh, D. Svinkin, G. Ricker, Russ R. Laher, S. R. Kulkarni, Gavin P. Lamb, D. A. Perley, S. B. Cenko, Igor Andreoni, Anna Y. Q. Ho, Eric C. Bellm, M. Ulanov, Thomas G. Brink, A. Goldstein, Erik C. Kool, R. Vanderspek, R. A. Perley, B. Rusholme, Tomás Ahumada, Roger M. Smith, M. M. Kasliwal, Eric Burns, Alexandra L. Lysenko
article en
13 citations

Abstract

ABSTRACT Classical gamma-ray bursts (GRBs) have two distinct emission episodes: prompt emission from ultrarelativistic ejecta and afterglow from shocked circumstellar material. While both components are extremely luminous in known GRBs, a variety of scenarios predict the existence of luminous afterglow emission with little or no associated high-energy prompt emission. We present AT 2019pim, the first spectroscopically confirmed afterglow with no observed high-energy emission to be identified. Serendipitously discovered during follow-up observations of a gravitational-wave trigger and located in a contemporaneous TESS sector, it is hallmarked by a fast-rising ($t \approx 2$ h), luminous ($M_{\rm UV,peak} \approx -24.4$ mag) optical transient with accompanying luminous X-ray and radio emission. No gamma-ray emission consistent with the time and location of the transient was detected by Fermi-GBM or by Konus, placing constraining limits on an accompanying GRB. We investigate several independent observational aspects of the afterglow in the context of constraints on relativistic motion and find all of them are consistent with an initial Lorentz factor of $\Gamma _0 \approx$ 10–30 for the on-axis material, significantly lower than in any well-observed GRB and consistent with the theoretically predicted ‘dirty fireball’ scenario in which the high-energy prompt emission is stifled by pair production. However, we cannot rule out a structured jet model in which only the line-of-sight material was ejected at low-$\Gamma$, off-axis from a classical high-$\Gamma$ jet core, and an on-axis GRB with below-average gamma-ray efficiency also remains a possibility. This event represents a milestone in orphan afterglow searches, demonstrating that luminous optical afterglows lacking detected GRB counterparts can be identified and spectroscopically confirmed in real time.

Monthly Notices of the Royal Astronomical SocietyVol. 537(3)
Louisiana State University (US), California Institute of Technology (US), Texas Tech University (US), Centre National de la Recherche Scientifique (FR), Goddard Space Flight Center (US), Universities Space Research Association (US), Radboud University Nijmegen (NL), Stockholm University (SE), Indian Institute of Technology Bombay (IN), University of Washington (US), Cornell University (US), National Radio Astronomy Observatory (US), Université Paris-Saclay (FR), Miller College (US), Joint Space Science Institute (US), Center for Astrophysics Harvard & Smithsonian (US), AlbaNova (SE), Deleted Institution, City University of Seattle (US), Liverpool John Moores University (GB), Massachusetts Institute of Technology (US), University of Maryland, College Park (US), University of California, Berkeley (US), Ioffe Institute (RU)
National Science Foundation, National Aeronautics and Space Administration, W. M. Keck Foundation, California Institute of Technology, San Diego State University, Aspen Center for Physics, University of Washington, National Radio Astronomy Observatory, Pomona College, European Commission, Indo-US Science and Technology Forum, Weizmann Institute of Science, Liverpool John Moores University, Vetenskapsrådet, National Central University, Indian Institute of Technology Bombay, Stockholms Universitet, Max-Planck-Institut für Astronomie, Science and Technology Facilities Council, Deutsches Elektronen-Synchrotron, Science and Engineering Research Board, Los Alamos National Laboratory
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Gamma-ray bursts and supernovae
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