CaltechAUTHORS
Published March 2025 | Version Published
Journal Article Open

Multiwavelength analysis of AT 2023sva: a luminous orphan afterglow with evidence for a structured jet

Creators

  • 1. ROR icon University of Maryland, College Park
  • 2. ROR icon Goddard Space Flight Center
  • 3. ROR icon Liverpool John Moores University
  • 4. ROR icon Cornell University
  • 5. ROR icon Carnegie Mellon University
  • 6. ROR icon Observatoire de la Côte d'Azur
  • 7. ROR icon Laboratoire d'Astrophysique de Marseille
  • 8. ROR icon Stockholm University
  • 9. Trottier Space Institute at McGill, 3550 Rue University, Montreal, Quebec H3A 2A7, Canada
  • 10. ROR icon McGill University
  • 11. ROR icon University of Cambridge
  • 12. ROR icon Ioffe Institute
  • 13. ROR icon Indian Institute of Technology Bombay
  • 14. ROR icon National Centre for Radio Astrophysics
  • 15. ROR icon National Radio Astronomy Observatory
  • 16. ROR icon University of Copenhagen
  • 17. ROR icon Radboud University Nijmegen
  • 18. ROR icon Perimeter Institute
  • 19. ROR icon University of Washington
  • 20. ROR icon Louisiana State University
  • 21. ROR icon University of Turku
  • 22. ROR icon California Institute of Technology
  • 23. ROR icon Indian Institute of Astrophysics
  • 24. ROR icon University of Minnesota
  • 25. ROR icon University of Oxford
  • 26. ROR icon Instituto de Astrofísica de Canarias
  • 27. ROR icon Harvard-Smithsonian Center for Astrophysics
  • 28. ROR icon Infrared Processing and Analysis Center
  • 29. ROR icon Drexel University
  • 30. ROR icon Astronomical Institute
  • 31. ROR icon University of Cagliari

Abstract

We present multiwavelength analysis of ZTF23abelseb (AT 2023sva), an optically discovered fast-fading (⁠Δm_r = 2.2 mag in Δt = 0.74d), luminous (M_r ~ -30.0⁠ mag), and red (⁠g - r = 0.50 mag) transient at z = 2.28 with accompanying luminous radio emission. AT 2023sva does not possess a γ-ray burst (GRB) counterpart to an isotropic equivalent energy limit of  Eγ,iso < 1.6 x 10⁵² erg, determined through searching γ-ray satellite archives between the last non-detection and first detection, making it the sixth example of an optically discovered afterglow with a redshift measurement and no detected GRB counterpart. We analyse AT 2023sva’s optical, radio, and X-ray observations to characterize the source. From radio analyses, we find the clear presence of strong interstellar scintillation (ISS) 72 d after the initial explosion, allowing us to place constraints on the source’s angular size and bulk Lorentz factor. When comparing the source sizes derived from ISS of orphan events to those of the classical GRB population, we find orphan events have statistically smaller source sizes. We also utilize Bayesian techniques to model the multiwavelength afterglow. Within this framework, we find evidence that AT 2023sva possesses a shallow power-law structured jet viewed slightly off-axis (⁠⁠θ_v = 0.07 ± 0.02) just outside of the jet’s core opening angle (⁠⁠θ_c = 0.06 ± 0.02). We determine this is likely the reason for the lack of a detected GRB counterpart, but also investigate other scenarios. AT 2023sva’s evidence for possessing a structured jet stresses the importance of broadening orphan afterglow search strategies to a diverse range of GRB jet angular energy profiles, to maximize the return of future optical surveys.

Copyright and License

© 2025 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.

Acknowledgement

GPS thanks Isiah Holt for useful discussions on nested sampling techniques, Tony Piro for reading the paper on request, and Simi Bhullar for her moral support through out the paper-writing process. AYQH was supported in part by NASA grant number 80NSSC23K1155. MWC acknowledges support from the National Science Foundation with grant numbers PHY-2308862 and PHY-2117997. MBS acknowledges the Finnish Cultural Foundation grant number 00231098 and Finnish Centre for Astronomy with ESO (FINCA) grant. GCA thanks the Indian National Science Academy for support under the INSA Senior Scientist Programme. MMK acknowledges generous support from the David and Lucille Packard Foundation. BO is supported by the McWilliams Postdoctoral Fellowship at Carnegie Mellon University. AT acknowledges the financial support from ‘ASI-INAF Accordo Attuativo HERMES Pathinder operazioni n.2022-25-HH.0’.

SED Machine is based upon work supported by the National Science Foundation under grant no. 1106171. Based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the ZTF project. ZTF is supported by the National Science Foundation under grant no. AST-2034437 and a collaboration including Caltech, IPAC, the Weizmann Institute of Science, the Oskar Klein Center at Stockholm University, the University of Maryland, Deutsches Elektronen-Synchrotron and Humboldt University, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, Trinity College Dublin, Lawrence Livermore National Laboratories, IN2P3, University of Warwick, Ruhr University Bochum and Northwestern University. Operations are conducted by COO, IPAC, and UW. The ZTF forced-photometry service was funded under the Heising–Simons Foundation grant #12540303 (PI: Graham). The Gordon and Betty Moore Foundation, through both the Data-Driven Investigator Programme and a dedicated grant, provided critical funding for SkyPortal.

The GROWTH India Telescope (GIT, Kumar et al. 2022) is a 70-cm telescope with a 0.7-degree FOV, set up by the Indian Institute of Astrophysics (IIA) and the Indian Institute of Technology Bombay (IITB) with funding from DST-SERB and IUSSTF. It is located at the Indian Astronomical Observatory (Hanle), operated by IIA. We acknowledge funding by the IITB alumni batch of 1994, which partially supports the operations of the telescope. Telescope technical details are available at https://sites.google.com/view/growthindia/. CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, URSC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project. This work is partially based on observations made with the Gran Telescopio Canarias (GTC), installed at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, on the island of La Palma. This work was also based on observations made with the Nordic Optical Telescope, owned in collaboration by the University of Turku and Aarhus University, and operated jointly by Aarhus University, the University of Turku and the University of Oslo, representing Denmark, Finland and Norway, the University of Iceland and Stockholm University at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. We thank the staff of the GMRT that made these observations possible. GMRT is run by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research. We thank the staff of the Mullard Radio Astronomy Observatory for their invaluable assistance in the operation of the Arcminute Microkelvin Imager.

Data Availability

All of AT 2023sva’s optical photometry, radio flux densities, and X-ray upper limits are available in this article. We will make the spectrum available in the supplementary material available online.

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Additional details

Related works

Is new version of
Discussion Paper: arXiv:2501.03337 (arXiv)

Funding

National Aeronautics and Space Administration
80NSSC23K1155
National Science Foundation
PHY-2308862
National Science Foundation
PHY-2117997
Finnish Cultural Foundation
00231098
Indian National Science Academy
David and Lucile Packard Foundation
Carnegie Mellon University
National Institute for Astrophysics
2022-25-HH.0
National Science Foundation
1106171
National Science Foundation
AST-2034437
Simons Foundation
12540303
Gordon and Betty Moore Foundation

Dates

Accepted
2025-02-06
Available
2025-02-18
Published
Available
2025-03-04
Corrected and typeset

Caltech Custom Metadata

Caltech groups
Astronomy Department, Infrared Processing and Analysis Center (IPAC), Zwicky Transient Facility, Division of Physics, Mathematics and Astronomy (PMA)
Publication Status
Published