AT2025ulz and S250818k: Deep X-ray and radio limits on off-axis afterglow emission and prospects for future discovery
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| Format: | Preprint |
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2025
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| _version_ | 1866912675180576768 |
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| author | O'Connor, Brendan Ricci, Roberto Troja, Eleonora Palmese, Antonella Yang, Yu-Han Ryan, Geoffrey van Eerten, Hendrik Yadav, Muskan Hall, Xander J. Amsellem, Ariel Becerra, Rosa L. Busmann, Malte Cabrera, Tomas Dichiara, Simone Hu, Lei Kaur, Ravjit Kunnumkai, Keerthi Hernandez, Ignacio Magana |
| author_facet | O'Connor, Brendan Ricci, Roberto Troja, Eleonora Palmese, Antonella Yang, Yu-Han Ryan, Geoffrey van Eerten, Hendrik Yadav, Muskan Hall, Xander J. Amsellem, Ariel Becerra, Rosa L. Busmann, Malte Cabrera, Tomas Dichiara, Simone Hu, Lei Kaur, Ravjit Kunnumkai, Keerthi Hernandez, Ignacio Magana |
| contents | The first joint electromagentic (EM) and gravitational wave (GW) detection, known as GW170817, marked a critical juncture in our collective understanding of compact object mergers. However, it has now been 8 years since this discovery, and the search for a second EM-GW detection has yielded no robust discoveries. Recently, on August 18, 2025, the LIGO-Virgo-KAGRA collaboration reported a low-significance (high false alarm rate) binary neutron star merger candidate S250818k. Rapid optical follow-up revealed a single optical candidate AT2025ulz ($z=0.08484$) that initially appeared consistent with kilonova emission. We quickly initiated a set of observations with \textit{Swift}, \textit{XMM-Newton}, \textit{Chandra}, and the Very Large Array to search for non-thermal afterglow emission. Our deep X-ray and radio search rules out that the optical rebrightening of AT2025ulz is related to the afterglow onset, reinforcing its classification as a stripped-envelope supernova (SN 2025ulz). We derive constraints on the afterglow parameters for a hypothetical binary neutron star merger at the distance of AT2025ulz ($\approx 400$ Mpc) based on our X-ray and radio limits. We conclude that our observational campaign could exclude a GW170817-like afterglow out to viewing angles of $θ_\textrm{v}\approx 12.5$ degrees. We briefly discuss the prospects for the future discovery of off-axis afterglows. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2510_23728 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | AT2025ulz and S250818k: Deep X-ray and radio limits on off-axis afterglow emission and prospects for future discovery O'Connor, Brendan Ricci, Roberto Troja, Eleonora Palmese, Antonella Yang, Yu-Han Ryan, Geoffrey van Eerten, Hendrik Yadav, Muskan Hall, Xander J. Amsellem, Ariel Becerra, Rosa L. Busmann, Malte Cabrera, Tomas Dichiara, Simone Hu, Lei Kaur, Ravjit Kunnumkai, Keerthi Hernandez, Ignacio Magana High Energy Astrophysical Phenomena The first joint electromagentic (EM) and gravitational wave (GW) detection, known as GW170817, marked a critical juncture in our collective understanding of compact object mergers. However, it has now been 8 years since this discovery, and the search for a second EM-GW detection has yielded no robust discoveries. Recently, on August 18, 2025, the LIGO-Virgo-KAGRA collaboration reported a low-significance (high false alarm rate) binary neutron star merger candidate S250818k. Rapid optical follow-up revealed a single optical candidate AT2025ulz ($z=0.08484$) that initially appeared consistent with kilonova emission. We quickly initiated a set of observations with \textit{Swift}, \textit{XMM-Newton}, \textit{Chandra}, and the Very Large Array to search for non-thermal afterglow emission. Our deep X-ray and radio search rules out that the optical rebrightening of AT2025ulz is related to the afterglow onset, reinforcing its classification as a stripped-envelope supernova (SN 2025ulz). We derive constraints on the afterglow parameters for a hypothetical binary neutron star merger at the distance of AT2025ulz ($\approx 400$ Mpc) based on our X-ray and radio limits. We conclude that our observational campaign could exclude a GW170817-like afterglow out to viewing angles of $θ_\textrm{v}\approx 12.5$ degrees. We briefly discuss the prospects for the future discovery of off-axis afterglows. |
| title | AT2025ulz and S250818k: Deep X-ray and radio limits on off-axis afterglow emission and prospects for future discovery |
| topic | High Energy Astrophysical Phenomena |
| url | https://arxiv.org/abs/2510.23728 |