Optical/infrared observations of the extraordinary GRB 250702B: a highly obscured afterglow in a massive galaxy consistent with multiple possible progenitors

Jonathan Carney; Igor Andreoni; Brendan O'Connor; James Freeburn; Hannah Skobe; Lewi Westcott; Malte Busmann; Antonella Palmese; Xander J. Hall; Ramandeep Gill; Paz Beniamini; Eric R. Coughlin; Charles D. Kilpatrick; Akash Anumarlapudi; Nicholas M. Law; Hank Corbett; Tomas Ahumada; Ping Chen; Christopher Conselice; Guillermo Damke; Kaustav K. Das; Avishay Gal-Yam; Daniel Gruen; Steve Heathcote; Lei Hu; Viraj Karambelkar; Mansi Kasliwal; Kathleen Labrie; Dheeraj Pasham; Arno Riffeser; Michael Schmidt; Kritti Sharma; Silona Wilke; Weicheng Zang

arXiv · 2509.22784

Optical/infrared observations of the extraordinary GRB 250702B: a highly obscured afterglow in a massive galaxy consistent with multiple possible progenitors

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Abstract

GRB 250702B was the longest gamma-ray burst ever detected, with a duration that challenges standard collapsar models and suggests an exotic progenitor. We collected a rich set of optical and infrared follow-up observations of its rapidly fading afterglow using a suite of telescopes including the W. M. Keck Observatory, the Gemini telescopes, the Magellan Baade Telescope, the Victor M. Blanco 4-meter telescope, and the Fraunhofer Telescope at Wendelstein Observatory. Our analysis reveals that the afterglow emission is well described by forward shock emission from a highly obscured relativistic jet. Deep photometric observations of the host galaxy reveal a massive (10^10.66 solar masses), dusty, and extremely asymmetric system that is consistent with two galaxies undergoing a major merger. The galactocentric offset, host galaxy properties, and jet characteristics disfavor a jetted TDE around a supermassive black hole but do not definitively distinguish between competing progenitor scenarios. We find that the afterglow and host are consistent with a range of progenitors including an atypical collapsar, a merger between a helium star and a stellar mass black hole, the disruption of a star by a stellar mass compact object (micro-TDE), and the tidal disruption of a star by an off-nuclear intermediate mass black hole.

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Jonathan Carney, Igor Andreoni, Brendan O'Connor, James Freeburn, Hannah Skobe, Lewi Westcott, Malte Busmann, Antonella Palmese, Xander J. Hall, Ramandeep Gill, Paz Beniamini, Eric R. Coughlin, Charles D. Kilpatrick, Akash Anumarlapudi, Nicholas M. Law, Hank Corbett, Tomas Ahumada, Ping Chen, Christopher Conselice, Guillermo Damke, Kaustav K. Das, Avishay Gal-Yam, Daniel Gruen, Steve Heathcote, Lei Hu, Viraj Karambelkar, Mansi Kasliwal, Kathleen Labrie, Dheeraj Pasham, Arno Riffeser, Michael Schmidt, Kritti Sharma, Silona Wilke, Weicheng Zang. 2025-09-26. Optical/infrared observations of the extraordinary GRB 250702B: a highly obscured afterglow in a massive galaxy consistent with multiple possible progenitors. https://arxiv.org/abs/2509.22784

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