Jonathan Carney, Igor Andreoni, Brendan O’Connor, James Freeburn, Hannah Skobe, Lewi Westcott, Malte Busmann, A. Palmese, Xander J. Hall, Ramandeep Gill, Paz Beniamini, Eric R. Coughlin, C. D. Kilpatrick, Akash Anumarlapudi, Nicholas M. Law, Hank Corbett, Tomás Ahumada, Ping Chen, Christopher J. Conselice, G. Damke, Kaustav K. Das, A. Gal‐Yam, D. Gruen, Steve Heathcote, Lei Hu, Viraj Karambelkar, M. M. Kasliwal, Kathleen Labrie, Dheeraj R. Pasham, A. Riffeser, M. J. Schmidt, Kritti Sharma, Silona Wilke, Weicheng Zang
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 m 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 M ⊙ ), 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 tidal disruption event (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.