EP250207b is not a collapsar fast X-ray transient. Is it due to a binary compact object merger?

Jonker, P. G.; Levan, A. J.; Liu, Xing; Xu, Dong; Liu, Yuan; Xu, Xinpeng; Li, An; Sarin, N.; Tanvir, N. R.; Lamb, G. P.; Ravasio, M. E.; Sánchez-Sierras, J.; Quirola-Vásquez, J. A.; Rayson, B. C.; van Dalen, J. N. D.; Malesani, D. B.; van Hoof, A. P. C.; Bauer, F. E.; Chacón, J.; Smartt, S. J.; Martin-Carrillo, A.; Corcoran, G.; Cotter, L.; Rossi, A.; Onori, F.; Fraser, M.; O'Brien, P. T.; Eyles-Ferris, R. A. J.; Hjorth, J.; Chen, T.-W.; Leloudas, G.; Tomasella, L.; Schulze, S.; De Pasquale, M.; Carotenuto, F.; Bright, J.; Wang, Chenwei; Xiong, Shaolin; Zhang, Jinpeng; Xue, Wangchen; Liu, Jiacong; Li, Chengkui; Mata Sánchez, D.; Torres, M. A. P.
Referencia bibliográfica

Monthly Notices of the Royal Astronomical Society

Fecha de publicación:
1
2026
Número de autores
44
Número de autores del IAC
2
Número de citas
4
Número de citas referidas
0
Descripción
Fast X-ray transients (FXTs) are short-lived extragalactic X-ray sources. Recent progress through multiwavelength follow-up of Einstein Probe-discovered FXTs has shown that several are related to collapsars, which can also produce $\gamma$-ray bursts (GRBs). In this paper, we investigate the nature of the FXT EP250207b. The Very Large Telescope/Multi Unit Spectroscopic Explorer spectra of a nearby (15.9 kpc in projection) lenticular galaxy reveal no signs of recent star formation. If this galaxy is indeed the host, EP250207b lies at a redshift $z=0.082$, implying a peak observed absolute magnitude for the optical counterpart of ${M_{\mathrm{ r}^\prime }=-14.5}$. At the time when supernovae (SNe) would peak, it is substantially fainter than all SN types. These results are inconsistent with a collapsar origin for EP250207b. The properties favour a binary compact object merger-driven origin. The X-ray, optical, and radio observations are compared with predictions of several types of extragalactic transients, including afterglow and kilonova models. The data can be fitted with a slightly off-axis viewing angle afterglow. However, the late-time (${\sim} 30$ d) optical/near-infrared counterpart is too bright for the afterglow and also for conventional kilonova models. This could be remedied if that late emission is due to a globular cluster or the core of a (tidally disrupted) dwarf galaxy. If confirmed, this would be the first case where the multiwavelength properties of an FXT are found to be consistent with a compact object merger origin, increasing the parallels between FXTs and GRBs. We finally discuss whether the source could originate in a higher redshift host galaxy.