EP250207b is not a collapsar fast X-ray transient. Is it due to a binary compact object merger?
/ Authors
P. Jonker, A. Levan, Xing Liu, Dong Xu, Yuan-Qi Liu, Xinpeng Xu, An Li, N. Sarin, N. Tanvir, G. Lamb
and 34 more authors
M. Ravasio, J. S'anchez-Sierras, J. Quirola-V'asquez, B. Rayson, J. V. Dalen, D. Malesani, A. V. Hoof, F. E. Bauer, J. Chac'on, S. Smartt, A. Martin-Carrillo, G. Corcoran, L. Cotter, A. Rossi, F. Onori, M. Fraser, P. O’Brien, R. Eyles-Ferris, J. Hjorth, T.-W. Chen, G. Leloudas, L. Tomasella, S. Schulze, M. D. Pasquale, F. Carotenuto, J. Bright, Chenwei Wang, S. Xiong, Jin-Peng Zhang, Wang-Chen Xue, Jia-Cong Liu, Chengkui Li, D. M. Sánchez, M. Torres
/ Abstract
Fast X-ray Transients (FXTs) are short-lived extra-galactic X-ray sources. Recent progress through multi-wavelength follow-up of Einstein Probe discovered FXTs has shown that several are related to collapsars, which can also produce γ-ray bursts (GRBs). In this paper we investigate the nature of the FXT EP250207b. The VLT/MUSE 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 of z = 0.082, implying a peak observed absolute magnitude for the optical counterpart of $\mathrm{M_{r^\prime }=-14.5}$. 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 extra-galactic transients, including afterglow and kilonova models. The data can be fit with a slightly off-axis viewing angle afterglow. However, the late-time (∼30 day) optical/NIR 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 multi-wavelength 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 if the source could originate in a higher redshift host galaxy.
Journal: Monthly Notices of the Royal Astronomical Society