Dynamical confirmation of a black hole in the X-ray transient Swift J1727.8-1613

TL;DR

This study dynamically confirms the presence of a black hole in the X-ray transient Swift J1727.8-1613 through optical spectroscopy, orbital analysis, and mass estimation, providing insights into its properties and origin.

Contribution

First dynamical confirmation of a black hole in Swift J1727.8-1613 using optical spectroscopy and radial velocity measurements.

Findings

Binary orbital period of approximately 10.8 hours.

Lower limit on black hole mass of about 3.12 solar masses.

Distance of approximately 3.7 kpc and a natal kick velocity of 220 km/s.

Abstract

The X-ray transient Swift J1727.8-1613 ended its 10-month discovery outburst on June of 2024, when it reached an optical brightness comparable to pre-discovery magnitudes. With the aim of performing a dynamical study, we launched an optical spectroscopy campaign with the GTC telescope. We detect the companion star and construct its radial velocity curve, yielding a binary orbital period of Porb = 10.8038 +- 0.0010 h and a radial velocity semi-amplitude of K2 = 390 +- 4 km/s. This results in a mass function of f(M1)=2.77 +- 0.09 Msun. Combined with constraints on the binary inclination, it sets a lower limit to the compact object mass of M1 > 3.12 +- 0.10 Msun, dynamically confirming the black hole nature of the accretor. Comparison of the average spectrum in the rest frame of the companion with synthetic stellar templates supports a K4V donor partially veiled (74%) by the accretion disc. A refined distance measurement of 3.7+- 0.3 kpc, together with the astrometric proper motion and the systemic velocity derived from the radial velocity curve (-181 +-4 km/s), supports a natal kick velocity of 220 +40 -50 km/s, at the upper end of the observed distribution.