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

¹ Instituto de Astrofísica de Canarias, E-38205 La Laguna, Tenerife, Spain
² Departamento de Astrofísica, Univ. de La Laguna, E-38206 La Laguna, Tenerife, Spain

^⋆ Corresponding author; dmata@iac.es

Received: 22 August 2024
Accepted: 12 November 2024

Abstract

The X-ray transient Swift J1727.8−1613 ended its 10-month discovery outburst in 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 detected the companion star and constructed its radial velocity curve, yielding a binary orbital period of P_orb = 10.8038 ± 0.0010 h and a radial velocity semi-amplitude of K₂ = 390 ± 4 km s⁻¹. This results in a mass function of f(M₁) = 2.77 ± 0.09 M_⊙. Combined with constraints on the binary inclination, it sets a lower limit on the compact object mass of M₁ > 3.12 ± 0.10 M_⊙, dynamically confirming the black hole nature of the accretor. A comparison of the average spectrum in the rest frame of the companion with synthetic stellar templates supports a K4V donor that is partially (74%) veiled by the accretion disc. A refined distance measurement of 3.4 ± 0.3 kpc, together with the astrometric proper motion and the systemic velocity derived from the radial velocity curve (γ = 181 ± 4 km s⁻¹), supports a natal kick velocity of v_kick = 210₋₅₀⁺⁴⁰ km s⁻¹, at the upper end of the observed distribution.