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

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.

6 (+2 appendix) pages, 3 (+1) figures, 2 tables. Submitted to A&A on the 22th of August 2024, accepted on the 12th of November 2024. Updated final distance values after fixing a bug affecting the spectroscopic distance determination

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Cited by in corpus (8)

• A broadband spectral-timing study of QPOs in the bright black hole X-ray binary Swift J1727.8-1613
• Timing analysis of the black-hole candidate Swift J1727.8-1613: detection of a dip-like feature in the high-energy cross spectrum
• Comprehensive Radio Monitoring of the Black Hole X-ray Binary Swift J1727.8$-$1613 during its 2023$-$2024 Outburst
• Black-hole X-ray binary Swift J1727.8$-$1613 shows simultaneous Type-B and Type-C quasi-periodic oscillations across the hard-intermediate and soft-intermediate states
• Ubiquitous yet forgotten: broad absorptions in the optical spectra of low-mass X-ray binaries
• Characteristics of the High-frequency Humps in the Black hole X-ray Binary Swift J1727.8--1613
• An Ejection Event Captured by VLBI During the Outburst of Swift J1727.8$-$1613
• Optical polarimetry of the accreting black hole X-ray binary Swift J1727.8$-$1613 over the state transition and radio ejections