Spectroscopic evidence for a low-mass black hole in SWIFT J1753.5-0127

TL;DR

This study provides spectroscopic evidence suggesting SWIFT J1753.5-0127 hosts a low-mass black hole, with a very short orbital period and a low mass function, challenging previous assumptions about its nature.

Contribution

The paper presents new optical and UV spectroscopic observations revealing a low-mass black hole in SWIFT J1753.5-0127, with a very short orbital period and low mass function, which was previously uncertain.

Findings

Detected broad, double-peaked emission lines in optical and UV spectra.

Measured a radial velocity semi-amplitude of 382 km/s for the donor star.

Identified a possible orbital period of 2.85 hours, shorter than previous reports.

Abstract

The black hole (BH) candidate SWIFT J1753.5-0127 has remained active since the onset of its 2005 outburst. Emission lines in the optical spectrum were observed at the very beginning of the outburst, but since then the spectrum has been featureless making a precise BH mass estimation impossible. Here we present results from our optical and UV observations of SWIFT J1753.5-0127 taken in 2012-2013. Our new observations show extremely broad, double-peaked emission lines in the optical and UV spectra. The optical data also show narrow absorption and emission features with nearly synchronous and significant Doppler motions. A radial velocity study of these lines which we associate with the secondary star, yields a semi-amplitude of K_2=382 km/s. A time-series analysis of the spectral and photometric data revealed a possible orbital periodicity of 2.85 h, significantly shorter than the reported 3.2 h periodic signal by Zurita et al. The observed variability properties argue against a low orbital inclination angle and we present several observational arguments in favour of the BH interpretation. However, the measured radial velocity semi-amplitude of the donor star and the short orbital period imply that SWIFT J1753.5-0127 has one of the lowest measured mass function for a BH in a low-mass X-ray binary. We show that the compact object mass in excess of 5 Msun is highly improbable. Thus, SWIFT J1753.5-0127 is a BH binary that has one of the shortest orbital period and hosts probably one of the smallest stellar-mass BH found to date.