A 420 day X-ray/optical modulation and extended X-ray dips in the short-period transient Swift J1753.5-0127

TL;DR

This paper reports the discovery of a 420-day modulation with X-ray dips in the short-period black-hole transient Swift J1753.5-0127, suggesting long-term accretion disc precession and providing insights into its structure and nature.

Contribution

It presents the first detection of a long-term periodicity and associated dips in Swift J1753.5-0127, indicating disc precession in a high-latitude, short-period black-hole transient.

Findings

A 420-day modulation with X-ray dips was discovered.

The modulation appears after a 3-year rebrightening phase.

The periodicity suggests disc precession or related phenomena.

Abstract

We have discovered a \sim420d modulation, with associated X-ray dips, in RXTE-ASM/MAXI/Swift-BAT archival light-curves of the short-period (3.2h) black-hole X-ray transient, Swift J1753.5-0127. This modulation only appeared at the end of a gradual rebrightening, approximately 3 years after the initial X-ray outburst in mid-2005. The same periodicity is present in both the 2-20 keV and 15-50 keV bands, but with a \sim0.1 phase offset (\sim40d). Contemporaneous photometry in the optical and near-IR reveals a weaker modulation, but consistent with the X-ray period. There are two substantial X-ray dips (very strong in the 15-50 keV band, weaker at lower energies) that are separated by an interval equal to the X-ray period. This likely indicates two physically separated emitting regions for the hard X-ray and lower energy emission. We interpret this periodicity as a property of the accretion disc, most likely a long-term precession, where the disc edge structure and X-ray irradiation is responsible for the hard X-ray dips and modulation, although we discuss other possible explanations, including Lense-Thirring precession in the inner disc region and spectral state variations. Such precession indicates a very high mass ratio LMXB, which even for a \sim10M_sun BH requires a brown dwarf donor (\sim0.02M_sun), making Swift J1753.5-0127 a possible analogue of millisecond X-ray pulsars.We compare the properties of Swift J1753.5-0127 with other recently discovered short-period transients, which are now forming a separate population of high latitude BH transients located in the galactic halo.