Dips and eclipses in the X-ray binary Swift J1858.6-0814 observed with NICER

TL;DR

This paper reports the discovery of eclipses and absorption dips in the X-ray binary Swift J1858.6-0814, constraining its orbital period, inclination, and variability characteristics, revealing insights into its accretion dynamics.

Contribution

First detection of eclipses in Swift J1858.6-0814, providing key orbital parameters and evidence for variable obscuration affecting its X-ray variability.

Findings

Orbital period of approximately 21.3 hours.

Inclination angle greater than 70 degrees.

Flares occur mainly post-eclipse due to obscuration effects.

Abstract

We present the discovery of eclipses in the X-ray light curves of the X-ray binary Swift J1858.6-0814. From these, we find an orbital period of $P=76841.3_{-1.4}^{+1.3}$ s ($\approx21.3$ hours) and an eclipse duration of $t_{\rm ec}=4098_{-18}^{+17}$ s ($\approx1.14$ hours). We also find several absorption dips during the pre-eclipse phase. From the eclipse duration to orbital period ratio, the inclination of the binary orbit is constrained to $i>70^\circ$. The most likely range for the companion mass suggests that the inclination is likely to be closer to this value than $90^\circ$. The eclipses are also consistent with earlier data, in which strong variability ('flares') and the long orbital period prevent clear detection of the period or eclipses. We also find that the bright flares occurred preferentially in the post-eclipse phase of the orbit, likely due to increased thickness at the disc-accretion stream interface preventing flares being visible during the pre-eclipse phase. This supports the notion that variable obscuration is responsible for the unusually strong variability in Swift J1858.6-0814.