Discovery of Eclipses from the Accreting Millisecond X-ray Pulsar SWIFT J1749.4-2807

TL;DR

This paper reports the first detection of X-ray eclipses in the accreting millisecond X-ray pulsar Swift J1749.4-2807, enabling precise orbital and mass measurements of the neutron star and its companion.

Contribution

It presents the discovery of X-ray eclipses in a millisecond pulsar system and provides a detailed timing solution for orbital parameters and companion mass constraints.

Findings

Eclipse duration is 2172 +/- 13 seconds.

Eclipses are symmetric about 90 degrees from the ascending node.

Companion mass estimated between 0.6-0.8 solar masses.

Abstract

We report the discovery of X-ray eclipses in the recently discovered accreting millisecond X-ray pulsar Swift J1749.4-2807. This is the first detection of X-ray eclipses in a system of this type and should enable a precise neutron star mass measurement once the companion star is identified and studied. We present a combined pulse and eclipse timing solution that enables tight constraints on the orbital parameters and inclination and shows that the companion mass is in the range 0.6-0.8 M_sun for a likely range of neutron star masses, and that it is larger than a main sequence star of the same mass. We observed two individual eclipse egresses and a single ingress. Our timing model shows that the eclipse features are symmetric about the time of 90 deg longitude from the ascending node, as expected. Our eclipse timing solution gives an eclipse duration (from the mid-points of ingress to egress) of 2172 +/- 13 s. This represents 6.85% of the 8.82 hr orbital period. This system also presents a potential measurement of "Shapiro" delay due to General Relativity; through this technique alone, we set an upper limit to the companion mass of 2.2 M_sun.