A new gamma-ray loud, eclipsing low-mass X-ray binary

TL;DR

This paper reports the discovery and detailed characterization of an eclipsing low-mass X-ray binary with a neutron star, exhibiting gamma-ray emission and eclipses, providing insights into gamma-ray origins in accreting neutron stars.

Contribution

It presents the first detailed study of an eclipsing gamma-ray loud low-mass X-ray binary with multi-wavelength observations and evidence for a massive neutron star primary.

Findings

Discovered an eclipsing low-mass X-ray binary with a neutron star primary.

Detected X-ray and tentative gamma-ray eclipses at the same phase.

Identified spectral features indicating an accretion disk and a massive neutron star.

Abstract

We report the discovery of an eclipsing low-mass X-ray binary at the center of the 3FGL error ellipse of the unassociated Fermi/Large Area Telescope gamma-ray source 3FGL J0427.9-6704. Photometry from OGLE and the SMARTS 1.3-m telescope and spectroscopy from the SOAR telescope have allowed us to classify the system as an eclipsing low-mass X-ray binary (P = 8.8 hr) with a main sequence donor and a neutron star accretor. Broad double-peaked H and He emission lines suggest the ongoing presence of an accretion disk. Remarkably, the system shows shows separate sets of absorption lines associated with the accretion disk and the secondary, and we use their radial velocities to find evidence for a massive (~ 1.8-1.9 M_sun) neutron star primary. In addition to a total X-ray eclipse of duration ~ 2200 s observed with NuSTAR, the X-ray light curve also shows properties similar to those observed among known transitional millisecond pulsars: short-term variability, a hard power-law spectrum (photon index ~ 1.7), and a comparable 0.5-10 keV luminosity (~ 2.4 x 10^33 erg/s). We find tentative evidence for a partial (~ 60%) gamma-ray eclipse at the same phase as the X-ray eclipse, suggesting the gamma-ray emission may not be confined to the immediate region of the compact object. The favorable inclination of this binary is promising for future efforts to determine the origin of gamma-rays among accreting neutron stars.