Constraining the inclination of the Low-Mass X-ray Binary Cen X-4

TL;DR

This study models the ellipsoidal light curves of Cen X-4 to accurately determine the system's inclination and neutron star mass, improving statistical uncertainties and exploring the effects of accretion disk assumptions.

Contribution

It provides a refined inclination measurement for Cen X-4 using near-IR data and ellipsoidal modeling, accounting for accretion disk parameters, which enhances previous estimates.

Findings

Inclination of 34.9 degrees with uncertainties

Neutron star mass estimated at 1.51 solar masses

Disk-free models suggest higher neutron star mass

Abstract

We present the results of ellipsoidal light curve modeling of the low mass X-ray binary Cen X-4 in order to constrain the inclination of the system and mass of the neutron star. Near-IR photometric monitoring was performed in May 2008 over a period of three nights at Magellan using PANIC. We obtain J, H and K lightcurves of Cen X-4 using differential photometry. An ellipsoidal modeling code was used to fit the phase folded light curves. The lightcurve fit which makes the least assumptions about the properties of the binary system yields an inclination of 34.9 (+4.9, -3.6) degrees, which is consistent with previous determinations of the system's inclination but with improved statistical uncertainties. When combined with the mass function and mass ratio, this inclination yields a neutron star mass of 1.51 (+0.40, -0.55) M_sun. This model allows accretion disk parameters to be free in the fitting process. Fits that do not allow for an accretion disk component in the near-IR flux gives a systematically lower inclination between approximately 33 and 34 degrees, leading to a higher mass neutron star between approximately 1.7 M_sun and 1.8 M_sun. We discuss the implications of other assumptions made during the modeling process as well as numerous free parameters and their effects on the resulting inclination.