Constraining the inclination of the low-mass X-ray binary Cen X-4

We present the results of ellipsoidal light-curve modelling of the low-mass X-ray binaryCen X-4 in order to constrain the inclination of the system and mass of the neutron star. Near-IR photometric monitoring was performed in 2008 May over a period of three nights atMagellan using PANIC. We obtain J, H, and K light curves of Cen X-4 using differential photometry. An ellipsoidal modelling code was used to fit the phase folded light curves. The light-curve fit that makes the least assumptions about the properties of the binary system yields an inclination of 34.9(-3.6)(+4.9) deg (1 sigma), 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(-0.55)(+0.40) M-circle dot. This model allows accretion disc parameters to be free in the fitting process. Fits that do not allow for an accretion disc component in the near-IR flux give a systematically lower inclination between approximately 33 and 34 deg, leading to a higher mass neutron star between approximately 1.7 and 1.8M(circle dot). We discuss the implications of other assumptions made during the modelling process as well as numerous free parameters and their effects on the resulting inclination.