Optical Photometry and Spectroscopy of the Accretion-Powered Millisecond Pulsar HETE J1900.1-2455

TL;DR

This study provides detailed optical observations of the accreting millisecond pulsar HETE J1900.1-2455, revealing superhump-induced modulation and constraining system parameters such as secondary mass and orbital inclination.

Contribution

It presents phase-resolved optical photometry and spectroscopy, identifying superhumps and deriving system parameters for HETE J1900.1-2455 for the first time.

Findings

Superhumps likely cause the observed optical modulation.

Estimated secondary mass is approximately 0.085 solar masses.

Orbital inclination is constrained to be less than 20 degrees.

Abstract

We present phase resolved optical photometry and spectroscopy of the accreting millisecond pulsar HETE J1900.1-2455. Our R-band light curves exhibit a sinusoidal modulation, at close to the orbital period, which we initially attributed to X-ray heating of the irradiated face of the secondary star. However, further analysis reveals that the source of the modulation is more likely due to superhumps caused by a precessing accretion disc. Doppler tomography of a broad Halpha emission line reveals an emission ring, consistent with that expected from an accretion disc. Using the velocity of the emission ring as an estimate for the projected outer disc velocity, we constrain the maximum projected velocity of the secondary to be 200 km/s, placing a lower limit of 0.05 Msun on the secondary mass. For a 1.4 Msun primary, this implies that the orbital inclination is low, < 20 degrees. Utilizing the observed relationship between the secondary mass and orbital period in short period cataclysmic variables, we estimate the secondary mass to be ~0.085 Msun, which implies an upper limit of ~2.4 Msun for the primary mass.