The nature of the X-ray transient MAXI J0556-332

TL;DR

This study investigates the orbital characteristics and composition of the X-ray transient MAXI J0556-332, revealing narrow emission lines, possible orbital periods, and a likely neutron star nature with high nitrogen abundance.

Contribution

It provides the first phase-resolved spectroscopy of MAXI J0556-332, identifies potential orbital periods, and constrains the system parameters suggesting a neutron star rather than a black hole.

Findings

Two candidate orbital periods identified (16.43 and 9.754 hrs)

System parameters consistent with a neutron star primary

High N/O abundance indicated by emission lines

Abstract

Phase-resolved spectroscopy of the newly discovered X-ray transient MAXI J0556-332 has revealed the presence of narrow emission lines in the Bowen region that most likely arise on the surface of the mass donor star in this low mass X-ray binary. A period search of the radial velocities of these lines provides two candidate orbital periods (16.43+/-0.12 and 9.754+/-0.048 hrs), which differ from any potential X-ray periods reported. Assuming that MAXI J0556-332 is a relatively high inclination system that harbors a precessing accretion disk in order to explain its X-ray properties, it is only possible to obtain a consistent set of system parameters for the longer period. These assumptions imply a mass ratio of q~0.45, a radial velocity semi-amplitude of the secondary of K_2~190 km/s and a compact object mass of the order of the canonical neutron star mass, making a black hole nature for MAXI J0556-332 unlikely. We also report the presence of strong N III emission lines in the spectrum, thereby inferring a high N/O abundance. Finally we note that the strength of all emission lines shows a continuing decay over the ~1 month of our observations.