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.
@article{arxiv.1111.6946,
title = {The nature of the X-ray transient MAXI J0556-332},
author = {R. Cornelisse and P. D'Avanzo and S. Campana and J. Casares and P. A. Charles and G. Israel and T. Muñoz-Darias and K. O'Brien and D. Steeghs and L. Stella and M. A. P. Torres},
journal= {arXiv preprint arXiv:1111.6946},
year = {2015}
}
Comments
8 pages, 6 figures, accepted for publication in the Monthly Notices of the Royal Astronomical Society