R. Cornelisse, P. D'Avanzo, S. Campana, J. Casares, P. A. Charles, G. Israel, T. Muñoz-Darias, K. O'Brien, D. Steeghs, L. Stella, M. A. P. Torres
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.
View original:
http://arxiv.org/abs/1111.6946
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