Journal Article

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

R. Cornelisse, P. D’Avanzo, S. Campana, J. Casares, P. A. Charles, G. Israel, T. Muñoz-Darias, K. O’Brien, D. Steeghs, L. Stella and M. A. P. Torres

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 420, issue 4, pages 3538-3544
Published in print March 2012 | ISSN: 0035-8711
Published online March 2012 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2011.20274.x
The nature of the X-ray transient MAXI J0556−332

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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 h), which differ from any potential X-ray periods reported. Assuming that MAXI J0556−332 is a relatively high-inclination system that harbours a precessing accretion disc 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 K2≃ 190 km s−1 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.

Keywords: accretion, accretion discs; stars: individual: MAXI J0556−332; X-rays: binaries

Journal Article.  4955 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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