PI: Jeroen Homan, Massachusetts Institute of Technology (Astrophysics), firstname.lastname@example.org
Address: Division of Astrophysics, Department of Physics, Cambridge, MA, 02139, USA
CoI: Maureen van den Berg, Harvard-Smithsonian Center for Astrophysics
CoI: Peter Jonker, Netherlands Institute for Space Research
Title: The puzzling mass donor in the Galactic halo X-ray binary MAXI J0556-332
Abstract: We request GMOS-S spectra of the recently discovered transient neutron-star low-mass X-ray binary (NS-LMXB) MAXI J0556-332. Apart from having an unusual location in the outer Galactic halo, the neutron star in this system has a puzzling donor. High temperatures inferred from photometry in quiescence ( 45K) rule out a dominant contribution from a late-type K/M donor typically found in NS-LMXBs. Two scenarios could explain the extremely blue colors, each with their own problems. The donor could be a hot subdwarf, which is further supported by the unusually high N/O abundance seen during outburst. This would make MAXI J0556-332 the first subdwarf with a neutron-star companion. However, a subdwarf donor does not fill its Roche lobe for the reported candidate orbital periods (9.8/16.4 hr). An alternative explanation could be that the quiescent emission is from the accretion disk; this requires substantial residual accretion in quiescence, which is ruled out by our X-ray observations. A GMOS-S spectrum allows us to test both scenarios for the quiescent emission. A spectral classification of the donor star would also provide us with an independent distance estimate.
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