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Constraints on the Compact Object Mass in the Eclipsing HMXB XMMU J013236.7+303228 in M33 | | Varun Bhalerao
; Marten H van Kerkwijk
; Fiona Harrison
; | | Date: |
29 Jun 2012 | | Abstract: | We present optical spectroscopic measurements of the eclipsing High Mass
X-ray Binary XMMU J013236.7+303228 in M33. Based on spectra taken at multiple
epochs of the 1.73d binary orbital period we determine physical as well as
orbital parameters for the donor star. We find the donor to be a B1.5IV
sub-giant with effective temperature T=22,000-23,000 K. From the luminosity,
temperature and known distance to M33 we derive a radius of R = 8.9 pm 0.5
R_sun. From the radial--velocity measurements, we determine a velocity
semi-amplitude of K_opt = 63 pm 12 km/sec. Using the physical properties of
the B-star determined from the optical spectrum, we estimate the star’s mass to
be M_opt = 11 pm 1 M_sun. Based on the X-ray spectrum, the compact companion
is likely a neutron star, although no pulsations have yet been detected. Using
the spectroscopically derived B-star mass we find the neutron star companion
mass to be M_X = 2.0 pm 0.4 M_sun, consistent with the neutron star mass in
the HMXB Vela X-1, but heavier than the canonical value of 1.4 M_sun found for
many millisecond pulsars. We attempt to use as an additional constraint that
the B star radius inferred from temperature, flux, and distance, should equate
the Roche radius, since the system accretes by Roche lobe overflow. This leads
to substantially larger masses, but from trying to apply the technique to known
systems, we find that the masses are consistently overestimated. Attempting to
account for that in our uncertainties, we derive M_X = 2.2^{+0.8}_{-0.6} M_sun
and M_opt =13 pm 4 M_sun. We conclude that precise constraints require
detailed modeling of the shape of the Roche surface. | | Source: | arXiv, 1207.0008 | | Services: | Forum | Review | PDF | Favorites | |
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