Physics
Scientific paper
Dec 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...455..614f&link_type=abstract
Astrophysical Journal v.455, p.614
Physics
74
Stars: Binaries: Close, Black Hole Physics, X-Rays: Stars, Stars: Individual Alphanumeric: Gro J0422+32, Stars: Novae, Cataclysmic Variables
Scientific paper
A series of 21 moderate-resolution (˜2.4 Å) spectra of the now quiescent Galactic X-ray nova GRO J0422+32, obtained in 1994 November and 1995 January with the W. M. Keck 10 m telescope, is used to derive the physical parameters of the binary system. The Hα emission-line profile exhibits large variations in consecutive half-hour exposures taken in 1994 November, but smaller variations in 1995 January. Cross-correlation of the 6000-6500 Å spectral region with that of late-type dwarf stars yields reliable absorption-line radial velocities for the secondary star. The orbital period is found to be 0d.21159±0d.00057, with a semiamplitude of 380.6±6.5 km s-1; the implied mass function is 1.21±0.06 Msun. Inspection of the averaged spectrum of GRO J0422+32 in the rest frame of the secondary star suggests that the secondary is an M2 V star, but the accretion disk contributes 30%-60% of the light at ˜6300 Å.
Fits to the wings of the strong, double-peaked Hα emission line yield approximate radial velocities for the compact primary; the velocity curve has a semi-amplitude of 41.6±3.2 km s-1, but with a phase offset by 253° (rather than 180°) from that of the secondary star. The offset, which is similar to that of several other X-ray novae and many dwarf novae, may be indicative of geometric distortions or additional emission components on the accretion disk; hence, the observed semi-amplitude does not necessarily reflect the true motion of the compact primary. Under the assumption that it does, however, we find q = 0.1093±0.0086, the mass ratio of the secondary to the primary.
If the secondary star is a normal M2 dwarf (M = 0.39±0.02 M0), as suggested by its spectrum and (independently) by the requirement that it fill its Roche lobe, the mass of the primary is 3.57±0.34 Msun, somewhat higher than the theoretical upper limit (˜3.2 Msun) for a slowly rotating neutron star with an extremely stiff equation of state, and considerably above the measured masses of neutron stars. We conclude that the compact object is probably a black hole, as suggested by its hard X-ray spectrum during outburst. The derived inclination angle of the system (48±3° ) is consistent with the apparent absence of eclipses of the accretion disk.
Filippenko Alexei V.
Ho Luis C.
Matheson Thomas
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