Sco X-1 is the brightest persistent X-ray in the sky. It is generally
believed that Sco X-1 is a low-mass X-ray binary containing a neutron star
accreting from a low-mass donor star where mass transfer is driven by the
magnetic braking. However, the mass transfer rate predicted by the standard
magnetic braking model is at least one order of magnitude lower than the one
inferred by X-ray luminosity. In this work, we investigate whether this source
could evolved from an intermediate-mass X-ray binary including Ap/Bp stars with
a slightly strong magnetic field of 300 - 1000 G. The coupling between the
magnetic field and an irradiation-driven wind induced by the X-ray flux from
the accretor can yield a strong magnetic braking, which could give rise to a
relatively high mass transfer rate. According to the observed orbital period,
the mass transfer rate, the mass ratio, and the donor star spectral type, the
progenitor of Sco X-1 should be an intermediate-mass X-ray binary including a
1.6 − 1.8 M⊙ Ap/Bp donor star in a 1.3 − 1.5 day orbit.
Therefore, we propose that anomalous magnetic braking of Ap/Bp stars provides
an alternative evolutionary channel to a part of luminous X-ray sources.Comment: 7 pages, 5 figures, accepted for publication in A&
