The evolutionary history of Hercules X-1.

Two evolutionary scenarios are discussed for Her X-1, i.e., massive and low mass binary scenarios. In a massive binary scenario, the present age of the neutron star in the system must be of the order of the main-sequence lifetime of the about 2 solar mass nondegenerate component (about 500 Myr). This large age is incompatible with the evidence of a high magnetic field strength observed in the X-ray source since within this time span the magnetic field should have decayed substantially. A low-mass binary scenario is adopted in which the neutron star was formed by accretion-induced collapse of a white dwarf. The neutron star can be considerably younger (about 10 Myr) which is more compatible with the observed strong magnetic field. In order to reach its present distance outside the galactic plane within its lifetime, the system must have acquired a runaway velocity over 70 km/sec. In a low-mass binary scenario such a velocity can have resulted from a supernova explosion in the system of which the ejection velocity of the supernova shell is about 20,000-30,000 km/sec. Alternatively, the ejection may have been slightly asymmetric.