Her X-1: A Processing Binary Pulsar?

Abstract

RECENT observations of the two pulsating binary X-ray sources Her X-1 (ref. 1) and Cen X-3 (ref. 2) have led to a great deal of speculation about the X-ray emission mechanism involved. Because orbital parameters have been derived from their respective Doppler curves implying orbital radii R₀ of only 10¹¹ to 10¹² cm, it is natural to suppose that matter is accreted on one star from its companion. In the case of Cen X-3 with one binary member a massive star³ (∼15 M_⊙), the low-mass companion (M≲0.1–1 M_⊙) is presumably a collapsed object with high surface gravity (either a white dwarf, neutron star or black hole). So a large source of gravitational energy is tantalizingly available for radiation. If this were released during accretion of gas on the collapsed star (with mass M and radius R) at a rate dm/dt, the resulting steady state luminosity L≈(GM/R) (dm/dt). Even for a modest mass transfer rate, 20²³ g yr⁻¹, the maximum probable X-ray luminosity L_x≈10³⁶ erg s⁻¹ can be accommodated (assuming GM/Rc²∼10⁻¹ as in a neutron star).