NS-Capture Theory - Explained

Since the term “Neutron Star Capture Theory” (NS-Capture) will come up again and again, this discussion is intended to clarify exactly what it refers to.

In the simplest terms I can think of, what it means is that: a neutron star traveling through space will occasionally pass near a normal star.

If it passes close enough to the star, the neutron star’s gravitational field will exert a pull on the atmosphere of the normal star, causing the atmosphere to bulge up in the direction of the neutron star. When the neutron star continues to pass by, the bulge will recede. This is exactly like the Moon’s pull on the oceans of the Earth that result in the daily tides we experience as the Earth goes through its daily rotation.

(When, say, the Pacific Ocean is facing the moon, it is pulled toward the moon, causing low tides. When the Earth turns such that the Pacific is on the other side of the Earth from the Moon, then the bulge recedes and the tide expands back resulting in a high tide. At least that’s how I think it works.)

In any event, what this means in the world of physics, is that energy is transferred from the orbital rotational energy of the Earth and Moon to the tidal energy of the oceans on Earth. This causes energy to be lost from the Moon’s orbital rotation around the Earth.

The same thing happens with the neutron star that passes by the regular star. Energy is transferred from the trajectory of the neutron star orbit to the atmosphere of the regular star. If enough energy is transferred during this first encounter, then the neutron star, rather than proceeding on, will turn back in a closed elliptical orbit. If not enough energy is transferred, then the neutron star will continue in a parabolic trajectory, never to return.

More information on capture can be found in G.W. Clark “X-ray stars in globular clusters” Scientific American, October 1977 (which you need to subscribe to sciam to access).

I originally came up with the idea of tidal capture on Oct 6, 1974, (the birth of NS-Capture theory, as well as the birth of my oldest son, Rob) when I realized that capture was the only mechanism that could explain the creation of Cen X-3. The first paper on the possibility of tidal capture of neutron stars by regular stars, I believe was by Fabian, Pringle, and Rees: “Tidal Capture Formation of Binary Systems and X-Ray Sources in Globular Clusters” in 1975, which provided the validation that I needed that the NS-Capture mechanism was feasible.

Globular Clusters is a subject that will be returned to in future posts and the above reference is critical for describing behavior in globular clusters.