A rare binary system, considered the “missing link” in the evolution of double stars, has been found for the first time. It is formed by a star in the process of becoming a white dwarf orbiting a neutron star that has just turned into a rapidly rotating pulsar.
Astronomers already imagined that systems of this type could exist. In fact, this configuration is the penultimate stage in the evolution of binary systems like this one. However, all other white dwarf-pulsar binaries discovered so far lie well beyond the rotation stage.
Astronomers led by Samuel Swihart of the US Naval Research Laboratory used the Goodman Spectrograph on the SOAR Telescope to investigate a mysterious gamma-ray source called 4FGL J1120.0-2204. They found it to be a binary system.
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One of the components of the system is a “millisecond pulsar”, that is, a neutron star with a magnetic field strong enough to transform its rotational energy into electromagnetic energy. As the pulsar rotates, this magnetic field induces an electric field at the surface.
The other component is a proto-white dwarf (that is, an object in the transition phase to become a white dwarf) of extremely low mass, a surface temperature of 8,200 °C and a mass of only 17% of the mass of the Sun. orbits its companion every 15 hours.
Located more than 2600 light-years away, the duo is nicknamed by astronomers as the “spider” because the pulsar tends to “eat” the outer parts of the companion star when it completes its transformation into a white dwarf. The process is similar to that of the PSR J2039–5617 system, formed by a “black widow” and a mate-donor companion.
Millisecond pulsars spin hundreds of times every second and are generated by the accumulation of matter from a companion. Most of them emit gamma rays and X-rays, often when the pulsar’s wind (a stream of charged particles emanating from the neutron star) collides with material emitted by its companion.