Two-faced star reveals a pulsar’s surprising bulk

A two-faced star just helped weigh an extra-massive pulsar.

The star takes about four hours to orbit its companion, a fast-spinning stellar corpse called a pulsar that’s about 10,000 light-years from Earth. That means the pair’s orbital dance is tight enough that the star always shows the same face to the pulsar, similar to how the moon is oriented to Earth.

Radiation from the pulsar has fried the near side of the companion star to a scorching 7800° Celsius, Manuel Linares of the Polytechnic University of Catalonia in Barcelona and colleagues report May 23 in the Astrophysical Journal. That’s as hot as an A-type star, which are typically around twice the mass of the sun and burn at higher temperatures. But the side facing away from the pulsar is just roughly 5400° Celsius, similar to stars like the sun.
Linares and colleagues also measured the Doppler shift — the change in the wavelength of the star’s light as it moves toward and away from the Earth — of the star’s two sides as it moved around the pulsar to calculate its orbit precisely. The team used that precision to estimate the mass of the pulsar, and found it was surprisingly heavy: around 2.3 times the mass of the sun.

A type of neutron star, pulsars are extremely dense, cramming the mass of a star into an orb the size of a planet. Previous studies of packing neutrons together at high pressures had suggested that a pulsar can’t be more massive than 2.2 solar masses without collapsing into a black hole (SN: 12/23/17, p. 7). The new finding may force a rethink of how these particles interact at high densities, Linares says.

The duo, named PSR J2215+5135, is called a “redback” binary, after the cannibalistic spider of the same name, because the pulsar is gradually stealing material from its neighbor. Eventually, the pulsar may gobble enough of the regular star to become a black hole. But at the rate the pulsar is eating, this pair seems to be stable. “They will keep dancing for a while,” Linares says.