A new method to measure the mass of a pulsar in a binary system

Artist's concept of PSR J2215+5135 irradiating its companion (Image courtesy G. Pérez-Díaz/IAC)
Artist’s concept of PSR J2215+5135 irradiating its companion (Image courtesy G. Pérez-Díaz/IAC)

An article published in “The Astrophysical Journal” describes the study of a pulsar cataloged as PSR J2215+5135 which is extreme even for this category of objects. A team of researchers from the Universitat Politècnica de Catalunya (UPC) and the Canary Islands Institute of Astrophysics (IAC) used an innovative method to measure the mass of one of the most massive neutron stars discovered, estimated in 2.3 times the Sun’s. This is a method that can also be used with other objects.

The pulsar PSR J2215+5135 was discovered in 2011. A pulsar is a neutron star, one of the possible remnants of a star that exploded in a supernova. It’s part of a binary system together with a companion, a star in the main sequence with a mass that’s about a third of the Sun’s, which means that it’s a red dwarf.

The red dwarf is much bigger than the pulsar because neutron stars are extremely compact, which means that their gravity is remarkable. The consequence is that the red dwarf orbits the pulsar, in this case in just over 4 hours. In these binary systems, the pulsar steals gas from its partner and is called “redback” in jargon. If the pulsar’s companion is much smaller in jargon it’s called a black widow, as in the case of the pulsar PSR B1957+20, which has a brown dwarf as a companion.

The team that studied the pulsar PSR J2215+5135 was led by Manuel Linares who, together with astronomers Tariq Shahbaz and Jorge Casares, put together data collected by the Great Telescope Canarias (GTC), the largest optical and infrared telescope in the world, the William Herschel Telescope (WHT), the Isaac Newton Telescope Group (ING) and the IAC-80 telescope. These data were combined with computer models to simulate the binary system.

These instruments allowed to detect the spectral lines of hydrogen and magnesium to measure the speed at which the red dwarf moves. The fact that it’s a redback system was helpful because a star’s face is irradiated by the pulsar, increasing its temperature, with an effect detectable in its spectral lines.

The surface of the red dwarf’s irradiated face has a temperature measured at about 8,080 Kelvin while that of the opposite face has a temperature measured at about 5,660 Kelvin. The rotation speed of the two stars around each other at about 412 km/s (921,618 mph). These measurements, together with other data such as the orbital inclination, allowed to measure the mass of the pulsar PSR J2215+5135 at 2.3 solar masses.

It’s one of the most massive neutron stars ever discovered, an extreme object even for this category. Astrophysicists are interested in objects of this kind because they can offer information on the interactions between nucleons, meaning the neutrons and protons that form the atoms’ nuclei, at enormously high densities, one of the great mysteries of today’s physics.

The method used by the team led by Manuel Linares could be used for other binary systems with neutron stars but also for the ones in which there’s a black hole or a white dwarf if the irradiation of their companion is relevant. Basically, it could provide a good help for various research in the field of astrophysics.

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