Astronomers Just Found the Largest Known Neutron Star

Neutron stars are the smallest, densest stars known to exist. But their small size, and the fact that they often don’t emit light in the visible spectrum, can make them difficult to see. The circumstances under which we can see neutron stars are rather complex — we know of about 2000 neutron stars in the Milky Way, most of which are classified as radio pulsars, but there are an estimated 100 million of them floating in our galaxy.

One of the fundamental questions surrounding neutron stars is just how large they can practically get. A “typical” neutron star has a radius on the order of 6.2 miles and a mass between 1.4 and 3 solar masses. But there’ s a lot of fuzziness in that estimate, and discoveries like this one help narrow down just how large a neutron star can be.

PSR J2215-5135 is what’s known as a “redback” pulsar, or a spinning neutron star with a low-mass, non-degenerate companion star in a binary orbit. Unfortunately, our efforts to measure the mass of the pulsar hit a snag. Attempts to measure Doppler shifts in absorption were initially thwarted until astronomers realized they needed to measure both sides of the companion binary. The pulsar is pouring so much radiation into its host star, it was skewing the results.

Here’s AASNova with additional information:

Linares and collaborators circumvent this problem by using high-quality optical spectra from the Gran Telescopio Canarias and other telescopes to identify, for the first time, absorption lines from both the cool side and the hot side of the companion star. The authors use these lines from opposite sides of the star to bracket the center-of-mass velocity. By jointly modeling both the radial-velocity data for the two star sides and the light curves in multiple bands, the authors are able to calculate the mass of the neutron star and its companion, respectively ~2.3 and ~0.33 solar masses.

If this finding holds, it would make PSR J2215-5135 the highest-mass neutron star known by a significant margin. Both PSR J0348-0432 and PSR J1614–2230 have been measured at ~2.01 solar masses, with a margin of error that means either could be slightly larger than the other. A finding of 2.3 solar masses would make this the largest known neutron star, which have an upper limit of 3 solar masses because neutron stars above this point are believed to collapse into black holes.

By sharpening our understanding of the properties and formation ranges of these stellar phenomena, we can better understand how they evolve and change over time. And finding the heaviest neutron star and measuring the way it interacts with its environment also offers clues to how the laws of physics behave in some of the most extreme environments that exist within the universe.

Feature image by G. Pérez-Díaz/IAC.

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