‘Black widow’ neutron star found to be heaviest known pulsar after it devoured neighbour

A collapsed spinning star has shredded and consumed nearly the entire mass of its stellar neighbour, and has grown into the heaviest ever known neutron star, astronomers have found.

The collapsed star – a pulsar – spinning 707 times per second, is also one of the fastest spinning neutron stars in the Milky Way galaxy, according to the study published on Tuesday in The Astrophysical Journal Letters.

It tops the charts as the heaviest observed neutron star to date at 2.35 times the mass of the sun, researchers, including those from the University of California (UC) Berkeley in the US, said.

The findings can help understand the weird state of matter inside such dense objects, which could collapse entirely into a black hole if they get much heavier.

Neutron stars are so dense, with 1 cubic inch weighing over 10 billion tons, that their cores are the densest matter in the universe, only next to black holes.

“A neutron star is like one giant nucleus, but when you have one-and-a-half solar masses of this stuff, which is about 500,000 Earth masses of nuclei all clinging together, it’s not at all clear how they will behave,” study co-author Alex Filippenko, pofessor of astronomy at the UC Berkeley, said.

The observed mass of the neutron star at 2.35 times that of the Sun is close to the upper limit of such stellar entities, scientists say.

Astronomers suspect that when a star with a core larger than about 1.4 solar masses collapses at the end of its life, it forms a dense, compact object whose interior is under such high pressure that all its atoms are smashed together to form a sea of neutrons and their subnuclear constituents, quarks.

Neutron stars are born spinning, and reveal themselves as pulsars, emitting beams of radiation as radio waves, X-rays, or even gamma rays that flash Earth as they spin, similar to the rotating beam of a lighthouse.

The observed neutron star, first discovered in 2017, is called a “black widow” pulsar in reference to the tendency of female black widow spiders to consume the much smaller male after mating.

Its companion star, about 20 times the mass of Jupiter, had its side facing the neutron star heated to 5,927°C (10,700°F) – a temperature that was hot and bright enough to be observed using a telescope.

As this neutron star becomes more and more energised while stripping away material from its companion, it pulsed hundreds of times per second.

“By combining this measurement with those of several other black widows, we show that neutron stars must reach at least this mass, 2.35 plus or minus 0.17 solar masses [before collapsing into black holes],” Roger W Romani, another co-author of the study, explained.

“In turn, this provides some of the strongest constraints on the property of matter at several times the density seen in atomic nuclei. Indeed, many otherwise popular models of dense-matter physics are excluded by this result,” he added.