PSR J0952-0607, a so-called millisecond pulsar, has shredded and consumed nearly the entire mass of its stellar companion and, in the process, grown into the heaviest neutron star observed to date.
This image shows PSR J0952-0607 and its companion. Image credit: W.M. Keck Observatory / Roger W. Romani / Alex Filippenko.
PSR J0952-0607 is located between 3,200 and 5,700 light-years away in the constellation of Sextans.
First discovered in 2017, it is referred to as a ‘black widow’ pulsar — an analogy to the tendency of female black widow spiders to consume the much smaller male after mating.
PSR J0952-0607 is in a 6.42 hr orbit around its very low-mass stellar companion.
At some point in the system’s history, matter began streaming from the companion and onto the pulsar, gradually raising its spin to 707 rotations a second and greatly increasing its emissions. Eventually, the pulsar began evaporating its companion, and this process continues today.
Weighing this record-setting neutron star, which tops the charts at 2.35 times the mass of the Sun, helps astronomers understand the weird quantum state of matter inside these dense objects, which collapse entirely and disappear as a black hole.
“We know roughly how matter behaves at nuclear densities, like in the nucleus of a uranium atom,” said University of California, Berkeley’s Professor Alex Filippenko.
“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.”
“Neutron stars are so dense — 1 cubic inch weighs over 10 billion tons — that their cores are the densest matter in the Universe short of black holes, which because they are hidden behind their event horizon are impossible to study,” said Stanford University’s Professor Roger Romani.
The measurement of PSR J0952-0607’s mass was possible thanks to the extreme sensitivity of the 10-m Keck I telescope on Maunakea in Hawai’i, which was just able to record a spectrum of visible light from the glowing companion star.
“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,” Professor Romani said.
“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.”
If 2.35 solar masses is close to the upper limit of neutron stars then the interior is likely to be a soup of neutrons as well as up and down quarks — the constituents of normal protons and neutrons — but not exotic matter, such as strange quarks or kaons, which are particles that contain a strange quark.
“A high maximum mass for neutron stars suggests that it is a mixture of nuclei and their dissolved up and down quarks all the way to the core,” Professor Romani said.
“This excludes many proposed states of matter, especially those with exotic interior composition.”
The results appear in the Astrophysical Journal Letters.
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Roger W. Romani et al. 2022. PSR J0952-0607: The Fastest and Heaviest Known Galactic Neutron Star. ApJL 934, L18; doi: 10.3847/2041-8213/ac8007
