A condensed collapsed star has chewed up and eaten almost the total mass of its stellar companion. In so doing, it has become the heaviest neutron star observed to date.
The star rotates 707 times per second, making it one of the fastest spinning neutron stars in the Milky Way galaxy. The neutron star, a pulsar designated PSR J0952-060, also tips the scales at 2.35 times the mass of the sun.
Astronomers are able to use the weight of this black widow neutron star to understand the quantum state of matter within the object. If neutron stars get much heavier than this one they 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. 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,”
said the University of California at Berkeley’s Alex Filippenko, professor of astronomy.
Black Widow Pulsar
Credit: W. M. Keck Observatory, Roger W. Romani, Alex Filippenko
Neutron stars are extremely dense; 1 cubic inch weighs over 10 billion tons. They are so dense that their cores are the densest matter in the universe except for black holes, which are impossible to study since they are concealed behind their event horizon, said Roger W. Romani, professor of astrophysics at Stanford University.
Measuring the neutron star’s mass was enabled by the extreme sensitivity of the 10-meter Keck I telescope on Maunakea in Hawaii.
The telescope was able to record a spectrum of visible light from the hot companion star, now reduced to the size of a large gaseous planet. The stars are about 20,000 light years from Earth in the direction of the constellation Sextans.
PSR J0952-0607, which was first discovered in 2017, is called a “black widow” neutron star, a reference to the habit of female black widow spiders to consume the male after mating. Filippenko and Romani have been studying black widow systems for more than a decade, hoping to establish the upper limit on how large neutron stars/pulsars can grow.
“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. 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,”
said Romani.
Black Widow Neutron Star Quarks And Kaons
If 2.35 solar masses is close to the upper limit of neutron stars, the researchers say, 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. This excludes many proposed states of matter, especially those with exotic interior composition,”
Romani said.
Born Spinning Pulsars
Most astronomers agree that when a star whose core is larger than about 1.4 solar masses collapses at the end of its life, it forms a dense, compact object with an interior under such high pressure that all atoms are smashed together to form a sea of neutrons and their subnuclear constituents, quarks.
These neutron stars are born spinning, and though too dim to be seen in visible light, reveal themselves as pulsars, emitting beams of light— radio waves, X-rays, or even gamma rays — that flash Earth as they spin, similar to the rotating beam of a lighthouse.
Typical pulsars spin and flash about once per second, on average, a speed that can easily be explained given the normal rotation of a star before it collapses.
But some pulsars repeat hundreds or up to 1,000 times per second, which is hard to explain unless matter has fallen onto the neutron star and spun it up. But for some millisecond pulsars, no companion is visible.
Lone Millisecond Pulsar Origin Stories
One possible explanation for isolated millisecond pulsars is that each did once have a companion, but it stripped it down to nothing.
“As the companion star evolves and starts becoming a red giant, material spills over to the neutron star, and that spins up the neutron star. By spinning up, it now becomes incredibly energized, and a wind of particles starts coming out from the neutron star. That wind then hits the donor star and starts stripping material off, and over time, the donor star’s mass decreases to that of a planet, and if even more time passes, it disappears altogether.
So, that’s how lone millisecond pulsars could be formed. They weren’t all alone to begin with—they had to be in a binary pair—but they gradually evaporated away their companions, and now they’re solitary,”
Filippenko explained.
The pulsar PSR J0952-0607 and its faint companion star support this origin story for millisecond pulsars.
10,700 Degrees Fahrenheit
Finding a black widow neutron star in which the companion is small, but not too small to detect, is one of few ways to weigh neutron stars. In the case of this binary system, the companion star — now only 20 times the mass of Jupiter — is distorted by the mass of the neutron star and tidally locked, similar to the way our moon is locked in orbit so that we see only one side.
The neutron star-facing side is heated to temperatures of about 6,200 Kelvin, or 10,700 degrees Fahrenheit, a bit hotter than our sun, and just bright enough to see with a large telescope.
Filippenko and Romani turned the Keck I telescope on PSR J0952-0607 on six occasions over the last four years, each time observing with the Low Resolution Imaging Spectrometer in 15-minute chunks to catch the faint companion at specific points in its 6.4-hour orbit of the pulsar. By comparing the spectra to that of similar sun-like stars, they were able to measure the orbital velocity of the companion star and calculate the mass of the neutron star.
They’re hoping to study more black widow pulsars, as well as their cousins: redbacks, named for the Australian equivalent of a black widow neutron star, which have companions closer to one-tenth the mass of the sun; and what Romani dubbed tidarrens — where the companion is around one-hundredth of a solar mass — after a relative of the black widow spider. The male of this species, Tidarren sisyphoides, is about 1% of the female’s size.
“We can keep looking for black widows and similar neutron stars that skate even closer to the black hole brink. But if we don’t find any, it tightens the argument that 2.3 solar masses is the true limit, beyond which they become black holes,”
Filippenko said.
