Heaviest neutron star to date is a 'black widow' eating its mate

Much faster than one would anticipate for a collapsing star, millisecond pulsars spin. Finding a black widow system where the pulsar has vaporized and consumed much of its companion star offers the finest chance to investigate these neutron stars. Astronomers were only able to weigh the pulsar of one of these companions because to the Keck I telescope's ability to catch its spectrum. It is the heaviest known object and may be getting close to the maximum limit for neutron stars.

One of the fastest spinning neutron stars in the Milky Way galaxy, a compact, collapsed star has virtually absorbed the whole mass of its stellar partner and developed into the heaviest neutron star yet seen. It spins 707 times per second.

This record-breaking neutron star, which weighs 2.35 times as much as the sun, allows scientists to better comprehend the peculiar quantum state of matter that exists within these dense objects, which, if they are any heavier, collapse completely and vanish as black holes.

In the nucleus of a uranium atom, for example, "we know basically how matter behaves at nuclear concentrations," said Alex Filippenko, Distinguished Professor of Astronomy at the University of California, Berkeley. "A neutron star is like one huge nucleus, but it's not at all apparent how they would behave when you have one and a half solar masses of this material, which is around 500,000 Earth masses of nuclei all clinging together."

According to Roger W. Romani, a professor of astrophysics at Stanford University, neutron stars are the densest objects in the universe aside from black holes, which are impossible to study because their event horizons are hidden from view. One cubic inch of a neutron star weighs over 10 billion tons. Thus, the neutron star, also known as pulsar PSR J0952-0607, is the densest object visible from Earth.

The 10-meter Keck I telescope on Maunakea in Hawaii, which was only able to record a spectrum of visible light from the furiously blazing companion star, now shrunk to the size of a big gaseous planet, made it feasible to determine the neutron star's mass. The stars are located in the direction of the constellation Sextans, some 3,000 light years away from Earth.

PSR J0952-0607, discovered in 2017, is known as a "black widow" pulsar, a reference to the female black widow spiders' propensity to eat the much smaller male after mating. For more than a decade, Filippenko and Romani have been researching black widow systems in an effort to determine the maximum size that can be reached by neutron stars and pulsars.

We demonstrate that neutron stars must attain at least this mass, 2.35 plus or minus 0.17 solar masses, by combining this measurement with those of several other black widows, said Romani, a professor of physics in Stanford's School of Humanities and Sciences and a member of the Kavli Institute for Particle Astrophysics and Cosmology. "This in turn offers some of the greatest limitations on the characteristics of matter at densities many times greater than those seen in atomic nuclei. In fact, this discovery excludes a large number of dense-matter physics models that were previously well-liked."

The interior is likely to be a soup of neutrons and up and down quarks, the building blocks of regular protons and neutrons, but not exotic matter like "strange" quarks or kaons, which are particles that contain a strange quark, if 2.35 solar masses is close to the upper limit of neutron stars, the researchers claim.

According to Romani, neutron stars with large maximum masses are made up of a combination of nuclei and their dissolved up and down quarks all the way to the core. This rule disqualifies a number of suggested states of matter, particularly those having unusual internal compositions.

The Astrophysical Journal Letters has approved the work written by Romani, Filippenko, and Stanford graduate student Dinesh Kandel for publication. The document describes the team's findings.

In general, astronomers concur that when a star with a core mass greater than 1.4 solar masses collapses at the end of its life, it creates a dense, compact object whose interior is under such intense pressure that all atoms are crushed together to create a sea of neutrons and their subnuclear byproducts, quarks. These neutron stars are born spinning and, while being too faint to be seen in visible light, expose themselves as pulsars by generating light beams that flash Earth as they rotate, much like the revolving beam of a lighthouse. These light beams include radio waves, X-rays, and even gamma rays.

The typical speed of "ordinary" pulsars is roughly once per second, which is readily explained given the star's usual rotation before it collapses. It is difficult to explain why some pulsars repeat hundreds or even 1,000 times per second unless stuff has dropped onto the neutron star and spun it up. However, no companion may be seen for certain millisecond pulsars.

Each single millisecond pulsar may have once had a partner, but it has been stripped away, which is one explanation for their isolation.

"The process of evolution is utterly fascinating. two exclamation marks, "said Filippenko. "Material leaks over to the neutron star as it develops and begins to turn into a red giant, spinning it up in the process. A jet of particles begins to emanate from the neutron star as it starts to spin up and become extremely energetic. The donor star is subsequently struck by that wind, which begins removing material from it. Eventually, the donor star's mass falls to that of a planet, and if more time passes, it vanishes completely. That explains how single millisecond pulsars might develop. They had to be in a binary pair, so they weren't wholly alone at first, but over time, their partners slowly vanished, leaving them alone."

This genesis theory for millisecond pulsars is supported by the pulsar PSR J0952-0607 and its dim companion star.

The remnants of regular stars that have given mass and angular momentum to their pulsar companions, spinning them up to millisecond periods and increasing their mass in the process, according to Romani, are what are known as these planet-like objects.

In a display of cosmic resentment, the black widow pulsar, which has already swallowed a sizable portion of its partner, is now heating and evaporating the companion to planetary masses, and perhaps causing full extinction, according to Filippenko.

One of the few techniques to weigh neutron stars is to find black widow pulsars when the partner is modest but not too small to detect. As in the case of our moon, which is locked in orbit so that we can only view one side, the neutron star's mass distorts and tidally locks the companion star in this binary system, which is now just 20 times the mass of Jupiter. About 6,200 Kelvin, or 10,700 degrees Fahrenheit, are reached on the neutron star-facing side, making it somewhat hotter than the sun and just brilliant enough to be visible via a powerful telescope.

Over the course of the previous four years, Filippenko and Romani turned the Keck I telescope on PSR J0952-0607 six times, each time using the Low Resolution Imaging Spectrometer to observe at 15-minute intervals to catch the dim companion at certain times in its 6.4-hour orbit around the pulsar. They were able to determine the mass of the neutron star and gauge the orbital velocity of the companion star by comparing the spectra to those of other sun-like stars.       

Only six of the black widow systems Filippenko and Romani have analyzed so far had partner stars luminous enough to allow them to determine a mass. Less massive neutron stars than the pulsar PSR J0952-060 were all involved. They want to learn more about black widow pulsars as well as their relatives, which are called tidarrens (after a relative of the black widow spider) and redbacks (after the Australian analogue of black widow pulsars), which have partners that are closer to one-hundredth the mass of the sun. This species' male, Tidarren sisyphoides, is around 1% the size of the female.

"We can continue to search for neutron stars that are comparable to black widows and skate even closer to the edge of the black hole. But if we don't discover any, it strengthens the case that the actual limit is 2.3 solar masses, after which they turn into black holes "explained Filippenko.

The tightening of the measurement of PSR J0952-0607, according to Romani, "likely awaits the 30-meter telescope era" since it is "exactly at the limit of what the Keck telescope can achieve."

University of California - Berkeley