The heaviest neutron star so far is a ‘black widow’ eating her companion

A rotating neutron star oscillates periodically through radio (green) and gamma rays (purple) across the Earth in this artist’s concept of the Black Widow pulsar. The neutron star/pulsar heats up the side facing its partner star (right) to temperatures twice the surface temperature of the Sun and slowly evaporates. Credit: NASA’s Goddard Space Flight Center

A dense collapsing star spinning 707 times per second — making it one of the fastest neutron stars in the Milky Way — tore and consumed nearly the mass of its stellar companion, and in the process, it grew to become the heaviest neutron star observed to date.

Measuring the weight of this record-breaking neutron star, which tops the charts at 2.35 times the mass of the Sun, is helping astronomers understand the peculiar quantum state of matter within these dense bodies, which — if they become much heavier than that — would completely collapse and disappear. Black hole.

“We know roughly how matter behaves at nuclear density, as it does in the nucleus of a uranium atom,” said Alex Filippenko, professor of astronomy at the University of California, Berkeley. “A neutron star is like one giant core, but when you have one and a half solar masses of that matter, which is about 500,000 Earth masses of cores all clinging to each other, it’s not at all clear how they’ll behave.”

A 2014 NASA video explains the Black Widow pulsar and how astronomers discovered one called PSR J1311−3430, the first of its kind discovered only through gamma-ray observations. Credit: NASA’s Goddard Space Flight Center

Roger W. Romani, professor of astrophysics at Stanford University, points out that neutron stars are so dense — one cubic inch weighing more than 10 billion tons — that their cores are the densest matter in the universe that lacks black holes, because hidden behind them is impossible to study. event horizon. A neutron star, a pulsar named PSR J0952-0607, is thus the densest object within sight of Earth.

The neutron star’s mass measurement was made possible by the extreme sensitivity of the 10-meter Keck I telescope at Maunakea, Hawaii, which was able to record a spectrum of visible light from the intensely glowing companion star, which has now been reduced to the size of a large gaseous planet. The stars are located about 3,000 light-years from Earth in the direction of the constellation Sixtans.

Discovered in 2017, PSR J0952-0607 is referred to as a “black widow” pulsar – an analogy to a female black widow spider’s tendency to consume a much smaller male after mating. Filippenko and Romani have been studying black widow systems for more than a decade, hoping to establish an upper bound on how large neutron stars/pulsars can grow.

“By combining this measurement with that of several other black widows, we show that neutron stars must have reached at least this mass, 2.35 plus or minus 0.17 solar masses,” said Romani, a professor of physics in Stanford University’s College of Humanities and Sciences. . He is a member of the Kavli Institute for Particle Physics and Cosmology. “This in turn provides some of the strongest constraints on the property of matter at many times the density visible in atomic nuclei. In fact, many common models of dense matter physics have been ruled out by this result.”

If 2.35 solar masses are close to the upper limit of neutron stars, the researchers say, the interior is likely a soup of neutrons as well as up and down quarks – components of ordinary protons and neutrons – but not exotic matter, such as “strange” quarks or kaons, which are particles It contains strange quarks.

PSR J0952-0607: The fastest and heaviest known neutron star in the galaxy

Astronomers have measured the velocity of a faint star (a green circle) that has been stripped of almost all of its mass by an unseen companion, a neutron star and a millisecond pulsar, which they determine is the most massive yet and possibly the upper limit of neutron stars. Image Credit: WM Keck Observatory, Roger W. Romani, Alex Filippenko

“The high maximum mass of neutron stars indicates that they are a mixture of cores and that their quarks are melted up and down to the core,” Romani said. “This excludes many of the proposed states of matter, especially those with a peculiar internal configuration.”

Romani and Filipenko and Stanford graduate student Dinesh Kandel are co-authors of a paper describing the team’s findings that have been accepted for publication by Astrophysical Journal Letters.

How far can it grow?

Astronomers generally agree that when a star with a core greater than about 1.4 solar masses collapses at the end of its life, it forms a dense, compact body within it under such high pressure that all the atoms collide together to form a sea of ​​neutrons. and its semi-nuclear components, quarks. These neutron stars are born spinning, and although they are too faint to be seen in visible light, they reveal themselves as pulsars, emitting beams of light – radio waves, X-rays or even gamma rays – that The Earth flashes as it spins, like a spinning. beacon beam.

“Normal” pulsars rotate and flash at a rate of about once per second, on average, a speed that can easily be explained given the star’s natural rotation before it collapses. But some pulsars repeat hundreds or as many as 1,000 times per second, which is difficult to explain unless matter falls on and damages the neutron star. But for some millisecond pulsars, no companion appears.

One possible explanation for isolated millisecond pulsars is that each once had a companion, but stripped it to nothing.

“The evolutionary path is very remarkable,” said Filippenko. “A double exclamation point.” “As the companion star evolves and begins to transform into a red giant, the material seeps into the neutron star, and that’s orbiting the neutron star. By spinning, it’s now incredibly energetic, and a wind of particles starts escaping from the star’s neutron. Then that wind collides with the star.” The donor starts stripping material, and over time, the donor star’s mass decreases to the mass of a planet, and if more time passes, it just disappears.So, this is how lonely millisecond pulsars can be they weren’t alone at first – They should have been in a pair—but they gradually evaporated away from their comrades, and are now aloof.”

PSR J0952-0607 and its fainter companion star support the millisecond pulsar origin story.

“These planet-like bodies are deposits of ordinary stars that have contributed to the mass and angular momentum, spinning their fellow pulsars to millisecond intervals and increasing their mass in the process,” Romani said.

“In the case of cosmic ingratitude, the Black Widow pulsar, which devoured a large part of its companion, is now heating up and evaporating into planetary masses and possibly complete annihilation,” said Filippenko.

Spider pulsars include redbacks and tidarrens

Finding black widow pulsars in which their companion is small, but not too small to detect, is one of the 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 warped by the mass of the neutron star and is gradually locking up, similar to the way our moon gets trapped in its orbit so that we see only one side. The side facing the neutron star is heated up to temperatures of about 6,200 Kelvin, or 10,700 degrees Fahrenheit, which is slightly hotter than our Sun, and bright enough to see with a large telescope.

Filippenko and Romani have turned the Keck I telescope on PSR J0952-0607 on six occasions over the past four years, each time observing using the Low Resolution Imaging Spectrometer in 15-minute segments to catch a faint companion at specific points in its 6.4-hour orbit of the pulsar. . By comparing the spectra with the spectra of Sun-like stars, they were able to measure the orbital velocity of the companion star and calculate the mass of the neutron star.

Filippenko and Romani have examined about a dozen Black Widow systems so far, although only six of their companion stars are bright enough to allow them to calculate mass. They all contained neutron stars less massive than PSR J0952-060. They hope to study more black widow pulsars, as well as their cousins: redbacks, named after the Australian equivalent of black pulsars, which have companions closer to one-tenth the mass of the Sun; And what gypsies call tidarrens – where the companion is about one hundredth of a solar mass – after a relative of the black widow spider. The size of the male of this species, Tidarren sisyphoides, is about 1% of the size of the female.

“We can continue to look for black widows and similar neutron stars skating close to the edge of the black hole. But if we don’t find any, it hardens the argument that 2.3 solar masses is the true limit, after which they become black holes,” Filipenko said.

“This is true within the limits of what the Keck telescope can do, so except in fantastic observational conditions, the tightening of the measurement of PSR J0952-0607 likely awaits the age of the 30-meter telescope,” Romani added.

Other co-authors of the ApJ Letters paper are UC Berkeley researchers Thomas Brink and Weikang Zheng.

Two millisecond pulsars discovered in globular cluster NGC 6440

more information:
PSR J0952-0607: The fastest and heaviest known neutron star in the galaxy, appears in ApJ رسائل messagesarXiv: 2207.05124 [astro-ph.HE]

Provided by University of California – Berkeley

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