The heaviest neutron star to date is a ‘black widow’ devouring its companion

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In this artistic concept of a black widow-like pulsar, a spinning neutron star periodically flashes its radio (green) and gamma (magenta) beams in front of Earth. The neutron/pulsar star heats the opposite side of its stellar partner (right) to temperatures twice as high as the surface of the sun and slowly evaporates it. Credit: NASA Goddard Space Flight Center

A dense, collapsed star spinning 707 times per second – making it one of the fastest spinning neutron stars in the Milky Way – shredded and consumed nearly all of its stellar companion’s mass and in doing so , became the heaviest neutron star observed to date.

The weight of this record-breaking neutron star, which reaches 2.35 times the mass of the sun, is helping astronomers understand the strange quantum state of matter inside these dense objects, which if they become much larger heavy, collapse entirely and disappear as a black hole.

“We basically know how matter behaves at nuclear densities, like in the nucleus of a uranium atom,” said Alex Filippenko, distinguished professor of astronomy at the University of California, Berkeley. “A neutron star is like a giant nucleus, but when you have a solar mass and a half of this material, which is about 500,000 Earth masses of nuclei all clinging to each other, it is not at all clear how they are going to behave. »

Roger W. Romani, a professor of astrophysics at Stanford University, has pointed out that neutron stars are so dense — one cubic inch weighs more than 10 billion tons — that their nuclei are the densest matter in the world. universe, with the exception of black holes which, because they are hidden behind their event horizon, are impossible to study. The neutron star, a pulsar called PSR J0952-0607, is therefore the densest object in sight of Earth.

The measurement of the mass of the neutron star was possible thanks to the extreme sensitivity of the 10-meter Keck I telescope located at the Maunakea site in Hawaii, which has just recorded a spectrum of visible light coming from the very bright companion star, now reduced to the size of a large gaseous planet. The stars lie about 3,000 light-years from Earth, in the direction of the constellation Sextan.

Discovered in 2017 , PSR J0952-0607 is referred to as a “black widow” type pulsar – an analogy to the tendency of female black widow spiders to consume the much smaller male after mating. Filippenko and Romani have been studying Black Widow systems for over a decade, hoping to establish the upper limit for the size of neutron/pulsar stars.

“Combining this measurement with those of several other black widows, we show that neutron stars must reach at least this mass, or 2.35 plus or minus 0.17 solar masses,” said Romani, who is a professor of physics. at the Stanford School of Humanities and a fellow at the Kavli Institute for Astrophysics and Particle Cosmology. “This provides some of the strongest constraints on the properties of matter at many times the density seen in atomic nuclei. Indeed, many otherwise popular models of dense matter physics are excluded by this result. »

According to the researchers, if the mass of 2.35 solar masses is close to the upper limit of neutron stars, it is likely that the interior is a soup of neutrons as well as up and down quarks – the constituents of protons and normal neutrons – but no exotic matter, such as “strange” quarks or kaons, which are particles containing a strange quark.

Astronomers have recorded the speed of a faint star (green circle) that has been stripped of almost all of its mass by an invisible companion, a neutron star and a millisecond pulsar. They determined it to be the most massive star yet discovered and possibly the upper limit of neutron stars. Credit: WM Keck Observatory, Roger W. Romani, Alex Filippenko

“A high peak mass for neutron stars suggests that it is a mixture of nuclei and their dissolved up and down quarks down to the nucleus,” Romani said. “This rules out many of the proposed states of matter, especially those with exotic interior composition.”
Romani, Filippenko and Stanford graduate student Dinesh Kandel are co-authors of a paper describing the team’s findings, which has been accepted for publication by The Astrophysical Journal Letters.

How big can they get?

Astronomers generally agree 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 subjected to a pressure so high that all atoms are crushed together to form a sea of ​​neutrons and their subnuclear constituents, quarks. These neutron stars are born in rotation and, although too faint to be seen in visible light, they turn out to be pulsars, emitting beams of light – radio waves, X-rays or even gamma rays – that cause the Earth to flash when they spin, much like the rotating beam of a lighthouse.

“Ordinary” pulsars spin and blink about once per second, on average, a rate that is easily explained by the normal rotation of a star before it collapses. But some pulsars repeat themselves hundreds or as many as 1,000 times per second, which is hard to explain unless material has fallen on the neutron star and caused it to spin. But for some millisecond pulsars, no companion is visible.

One possible explanation for the isolated millisecond pulsars is that each of them once had a companion, but that companion has been wiped out.

“The path of evolution is absolutely fascinating. Double exclamation mark,” Filippenko said. “As the companion star evolves and begins to become a red giant, material pours onto the neutron star, causing the neutron star to spin. As it spins, it becomes incredibly energetic, and a wind of particles begins to shoot out of the neutron star. This wind then hits the donor star and begins to remove material, and over time the mass of the donor star decreases to that of a planet, and if time passes even more, it completely disappears . So that’s how solitary millisecond pulsars were able to form. They weren’t alone at first – they were supposed to be part of a binary pair – but they gradually evaporated their companions, and now they are lonely. »

The PSR J0952-0607 pulsar and its faint companion star confirm this origin story of millisecond pulsars.

“These planet-like objects are the dregs of normal stars that have contributed their mass and angular momentum, spinning their pulsar companions at millisecond periods and increasing their mass in the process,” said Romani.

“In a case of cosmic ingratitude, the Black Widow’s pulsar, which has devoured much of its companion, heats it up and evaporates it until it reaches planetary masses, or even complete annihilation,” Filippenko said.

Spider pulsars include red widows and tidarrens

Finding black widow pulsars in which the 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 distorted by the mass of the neutron star and locked tidally, much like our moon is locked. orbiting so that we only see one side. The side facing the neutron star is heated to temperatures of around 6,200 Kelvin, or 10,700 degrees Fahrenheit, a little hotter than our sun, and just bright enough to be seen with a large telescope.

Filippenko and Romani have rotated the Keck I telescope to PSR J0952-0607 six times over the past four years, each time observing with the low-resolution imaging spectrometer in 15-minute increments to capture the faint companion at specific times of its 6.4 hour orbit around the pulsar. By comparing the spectra to those of stars similar to the sun, they were able to measure the orbital speed of the companion star and calculate the mass of the neutron star.

Filippenko and Romani have looked at a dozen Black Widow systems so far, but only six had companion stars bright enough for them to calculate mass. All involved neutron stars less massive than the PSR J0952-060 pulsar. Researchers hope to be able to study other black widow pulsars, as well as their cousins: redbacks, named after the Australian equivalent of black widow pulsars, whose companions have a mass close to a tenth of that of the sun, and what Romani called tidarrens – where the companion has a mass about one-hundredth the mass of the sun – after a relative of the black widow spider. The male of this species, Tidarren sisyphoides, is about 1% the size of the female.

“We can continue to search for black widows and similar neutron stars that come even closer to the edge of the black hole. But if we don’t find any, it strengthens the argument that 2.3 solar masses is the real limit, beyond which they become black holes,” Filippenko said.

“It’s just at the limit of what the Keck telescope can do, so unless there are fantastic observing conditions, the strengthening of the PSR J0952-0607 measurement will likely wait until the 30-meter telescope era,” Romani added.

The other co-authors of the ApJ Letters paper are UC Berkeley researchers Thomas Brink and WeiKang Zheng.

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