The heaviest neutron star yet is a ‘black widow’ eating its companion – Eurasia Review

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

Weighing this record-breaking neutron star, which dominates charts at 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 heavier than that – collapse entirely and disappear like a black hole.

“We know roughly how matter behaves at nuclear densities, like in the nucleus of a uranium atom,” said Alex Filippenko, professor emeritus 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 that stuff, or about 500,000 Earth masses of nuclei all hooked together, there’s no telling how they’re going to behave. “

Roger W. Romani, professor of astrophysics at Stanford University, noted that neutron stars are so dense – 1 cubic inch weighs more than 10 billion tons – that their cores are the densest matter in the world. universe except for black holes, which because they are hidden behind their event horizons are impossible to study. The neutron star, a pulsar designated PSR J0952-0607, is therefore the densest object in view of Earth.

Measuring the neutron star’s mass was possible thanks to the extreme sensitivity of the 10-meter Keck I telescope on Maunakea in Hawai’i, which was just able to record a visible light spectrum of the companion star hot, now reduced to the size of a large gaseous planet. The stars are about 3,000 light-years from Earth in the direction of the constellation Sextans.

Discovered in 2017, PSR J0952-0607 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. Filippenko and Romani have been studying Black Widow systems for more than a decade, hoping to establish the upper limit for the growth of large neutron/pulsar stars.

“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,” said Romani, professor of physics at Stanford’s School of Humanities and Sciences. and Fellow of the Kavli Institute for Particle Astrophysics and Cosmology. “In turn, this provides some of the strongest constraints on the property of matter at many times the density seen in atomic nuclei. Indeed, many otherwise popular models of dense matter physics are precluded by this result.

If 2.35 solar masses is near the upper limit for neutron stars, the researchers say, then the interior is likely to be neutron soup as well as up and down quarks – the constituents of normal protons and neutrons. – but no exotic matter, such as “strange” quarks or kaons, which are particles that contain a strange quark.

“A high peak mass for neutron stars suggests that it is a mixture of nuclei and their dissolved quarks from top to bottom to the nucleus,” Romani said. “This rules out many 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 results which has been accepted for publication by Letters from the Astrophysical Journal.

How high can they grow?

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 with an interior under such high pressure that all of the atoms are crushed to form a sea of ​​neutrons. and their subnuclear constituents, the quarks. These neutron stars are born rotating, and although too faint to be seen in visible light, reveal themselves as pulsars, emitting beams of light – radio waves, X-rays or even gamma rays – that cause the Earth to flash while ‘they turn, a bit like the rotating beam of a lighthouse.

“Ordinary” pulsars spin and blink about once per second, on average, a rate that is easily explained given 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 once had a companion, but this has wiped it out.

“The evolutionary path is absolutely fascinating. Double exclamation mark, Filippenko said. “As the companion star evolves and begins to become a red giant, matter spills over the neutron star, and this spins the neutron star. As it spins, it now becomes incredibly energized and a wind of particles begins to come 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 even more time passes, it disappears completely. This is how solitary millisecond pulsars could form. They weren’t all alone at first – they had to be in a binary pair – but they gradually made their companions disappear, and now they are lonely.

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

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

“In a case of cosmic ingratitude, the black widow pulsar, which devoured much of its companion, is now heating up and evaporating the companion to planetary masses and possibly complete annihilation,” Filippenko said.

Spider pulsars include redbacks 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 tidally locked, much like our moon is orbit locked. 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 on PSR J0952-0607 six times over the past four years, each time observing with the low-resolution imaging spectrometer in 15-minute increments to catch the faint companion at specific points in its 6.4 hour orbit. of the pulsar. By comparing the spectra to those 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 so far examined a dozen black widow systems, although only six have companion stars bright enough to allow them to calculate mass. All involved neutron stars less massive than the PSR J0952-060 pulsar. They hope to study more black widow pulsars, as well as their cousins: red backs, named for the Australian equivalent of black widow pulsars, which have companions closer to a tenth the mass of the sun; and what Romani dubbed tidarrens – where the companion is about one-hundredth of a solar mass – after a relative of the black widow spider. The male of this species, Sisyphoid Tidarren, is about 1% the size of the female.

“We can continue to search for black widows and similar neutron stars skating 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 true limit, beyond which they become black holes,” Filippenko said.

“It’s just on the edge of what the Keck telescope can do, so barring fantastic observing conditions, the measurement crunch of PSR J0952-0607 probably awaits the era of the 30-meter telescope,” said added Romani.

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