A dense, collapsed star spinning 707 times per second has shredded and eaten nearly the entire mass of its stellar companion and grown into the heaviest neutron star observed to date.
Weighing this record-setting neutron star, which tops the charts at 2.35 times the mass of the sun, helps astronomy understand the weird quantum state of matter inside these dense objects, which if they get much heavier than that, collapse completely and disappear as a black hole.
Alex Filippenko is a professor of astronomy at the University of California, Berkeley. When you have one-and-a-half solar mass of this stuff, it's not at all clear how they will behave.
The densest matter in the universe is the core of a neutron star, which is hidden behind its event horizon, making it impossible to see. The densest object in sight of Earth is the neutron star.
The measurement of 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 spectrum of visible light from the bright companion star. Sextans is located in the direction of the stars.
The term "black widow" refers to the tendency of female black widow spiders to consume the smaller male after they mate. Filippenko and Romani have been studying black widow systems for more than a decade in hopes of establishing the upper limit on how large stars can grow.
Romani is a professor of physics in the School of Humanities and Sciences and member of the Kavli. Some of the strongest constraints on the property of matter are provided by this. Many popular models of dense-matter physics are not included.
The researchers say that if 2.35 solar mass is close to the upper limit of neutron stars, the interior is likely to be a soup of neutrons and up and down quarks.
Romani said that a high maximum mass suggests that it is a mixture of nucleus and dissolved up and down quarks all the way to the core. Many proposed states of matter are not included.
The results of the team's work have been accepted for publication in The Astrophysical Journal Letters.
What size can they grow?
Astronomers agree that when a star with a core larger than about 1.4 solar mass 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 quarks. Even though they are too dim to be seen in visible light, these stars emit beams of light that flash Earth as they spin.
"Ordinary" pulsars spin and flash about once per second, on average, a speed that can be explained by the normal rotation of a star before it collapses. It's difficult to explain why some pulsars repeat hundreds or up to 1000 times per second. No companion can be seen for some millisecond pulsars.
One explanation for isolated millisecond pulsars is that they once had a companion, but it was stripped away.
The evolutionary pathway is really interesting. Filippenko said there were two exclamation points. Material spills over to the neutron star when the companion star becomes a red giant. A wind of particles comes out from the neutron star when it spins up. Over time, the donor star's mass decreases to that of a planet, and if even more time passes, it disappears entirely. That is how lone millisecond pulsars could be created. They weren't all alone to begin with, they had to be in a pair, and now they're alone.
The origin story for pulsars is supported by the PSR J0952-0607.
"These planet-like objects are the dregs of normal stars, spinning up their pulsar mates to millisecond periods and increasing their mass in the process," Romani said.
Filippenko said that the black widow pulsar, which has devoured a large part of its mate, now has a companion and may be complete.
Redbacks and Tidarrens are included in spider pulsars.
Finding black widow pulsars in which the companion is small but not too small is one of the few ways to weigh them. The companion star is distorted by the mass of the neutron star and locked in a position so that we can only see one side of it. The neutron star-facing side is a bit hotter than the sun and bright enough to be seen with a telescope.
Filippenko and Romani have turned the Keck I telescope on six occasions in the last four years to catch a faint companion in its 6.4-hour rotation. They were able to calculate the mass of the companion star and measure the orbital velocities of the sun-like stars.
Filippenko and Romani have looked at about a dozen black widow systems and only six had companion stars that were bright enough to allow them to calculate a mass. The stars were less massive than the one in the pulsar. They are hoping to study more black widow pulsars, as well as their cousins, redbacks, which have companions closer to one-tenth the mass of the sun. The male of Tidarren sisyphoides is small.
We can look for black widows and similar stars that are close to the black hole's edge. Filippenko said that if we don't find any, it will tighten the argument that 2.3 solar mass is the true limit.
"This is at the limit of what the Keck telescope can do, so barring fantastic observing conditions, tighten the measurement of PSR J0952-0607 likely awaits the 30-meter telescope era," said Romani.
The UC Berkeley researchers are one of the co-authors of the paper.
More information: PSR J0952-0607: The Fastest and Heaviest Known Galactic Neutron Star, To Appear in ApJ Letters, arXiv:2207.05124 [astro-ph.HE] arxiv.org/abs/2207.05124 Journal information: Astrophysical Journal Letters
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