Scientists Identify Heaviest Known Neutron Star To Date. It Is A 'Black Widow' Eating Its Stellar Companion

The neutron star has a mass 2.35 times that of the Sun. It is a dense, collapsed star spinning 707 times per second that shredded and consumed nearly the entire mass of its stellar companion.

An artist's concept of a black widow pulsar. A spinning neutron periodically swings its radio and gamma-ray beams past Earth. The radio waves are depicted in green and the gamma rays are depicted in magenta.

Source : NASA's Goddard Space Flight Center

Astronomers have identified the heaviest known neutron star to date. It is a dense, collapsed star spinning 707 times per second that shredded and consumed nearly the entire mass of its stellar companion. As a result of this process, the dense star has grown into the largest neutron star observed to date. It is one of the fastest spinning neutron stars in the Milky Way galaxy.

The neutron star has a mass 2.35 times that of the Sun. Weighing the neutron star helps astronomers understand the weird quantum state of matter inside these dense objects. If these dense objects get much heavier than what they already are, they collapse entirely and disappear as a black hole. The neutron star is a "black widow" pulsar consuming its much smaller mate. 

The study describing the findings has been accepted for publication in The Astrophysical Journal Letters.

In a statement released by University of California Berkeley, Alex Filippenko, one of the authors on the paper, said "we know roughly how matter behaves at nuclear densities, like in the nucleus of a uranium atom".

Neutron Star, Pulsar, And Black Widow

A neutron star is the dense, collapsed core of a massive star that exploded as a supernova, and is the densest object astronomers can observe directly. According to NASA, a neutron star contains about a Sun's worth of mass packed in a sphere the size of a large city. Pulsars are rotating neutron stars observed to have pulses of radiation at very regular intervals that range from milliseconds to seconds, and have very strong magnetic fields which funnel jets of particles out along the two magnetic poles. 

A black widow binary is a rapidly spinning neutron star, or pulsar, that is circling and slowly consuming a smaller companion star. This is what the arachnid "black widow" does to its mate. The pulsar in the binary is known as a "black widow pulsar".

Filippenko said that a neutron star is like one giant nucleus, but when there are one-and-a-half solar masses of this stuff, which is about 500,000 Earth masses of nuclei all clinging together, it is not clear how they will behave, he added.

PSR J0952-0607 Is The Densest Object Within Sight Of Earth

According to Roger W Romani, the lead author of the study, neutron stars are so dense that their cores are the densest matter in the universe short of black holes. One cubic inch of a neutron star weighs over 10 billion tonnes. Since black holes are hidden behind their event horizon, they are impossible to study. According to the study, the neutron star is a pulsar designated PSR J0952-0607. It is the densest object within sight of Earth. 

With the help of the extreme sensitivity of the 10-metre Keck I telescope on Mauna Kea in Hawaii, the astronomers were able to measure the neutron star's mass. The telescope was able to record a spectrum of visible light from the hotly glowing companion star. The star has now been reduced to the size of a large gaseous planet. The neutron star and its companion are located about 3,000 light years from Earth in the direction of the constellation Sextans.

The pulsar was discovered in 2017. It is referred to as a "black widow" pulsar. This is an analogy to the tendency of female black widow spiders to consume the much smaller male after mating. 

In the statement, Romani said that by combining the measurement of the neutron star's mass with those of several other black widows, the astronomers have shown that neutron stars must reach at least this mass, 2.35 plus or minus 0.17 solar masses. 

Interior Of A Neutron Star Having A Mass Equivalent To 2.35 Suns

The researchers say that if 2.35 solar masses is close to the upper limit of neutron stars, then the interior is likely to be a soup of neutrons as well as up and down quarks, which are constituents of normal protons and neutrons. The interior is likely to not contain exotic matter, such as "strange" quarks or kaons, the study said. Strange quarks or kaons are particles that contain a strange quark.

Romani said that a high maximum mass for neutron stars suggests that it is a mixture of nuclei and their dissolved up and down quarks all the way to the core. He added that this excludes many proposed states of matter, especially those with exotic interior composition.

How Large Can Neutron Stars Grow?

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 high pressure. Due to this pressure, all the atoms are smashed together to form a sea of neutrons and their subnuclear constituents, quarks. 

Neutron stars are born spinning, and reveal themselves as pulsars, emitting beams of electromagnetic radiation including radio waves, X-rays or gamma rays. These rays flash Earth as they spin, much like the rotating beam of a lighthouse. 

On average, "ordinary" pulsars spin and flash about once per second. This is a speed that can easily be explained given the normal rotation of a star before it collapses. Some pulsars spin hundreds or up to 1,000 times per second. This is hard to explain unless matter has fallen into the neutron star and spun it up, the statement said. However, for some millisecond pulsars, no companion is visible. 

How Are Lone Millisecond Pulsars Formed?

According to the study, a possible explanation for isolated millisecond pulsars is that each did once have a companion, but it stripped it down to nothing. In other words, the pulsar ate its mate. 

Filippenko exclaimed that the evolutionary pathway is "absolutely fascinating". He also said that as the companion star evolves and starts becoming a red giant, material spills over to the neutron star, and that spins up the neutron star. The neutron star becomes incredibly energised by spinning up, and a wind of particles starts coming out from the neutron star. Then, the wind hits the donor star and starts stripping material off. Over time, the donor star's mass decreases to that of a planet, and if even more time passes, it disappears altogether. That is how lone millisecond pulsars could be formed, Filippenko explained.

He added that neutron stars were not all alone to begin with, and had to be in a binary pair. Gradually, they evaporated away their companions, and now they are solitary.

The pulsar and its faint companion star described in the study support this origin story for millisecond pulsars, the statement said.

Romani said that these planet-like objects are the dregs of normal stars which have contributed mass and angular momentum, spinning up their pulsar mates to millisecond periods and increasing their mass in the process. 

Filippenko said that in a case of cosmic ingratitude, the black widow pulsar, which has devoured a large part of its mate, now heats and evaporates the companion down to planetary masses and perhaps "complete annihilation". 

How Are Neutron Stars Weighed?

One of the few ways to weigh neutron stars is finding black widow pulsars in which the companion is small, but not too small to detect. In the case of the binary system considered in the study, the companion star is now only 20 times the mass of Jupiter, and is distorted by the mass of the neutron star and tidally locked. Tidal locking is the phenomenon by which an astronomical object has the same rotational period as its orbital period around a partner. Since the Moon rotates in exactly the same time as it takes to orbit Earth, it is said to be tidally locked. We see only one side of the Moon because it is locked in orbit. 

According to the study, the neutron star-facing side of the companion star is heated to temperatures of about 6,200 Kelvin, or 5727 degrees Celsius, a bit hotter than the Sun. The companion star is bright enough to be clearly seen with a large telescope. 

Black Widow Systems Can Have Redbacks And Tidarrens

Over the last four years, Filippenko and Romani used the Keck I telescope to observe the black widow system on six occasions. They observed the black widow system using the Low Resolution Imaging Spectrometer in 15-minute chunks to catch the faint companion at specific points in its 6.4-hour orbit of the pulsar.

The astronomers compared the spectra of the companion star to that of similar Sun-like stars. This helped them measure the orbital velocity of the companion star and calculate the mass of the neutron star.

According to the University of California Berkeley, Filippenko and Romani have examined about a dozen black widow systems so far. Of these, only six black widow systems had companion stars bright enough to let them calculate a mass. The neutron stars in these systems are less massive than the pulsar PSR J0952-060. 

The astronomers aim to study more black widow pulsars, and their cousins, called redbacks. These have companions closer to one-tenth the mass of the Sun. The redback spider is also known as the Australian black widow. 

There are some black widow systems in which the companion star is around one-hundredth of a solar mass. Romani calls these stars tidarrens, after a relative of the black widow spider. The male tidarren is about one per cent of the female's size.

Filippenko said that they can keep looking for black widows and similar neutron stars that skate even closer to the black hole brink, but if they don't find any, it tightens the argument that 2.3 solar masses is the true limit, beyond which they become black holes.