Pulsars murder their companion stars, X-rays reveal
Nearly half of all stars are born in binary systems, with the most massive ones dying the fastest. It's not pretty for the "second" star.
When a pulsar, a rapidly-rotating neutron star, finds itself in a tight orbit with another star, it will siphon mass from it, leading to electromagnetic emissions from the pulsar-star system. When the star is low in mass, forming an X-ray binary, the pulsar will heat and slowly evaporate the companion star, leading to the creation of "black widow" systems when the companion star loses enough mass.
Credit: ESA
Key Takeaways
- Wherever stars form, they come not only in a variety of masses, but also in a variety of configurations, with half of all stars having at least one "companion" in their stellar system.
- The most massive stars burn through their fuel the fastest, and die the most quickly: in core-collapse supernova events, with their corpses becoming either black holes or neutron stars.
- For the ones that become neutron stars, their companions are often in for a gruesome fate. Thanks to a new X-ray study, we're learning just how these "spider pulsars" murder their companion stars.
To survive in this Universe, you must avoid pulsars.
An illustrated view of a black widow pulsar and its stellar companion. The pulsar’s gamma-ray emissions (magenta) strongly heat the facing side of the star (orange). The pulsar is gradually evaporating its partner.
Formed when massive stars die in a core-collapse supernova, pulsars are rapidly spinning neutron stars.
This side-by-side set of images shows a series of views of the Crab Pulsar and its surrounding environment taken by NASA’s Chandra X-ray telescope (left) and NASA’s Hubble space telescope (right) over the 6-month period from November 2000 to April 2001. Formed from a star that went supernova in 1054, the Crab pulsar is one of the youngest known neutron stars, and the ringed feature around the pulsar was only discovered due to Chandra’s revolutionary X-ray capabilities.
The fastest rotators — millisecond pulsars — “spin up” from siphoning matter off of nearby stars.
This image shows the illustration of a massive neutron star, along with the distorted gravitational effects an observer might see if they had the capability of viewing this neutron star at such a close distance. While neutron stars are famous for pulsing, not every neutron star is a pulsar. The fastest pulsars, known as millisecond pulsars, rotate at more than 100 times per second, with the current record holder completing a whopping 766 rotations each second.
Whenever stars form, they aren’t always singlets, but often possess a companion.
An X-ray binary is formed when a neutron star or black hole is orbited by a much larger, less dense, massive star. The material accretes onto the dense stellar remnant, heats up and ionizes, and emits X-rays. Many pulsing neutron stars are known to have binary companions, with a wide variety of distances and masses possible for the companion star. Farther out, additional planetary or stellar companions are possible.
Furthermore, in dense stellar environments — like globular clusters — gravitational ejection and capture are common.
Here in the heart of Omega Centauri, one of the largest, richest globular clusters visible from Earth’s location within the Milky Way, lots of stars of various colors have been imaged. Owing to the dense nature of this environment, gravitational interactions between stars and stellar systems are common, often resulting in ejections, gravitational captures, and sometimes, low-mass stars (or even failed stars) winding up in tight orbits with millisecond pulsars.
Many millisecond pulsars come to have low-mass companion stars, forming LMXBs: low-mass X-ray binaries.
This illustration shows a neutron star with an accretion disk, siphoning mass off of a low-mass companion star. Many of these systems with neutron stars will have millisecond pulsars for their neutron stars, and the neutron star’s pulsing “jets” will strike, and slowly destroy, the companion star.
In close-in LMXB systems, these pulsars strip their companion stars of atmospheres through energetic winds.
Using the combined data from NASA’s Chandra (X-ray), Hubble (visible light), and IXPE (X-ray polarization, in light blue), pulsar winds coming off of the Vela pulsar, a neutron star just ~10,000 years old, can easily be seen. These jets, if the pulsar has a binary companion (Vela has a high-mass one), can damage or even potentially destroy the companion star.
To study these “spider pulsars,” astronomers looked at nearby globular cluster Omega Centauri with the Chandra X-ray telescope.
By viewing the globular cluster Omega Centauri in both optical and X-ray light, scientists were able to not only identify low-mass X-ray binary systems, but to measure the X-ray flux and how it correlates with the companion mass of the pulsar. There is a relationship, after all.
The new study has bad news for those stellar companions: they’re being murdered by these X-ray emitting pulsars.
Within the globular cluster Omega Centauri, the largest known within the Milky Way, some 10 million stars occupy the space within a diameter of just ~150 light-years. Many millisecond pulsars can be found inside, and the ones with either red dwarf or brown dwarf companions are actively preying on these doomed stars.
Some companions are red dwarf stars, while others are lower-mass failed stars: brown dwarfs.
By observing a large number of low-mass X-ray binary (LMXB) systems with NASA’s Chandra, researchers were able to show that two distinct populations, the “redbacks” (with red dwarf companions) and “black widows” (with sub-stellar mass companions) obey an X-ray brightness-to-companion mass correlation. The heavier the companion, the greater the X-ray energy coming from the system.
The higher the companion star’s mass, the more strongly the system emits X-rays.
When millisecond pulsars are in a tight orbit with a low-mass companion, its emitted “beams” of radiation strike one side of the companion star, heating it up and stripping material away. A new study shows that the higher the mass of the companion, the stronger the pulsar’s X-ray emissions are, which likely lead to a greater rate of mass loss.
And with stronger emitted X-rays from the pulsar, the faster the stellar companion loses mass.
This composite X-ray (red/white) and optical (green/blue) image shows Chandra (X-ray) data superimposed atop visible light observations taken with the Anglo-Australian Telescope. The green bow shock is caused by the pulsar’s magnetic fields and winds as it speeds through the interstellar medium, while the X-rays, emitted by the pulsar, smash into the binary companion, causing it to lose mass.
Pulsars don’t just “steal mass” from their companions, but fire particles back out, further damaging their victims.
Many pulsars with close-in, low-mass binary companions move very quickly through the interstellar medium. The rapid rotation indicates that an elongated, cocoon-like cloud of high-energy particles surrounds these pulsar-companion systems, creating a bow shock along its direction of motion.
For low-mass companions, the term “black widow” has never been more fitting.
This artist’s impression shows the optical features of a millisecond pulsar-brown dwarf system. Pulsars in these systems are known as “black widows” because they prey on their companions, with pulsar winds firing particles and ablating material off of the low-mass companion.
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