Our sun is alone in this galaxy, with no close companion orbiting it. But binary star systems are very common, and our closest neighbors appear to be triple star systems. Given the many different types of stars, many multi-star systems have an odd organic mix, with unstable giant stars orbiting alongside relatively regular stars.
In Wednesday’s issue of Nature, researchers report a rarity: a “black widow” neutron star that is close enough to its companion to blast it with radiation. If the process continues, it will eventually cause the star to evaporate and die. And for good measure, this pair also has a distant companion which is an ancient and rare dwarf star.
Looking for anomalies
Work started on archive Zwicky Transit Facilities. ZTF is designed to survey the entire sky in the Northern Hemisphere every two days and use software to pick out anything that changes. Oftentimes, this can mean something exploded: a star suddenly lit up (in some cases becoming visible from Earth for the first time) because it had exploded as a supernova.
But this quest looks for temporary changes in brightness: things that will periodically turn on and fade away again. Often this is due to orbiting pairs, and researchers have used their search to specifically look for close binaries, where two stars orbit each other at a distance that fits within our solar system. Because the two stars eclipse each other from Earth’s perspective, the total amount of light reaching Earth will change periodically.
One of the things that came out of that poll was called ZTF J1406+1222, and it was…weird. Follow-up observations confirmed that the light from the system exhibited a sine wave-like pattern, rising and falling regularly. But he did so on a short schedule, with a frequency of barely more than an hour. This behavior is not caused by the eclipse because some wavelengths of light exhibit more significant changes than others – some wavelengths experience a 13-fold difference in intensity over the course of a one-hour cycle. If ZTF J1406 + 1222 included a stellar eclipse, most of the wavelengths would experience a similar change in intensity.
Given that the obvious explanation didn’t seem to work, the researchers turned to a less obvious but still plausible explanation. And what they love also includes a star surrounding them with close, invisible friends. But in this case, the invisible companion produces a large amount of radiation that heats the star. This process essentially results in a star that has a “day” side that is bathed in radiation, making it more energetic and brighter, and a “night” side that radiates the inner brightness of the star.
How much energy would it take to get this difference in luminosity? The researchers estimated it in completely useless ergs/sec units; Put the unit that is at least somewhat understandable, can be approximately 1012 megatons per second. Which, by most standards, is a lot of radiation.
spider without web
There are only a few things that can produce this type of radiation. The researchers excluded white dwarfs, which produce a lot of radiation in the ultraviolet region of the spectrum. The ZTF J1406+1222 doesn’t seem to have much of a surplus in there, which means there’s no way a white dwarf could exist. This leaves a neutron star as the most likely explanation.
This is not the first time a system close to a neutron star has been observed. We’ve seen enough that they take their own terminology. The first person identified took the name.Black Widow Pulsar‘ where the neutron star drowns its companion in enough radiation to destroy it. Subsequent discoveries of similar systems were grouped together in the black widow binary class, which forms part of the general classification of spider binaries.
A closer look at ZTF J1406 + 1222 shows that the star has hydrogen absorption lines in its spectrum. This is very unusual, considering that most stars are made up of high-energy hydrogen that produces a lot of emissions. But in this case, the radiation appears to have pushed a large amount of hydrogen away from the star, where it could absorb radiation from the surroundings. This is consistent with the idea that this is a Black Widow System, where stars will evaporate.
ZTF J1406+1222 happens to be the closest black widow duo identified to date and raises questions about how it came to be. But these questions go beyond the black widow binary part of the system. Observations also reveal that the neighboring star is likely gravitationally bound, making it a triple star system. And of course, this star is also on the weird side, belonging to a class called (I’m not making this up) the cool sub-dwarfs. It is very old and contains very low levels of elements other than hydrogen and helium.
Finally, not only are the individual components of this system weird, but the system as a whole is pretty weird too. The outer companion orbits about 600 astronomical units (one AU is the average distance between the Earth and the Sun). At this distance, the gravitational force is small, and any disturbance can damage the three-star system. Which is especially odd because the system orbits close to the galactic core, and may have seen a supernova explosion when the neutron star formed, meaning ZTF J1406+1222 has plenty of reason to break up now.
All this reinforces the main conclusion of those who discovered it: The ZTF J1406 + 1222 is an interesting system that requires a lot of additional monitoring.
temper tantrums2022. DOI: 10.1038 / s41586-022-04551-1 (About DOI).
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