Our sun is alone in this galaxy, with no close companion orbiting with it. But binary star systems are very common, and our closest neighbor seems to be a triple star system. Given how many different types of stars there are, many multi-star systems have a peculiar organic mixture, 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’s 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, the pair also has a distant companion which is an ancient and rare dwarf star.
Searching for anomalies
Work started on archives Zwicky Transit Facility. ZTF is designed to survey the entire sky in the northern hemisphere every two days and uses software to pick out anything that changes. Oftentimes, this could mean that something is exploding: the star suddenly lights up (in some cases becoming visible from Earth for the first time) because it has exploded as a supernova.
But this search looked for transient changes in brightness: things that would periodically light up and fade again. Often this is due to orbiting mates, and researchers have been using their search to search specifically for close binaries, in which two stars orbit each other at distances that fit snugly with our solar system. Since the two stars eclipse each other from Earth’s perspective, the total amount of light reaching Earth will periodically change.
One of the things that came out of the poll was called a 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 she did it on a brief schedule, with a frequency of barely more than an hour. This behavior was not due to the eclipse because some wavelengths of light showed a more significant change than others – some wavelengths experienced a 13-fold difference in intensity over a one-hour cycle. If ZTF J1406 + 1222 included a stellar eclipse, most wavelengths would experience similar changes in intensity.
Given that the obvious explanation doesn’t seem to work, researchers have turned to the less obvious but still plausible explanations. And the one they favored also included a star around which an invisible close companion orbited. But in this case, the invisible companion was producing copious amounts of radiation that heated the star. This process essentially produces a star that has a “day” side that is bathed in radiation, so it is more energetic and brighter, and the “night” side emits the inner brightness of the star.
How much energy is needed to get this kind of difference in luminosity? Researchers estimate it in completely useless units of ergs/sec; Put the units that are at least somewhat understandable, it can be approximately 1012 megatons per second. Which, by most standards, is a lot of radiation.
spiders 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. ZTF J1406 + 1222 doesn’t seem to have much of a surplus there, which means a white dwarf is unlikely. This leaves a neutron star as the most likely explanation.
This is not the first time that a system close to a neutron star has been observed. We’ve seen enough that they picked up their own terminology. The first person identified picked up the name.Black Widow Pulsar‘in which a neutron star was immersing its companion in enough radiation to destroy it. Subsequent discoveries of similar systems were grouped together in the class of black widow binaries, which became a subset of the general classification of spider binaries.
A closer look at ZTF J1406 + 1222 showed that the star has hydrogen absorption lines in its spectrum. This is quite unusual, given that most stars are made up of high-energy hydrogen that does 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 can absorb radiation from the environment. This is consistent with the idea that this is the Black Widow System, where the star is due to evaporate.
ZTF J1406 + 1222 happens to be the closest black widow duo identified to date and raises questions about how it was formed. But these questions go beyond the black widow’s binary part of the system. The observations also revealed that a neighboring star is likely gravitationally bound, making it a three-star system. And of course, this star is on the weird side too, belonging to a class called (I’m not making this up) cool sub-dwarfs. These are very old and contain 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. The outer companion orbits around 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 break the three star system. Which is especially strange because the system’s orbit approaches the galactic core, and may have seen a supernova explosion when the neutron star formed, which means ZTF J1406 + 1222 has plenty of excuses to break up now.
All this reinforces the main conclusion of those who discovered it: the ZTF J1406 + 1222 is an interesting system that deserves a lot of additional monitoring.