“Black widow” neutron star takes an hour to orbit the star it’s roasting

Our Sun is on its own in this galaxy, without a nearby companion orbiting with it. But binary star systems are pretty common, and our nearest neighbor appears to be a three-star system. Given how many different types of stars there are, many multistar systems have a strange membership mix, with giant, unstable stars orbiting next to relatively mundane ones.

In Wednesday's issue of Nature, researchers report on a rarity: a "black widow" neutron star that's close enough to its companion to be blasting it with radiation. Should the process continue, it will ultimately lead to the star's evaporation and death. And, just for good measure, the pair also has a distant companion that's an old and rare dwarf star.

Searching for oddities

The work started in the archives of the Zwicky Transient Facility. The ZTF is designed to scan the entire sky in the Northern Hemisphere every two days and uses software to pick out anything that changes. Often, this would mean something blew up: A star suddenly brightens (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: objects that would periodically brighten and fade again. This will often be due to orbiting companions, and the researchers were using their search to specifically look for close-in binaries, where two stars orbit each other at distances that would comfortably fit both within our Solar System. As 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 survey was called ZTF J1406+1222, and it was... odd. Follow-up observations confirmed that the light from the system showed a sine-wave-like pattern, regularly rising and falling. But it did so on a concise schedule, with a periodicity of barely over an hour. And the behavior wasn't due to an eclipse since some wavelengths of light showed a much more significant change than others—some wavelengths saw a 13-fold difference in intensity during an hour-long cycle. If ZTF J1406+1222 involved eclipsing stars, then most wavelengths would see similar changes in their intensity.

Given that the obvious explanation didn't seem to work, the researchers turned to the less-obvious-but-still-plausible explanations. And the one they favored also involved a star orbited by a nearby, not visible companion. But in this case, the unseen companion was producing copious amounts of radiation that was heating the star. This process essentially produces a star with a "daytime" side bathed in radiation, so it's more energetic and brighter, and a "nighttime" side that emits the star's intrinsic brightness.

How much energy is needed to get this sort of luminosity difference? The researchers estimate it in the utterly useless units of ergs/second; put in units that are at least somewhat comprehensible, it works out to be roughly 1012 Megatons per second. Which is, by most standards, a whole lot of radiation.

Webless spiders

There are only a few objects that can produce that sort of radiation. The researchers rule out white dwarfs, which produce lots of radiation in the ultraviolet area of the spectrum; ZTF J1406+1222 doesn't seem to have much in the way of an excess there, which means a white dwarf is unlikely. That leaves us with a neutron star as the most likely explanation.

This is not the first time a system with a close neutron star has been observed. Enough are seen that they've picked up their own terminology. The first one identified picked up the name "black widow pulsar," as the neutron star was bathing its companion in enough radiation to destroy it. Later discoveries of similar systems were lumped together in the category 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. That's quite unusual, given that most stars are composed of highly energetic hydrogen that's doing a lot of emitting. But in this case, it appears that the radiation has driven a significant amount of hydrogen away from the star, where it can absorb radiation from the environment. That's keeping with the idea that this is a black widow system, where the star is destined to evaporate.

ZTF J1406+1222 happens to be the closest black widow binary we've yet identified and raises questions about how it could have formed. But those questions go beyond the black widow binary portion of the system. The observations also revealed that there's a neighboring star that's likely to be gravitationally bound, making it a three-star system. And naturally, that star's a bit on the weird side, too, belonging to a category called (I am not making this up) cool sub-dwarfs. These are very old, and have 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 strange. The outer companion orbits at about 600 Astronomical Units (one AU is the average distance between Earth and the Sun). At this distance, the gravitational attraction is small, and any upset could break up the three-star system. Which is especially strange since the system's orbit takes it near the galactic core, and it probably saw a supernova explosion when the neutron star was formed, meaning ZTF J1406+1222 has had plenty of excuses to break up now.

All of which reinforces the main conclusion of those who discovered it: ZTF J1406+1222 is an interesting system that merits a lot of further observation.

Nature, 2022. DOI: 10.1038/s41586-022-04551-1  (About DOIs).