A pulsar and a star orbit every 62 minutes. The Fastest “Black Widow” Binary Ever

Caption: An illustrated view of a Black Widow pulsar and its stellar companion. Gamma ray emissions from the pulsar (magenta) strongly heat the opposite side of the star (orange). The pulsar gradually evaporates its partner. Courtesy of NASA Goddard Space Flight Center/Cruz deWilde

The Milky Way galaxy has its share of oddities, from black holes and magnetars to bright blue variable stars and strange new worlds. But have you ever heard of a “black widow binary?” It’s not exactly an easy name to understand, especially if you’re afraid of spiders. But, these things actually exist in our galaxy and they are fascinating.

Imagine a binary pair in space – a regular star dancing in orbit with a pulsar (a rapidly spinning neutron star). Now visualize the pulsar with its strong gravitational pull cannibalizing its companion star. It literally shoots tufts of gas out of the star so it can have something to eat. Since pulsars tend to slow down over time, this stellar “happy meal” breathes new life into the pulsar. Unfortunately, the companion star is destroyed in the process.

Why the name “black widow”? This cannibalizing action is quite similar to what a black widow spider here on Earth does to her male mate after they mate. Astronomers liked the analogy, and lo and behold, we now have black widow binaries.

Find an Unusual

The latest Black Widow binary is even weirder than its two dozen siblings in the galaxy. Not only does it gobble up its companion, but the hapless nearby star suffers its fate as it finds itself in a rapid 62-minute orbit with the pulsar. It’s one of the fastest orbital dances ever seen in any of these binaries.

That’s not all. As they say in late night TV commercials, wait, there’s more. An additional star is part of this system. It sits quite far from the central pair and moves in a very large orbit that takes it into the vicinity of the pulsar and the slowly eroding companion star once every 10,000 years. The presence of the second star makes it a triple black widow system. As you can imagine, the very existence of this oddly interactive triplet raises many questions, primarily aimed at finding it in the first place and then understanding how the group formed in the first place.

Searching for that rare binary

The system has a catalog name that looks like a secret code: ZTF J1406+1222. The numbers actually tell you its coordinates in the sky. The cluster is about 3,000 light-years from Earth, actually relatively close in terms of galactic distance. Astronomers at MIT first observed it and its dying companion by tracking the gamma-ray and X-ray emissions emitted by the pulsar. It turns out that every binary black widow astronomers have found so far has been detected by the gamma-ray and X-ray emissions it emits. The twist to finding this one was to look for something a little different as it’s not very bright in those regions of light.

In this case, the rapidly disappearing star was an obvious clue. Since astronomers did not directly detect gamma and X-ray emissions from the companion pulsar, they got creative and focused on finding the nearest companion star. According to astronomer Kevin Burdge, a postdoctoral researcher at MIT and first author of an article in Nature About this wacky system, the overheated side of the cannibalized star made it easier to find. “I thought, instead of looking directly for the pulsar, try looking for the star it’s cooking,” he said in a recent press release.

It turns out that the “day side” of the star – the hemisphere that constantly faces the pulsar – is crowded with high-energy pulsar radiation. It gnaws at the star. This bath of radiation makes the day side of the companion star much hotter than its night side.

Using all data to find the system

Burdge and his team at MIT used observations from a number of sources to locate this system. This included the Gaia Space Telescope database, which regularly explored the galaxy for us, the Sloan Digital Sky Survey, and the optical information of the Transitional installation of Zwicky in California search for the system and characterize its orbit. They found the dozen known binary widows by testing their observing methods, then spotted a star that varied in brightness by a factor of 13 every 62 minutes. It was an “ah!” moment and that’s how they found ZTF J1406+1222.

Illustration of the Gaia space telescope.  one of the sources used by astronomers at MIT to search for the black widow binary.  Credit: Kavli Institute for Cosmology, Cambridge.
Illustration of the Gaia space telescope. one of the sources used by astronomers at MIT to search for black widow binaries. Credit: Kavli Institute for Cosmology, Cambridge.

The origin of this Triple Black Widow binary

The evolutionary history of this unusual grouping is complex. According to Burdge and his team, getting a Triple Black Widow like this requires three things: a globular cluster, the Milky Way’s core, and our galaxy’s central black hole (called Sagittarius A*). “It’s a complicated birth scenario,” Kevin Burge said. “This system has probably been floating around in the Milky Way for longer than the Sun has existed.”

The ZTF J1506+1222 Triple Black Widow System likely originated inside a globular cluster that was one of many encircling the Milky Way’s core billions of years ago. The Milky Way currently has about 150 (and possibly more) of these ancient, dense star systems orbiting the bulge. This is not unusual, as many galaxies have these clusters and they have played an important role in galactic history. Anyway, the original cluster of ZTR J1506+1222 had the misfortune to wander too close to Sagittarius A* and its supermassive black hole. This tore the cluster apart, but somehow the Triple Black Widow System survived this encounter. Since then, he travels across the galaxy.

Could there be more?

Are there other such systems? Burge does not rule them out. “The only thing we know for sure is that we see a star with a much hotter dayside than the nightside, orbiting something every 62 minutes,” he said. “Everything seems to indicate that this is a black widow binary. But there are a few weird things about it, so it’s possible this is something entirely new.

The team will continue to monitor this new system to see what it does next. What they learned from this discovery will help them in their search for more black widow binaries in the galaxy.

This work was supported in part by the National Science Foundation.

For more information:

Astronomers discover a rare “black widow” binary with the shortest orbit to date.

A black widow binary with an orbital period of 62 minutes in a broad hierarchical triplet

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