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A New Spider Joins a Deadly Club

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An artists depiction of a star around a pulsar slowly being torn apart.
A black widow pulsar begins consuming its companion. [NASA's Goddard Space Flight Center]

Tiny but deadly, black widow pulsars are some of the cruelest astronomical objects in the galaxy: first they consume most of their companion, then they destroy the remains. A recent study has caught yet another in the final act of this gruesome sequence and draws insights from the population of these celestial arachnids as a whole.

New Specimen

As tranquil as the night sky can be, some truly vicious monsters lurk above us. Pulsars, the highly magnetized zombie remains of a supernova, are scary enough, but certain subpopulations take it a step further. Black widow pulsars, keeping with their terrestrial namesakes, prey upon larger nearby stars first with extreme gravitational tides and later with venomous doses of high-energy radiation. Stars partially devoured by these beasts must end their lives in a doomed fight to avoid dissolution by these high-energy winds, and their struggle releases gamma rays, X-rays, and occasional optical signatures detectable here on Earth.

The list of systems in the midst of such death throes is short but growing, and recently a study led by Samuel J. Swihart (National Academy of Sciences, US Naval Research Laboratory) has lengthened it further. They report the discovery of J1408, the forty-first known black widow pulsar, caught in the act of destroying a stellar companion on a blisteringly fast 3-hour orbit.

Two telescope images, one an enlargement of a portion of the other

Images of the newly discovered black widow pulsar, along with representative ellipses marking the resolution of various high-energy telescopes used in the analysis. [Swihart et al. 2022]

Confirming this murder-in-progress required a fleet of telescopes spanning the entire electromagnetic spectrum. After the broad region surrounding J1408 appeared in a catalog of Fermi gamma-ray sources, Swihart and collaborators narrowed in on the precise source and its nature using two X-ray telescopes, two optical telescopes, a radio observatory, and data from the Gaia spacecraft. By combining data from these disparate tools and techniques, the team conclusively showed that all measurements could be explained by a black widow on the hunt.

Insights from the Collection

A scatter plot of mass on the X axis and period on the Y.

The mass and period of many known millisecond pulsars, colored by subpopulation. Note the lack of objects between 0.07 and 0.1 solar mass. [Swihart et al. 2022]

Following this discovery, the team took a step back from their new object and considered the population of spider pulsars as a whole. They started by curating a collection of measurements for all black widows known to date, then compared these to a closely related species known as “redback” pulsars. This other group of venomous neutron stars has a taste for higher-mass companions, but interestingly, the transition between black widow and redback companions is not smooth. Instead, this comparison emphasized a previously noted discontinuity: while black widows prey on everything below 0.07 solar mass and redbacks hunt stars above 0.1 solar mass, astronomers have yet to find a companion that falls between those values.

Why that might be remains a mystery, since redbacks and black widows are thought to form via the same pathway. Regardless of the reason, it seems that at least some stars might be immune to spider bites.

Citation

“A New Flaring Black Widow Candidate and Demographics of Black Widow Millisecond Pulsars in the Galactic Field,” Samuel J. Swihart et al 2022 ApJ 941 199. doi:10.3847/1538-4357/aca2ac