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When White Dwarf Is on the Menu

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Illustration of a star being torn apart by a black hole
Illustration of a star being torn apart by a black hole. [Adapted from NASA/CXC/U. Michigan/J. Miller et al.; Illustration: NASA/CXC/M. Weiss]

Not everything astronomers observe has firmly supported explanations. Recently, however, advanced simulations have supported the hypothesis that certain flashes are the sign of a white dwarf in trouble.

Intermediate Mass, Extreme Danger

Intermediate mass black holes, though several thousand times smaller than their supermassive cousins, share many of the same egotistical personality traits. The more famous gargantuans tend to make themselves the center of attention by living in the middle of large galaxies and surrounding themselves with a dense core of stellar sycophants. Intermediate mass black holes similarly enjoy the spotlight, but on a smaller scale: they inhabit the centers of dwarf galaxies, or even smaller stellar clusters, but also surround themselves with many tightly-packed stars.

As a result of this dense environment, every now and then a star will get gravitationally bumped by its neighbors onto a trajectory that will carry it too close to the central black hole. Once within a certain distance, the star is doomed: as punishment for crossing an unseen barrier, the black hole will stretch the star into a long string of gas, which it will then consume. An even grislier fate awaits hardy white dwarf stars bumped onto very special trajectories that only graze this minimum distance. These stars will continue to circle the black hole on elongated, eccentric orbits, but each time they reach their closest distance, their outermost material will be peeled off and stripped away. Instead of destroying them quickly, the black hole will extend their suffering, slowly consuming them layer by layer, all the while burping out X-rays with each snack.

A Simulated Feast

A 2D heatmap displaying gas density. Stripped gas traces out a figure shaped like the numeral "6", while the dense, still bound gas concentrates at a point along the path.

A snapshot of a hydrodynamical simulation. The white dwarf core is shown in the inset; the long, spiraled streamer of gas represents material that has already been tidally stripped. [Chen et al. 2023]

That’s the story, anyway. Although astronomers have guessed that some strange X-ray flashes and quasi-repeating flares are the signs of the drawn-out ends to white dwarfs, they’ve never been sure since the process has mostly been studied only with analytic approximations. To more confidently attribute these strange observations to the slow deaths of white dwarfs near intermediate mass black holes, a team led by Jin-Hong Chen (Sun Yat-sen University) completed detailed hydrodynamical simulations that more accurately mimic the gruesome process.

A log-log plot of mass loss rate vs. time. The the line appears linear for nearly 1 year, following a t proportional to 5/2 slope, but the diverges towards infinity at the time when the star is destroyed.

The rate at which a white dwarf loses mass to the black hole. Over time, tidal stripping becomes more and more effective, until a certain point at which the white dwarf cannot maintain its structural integrity and is completely disrupted. [Chen et al. 2023]

The team found that yes, if intermediate mass black holes really were feasting on unsuspecting white dwarfs, they would periodically emit bright bursts of X-rays that we could detect with specialized space-based telescopes. Equally exciting, the team also found that if the dance of death were close enough to Earth (within about 100 million light-years, “nearby” by cosmic standards), next-generation gravitational wave detectors could also likely record the inspiral.

Though the instruments needed to record such a signal are still several years away, these accurate simulations of white dwarf tidal stripping will help future astronomers make sense of the strange, somewhat frightening processes that make things flash in the night.

Citation

“Tidal Stripping of a White Dwarf by an Intermediate-mass Black Hole,” Jin-Hong Chen et al 2023 ApJ 947 32. doi:10.3847/1538-4357/acbfb6