When Black Holes Shred White Dwarfs, the Result Can Be a Lopsided Supernova

New simulations show that a white dwarf torn apart by an intermediate-mass black hole can detonate into a thermonuclear supernova. This explosion resembles those from double-white-dwarf mergers, but its brightness and spectra vary dramatically depending on viewing angle.

The study, recently published in Astronomy & Astrophysics, was led by CITA Postdoctoral Fellow Pavan Vynatheya and Luc Dessart from the Institut d’Astrophysique de Paris, as well as co-authored by Taeho Ryu and Rüdiger Pakmor. The team combined high-resolution hydrodynamic simulations with detailed radiative-transfer modelling to predict, for the first time at this level of detail, what these rare events would actually look like from Earth.

Collage of debris resulting from white dwarfs tidal disruption events for eight different values of impact parameter (closest approach distance to black hole). The orange, blue, and pink colours represent mass densities of three selected isotopes: oxygen-16, silicon-28, and nickel-56, respectively. The diverse array of isotope densities arises from varying extents of nuclear burning in the white dwarfs, resulting in onion-shell-layered structures of isotopes.

A white dwarf’s close call

Intermediate-mass black holes — weighing in at somewhere between 100 and 100,000 times the mass of the Sun — are thought to lurk in the dense cores of some star clusters, but they are notoriously difficult to detect directly. One promising way to find them is to catch them in the act of tidally disrupting a passing star. Because white dwarfs are so dense, they can only be torn apart by black holes in this intermediate mass range; if a white dwarf strays too close to a much larger supermassive black hole, it disappears without a trace.

Using the moving-mesh hydrodynamics code AREPO, equipped with a 55-isotope nuclear reaction network, the team modeled the disruption of a compact carbon-oxygen white dwarf by a black hole roughly 500 times the Sun’s mass, varying how close the encounter was. This was followed by radiative-transfer calculations using the codes CMFGEN and LONG_POL to understand the observations of such events. Their key findings:

• A vast array of outcomes: The closest encounters compress the white dwarf so violently that runaway nuclear fusion ignites, converting anywhere from about 1% to as much as 82% of the star’s material into radioactive nickel-56 — the same isotope that powers ordinary Type Ia supernovae — depending on exactly how close the pass was. More distant encounters simply disrupt the star without triggering an explosion.
• A spectrally-diverse, lopsided explosion:  The disrupted debris has an onion-shell-layered structure of isotopes and fans out towards one direction, material spreading across an extremely wide range of velocities.
• It depends where you’re standing: Radiative-transfer modeling of the resulting light shows rise times and peak brightness comparable to normal Type Ia supernovae — but with an unusually strong dependence on viewing angle, meaning the same explosion could look very different to observers positioned in different directions. Models of spectra reveal a diverse range of potentially observable spectral lines depending on the extent of nuclear burning in the disrupted white dwarf. Months later, characteristic emission lines in the fading afterglow can appear shifted and distorted by thousands of kilometers per second, offering a potential telltale signature of these events.

Why it matters

Because white dwarfs cannot survive close encounters with far larger supermassive black holes, catching one of these disruption events in progress would be strong, direct evidence for the presence of an elusive intermediate-mass black hole — the missing link between stellar-mass and supermassive black holes. With next-generation surveys such as the Vera C. Rubin Observatory expected to discover far more tidal disruption events than ever before, the distinctive light curves, spectra, and viewing-angle dependence identified in this study could help astronomers pick out these rare white dwarf disruptions from the broader population of transients — and, in the process, help pin down where intermediate-mass black holes are hiding.

Publication details

Pavan Vynatheya, Luc Dessart, Taeho Ryu, and Rüdiger Pakmor, “Asymmetrical thermonuclear supernovae triggered by the tidal disruption of white dwarfs,” accepted for publication in Astronomy & Astrophysics, 2026. https://www.aanda.org/articles/aa/full_html/2026/05/aa59171-26/aa59171-26.html

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