AAS Nova

When Stars, Disks, and Black Holes Meet, Outbursts Follow

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Illustration of an accretion disk swirling around a supermassive black hole
Illustration of an accretion disk swirling around a supermassive black hole. Accretion disks around supermassive black holes may provide the setting for quasi-periodic eruptions. [NOIRLab/AURA/NSF/P. Marenfeld; CC BY 4.0]

Astronomers have discovered a handful of supermassive black holes that are associated with bursts of X-rays that recur every few hours or days. New work expands on a theory that explains these X-ray outbursts and explores how we can spot them at other wavelengths.

Investigating Quasi-periodic Eruptions

For the past few decades, researchers have seen X-ray flares coming from the centers of certain galaxies. These flares, dubbed quasi-periodic eruptions, last about an hour, and individual flares are separated by a few hours to a few days. The brightness changes from flare to flare, and the flares don’t recur at precise intervals, giving the events their name. The cause of quasi-periodic eruptions isn’t yet known, but leading models involve a star being partially torn apart by a supermassive black hole or an object interacting with gas accreting onto a black hole.

diagram showing the star–disk–black hole model proposed by the authors

A simple diagram showing the proposed model. A star orbits a supermassive black hole on an orbit that is slightly elliptical (exaggerated here for clarity) and inclined relative to the accretion disk (yellow). The X’s mark the places where the star passes through the disk. The time between crossings at A and B is shorter than the time between crossings at B and A. [AAS Nova/Kerry Hensley]

Previously, Itai Linial (Institute for Advanced Study and Columbia University) and Brian Metzger (Columbia University and Flatiron Insitute) proposed a model that explained a curious feature seen in two quasi-periodic eruption sources: a repeating pattern in which pairs of flares are separated by alternating long and short intervals. In this model, a star near a supermassive black hole collides repeatedly with an accretion disk. The disk might form when the star loses gas to the black hole’s gravitational pull, or it might come from an unrelated star that’s being ripped apart. Each time the star plunges through the disk, shocks heat a spray of gas that emits an X-ray flare. If the star’s orbit is elliptical and inclined relative to the accretion disk, this model naturally explains why the flares aren’t precisely periodic: flares are more closely spaced when the star travels a shorter distance between disk crossings, and they’re farther apart when the star travels farther between crossings.

An Ultraviolet Possibility

Now, Linial and Metzger have explored the implications of this model further, delving into the properties of quasi-periodic eruptions to understand if these events can only be observed at X-ray wavelengths, or if it’s possible to see them at other wavelengths as well. Because there are several planned and proposed ultraviolet survey missions, the team focused on the possibility of observing quasi-periodic eruptions in the ultraviolet. By outlining the equations that describe a star punching through a thin disk of gas around a supermassive black hole, the authors predicted how the brightness and timing of the flares depend on the size and mass of the star, how quickly the black hole is accreting gas from the disk, and other factors.

plot summarizing the conditions under which quasi-periodic eruptions emerge in the X-ray versus the ultraviolet

Luminosity of quasi-periodic eruptions as a function of accretion rate. This plot demonstrates how a system might transition from producing X-ray flares to ultraviolet flares, or vice versa, as the accretion rate changes. Click to enlarge. [Linial & Metzger 2024]

To be able to spot a flare, it must outshine the hot, glowing gas of the disk, which may be possible in two cases. In the first, stars on short orbits around relatively massive black holes — in the few-million-solar-mass range — produce bright X-ray flares. In the second, stars on long orbits around less-massive black holes — in the few-hundred-thousand-solar-mass range — produce bright ultraviolet flares. It may even be possible for X-ray flares to reinvent themselves as ultraviolet flares after fading from view at X-ray wavelengths, or vice versa, as the system evolves: if the accretion disk formed out of a disrupted star, the accretion rate will eventually drop, shifting the peak of the flare emission from X-rays to ultraviolet over time. It’ll be fascinating to see these predictions tested when future ultraviolet surveys get underway!

Citation

“Ultraviolet Quasiperiodic Eruptions from Star–Disk Collisions in Galactic Nuclei,” Itai Linial and Brian D. Metzger 2024 ApJL 963 L1. doi:10.3847/2041-8213/ad2464