A star’s demise is connected to a neutrino outburst

On 1 October 2019, the IceCube Neutrino Observatory in Antarctica detected an exceptionally energetic 0.2 PeV neutrino. The Zwicky Transient Facility in California followed up seven hours later with wide-field observations of the sky at optical wavelengths. The facility observed optical emission in the 90% uncertainty region of the incoming neutrino.

After studying the large energy flux of the optical emission, its location within the reported uncertainty region of the sky where the high-energy neutrino came from, and some modeling results, researchers concluded that the two observations could be connected. ¹ The optical emission was caused by a bright transient phenomenon known as a tidal disruption event (TDE) that had first been observed one year before the neutrino.

TDEs occur when stars get close enough to supermassive black holes to experience spaghettification—the stretching and compressing of an object into a long, thin strand that is caused by the black hole’s extreme tidal forces. (See the article by Suvi Gezari, Physics Today, May 2014, page 37 .)

Two theory papers proposed that neutrinos with energies above 100 TeV, like the 2019 sighting, could be produced in relativistic jets of plasma, which are composed of stellar debris that’s flung outward after such an event. ² Active galactic nuclei (AGNs) and other possible emitters of high-energy neutrinos have been debated in the literature before IceCube detected the first extragalactic ones in 2013 (see the article by Peter Mészáros, Physics Today, October 2018, page 36 ). But with only the one reported TDE–neutrino association from 2019, researchers haven’t been able to conclusively establish TDEs as high-energy neutrino sources.

Now the Zwicky Transient Facility observed another TDE that was located within the uncertainty region of a neutrino detected by IceCube. With their colleagues, Simeon Reusch and Marek Kowalski (German Electron Synchrotron and Humboldt University of Berlin) estimated that the probability of a second such pairing happening by chance is 0.034%, lending more credence to TDEs as sources of high-energy neutrinos. ³

Star light, star bright

The newly observed TDE is located in an AGN—the luminous, compact center of a galaxy. The galaxy in question is 4.4 billion light-years from Earth and has at its center a black hole with a mass of 31.5 million Suns. After a star got close enough to be ripped apart, its remains likely swirled around the black hole, accreted, and began shining brightly across many wavelengths.

That transient flare was first discovered at the Zwicky Transient Facility in May 2019 and reached peak luminosity in August 2019. The associated neutrino was detected by IceCube nine months later, by which time the flare’s flux had decreased by about 30%. Such flares often last several months, although this one was still detectable as of June 2022.

A TDE isn’t the only possible source of the flare. It could have come directly from the AGN. Because AGNs are far more numerous than TDEs, their emission is more common. And the first data of the flare suggested that it could have been a superluminous supernova—a stellar explosion with a luminosity that’s at least 10 times as bright as a typical supernova. To better establish whether a TDE was, in fact, the source of the optical emission, Reusch, Kowalski, and colleagues looked at measurements of the flare that spanned nearly the entire electromagnetic spectrum.

Some of the most useful evidence came from the eROSITA telescope, which is part of the Russian–German Spektr-RG satellite. The instrument scanned the sky location of the putative TDE four times. On the third scan in March 2021, after the peak luminosity of the flare had declined, it detected low-energy, or soft, x-ray emission, which would be quite uncommon for a superluminous supernova.

Sluggish IR

A second critical piece of evidence in support of the TDE came from mid-IR observations collected by the NEOWISE space telescope—NASA’s reactivated Wide-Field Infrared Survey Explorer whose current mission is to identify and characterize near-Earth objects (see Physics Today, March 2015, page 19 ). The mid-IR observations showed a peak IR luminosity that curiously lagged the peak of the optical emission by a year.

“The time delay led us to the dust-echo interpretation,” says Reusch. Figure 1 shows an artistic illustration of the AGN surrounded by a preexisting dust cloud; it heated up and started to glow as light traveled through it. The dust in the immediate vicinity of the disrupted star was destroyed by the TDE’s radiation, which left only the far-flung dust surrounding the TDE.

Figure 1.

In the dust-echo interpretation, some of the IR light emitted from the TDE was absorbed and reemitted by the surrounding dust. Light traveling directly along the line of sight to the TDE arrived at Earth first. IR light from the heated dust that was initially emitted perpendicular to the direct line of sight or from the far side of the system must travel farther and thus arrived months later than the optical emission.

Reusch, Kowalski, and their colleagues first modeled the combined IR, optical, and UV light coming from the TDE using a single blackbody as the radiation source. But the results were inconsistent with the spectral shape of the observations. The best fit, they found, came from a model composed of two blackbodies at different temperatures: one for the TDE emission and the other for the IR dust-echo emission.

From jets, the disk, or wind?

To better understand how the unusually long-lasting TDE may have produced high-energy neutrinos, the research team simulated three possible mechanisms. Besides relativistic jets, a TDE could also generate a disk of gaseous material accreted from the remains of a star. With sufficiently high accretion, collisional plasma in the coronal region of the disk may accelerate particles and produce neutrinos. Such an accretion disk could also launch a subrelativistic wind of ejected material that’s energetic enough for generating neutrinos.

Figure 2 shows the predicted neutrino flux for each of the possible mechanisms as a function of energy. Any of the three mechanisms could reasonably generate a neutrino with the energy (vertical dotted line) observed by IceCube.

Figure 2.

Other details of the TDE–neutrino association remain murky. For example, in their statistical analysis, IceCube researchers couldn’t rule out the possibility that the neutrino may have formed from atmospheric processes on Earth. They concluded that the neutrino had a 59% probability of having an astrophysical origin. ³

Multimessenger astrophysics

Detecting more TDEs and better establishing their relationship to high-energy neutrinos should be possible as early as next year. The Vera C. Rubin Observatory, previously named the Large Synoptic Survey Telescope, is currently being built in Chile. Once it sees first light, its wide-field Simonyi Survey Telescope will have the capability of photographing the entire sky every few nights.

If the TDE–neutrino association is true, TDEs would have to be extremely efficient particle accelerators. The energies of high-energy neutrinos are many orders of magnitude higher than can be reached in even the most impressive terrestrial particle accelerators, and they reach Earth largely unperturbed.

High-energy neutrinos, therefore, are a natural part of a multimessenger astrophysical laboratory. They can’t be controlled or replicated as in a traditional lab experiment, but they can be used to study high-energy processes and test fundamental ideas about particle physics.

For example, some of the densest and most energetic conditions in the universe are found in supernovae. Because neutrinos are very light and only interact by the weak nuclear force, they can pass through the dense core of a supernova and probe the conditions there.

Neutrinos are also theoretically expected to be produced during neutron star mergers. None were seen after the 2017 binary neutron star merger in which gravitational waves and a gamma-ray burst were observed (see “The era of multimessenger astronomy begins ,” Physics Today online, 16 October 2017). Nevertheless, the hunt for them continues. And should any be detected, they may offer bits of information about the density of a neutron star merger and how energy is dissipated from it.