From “Physics Today” : “A star’s demise is connected to a neutrino outburst”

Physics Today bloc

From “Physics Today”

23 Jun 2022
Alex Lopatka

The prospect of high-energy neutrinos being formed by tidal forces ripping apart a star near a supermassive black hole has garnered new support.

(S. Reusch et al., Phys. Rev. Lett. 128, 221101, 2022.)

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Technicians install a camera at the Zwicky Transient Facility. Credit: Caltech/Palomar.

On 1 October 2019 the IceCube Neutrino Observatory in Antarctica detected a 0.2 PeV neutrino.

Seven hours later the Zwicky Transient Facility in California followed up with a wide-field survey of the sky at optical and IR wavelengths. The facility observed optical emission in the direction of the incoming neutrino.

Researchers concluded [Nature Astronomy] that the two observations could be connected after studying the exceptional energy flux of the emission, its location within the reported uncertainty region of the high-energy neutrino, and some modeling results. The optical emission was caused by a bright transient phenomenon known as a tidal disruption event (TDE), and that particular one had first been observed one year before the neutrino. Such events occur when stars get close enough to supermassive black holes to experience spaghettification—the stretching and compression of an object into a long, thin shape due to the black hole’s extreme tidal forces. (See the article by Suvi Gezari, Physics Today, May 2014, page 37.)

A theory paper [Nature Astronomy] 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. TDEs and many other sources for high-energy neutrinos have been debated in the literature. But with only one reported TDE–neutrino association researchers haven’t been able to conclusively establish TDEs as high-energy neutrino sources.

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Credit: S. Reusch et al., Phys. Rev. Lett. 128, 221101 (2022)

Recently the Zwicky Transient Facility observed another TDE that was coincident with a high-energy neutrino detected by IceCube. Simeon Reusch, Marek Kowalski, and their colleagues estimated that the probability of a second such pairing happening by chance is 0.034%, lending more credence to TDEs as a source for high-energy neutrinos.

The second TDE caused a long-duration optical flare which reached its peak luminosity in August 2019. The neutrino was detected by IceCube in May 2020, by which point the flare’s flux had decreased by about 30% from its peak. Such flares often last several months, though this one was still detectable as of June 2022.

To better understand how the unusually long-lasting TDE may have produced high-energy neutrinos, the research team simulated three mechanisms. The figure shows the predicted neutrino flux as a function of energy, and the vertical dotted line indicates the energy of the neutrino observed by IceCube. Any of the three mechanisms could reasonably explain the neutrino. Besides relativistic jets, a TDE could also generate an accretion disk, and emission from its corona or a subrelativistic wind of ejected material may generate neutrinos too.

Other uncertainties remain. The radio-emission measurements of the flare, for example, mean that it could have originated from an active galactic nucleus instead of a TDE. In addition, IceCube’s statistical analysis cannot rule out that the neutrino may have formed from atmospheric processes on Earth.

Although it’ll take more observations to lower those uncertainties, the latest detection of a TDE–neutrino pairing reinforces the significance of TDEs as neutrino sources. And if the association is true, TDEs would have to be surprisingly efficient particle accelerators, a possibility that could only be further studied with more comprehensive multimessenger data.

See the full article here .

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