A single high-energy neutrino could have supplied astrophysicists with telltale details about the tidal disruption of a star by a supermassive black gap some 750 million light-years away.
The Neutrino and the Star-nabbing Black Gap
The sub-surface IceCube Neutrino Observatory in Antarctica detected the neutrino on October 1, 2019. The tiny, ghostlike particle was packing a punch of about 200 tera-electronvolts (15 instances extra energetic than what might be achieved in CERN’s Massive Hadron Collider). It got here from the route of the small constellation Delphinus. Whereas IceCube can’t exactly pinpoint the particle’s origin, the detector can slim down the supply to inside an space of roughly 10 sq. levels on the sky.
“Inside seven hours, we have been observing the world with the Zwicky Transient Facility (ZTF),” says Robert Stein (DESY, Germany). ZTF is a wide-field sky survey using the refurbished Samuel Oschin 1.2-meter Schmidt telescope at Palomar Observatory.
Earlier than lengthy, Stein and his colleagues discovered the still-fading glow of a tidal disruption occasion (TDE) that had been detected on April 9, 2019, additionally by ZTF. These occasions happen when stars enterprise too near a supermassive black gap; they flare as they’re torn to shreds within the robust gravitational area.
“Clearly, this was probably the most promising candidate” for the neutrino’s origin,” Stein says. Theorists had already advised that TDEs would possibly produce high-energy neutrinos, probably in relativistic jets of plasma that shoot out from close to the black gap.
The April ninth TDE, catalogued as AT2019dsg, was a relatively particular beast. The optical/ultraviolet outburst, which peaked in Might, occurred within the nucleus of a comparatively luminous galaxy, suggesting that a star had been destroyed by the tidal forces of a large central black gap, most likely weighing in at some 30 million photo voltaic lots.
“I’m excited and pleased that individuals are discovering this potential affiliation,” feedback Dorothea Samtleben (Dutch Nationwide Institute for Subatomic Physics, Amsterdam), who was not concerned with the discover.
Nonetheless, she warns that it’s not a closed case — because the researchers word, there’s nonetheless a zero.5% likelihood that the neutrino and the TDE don’t have anything to do with one another.
In keeping with Sjoert van Velzen (Leiden Observatory, The Netherlands), who led the staff that originally found the tidal disruption occasion, the 18th-magnitude supply began to provide brilliant however quickly fading X-rays after 5 weeks, whereas emitting growing quantities of radio waves for a lot of months after the preliminary explosion.
The radio waves end result from high-energy charged particles spiraling round magnetic area traces. Apparently, the TDE is an environment friendly particle accelerator. “The radio knowledge inform us that the particle acceleration course of have to be fairly steady,” says Stein. “It’s not like an explosive engine,” provides van Velzen, “however extra like a locomotive.”
This could clarify how a TDE can produce a high-energy neutrino virtually half a yr after the beginning of the occasion. To create a neutrino, a high-energy proton should slam into one other proton or right into a photon. If particles are accelerated over an extended time period, there’s ample alternative for neutrino manufacturing at a late stage.
How Do Black Holes Make Neutrinos?
Within the February 22nd concern of Nature Astronomy, Stein, van Velzen, and 56 coauthors describe a “multi-zone” state of affairs for the TDE. In keeping with this state of affairs, the extraordinarily sizzling internal a part of the transient accretion disk that kinds from the torn-apart star’s stays generates X-rays near the black gap’s occasion horizon. In the meantime, at bigger distances, 40,000-degree gasoline, additionally from the star, emits ultraviolet radiation. Radio waves are produced even additional out, in a broad outflow. Curiously, the researchers didn’t discover any proof for relativistic jets.
Of their view, shock waves or magnetic fields speed up the protons which are near the central black gap. The energized protons then collide with the plentiful ultraviolet photons farther out to create high-energy neutrinos that escape the system. One of many neutrinos touring in Earth’s route occurred to get caught within the IceCube detector some 750 million years later.
Writing in the identical concern of Nature Astronomy, Walter Winter (DESY) and Cecilia Lunardini (Arizona State College) recommend a special state of affairs, partly primarily based on the truth that the TDE’s X-ray emission light so quickly. Key to their state of affairs is the concept the TDE did produce jets of charged particles rushing away from the black gap.
“The thought is that the [UV-producing] outflow expands and begins to trigger a partial obscuration of the X-rays emitted by the accretion disk,” explains Lunardini. “The X-ray photons which are blocked by the outflow are re-emitted in all instructions, and a few of them find yourself contained in the jets.” When high-velocity protons within the jets collide with these X-ray photons, they produce high-energy neutrinos.
A lot stays unsure, although. “With just one neutrino noticed from a TDE, you can’t actually draw agency conclusions, and the information stay open to interpretation,” admits Lunardini. Stein warns that not everybody agrees on why the X-rays disappeared so shortly. As for the existence of relativistic jets, there’s no corroborating gamma-ray proof.
Thus far, the Zwicky Transient Facility has discovered a pair dozens of tidal disruption occasions, however a few of their options elude rationalization. “Neutrinos can assist to be taught extra about TDEs and their variety sooner or later,” Lunardini notes.
“Crucial conclusion,” says Stein, “is that TDEs can speed up particles for such a very long time,” as evidenced each by the radio observations and by the late-time manufacturing of the high-energy neutrino.
Samtleben, who’s concerned with the KM3NeT neutrino detector that’s being constructed within the Mediterranean Sea, says she seems to be ahead to a extra strong hyperlink between TDEs and high-energy neutrinos. Each KM3NeT and the upcoming improve of IceCube may present extra examples with higher pointing accuracy.
Different neutrino detectors will proceed to play a task, too. Already, scientists have reported a second potential TDE-neutrino affiliation, primarily based on a neutrino detected in Might 2020, she says.