Identified as neutrinos, these tiny subatomic particles carry minimal mass and lack electric charge. Often referred to as “phantom particles,” this is due to their ability to glide without leaving a mark through gases, dust, and even celestial bodies. High-energy neutrinos traverse indiscriminately across the universe, relaying details about far-off locations. Yet, the origins of these particles have usually been enigmatic.
Now, scientists have detected the initial evidence of high-energy neutrinos originating from within our Milky Way. They charted the particles to construct a novel portrait of our galaxy. It’s the inaugural image crafted with something other than photons. The chart also provides clues concerning potential origins for these high-energy neutrinos. They may be the residual effects of previous supernovas — stellar detonations. Alternatively, they might emerge from the centers of imploded supergiant stars or other unclassified celestial objects. Further investigation is required to determine the precise origins of these neutrinos.
In the past, only a handful of high-energy neutrinos were tracked to their conceivable origin. They all emanated from beyond the Milky Way. A pair seemed to originate from black holes tearing apart their partner stars. Some were sourced from a category of galaxy termed a blazar. It’s now evident that scientists are detecting neutrinos from both within and outside our galaxy, asserts Kate Scholberg. A physicist at Duke University in Durham, N.C., who did not contribute to the recent charting endeavor, she remarks, “There’s an abundance yet to discover,” she articulates. “It can be immensely enjoyable to unravel how to perceive the cosmos through the lens of neutrinos.”
The neutrino “visual receptors” may someday enable us to perceive remote objects in a manner that surpasses all other telescopes. While some telescopes depend on observable light, others detect X-rays, gamma rays, or the ionized particles constituting cosmic rays. All these light forms may be diverted or soaked up as they journey through the cosmos. Neutrinos, however, can traverse vast distances without deviation. This permits the particles to convey information about objects far away.
The characteristic of neutrinos to penetrate objects with ease also renders them exceptionally difficult to discover. Scientists identified particles from the Milky Way using a neutrino detection device in Antarctica, named IceCube, which is buried deep within the ice. In order to more effectively sense these elusive neutrinos, it is gigantic, consisting of 5,160 sensors configured in a one-kilometer cube (3,281 feet) on every side.
Nevertheless, the experiment only captures a minuscule fraction of the neutrinos that dart through space. IceCube researchers register approximately 100,000 neutrinos annually. Some of these neutrinos create paths within the detector, and scientists can occasionally trace these paths to their source. The majority of neutrino signals that IceCube detects, however, are classified as cascade events. These produce flashes of light within the detector but do not disclose a neutrino’s origin as precisely as paths do.
Astronomers once discarded information concerning cascade events, informs Naoko Kurahashi Neilson, a physicist at Drexel University in Philadelphia, Pa. Such data may contain valuable insights into the neutrinos’ origins, but it’s challenging to distinguish the most significant ones among the tens of thousands of cascade events.
Kurahashi Neilson and her group accepted this daunting task, sifting through ten years of IceCube cascade-event records. They recruited the assistance of a machine-learning system referred to as a neural network. “You can educate the neural networks to discern which events are worth retaining,” explains Kurahashi Neilson.
She initiated this method in 2017 and has progressively refined it over time. Kurahashi Neilson and her peers have recently applied it to pinpoint the neutrinos employed to create the new chart. “This is an admirable analysis,” Scholberg comments, with the method possibly having further room for development. “Certainly, substantial additional effort is required,” she notes, “but it’s thrilling to witness the fundamental anticipation [of Milky Way neutrinos] confirmed.”