Almost two years ago, the IceCube Neutrino Observatory at the South Pole announced evidence for high-energy neutrinos spewing from the “nearby” active galaxy NGC 1068. NGC 1068 is an active galactic nucleus (AGN) with a central, supermassive black hole that actively gobbles up surrounding matter. Classified as a Seyfert galaxy, NGC 1068 is not dominated by a beam of matter shooting out of its supermassive black hole. Even though the first source of high-energy neutrinos identified by IceCube was the blazar TXS 0506+056, a jetted AGN with a jet pointing directly at Earth, IceCube data suggests that gamma-ray emitting blazars are unlikely to explain the majority of the astrophysical neutrinos that IceCube detects every year.
Since the 1970s, AGNs have been discussed as potential neutrino emitters, with the recent NGC 1068 results making a stronger case for them. Unlike gamma rays, which are typically used as a guide in IceCube’s searches for the sources of neutrinos, X-rays can escape the environment around the supermassive black hole and provide information about its properties. Neutrinos could be produced if particles are accelerated and subsequently interact with the X-rays in this environment. Because NGC 1068 is believed to produce the brightest intrinsic X-ray flux of all non-jetted AGNs in the northern sky where IceCube is most sensitive to neutrino sources, researchers shifted focus from gamma-ray emitting blazars to non-jetted AGNs with a high intrinsic flux of X-rays.
In a new study, the IceCube Collaboration searched for additional non-jetted AGNs with high intrinsic X-ray flux that emit high-energy neutrinos. The findings revealed a subset of non-jetted AGNs showing an excess of neutrinos associated with the objects NGC 4151 and CGCG 420-015, resulting in a 2.7σ significance with respect to background expectations. These results are presented in a paper submitted to The Astrophysical Journal alongside a companion IceCube paper searching for neutrino emission from hard X-ray bright AGNs.
“There is more to the story than just identifying a potential main contributor to the astrophysical neutrinos observed by IceCube for over a decade,” says Hans Niederhausen, a postdoctoral research associate at Michigan State University and a study lead. “If non-jetted AGNs commonly produce neutrinos in the universe, neutrino astronomy may provide an independent and complementary probe of the difficult-to-study vicinity of the supermassive black hole, where particles may be accelerated.”
Niederhausen led the study along with Qinrui Liu, postdoctoral fellow at Queen’s University; Tomas Kontrimas, PhD student at Technical University of Munich (TUM); Theo Glauch, who was a postdoctoral research associate at TUM when he contributed to the work; and Ali Kheirandish, assistant professor at the University of Nevada, Las Vegas.
Using 10 years of IceCube data that recorded throughgoing neutrino events from the northern sky, the team searched for individual and aggregated neutrino signals from 27 additional Seyfert galaxies—a subset of non-jetted AGNs—that are contained in the Swift-BAT AGN Spectroscopic Survey (BASS).
Each candidate source was searched individually for signs of point-like neutrino emission above background expectations, with the Seyfert galaxies NGC 4151 and CGCG 420-015 potentially showing just that. A statistical combination of all 27 individual results quantified the excess as 2.7σ, which falls just shy of the 3.0σ threshold required for evidence in the field of particle physics. Future work will show whether this is a statistical fluctuation or a first sign of a genuine astrophysical signal.
“While we cannot claim evidence for neutrinos from additional AGNs, our results encourage further studies of non-jetted, X-ray bright AGNs not only with neutrino analyses but also through multimessenger astronomy,” says Liu.
These results also support the idea that AGNs could be one of the primary contributors to the high-energy neutrino flux.
“Our findings demonstrate that identification of sources similar to NGC 1068 is feasible with IceCube,” says Ali Kheirandish. ”While this search focused on the sources in the Northern Hemisphere, there are prominent bright Seyfert galaxies in the southern sky, which we are going to be looking at next.”
An upcoming IceCube analysis will enable the development of new event selection techniques with enhanced sensitivity to the southern sky, which will provide further insight into the origin of high-energy neutrinos.
“Improvements to track reconstruction will also enhance the sensitivity to the sources discussed in the current work,” says Kontrimas. ”And of course, IceCube will keep running, supplying us with more and more data.”
+ info “IceCube Search for Neutrino Emission from X-ray Bright Seyfert Galaxies,” IceCube Collaboration: R. Abbasi et al., Submitted to The Astrophysical Journal Letters, arxiv.org/abs/2406.07601