Many galaxies, including our own, have a central supermassive black hole. In certain galaxies, gas becomes ensnared in the black hole’s gravitational pull and collects in a disk that feeds the black hole, forming an active galactic nucleus (AGN). An AGN can feast on this disk of gas and dust for millions to billions of years — glowing across the electromagnetic spectrum, brandishing relativistic jets, and creating a brilliant, variable light show visible from billions of light-years away.

Today’s Monthly Roundup investigates the connection between AGNs and neutrinos, considers the question of AGN variability, and explores modeling techniques for relativistic environments.

A Search for Neutrinos from X-Ray-Bright AGNs

Where do neutrinos — neutral, nearly massless elementary particles — come from? AGNs have emerged as one likely source; in 2022, the IceCube collaboration reported evidence for neutrinos from the nearby galaxy Messier 77, which hosts an X-ray-bright AGN, and earlier observations potentially linked a neutrino to the blazar TXS 0506+056.

To investigate the possible connection between neutrinos and X-ray-bright AGNs, the IceCube collaboration performed a dedicated search for neutrinos toward a collection of X-ray-bright AGNs in the northern sky. The team’s initial analysis confirmed that Messier 77 is the strongest source of neutrinos in that area. Focusing specifically on X-ray-bright AGNs with hard X-ray fluxes at least 20% of Messier 77’s flux — 47 AGNs in total — the team found evidence for a possible neutrino excess toward 11 of the galaxies, with NGC 7469 being the strongest candidate.

This result suggests that X-ray-bright AGNs are indeed a plausible source of neutrinos. As for the specific physical origin of these AGN-generated neutrinos, the data provide a few clues. The X-ray emission of an AGN is thought to arise from a billion-degree cloud of plasma called the corona. The interaction of coronal X-rays with high-energy protons is thought to produce neutrinos in the energy range of 1–10 teraelectronvolts — roughly the energies of the neutrinos associated with Messier 77 in this study. However, NGC 7469 was associated with neutrinos more energetic than this range, suggesting that not all AGN-produced neutrinos have a coronal origin. Thus, though the body of evidence suggesting that X-ray-bright AGN are a source of neutrinos, the precise mechanism through which these neutrinos are produced remains unknown.

Disk or Jet: Which Varies More?

If there’s one constant when it comes to AGNs, it’s change; observations show that the emission from active galaxies varies on timescales from minutes to decades. Though both the accretion disk and the jet contribute to the overall multiwavelength behavior of an AGN, it’s not yet clear which of these structures is a greater contributor to an AGN’s variability.

Vineet Ojha (Peking University) and coauthors considered AGN variability at optical and mid-infrared wavelengths. Ojha’s team collected a sample of AGN that lie at similar redshifts and fall into one of three categories: narrow-line Seyfert 1 galaxies detected in gamma rays, narrow-line Seyfert 1 galaxies not detected in gamma rays, and broad-line Seyfert 1 galaxies detected in gamma rays. These three classes are distinguished by the presence or absence of relativistic jets as well as their accretion rate; gamma-ray-detected Seyfert 1s likely host jets, and narrow-line Seyfert 1s likely have higher accretion rates than broad-line Seyfert 1s.

Using optical data from the Zwicky Transient Facility and mid-infrared data from the Wide-field Infrared Survey Explorer, the team separated statistical wiggles from true variability and determined which sample of AGNs was most variable. They found that broad-line AGNs detected in gamma rays are the most variable, suggesting that jets are major contributors to AGN variability. Narrow-line AGNs detected in gamma rays are next in line, likely because these AGNs have jets but also have a strong thermal emission component from their accretion disks due to their high rate of accretion. Narrow-line AGNs not detected in gamma rays are the least variable, lacking a jet and dominated by thermal emission. Taken together, these results suggest that AGN variability is mostly jet driven, with some contribution from accretion disk instabilities.

More on AGN Jets: The Slow-Light Effect

The galaxy Messier 87 (M87) hosts one of the most studied AGN jets. M87’s jet is visibly structured, and it exhibits superluminal motion, in which the relativistic plasma appears to move faster than the speed of light. Because the plasma accelerated by an AGN moves so quickly, researchers attempting to model this plasma may opt to use the slow-light approach, in which the evolution of the ambient plasma is modeled simultaneously with the propagation of light through the medium. Though more computationally intensive than the fast-light approach, in which photons zip through a static medium, slow-light techniques are needed in regions of relativistic flows or strong acceleration.

Yuh Tsunetoe (Chinese Academy of Sciences; University of Tsukuba) and collaborators demonstrated their use of the slow-light approach in models of M87’s jet-launching region, where plasma is accelerated to relativistic velocities. The team performed general relativistic magnetohydrodynamics simulations of the accretion disk and jet from M87’s supermassive black hole and compared the results from slow-light and fast-light methods.

The team found that slow-light images tended to be smoother, lacking the looping, helical structures within the jet seen in fast-light images. Slow-light images show more evidence for the superluminal motion that is observed in relativistic jets like M87’s, as well as greater limb brightening and less wobbling motion. The team also investigated the effects of changing the black hole spin, finding greater wobbling in the jet for slow black hole spins and a straighter, wider jet for rapid black hole spins. Overall, Tsunetoe and coauthors found that the slow-light approach generated images that are more consistent with the properties of M87’s jet, demonstrating that the slow-light approach is necessary to capture the behavior of relativistic jets.

Citation

“Evidence for Neutrino Emission from X-Ray-Bright Active Galactic Nuclei with IceCube,” R. Abbasi et al 2026 ApJL 1000 L26. doi:10.3847/2041-8213/ae4aad

“The Relative Contributions of Accretion Disk Versus Jet to the Optical and Mid-Infrared Variability of Seyfert Galaxies,” Vineet Ojha et al 2025 ApJ 994 84. doi:10.3847/1538-4357/ae0a38

“Slow-Light Effect in the Jet-Launching Region of M87,” Yuh Tsunetoe et al 2026 ApJ 1000 29. doi:10.3847/1538-4357/ae43e7

The post Monthly Roundup: Investigating Active Galactic Nuclei appeared first on AAS Nova.

Where do the mysterious cosmic messengers called neutrinos come from? Researchers search for a connection between neutrinos and accretion flares from black holes.

Seeking Connections

Neutrinos — neutral, nearly massless elementary particles — are generated by natural particle accelerators across the universe. Facilities like the IceCube Neutrino Observatory, which employs detectors buried a mile deep in Antarctic ice, spot neutrinos from distant astrophysical sources, but matching these neutrinos to their sources is no easy task.

Thus far, researchers have discovered a compelling connection between neutrinos and distant active galactic nuclei, which are powered by the accretion of gas onto supermassive black holes. The active galactic nucleus phase of a black hole tends to be long lived, with activity lasting hundreds of thousands of years or more, making these nuclei lasting sources of neutrinos.

Sampling Flares

Recently, researchers have found tentative evidence for coincident neutrinos from short-lived accretion events like tidal disruption events, in which a star is torn apart and accreted by a black hole. These studies have uncovered a potential connection between neutrinos and tidal disruption events with mid-infrared echoes, signaling the presence of obscuring dust.

To investigate whether there is any connection between neutrinos and short-lived accretion events, Megan Wang (Massachusetts Institute of Technology) and collaborators first compiled a sample of 99 black hole flares observed by NEOWISE. These flares were selected for their strong mid-infrared emission, location in the nuclei of galaxies, and a fast rise and slow decline. The team applied additional selection criteria to remove other bright transients, such as supernovae, and retain only tidal disruption events and active galactic nucleus accretion flares.

For the neutrino side of the equation, Wang’s team turned to IceCube’s most recent catalog of neutrino events from astrophysical sources. In total, they compiled a sample of 68 events for which the odds of the source being astrophysical was greater than 50%, and for which the directional uncertainty was below 50.

No Coincidences

For a flare and a neutrino to be considered spatially and temporally coincident, the neutrino must arrive within one year of the flare, and the flare must fall within the 90% certainty contours of the neutrino. Ultimately, Wang’s team found no neutrinos that fulfilled both criteria and only one that was spatially but not temporally coincident with a black hole flare.

This result appears to be at odds with previous studies that found several neutrinos that were coincident with tidal disruption events. Wang and collaborators noted that while this work finds no association between neutrinos and black hole flares, it’s possible that the team is studying a different population of accretion events.

For example, while the previous neutrino-associated flares were flagged for having strong mid-infrared emission, they differ from the current sample in that they were detected at optical wavelengths. The flares sampled in this work were detected in the mid-infrared and have little to no optical emission, potentially highlighting intrinsic differences compared to previous samples.

Future work, including studies that leverage upcoming datasets from the Vera C. Rubin Observatory, the Roman Space Telescope, and the NEO Surveyor, can address this question further and search for connections with specific populations of black hole flares.

Citation

“Testing the Association of Supermassive Black Hole Infrared Flares and High-Energy Neutrinos,” Megan Wang et al 2026 ApJL 998 L29. doi:10.3847/2041-8213/ae3f90

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Title: ASKAP and VLASS Search for a Radio-Continuum Counterpart of Ultra-High-Energy Neutrino Event KM3–230213A
Authors: M. D. Filipović et al.
First Author’s Institution: Western Sydney University
Status: Published in ApJL

The Scene of the Crime

On Galentine’s Day this year, an ultra-high-energy neutrino attempted to sneak through the Mediterranean Sea, likely expecting she wouldn’t be caught. The odds were in her favor; neutrinos, ghostly particles with no electric charge and infinitesimal mass, only very rarely interact with matter. However, what she failed to account for was the awaiting undersea neutrino detector, KM3NeT, and the clever lepton within who would finally notice her. She slammed into the lepton, spewing charged particles everywhere at speeds greater than the speed of light in the water. While no particle can outrun a photon in a vacuum, water slows light down, giving us the familiar effect of refraction; similar to supersonic jets creating a boom when they break the sound barrier, these charged particles produced a distinctive blue light, known as Cherenkov light, exposing the neutrino’s position to astronomers and physicists everywhere. Busted.

The Investigation Begins

However, the neutrino was only the messenger; of even more interest is the astrophysical object that produced her. It’s not easy to generate such a high-energy particle, and no one can create a neutrino from thermal emission alone, indicating that wherever she originated, something extreme was going on. To date, only three astrophysical sources have been caught emitting neutrinos at all, and none of them are extragalactic: the Sun, although this is old news (in the 1960s, detections of solar neutrinos showed definitively that the Sun is powered by nuclear fusion, resolving the issue of how the Sun has burned long enough for life to evolve on Earth); the nearest core-collapse supernova to our galaxy in modern times, SN 1987A; and the galactic plane.

Theoretical models predict a much wider variety of objects, including extragalactic sources, to produce neutrinos, usually via cosmic-ray production: supernova remnants, star-forming galaxies, gamma-ray bursts, supermassive black holes (which are found at the centers of most galaxies), active galactic nuclei (a particularly fussy subset of supermassive black holes that are eating their host galaxies), and blazars (an extreme subset of active galactic nuclei that emit jets of radio light directly at Earth). The reason we have not detected their predicted neutrino emission is that neutrino astronomy is a new field, extragalactic sources are super far away, and neutrinos are both difficult to detect and difficult to trace back to their origin.

Rounding Up Suspects

With this in mind, today’s authors embark on a quest to catch the culprit, starting in the radio band. Radio emission, like neutrino emission, is usually an indicator of non-thermal radiative processes, and one such process, synchrotron radiation (emitted by relativistic electrons getting spun around in powerful magnetic fields), can be distinguished from other types of radiation based on its radio characteristics. Conveniently, the region our neutrino hails from is spanned by multiple radio surveys conducted with the Very Large Array (VLA) and the Australian Sub-Kilometer Compact Array Pathfinder (ASKAP), and so our authors use these surveys to round up all the radio riffraff. Unfortunately, the long wavelengths of radio photons and the scarcity of neutrinos result in reduced resolution for both compared to traditional optical telescopes, and our authors find over a thousand radio emitters in the region. Of course, no one can question that many sources, so our authors limit their investigation to objects with at least two radio brightness measurements, which can be used to calculate the brightness as a function of radio wavelength (the spectral energy distribution, which tells us about what type of radiation we see) and/or as a function of time (a light curve, which tells us if our source is variable). Our authors settle on a lineup of 10 likely blazars, any of whom could have emitted our ultra-high-energy neutrino, as well as a shortlist of prime suspects warranting further investigation: Phaedra, a spiral galaxy; Hebe, a radio galaxy; and Narcissus, an unusual compact radio emitter (see Figure 1).

Phaedra: A Spiral Galaxy with a Secret?

Phaedra (Figure 2), the most radio-luminous in the area, exhibits plenty of behavior typical of a galaxy guilty of neutrino emission. For starters, she has two regions of highly concentrated radio emission, and these regions are offset from her center, making them look suspiciously like active galactic nucleus jets, which are excellent particle accelerators. Furthermore, infrared observations suggest she is a starburst galaxy, churning out stars faster than a bestselling author with a team of ghostwriters churns out books. This intense star formation could have easily been triggered by jet activity. Even more suspiciously, she is closely associated with an X-ray binary, and where there are high-energy photons, there are likely to be other high-energy particles like neutrinos and cosmic rays. Phaedra’s prospects of beating the neutrino emission allegations are not looking good; these high-energy phenomena produce buckets of high-energy particles, and even if they produce only cosmic rays, the cosmic rays are bound to crash into the surrounding dense gas and photons, creating neutrinos anyway.

Hebe: A Simple Radio Galaxy, or Something More?

Hebe (Figure 3), the nearest extended radio source, isn’t exactly innocent-looking either. She is one of a triplet of galaxies sharing a common envelope, like peas in an extragalactic pod. Galaxies, unlike peas, however, are so massive that they can’t help but interact dynamically in such close quarters, causing a commotion that could totally produce ultra-high-energy neutrinos. She likely also has an active galactic nucleus jet, giving her the same neutrino-wielding powers as Phaedra.

Narcissus: Double Active Galactic Nucleus?

Our final suspect, Narcissus (Figure 4), consists of not one, but two active galactic nuclei. One appears to exhibit the classic synchrotron spectral energy distribution, and the other is likely a blazar, based on his notable radio variability and infrared observations.

Solving the Mystery

So, who really emitted the ultra-high-energy neutrino? For now, our authors can’t jump to any firm conclusions — they’d never risk condemning an innocent galaxy — but they will continue to closely monitor the suspects and gather more evidence. In the meantime, Phaedra, Hebe, and Narcissus should find themselves a good defense attorney experienced in neutrino emission cases.

Original astrobite edited by Sandy Chiu.

About the author, Chloe Klare:

I’m a PhD student in astronomy and astrophysics at Penn State (with a physics minor, so I get to use my semester spent in QFT for something!). I study active galactic nuclei (in the radio!), and I’m currently looking for baby synchrotron jets in active galactic nuclei.

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