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Alessandra Azzollini

Publications and source records attributed to Alessandra Azzollini.

9 recordsLinked to original sources
arXiv2026

An Accretion Flare Interpretation for the Ultra-High-Energy Neutrino Event KM3-230213A

We study the origin of the ultra-high-energy (UHE) neutrino event KM3-230213A detected by KM3NeT, focusing on MRC 0614-083 which has been pinpointed as the closest blazar to the neutrino localization exhibiting variable multi-wavelength emission. A joint interpretation of the optical, infrared, and X-ray light curves suggests that MRC 0614-083 has undergone a super-Eddington accretion flare accompanied by efficient proton acceleration. That flare has initiated a delayed infrared echo within the surrounding dust torus, which serves as a target for photomeson ($pγ$) interactions such that a self-consistent picture emerges that complements the blazar jet scenario: the predicted UHE neutrino flux is at the level expected from joint $E^{-2}$ fit with the IceCube measurements at lower energies, the variable nature of the event alleviates the tension with IceCube limits, and the accompanying electromagnetic cascade describes the X-ray flare around the neutrino detection time. Since a key remaining uncertainty is the unknown redshift of the source, we strongly encourage optical/ultraviolet spectroscopic measurements to determine its redshift.

astro-ph.HE↗
arXiv2025

The changing look of the neutrino-emitter blazar candidate 5BZQ J1243+4043

In recent years, changing-look blazars have called the traditional view of BL Lacs-flat spectrum radio quasars into question within the empirical classification of blazars. Based on the intensity of optical lines, they appear to transition between the two classes over time. We focus on the blazar 5BZQ J1243+4043, recently proposed as a promising candidate for the emission of high-energy neutrinos observed by the IceCube Neutrino Observatory and reported as a changing-look blazar in the literature. We study the spectral properties of this blazar, inferring its radiation field and accretion regime across different epochs. This study presents new optical spectroscopy observations of 5BZQ J1243+4043 taken with Gran Telescopio Canarias. We used this new dataset and two optical spectra available from the literature to investigate the continuum and line emissions and pinpoint the physical properties of the source. In particular, we used the emission lines to probe the accretion regime. The newly collected data for 5BZQ J1243+4043 shows broad emission lines, consistent with the spectrum of the first epoch and the redshift z = 1.5181$\pm$0.0002 known from the literature. For the second epoch, the spectrum appears featureless and so, we placed limits on the emission lines and related properties. We observed spectral variability for both the continuum and line emissions among the three spectra. Nonetheless, the accretion properties of the blazar generally remain unvaried, indicating that the intrinsic physics stays the same across the three epochs. In the broader multi-messenger context, this suggests that, despite the changing look in the optical band, the candidate neutrino-emitter blazar 5BZQ J1243+4043 is still characterized by the presence of intense external radiation fields and radiatively efficient accretion, typical of high-excitation radio galaxies, which may foster neutrino production.

astro-ph.HE↗
arXiv2025

The physical properties of candidate neutrino-emitter blazars

The processes governing the production of astrophysical high-energy neutrinos are still debated, and the sources originating them remain an open question. Among the putative emitters, active galactic nuclei have gained increasing attention. Blazars, in particular, stand out due to their ability to accelerate particles in environments with external radiation fields. Recent observations suggest they may contribute to the neutrino flux detected by IceCube. We study the physical properties of a subsample of 52 blazars proposed as candidate neutrino emitters, based on a positional cross-correlation analysis between IceCube hotspots and the 5BZCat catalog. We aim to provide a first characterization of their central engines and physical nature, to explore the potential link with neutrino production. We analyze the optical spectroscopic properties of the 52 candidate neutrino-emitter blazars to infer their accretion regime. The study is complemented by radio and $γ$-ray data, which trace the intrinsic jet power. We compare the sample to other blazar populations in the literature, perform statistical tests, and explore, through simulations, the applicability of methods that include censored data. Overall, the target sample shows properties compatible with the reference samples. We observe a mild tendency to prefer objects with intense radiation fields, typical of radiatively efficient accretors, and high radio power. Among them, 24 are detected by Fermi-LAT, spanning various $γ$-ray luminosities. We also show that statistical tests commonly used in the literature need to be handled with caution, as they are sensitive to the number of censored data and the sample size.

astro-ph.HE↗
arXiv2025

X-ray Spectral Variability as Probe of Multimessenger Emission in Blazar 5BZB J0630-24064

X-ray observations are essential for understanding the multimessenger emission mechanisms of active galactic nuclei (AGN). Blazars, a subset of AGN whose X-ray emission predominantly originates from relativistic jets, have been proposed as promising high-energy neutrino sources. In this work, we study the candidate neutrino-emitting blazar 5BZB J0630-24064, which has been observed over multiple epochs with the XMM-Newton, NuSTAR, Neil Gehrels Swift-XRT, and eROSITA observatories. Analysis of the X-ray spectra in the 2.0-10.0 keV band shows significant variability, with high flux states adhering to a power-law model indicative of jet emission. However, during low-flux states, the spectrum reveals an additional component at hard-X-rays, indicating a transition from jet-dominated to multi-component X-ray emission, possibly associated with hadronic processes. To investigate this spectral evolution, we tested various models and found it to be consistent with corona emission or photoionised absorption processes typically observed in obscured AGN. The identification of the X-ray spectral variability in 5BZB J0630-24064, combined with its potential for neutrino production, opens new perspectives in multimessenger astrophysics of blazars highlighting the synergies between the mechanisms of the jet and the nuclear environment.

astro-ph.HE↗
arXiv2023

Hadronic processes at work in 5BZB J0630-2406

Recent observations are shedding light on the important role that active galactic nuclei (AGN) play in the production of high-energy neutrinos. In this study, we focus on one object, 5BZB J0630-2406, which is among the blazars recently proposed as associated with neutrino emission during the first 7-yr IceCube observations. Modelling the quasi-simultaneous, broad-band spectral energy distribution, we explore various scenarios from purely leptonic to lepto-hadronic models, testing the inclusion of external photon fields. This theoretical study provides a complementary testing ground for the proposed neutrino-blazar association. Despite being historically classified as a BL Lac, our study shows that 5BZB J0630-2406 belongs to the relatively rare sub-class of high-power flat-spectrum radio quasars (FSRQs). Our results indicate that interactions between protons and external radiation fields can produce a neutrino flux that is within the reach of the IceCube detector. Furthermore, the spectral shape of the X-ray emission suggests the imprint of hadronic processes related to very energetic protons.

astro-ph.HE↗
arXiv2023

Multiwavelength spectroscopic study of shock driven phenomena in explosive outbursts in symbiotic-like recurrent novae with emphasis on RS Ophiuchi

To detail the development of RS Ophiuchi and the other Galactic Symbiotic-like Recurrent Novae throughout their outburst and quiescence, with a particular emphasis on the propagation of the shock wave during the outburst of the binaries. The spectral analysis has been performed using archival data according to the features of the individual datasets. Swift grism spectra were reduced and extracted using a combination of the pre-existing UVOTPY Python routine and newly written pipelines in Matlab. Other datasets were directly available in reduced form, already corrected for instrumental or background contamination, calibrated in wavelength and flux or intensity. The work on these was done through pipelines suited for reading the data and elaborating them to extract quantities of interest for the analysis. We find striking similarities in different outbursts of the same object and for different novae. For example, RS Oph 2021 was almost identical to the 2006 outburst, despite having occurred at a different orbital phase with the observations made from a different line of sight through the red giant wind. Despite the intrinsically different properties of the binaries, striking similarities are found for different systems of the same class, for instance, that the trend of the electron density over time during outburst appears to follow a general temporal development.

astro-ph.SR↗
arXiv2023

Extragalactic neutrino factories

Identifying the astrophysical sources responsible for the high-energy cosmic neutrinos has been a longstanding challenge. In a previous work, we report evidence for a spatial correlation between blazars from the 5th Roma-BZCat catalog and neutrino data of the highest detectable energies, i.e. >0.1 PeV, collected by the IceCube Observatory in the southern celestial hemisphere. The statistical significance is found at the level of 2 x 10^{-6} post-trial. In this work we test whether a similar correlation exists in the northern hemisphere, were IceCube is mostly sensitive to <0.1 PeV energies. We find a consistent correlation between blazars and northern neutrino data at the pre-trial p-value of 5.12 x 10^{-4}, and a post-trial chance probability of 6.79 x 10^{-3}. Combining the post-trial probabilities observed for the southern and northern experiments yields a global post-trial chance probability of 2.59 x 10^{-7} for the genuineness of such correlation. This implies that the spatial correlation is highly unlikely to arise by chance. Our studies push forward an all-sky subset of 52 objects as highly likely PeVatron extragalactic accelerators.

astro-ph.HE↗
arXiv2022

Beginning a journey across the universe: the discovery of extragalactic neutrino factories

Neutrinos are the most elusive particles in the Universe, capable of traveling nearly unimpeded across it. Despite the vast amount of data collected, a long standing and unsolved issue is still the association of high-energy neutrinos with the astrophysical sources that originate them. Amongst the candidate sources of neutrinos there are blazars, a class of extragalactic sources powered by supermassive black holes that feed highly relativistic jets, pointed towards the Earth. Previous studies appear controversial, with several efforts claiming a tentative link between high-energy neutrino events and individual blazars, and others putting into question such relation. In this work we show that blazars are unambiguously associated with high-energy astrophysical neutrinos at unprecedented level of confidence, i.e. chance probability of 6 x 10^{-7}. Our statistical analysis provides the observational evidence that blazars are astrophysical neutrino factories and hence, extragalactic cosmic-ray accelerators.

astro-ph.HE↗