The physics of blazar jets in the context of multi-wavelength and multi-messenger astronomy
School
Università di Bologna - Istituto Nazionale di Astrofisica
Abstract:
The theoretical expectations on blazar jets as neutrino sources were observationally confirmed when the gamma-ray blazar TXS 0506+056 was detected in spatial coincidence with the IceCube170922A (IC 170922A) neutrino event, highlighting one of the most significant connections between high-energy neutrinos and astrophysical sources to date. Once the first milestone has been reached, now it is crucial to provide more observational insights to support or discard the neutrino-blazar connection hypothesis. To delve into potential blazar counterparts of highenergy neutrinos, this Thesis focuses on the parsec-scale regions of gamma-ray blazars spatially coinciding with neutrino detections from the IceCube Neutrino Observatory. The project aims to explore distinctive radio properties related to neutrino production, such as coincident radio flares or features in jet morphology like limb brightening and transverse structures which have been observed in TXS 0506+056. The study involved detailed multi-frequency VLBI follow-up observations of candidate counterparts associated with four high-energy neutrino events detected by IceCube: IC 190704A, IC 200109A, IC 201021A and IC 201114A. We identified and examined ten potential neutrino-emitting blazars, with emphasis on five of them because of the gammaray association, similar to TXS 0506+056. We investigated their radio characteristics shortly after neutrino arrival, comparing them with archival VLBI observations and low-frequency radio data. The results were discussed in the context of previous statistical works and studies on the case of TXS 0506+056. Two of the sources, PKS 1725+123 and TXS 1100+122, show hints of an elevated state of activity emerging as promising neutrino source candidates. While our results to date can not establish a robust connection between the radio activity state and neutrino arrival for the other sources, they stress the need for more extensive VLBI and MWL followup observations of neutrino events. Such efforts are crucial to advancing our understanding of neutrino production mechanisms in astrophysical sources. A crucial tool for unravelling the fundamental processes at play in blazar jets, whether they are connected to neutrino emission or not, is the investigation of their broadband Spectral Energy Distribution (SED). Blazar SED is interpreted as synchrotron radiation in the lowenergy regime and radiation produced through inverse Compton scattering (leptonic models) or hadronic processes at high energy. The latter processes are also involved in the neutrino production. Leptonic and hadronic models offer different perspectives, with SED modelling playing a pivotal role in discerning the microphysics of the system. To this purpose, the MAGIC Collaboration coordinates long-term monitoring of blazars in collaboration with lower energy facilities. The blazar 1ES1959+650 serves as an ideal laboratory for the study of the MWL behaviour of blazars, due to its brightness at all wavelengths and peculiar flare episodes. During the last few years, the source has experienced its lowest state ever reached, mainly at VHE. Yet it remained significantly detected on hour time scales by MAGIC. In this Thesis, we discussed the MWL spectral features and variability patterns during this low-state period, with a focus on the VHE data. Taking into account the findings on the VHE and MWL variability and spectral properties, we modelled the broadband SED of the source under the assumption of a leptonic scenario. In particular, to study the source evolution during the 2020-2022 period, we focused on three different states, a high and low state at VHE and a high state in the optical band, one at the beginning, the end and the middle of the period, respectively. We found that the leptonic explanation satisfactorily describes the data during this low-state period. We discussed the differences in the model parameters among the three days of the 2020-2022 low-state period and compared them with past flaring states. The MWL monitoring of 1ES1959+650 is ongoing and the view of the quiescent state that we provided will be useful as a baseline for the investigation of the source evolution with future new data. This doctoral project aims to enhance our comprehension of particle acceleration in astrophysical settings, with a specific focus on blazars and their potential role as sources of high-energy neutrinos. The research takes advantage of the parallel and complementary insights derived from radio and gamma-ray observations, serving as indicators of relativistic particle acceleration in extreme cosmic environments. Looking ahead, both radio and gamma-ray astronomy fields are experiencing remarkable and innovative developments. The advent of the Square Kilometre Array Observatory (SKAO) in the radio astronomy field and the ongoing construction of the Cherenkov Telescope Array Observatory (CTAO) in the gamma-ray astronomy field promise significant advancements. In this context, this Thesis provides an overview of the use of radio and gamma-ray observations in the study of cosmic accelerators, utilizing data from existing facilities and anticipating the improvements that future instruments will bring to this field. The Thesis starts with an introduction to blazar physics, discussing blazar characteristics, classification, hypotheses on emission mechanisms, and the radio and gamma-ray view of these sources. It is then divided into two independent parts. Part I describes the radio astronomy approach for studying acceleration processes in blazars, starting with interferometry and data analysis techniques. The results from radio observations of the sample of blazar candidate neutrino emitters are then presented. Part II addresses the gamma-ray astronomy approach for investigating blazar emission mechanisms. The observing technique and data analysis are illustrated, followed by a description of current and future instruments. The study of the blazar 1ES 1959+650 represents the application of the MWL approach (driven by the observations in the gamma-ray band). The conclusions close the Thesis with a summary of the results and considerations for future instrument improvements and scientific outputs.