The recent IceCube discovery of 0.1-1 PeV neutrinos of astrophysical origin opens up a new era for high-energy astrophysics. Although there are various astrophysical candidate sources, a firm ...association of the detected neutrinos with one (or more) of them is still lacking. A recent analysis of plausible astrophysical counterparts within the error circles of IceCube events showed that likely counterparts for nine of the IceCube neutrinos include mostly BL Lacs, among which Mrk 421. Motivated by this result and a previous independent analysis on the neutrino emission from Mrk 421, we test the BL Lac-neutrino connection in the context of a specific theoretical model for BL Lac emission. We model the spectral energy distribution (SED) of the BL Lacs selected as counterparts of the IceCube neutrinos using a one-zone leptohadronic model and mostly nearly simultaneous data. The neutrino flux for each BL Lac is self-consistently calculated, using photon and proton distributions specifically derived for every individual source. We find that the SEDs of the sample, although different in shape and flux, are all well fitted by the model using reasonable parameter values. Moreover, the model-predicted neutrino flux and energy for these sources are of the same order of magnitude as those of the IceCube neutrinos. In two cases, namely Mrk 421 and 1H 1914-194, we find a suggestively good agreement between the model prediction and the detected neutrino flux. Our predictions for all the BL Lacs of the sample are in the range to be confirmed or disputed by IceCube in the next few years of data sampling.
Detection of the IceCube-170922A neutrino coincident with the flaring blazar TXS 0506+056, the first and only ∼3 high-energy neutrino source association to date, offers a potential breakthrough in ...our understanding of high-energy cosmic particles and blazar physics. We present a comprehensive analysis of TXS 0506+056 during its flaring state, using newly collected Swift, NuSTAR, and X-shooter data with Fermi observations and numerical models to constrain the blazar's particle acceleration processes and multimessenger (electromagnetic (EM) and high-energy neutrino) emissions. Accounting properly for EM cascades in the emission region, we find a physically consistent picture only within a hybrid leptonic scenario, with γ-rays produced by external inverse-Compton processes and high-energy neutrinos via a radiatively subdominant hadronic component. We derive robust constraints on the blazar's neutrino and cosmic-ray emissions and demonstrate that, because of cascade effects, the 0.1-100 keV emissions of TXS 0506+056 serve as a better probe of its hadronic acceleration and high-energy neutrino production processes than its GeV-TeV emissions. If the IceCube neutrino association holds, physical conditions in the TXS 0506+056 jet must be close to optimal for high-energy neutrino production, and are not favorable for ultrahigh-energy cosmic-ray acceleration. Alternatively, the challenges we identify in generating a significant rate of IceCube neutrino detections from TXS 0506+056 may disfavor single-zone models, in which γ-rays and high-energy neutrinos are produced in a single emission region. In concert with continued operations of the high-energy neutrino observatories, we advocate regular X-ray monitoring of TXS 0506+056 and other blazars in order to test single-zone blazar emission models, clarify the nature and extent of their hadronic acceleration processes, and carry out the most sensitive possible search for additional multimessenger sources.
We investigate the origin of high-energy emission in blazars within the context of the leptohadronic one-zone model. We find that γ-ray emission can be attributed to synchrotron radiation either from ...protons or from secondary leptons produced via photohadronic processes. These possibilities imply differences not only in the spectral energy distribution (SED) but also in the variability signatures, especially in the X- and γ-ray regime. Thus, the temporal behaviour of each leptohadronic scenario can be used to probe the particle population responsible for the high-energy emission as it can give extra information not available by spectral fits. In this work, we apply these ideas to the non-thermal emission of Mrk 421, which is one of the best monitored TeV blazars. We focus on the observations of 2001 March, since during that period Mrk 421 showed multiple flares that have been observed in detail both in X-rays and γ-rays. First, we obtain pre-flaring fits to the SED using the different types of leptohadronic scenarios. Then, we introduce random-walk-type, small-amplitude variations on the injection compactness or on the maximum energy of radiating particles and follow the subsequent response of the radiated photon spectrum. For each leptohadronic scenario, we calculate the X-ray and γ-ray fluxes and investigate their possible correlation. Whenever the 'input' variations lead, apart from flux variability, also to spectral variability, we present the resulting relations between the spectral index and the flux, both in X-rays and γ-rays. We find that proton synchrotron models are favoured energetically but require fine tuning between electron and proton parameters to reproduce the observed quadratic behaviour between X-rays and TeV γ-rays. On the other hand, models based on pion decay can reproduce this behaviour in a much more natural way.
We present a method of constraining the properties of the γ-ray emitting region in flat spectrum radio quasars in the one-zone proton synchrotron model, where the γ-rays are produced by synchrotron ...radiation of relativistic protons. We show that for low enough values of the Doppler factor δ, the emission from the electromagnetic (EM) cascade which is initiated by the internal absorption of high-energy photons from photohadronic interactions may exceed the observed ∼GeV flux. We use that effect to derive an absolute lower limit of δ; first, an analytical one, in the asymptotic limit where the external radiation from the broad-line region (BLR) is negligible, and then a numerical one in the more general case that includes BLR radiation. As its energy density in the emission region depends on δ and the region's distance from the galactic centre, we use the EM cascade to determine a minimum distance for each value of δ. We complement the EM cascade constraint with one derived from variability arguments and apply our method to the FSRQ 3C 273. We find that δ ≳ 18–20 for B ≲ 30 G and ∼day time-scale variability; the emission region is located outside the BLR, namely at r ≳ 10R
BLR ∼ 3 pc; the model requires at pc-scale distances stronger magnetic fields than those inferred from core shift observations; while the jet power exceeds by at least one order of magnitude the accretion power. In short, our results disfavour the proton synchrotron model for the FSRQ 3C 273.
The recent discovery of extragalactic PeV neutrinos opens a new window to the exploration of cosmic ray accelerators. The observed PeV neutrino flux is close to the Waxman–Bahcall upper bound ...implying that gamma-ray bursts (GRBs) may be the source of ultrahigh energy cosmic rays (UHECRs). Starting with the assumption of the GRB–UHECR connection, we show using both analytical estimates and numerical simulations that the observed neutrinos can originate at the jet as a result of photopion interactions with the following implications: the neutrino spectra are predicted to have a cut-off at energy ≲10 PeV; the dissipation responsible for the GRB emission and cosmic ray acceleration takes place at distances r
diss ≃ 3 × 1011–3 × 1013 cm from the central engine; the Thomson optical depth at the dissipation region is τT ∼ 1; the jet carries a substantial fraction of its energy in the form of Poynting flux at the dissipation region, and has a Lorentz factor Γ ≃ 100–500. The non-detection of PeV neutrinos coincident with GRBs will indicate that GRBs are either poor cosmic accelerators or the dissipation takes place at small optical depths in the jet.
The sequence of events associated with the triggering of energy release during substorm expansion phase onset is still not well‐understood. Oberhagemann and Mann (2020b, ...https://doi.org/10.1029/2019gl085271) proposed a new substorm onset mechanism, where the transition toward parallel proton pressure anisotropy during tail stretching in the late growth phase could trigger a pressure anisotropic ballooning instability. Here we examine the evolution of energetic proton parallel pressure anisotropy at geosynchronous altitudes, seeking evidence in support of the proposed substorm onset mechanism. We use the Geostationary Operational Environment Satellite (GOES) proton flux and magnetometer data combined with substorm onset indicators derived from ground‐based magnetometers. Superposed epoch analysis of substorm onset times for 2014 using the isolated substorm list (Ohtani & Gjerloev, 2020, https://doi.org/10.1029/2020ja027902) clearly shows signatures of energetic proton parallel pressure anisotropy immediately before substorm onset, potentially supportive of the Oberhagemann and Mann theory.
Plain Language Summary
Substorms are disturbances in the nightside region of the geospace associated with the rapid release of stored magnetic energy. In the ionosphere, the signatures of this energy release are the spectacular dancing lights known as aurorae (northern and southern lights). The processes that lead to energy storage are well‐known. However, there are competing theories on what triggers the release of this significant amount of energy at substorm onset. According to a new substorm onset theory proposed by Oberhagemann and Mann, when the magnetic field stretches in the nightside during the energy storage, the pressure becomes more parallel to the magnetic field, leading to a ballooning instability at substorm onset. Here, we look for observational support for the association of such pressure profile at geosynchronous altitudes with substorm onset to examine the proposed model. Superposed epoch analysis of isolated substorms in 2014 shows increasing energetic proton parallel pressure anisotropy at the onset, providing evidence to support the Oberhagemann and Mann theory.
Key Points
Oberhagemann and Mann theory proposes that proton parallel temperature anisotropy triggers ballooning instability leading to substorm onset
We use pitch angle resolved energetic proton fluxes at geosynchronous altitudes seeking observational evidence in support of the model
Superposed epoch analysis of isolated substorms shows signatures of increasing energetic proton parallel anisotropy which peaks near onset
Aims. We investigate the role of the second synchrotron self-Compton (SSC) photon generation to the multiwavelength emission from the compact regions of sources that are characterized as misaligned ...blazars. For this, we focus on the nearest high-energy emitting radio galaxy Centaurus A and we revisit the one-zone SSC model for its core emission. Methods. We have calculated analytically the peak luminosities of the first and second SSC components by first deriving the steady-state electron distribution in the presence of synchrotron and SSC cooling, and then by using appropriate expressions for the positions of the spectral peaks. We have also tested our analytical results against those derived from a numerical code where the full emissivities and cross-sections were used. Results. We show that the one-zone SSC model cannot account for the core emission of Centaurus A above a few GeV, where the peak of the second SSC component appears. We thus propose an alternative explanation for the origin of the high-energy (≳0.4 GeV) and TeV emission, where these are attributed to the radiation emitted by a relativistic proton component through photohadronic interactions with the photons produced by the primary leptonic component. We show that the required proton luminosities are not extremely high, i.e. ~1043 erg/s, provided that the injection spectra are modelled by a power law with a high value of the lower energy cutoff. Finally, we find that the contribution of the core emitting region of Cen A to the observed neutrino and ultra-high-energy cosmic-ray fluxes is negligible.
Since the discovery of the Van Allen radiation belts over 50 years ago, an explanation for their complete dynamics has remained elusive. Especially challenging is understanding the recently ...discovered ultra-relativistic third electron radiation belt. Current theory asserts that loss in the heart of the outer belt, essential to the formation of the third belt, must be controlled by high-frequency plasma waveparticle scattering into the atmosphere, via whistler mode chorus, plasmaspheric hiss, or electromagnetic ion cyclotron waves. However, this has failed to accurately reproduce the third belt. Using a data-driven, time-dependent specication of ultra-low-frequency (ULF) waves we show for the first time how the third radiation belt is established as a simple, elegant consequence of storm-time extremely fast outward ULF wave transport. High-frequency waveparticle scattering loss into the atmosphere is not needed in this case. When rapid ULF wave transport coupled to a dynamic boundary is accurately specied, the sensitive dynamics controlling the enigmatic ultra-relativistic third radiation belt are naturally explained.
The relationship between auroral, ground, and plasma sheet signatures in the late growth phase is crucial for understanding the sequence of events during a substorm expansion phase onset. Here we ...show conjugate ground‐auroral‐satellite observations of a substorm that occurred on 18 September 2021, between 04:45 and 05:00 UT, where four auroral activations were detected in the all‐sky imagers. An initial activation showed the brightening of an equatorward arc within the cutoff of the 630 nm emissions, indicating activity on closed field lines well inside the open‐closed field line boundary (OCFLB). During a second activation, auroral beads were observed on a brightening arc, equatorward and within the OCFLB, followed by the transformation from small‐scale to large‐scale vortices. The tail current sheet was highly disturbed during the auroral vortex evolution, including pressure and magnetic disturbances, an apparent broadening of a previously thin current sheet, and a breakdown of the frozen‐in condition. Our observations clearly show late growth phase dynamics, including arc brightenings, the formation of auroral beads, and auroral vortex development, can occur well in advance of fast Earthward flows in the tail. Indeed, it is only during that later activity that auroral breakup and strong Earthward flows, which we associate with magnetic reconnection further down the tail, are observed together with strong magnetic bays on the ground. The sequence of events is consistent with an inside‐to‐outside model at substorm expansion phase onset, most likely via a shear‐flow ballooning instability in the transition region from dipole to tail‐like fields in the near‐Earth plasma sheet.
Plain Language Summary
Substorm onset is associated with the explosive release of stored magnetic energy, which can be visualized as auroral activity in the ionosphere, magnetic‐field disturbances in the ground‐based magnetometers, and plasma sheet disturbances in the magnetosphere. Even though the processes that lead to energy storage are well known, the exact sequence of events and the triggering factors that lead to the release of this stored energy are poorly understood. In this study, we show conjugate auroral‐ground‐satellite observations of a substorm event that occurred on 18 September 2021. Four auroral activations were observed in the all‐sky imagers, all of which can be associated with plasma sheet disturbances observed in the satellites. Our observations show an initial activation and bead‐like structures on a brightening arc, followed by the formation and expansion of vortex‐like auroral forms, all of which can be associated with magnetic field and pressure fluctuations in the near‐Earth nightside magnetosphere on closed field lines. Auroral breakup and strong magnetic bays on the ground are only observed after the arrival of fast Earthward flows in the magnetosphere. Overall, this paper identifies disturbances in the near‐Earth plasma environment which are the counterparts to the evolving auroral forms seen leading up to the substorm expansion phase onset.
Key Points
Plasma sheet dynamical counterparts are reported for an evolving sequence of late growth phase auroral forms
Plasma sheet current disruption and ion kinetic scale perturbations occur in advance of fast Earthward flows and magnetic reconnection
One‐to‐one correspondence between plasma sheet disturbances and auroral forms implies ballooning instability in advance of auroral breakup
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