IceCube has recently reported the discovery of high-energy neutrinos of astrophysical origin, opening up the PeV (1015 eV) sky. Because of their large positional uncertainties, these events have not ...yet been associated to any astrophysical source. We have found plausible astronomical counterparts in the GeV–TeV bands by looking for sources in the available large area high-energy γ-ray catalogues within the error circles of the IceCube events. We then built the spectral energy distribution of these sources and compared it with the energy and flux of the corresponding neutrino. Likely counterparts include mostly BL Lacs and two Galactic pulsar wind nebulae. On the one hand many objects, including the starburst galaxy NGC 253 and Centaurus A, despite being spatially coincident with neutrino events, are too weak to be reconciled with the neutrino flux. On the other hand, various GeV powerful objects cannot be assessed as possible counterparts due to their lack of TeV data. The definitive association between high-energy astrophysical neutrinos and our candidates will be significantly helped by new TeV observations, but will be confirmed or disproved only by further IceCube data. Either way, this will have momentous implications for blazar jets, high-energy astrophysics, and cosmic ray and neutrino astronomy.
ABSTRACT
The association of two IceCube detections, the IceCube-170922A event and a neutrino flare, with the blazar TXS 0506+056, has paved the way for the multimessenger quest for cosmic ...accelerators. IceCube has observed many other neutrinos but their origin remains unknown. To better understand the reason for the apparent lack of neutrino counterparts, we have extended the comprehensive dissection of the sky area performed for the IceCube-170922A event to all 70 public IceCube high-energy neutrinos that are well reconstructed and off the Galactic plane. Using the multifrequency data available through the Open Universe platform, we have identified numerous candidate counterparts of IceCube events. We report here the classification of all the γ-ray blazars found and the results of subsequent statistical tests. In addition, we have checked the 4LAC, 3FHL, and 3HSP catalogues for potential counterparts. Following the dissection of all areas associated with IceCube neutrinos, we evaluate the data using a likelihood-ratio test and find a $3.23\, \sigma$ (post-trial) excess of HBLs and IBLs with a best fit of 15 ± 3.6 signal sources. This result, together with previous findings, consistently points to a growing evidence for a connection between IceCube neutrinos and blazars, the most energetic particle accelerators known in the Universe.
ABSTRACT
We present evidence that TXS 0506+056, the first plausible non-stellar neutrino source, despite appearances, is not a blazar of the BL Lac type but is instead a masquerading BL Lac, i.e. ...intrinsically a flat-spectrum radio quasar with hidden broad lines and a standard accretion disc. This reclassification is based on: (1) its radio and $\rm {O \,{\small {II}}}$ luminosities; (2) its emission line ratios; (3) its Eddington ratio. We also point out that the synchrotron peak frequency of TXS 0506+056 is more than two orders of magnitude larger than expected by the so-called ‘blazar sequence’, a scenario which has been assumed by some theoretical models predicting neutrino (and cosmic ray) emission from blazars. Finally, we comment on the theoretical implications this reclassification has on the location of the γ-ray emitting region and our understanding of neutrino emission in blazars.
Blazars have been suggested as possible neutrino sources long before the recent IceCube discovery of high-energy neutrinos. We re-examine this possibility within a new framework built upon the blazar ...simplified view and a self-consistent modelling of neutrino emission from individual sources. The former is a recently proposed paradigm that explains the diverse statistical properties of blazars adopting minimal assumptions on blazars’ physical and geometrical properties. This view, tested through detailed Monte Carlo simulations, reproduces the main features of radio, X-ray, and γ-ray blazar surveys and also the extragalactic γ-ray background at energies ≳ 10 GeV. Here, we add a hadronic component for neutrino production and estimate the neutrino emission from BL Lacertae objects as a class, ‘calibrated’ by fitting the spectral energy distributions of a preselected sample of such objects and their (putative) neutrino spectra. Unlike all previous papers on this topic, the neutrino background is then derived by summing up at a given energy the fluxes of each BL Lac in the simulation, all characterized by their own redshift, synchrotron peak energy, γ-ray flux, etc. Our main result is that BL Lacs as a class can explain the neutrino background seen by IceCube above ∼0.5 PeV while they only contribute ∼10 per cent at lower energies, leaving room to some other population(s)/physical mechanism. However, one cannot also exclude the possibility that individual BL Lacs still make a contribution at the ≈20 per cent level to the IceCube low-energy events. Our scenario makes specific predictions, which are testable in the next few years.
We explore the correlation of γ-ray emitting blazars with IceCube neutrinos by using three very recently completed, and independently built, catalogues and the latest neutrino lists. We introduce a ...new observable, namely the number of neutrino events with at least one γ-ray counterpart, N
ν. In all three catalogues we consistently observe a positive fluctuation of N
ν with respect to the mean random expectation at a significance level of 0.4–1.3 per cent. This applies only to extreme blazars, namely strong, very high energy γ-ray sources of the high energy peaked type, and implies a model-independent fraction of the current IceCube signal ∼10–20 per cent. An investigation of the hybrid photon – neutrino spectral energy distributions of the most likely candidates reveals a set of ≈5 such sources, which could be linked to the corresponding IceCube neutrinos. Other types of blazars, when testable, give null correlation results. Although we could not perform a similar correlation study for Galactic sources, we have also identified two (further) strong Galactic γ-ray sources as most probable counterparts of IceCube neutrinos through their hybrid spectral energy distributions. We have reasons to believe that our blazar results are not constrained by the γ-ray samples but by the neutrino statistics, which means that the detection of more astrophysical neutrinos could turn this first hint into a discovery.
We propose a scenario where blazars are classified into flat-spectrum radio quasars (FSRQs), BL Lacertae (BL Lac) objects, low-synchrotron, or high-synchrotron peaked objects according to a varying ...mix of the Doppler-boosted radiation from the jet, the emission from the accretion disc, the broad-line region, and the light from the host galaxy. In this framework, the peak energy of the synchrotron power (
) in blazars is independent of source type and radio luminosity. We test this new approach, which builds upon unified schemes, using extensive Monte Carlo simulations, and show that it can provide simple answers to a number of long-standing issues, including, amongst others, the different cosmological evolution of BL Lac objects selected in the radio and X-ray bands, the larger
values observed in BL Lac objects, the fact that high-synchrotron peaked blazars are always of BL Lac type, and the existence of FSRQ-BL Lac transition objects. Objects so far classified as BL Lac objects on the basis of their observed weak, or undetectable, emission lines are of two physically different classes: intrinsically weak lined objects, more common in X-ray-selected samples, and heavily diluted broad-lined sources, more frequent in radio-selected samples, which explains some of the confusion in the literature. We also show that strong selection effects are the main cause of the diversity observed in radio and X-ray samples, and that the correlation between luminosity and
, which led to the proposal of the 'blazar sequence', is also a selection effect arising from the comparison between shallow radio and X-ray surveys, and to the fact that high-
-high-radio-power objects have never been considered because their redshift is not measurable.
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.
The blazar-simplified view is a new paradigm that explains well the diverse statistical properties of blazars observed over the entire electromagnetic spectrum on the basis of minimal assumptions on ...blazars' physical and geometrical properties. In this paper, the fourth in a series, we extend the predictions of this paradigm below the sensitivity of existing surveys and estimate the contribution of blazars to the X-ray and gamma -ray extragalactic backgrounds. We find that the integrated light from blazars can explain up to 100 per cent of the cosmic background at energies larger than ~10 GeV, and contribute ...40-70 per cent of the gamma -ray diffuse radiation between 100 MeV and 10 GeV. The contribution of blazars to the X-ray background, between 1 and 50 keV, is approximately constant and of the order of 4-5 per cent. On the basis of an interpolation between the estimated flux at X-ray and gamma -ray energies, we can expect that the contribution of blazars raises to ~10 per cent at 100 keV, and continues to increase with energy until it becomes the dominant component at a few MeV. Finally, we show that a strong dependence of the synchrotron peak frequency on luminosity, as postulated by the blazar sequence, is ruled out by the observational data as it predicts a gamma -ray background above a few GeV that is far in excess of the observed value. (ProQuest: ... denotes formulae/symbols omitted.)
The uncertainty region of the highly energetic neutrino IceCube200107A includes 3HSP J095507.9+355101 (
z
= 0.557), an extreme blazar, which was detected in a high, very hard, and variable X-ray ...state shortly after the neutrino arrival. Following a detailed multiwavelength investigation, we confirm that the source is a genuine BL Lac. This new detection differs from TXS 0506+056, which is thus far the first source associated with IceCube neutrinos, and is considered a “masquerading” BL Lac. As in the case of TXS 0506+056, 3HSP J095507.9+355101 is also way off the so-called blazar sequence. We consider 3HSP J095507.9+355101 a possible counterpart to the IceCube neutrino. Finally, we discuss some theoretical implications in terms of neutrino production.