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.
A 13-yr-long broad-band view of BL Lac Sahakyan, N; Giommi, P
Monthly notices of the Royal Astronomical Society,
05/2022, Letnik:
513, Številka:
3
Journal Article
Recenzirano
Odprti dostop
ABSTRACT
We present the results of an extensive analysis of the optical, ultraviolet, X-ray, and γ-ray data collected from the observations of the BL Lac objects prototype BL Lacertae carried out ...over a period of nearly 13 yr, between 2008 August and 2021 March. The source is characterized by strongly variable emission at all frequencies, often accompanied by spectral changes. In the γ-ray band several prominent flares have been detected, the largest one reaching the flux of Fγ(> 196.7 MeV) = (4.39 ± 1.01) × 10−6 photon cm−2 s−1. The X-ray spectral variability of the source during the brightest flare on MJD 59128.18 (2020 October 6) was characterized by a softer-when-brighter trend due to a shift of the synchrotron peak to ∼1016 Hz, well into the HBL domain. The widely changing multiwavelength emission of BL Lacertae was systematically investigated by fitting leptonic models that include synchrotron self-Compton and external Compton components to 511 high-quality and quasi-simultaneous broad-band spectral energy distributions (SEDs). The majority of selected SEDs can be adequately fitted within a one-zone model with reasonable parameters. Only 46 SEDs with soft and bright X-ray spectra and when the source was observed in very high energy γ-ray bands can be explained in a two-zone leptonic scenario. The HBL behaviour observed during the brightest X-ray flare is interpreted as due to the emergence of synchrotron emission from freshly accelerated particles in a second emission zone located beyond the broad-line region.
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.
ABSTRACT
We present a multifrequency study of the transient γ-ray source 4FGL J1544.3−0649, a blazar that exhibited a remarkable behaviour raising from the state of an anonymous mid-intensity radio ...source, never detected at high energies, to that of one of the brightest extreme blazars in the X-ray and γ-ray sky. Our analysis shows that the averaged γ-ray spectrum is well described by a power law with a photon index of 1.87 ± 0.04, while the flux above 100 MeV is (8.0 ± 0.9) × 10−9 photon cm−2 s−1, which increases during the active state of the source. The X-ray flux and spectral slope are both highly variable, with the highest 2–10 keV flux reaching (1.28 ± 0.05) × 10−10 erg cm−2 s−1. On several observations, the X-ray spectrum hardened to the point implying as SED peak moving to energies larger than 10 keV. As in many extreme blazars the broad-band spectral energy distribution can be described by a homogeneous one-zone synchrotron-self-Compton leptonic model. We briefly discuss the potential implications for high-energy multimessenger astrophysics in case the dual behaviour shown by 4FGL J1544.3−0649 does not represent an isolated case, but rather a manifestation of a so far unnoticed relatively common phenomenon.
The 5th edition of the
Roma-BZCAT
Multifrequency Catalogue of Blazars is available in a printed version and online at the ASDC website (
http://www.asdc.asi.it/bzcat
); it is also in the NED ...database. It presents several relevant changes with respect to the past editions which are briefly described in this paper.
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 γ-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 γ-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 γ-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 γ-ray background above a few GeV that is far in excess of the observed value.
ABSTRACT
The blazar PKS 0735+178 is possibly associated with multiple neutrino events observed by the IceCube, Baikal, Baksan, and KM3NeT neutrino telescopes while it was flaring in the γ-ray, X-ray, ...ultraviolet, and optical bands. We present a detailed study of this peculiar blazar to investigate the temporal and spectral changes in the multiwavelength emission when the neutrino events were observed. The analysis of Swift-XRT snapshots reveal a flux variability of more than a factor 2 in about 5 × 103 s during the observation on 2021 December 17. In the γ-ray band, the source was in its historical highest flux level at the time of the arrival of the neutrinos. The observational comparison between PKS 0735+178 and other neutrino source candidates, such as TXS 0506+056, PKS 1424+240, and GB6 J1542+6129, shows that all these sources share similar spectral energy distributions, very high radio and γ-ray powers, and parsec scale jet properties. Moreover, we present strong supporting evidence for PKS 0735+178 to be, like all the others, a masquerading BL Lac. We perform comprehensive modelling of the multiwavelength emission from PKS 0735+178 within one-zone lepto-hadronic models considering both internal and external photon fields and estimate the expected accompanying neutrino flux. The most optimistic scenario invokes a jet with luminosity close to the Eddington value and the interactions of ∼ PeV protons with an external UV photon field. This scenario predicts ∼0.067 muon and anti-muon neutrinos over the observed 3-week flare. Our results are consistent with the detection of one very high-energy neutrino like IceCube-211208A.
PDF
