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Register a new userDeep Learning Blazar Classification based on Multi-frequency Spectral Energy Distribution Data
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Blazars are among the most studied sources in high-energy astrophysics as they form the largest fraction of extragalactic gamma-ray sources and are considered prime candidates for being the counterparts of high-energy astrophysical neutrinos. Their reliable identification amid the many faint radio sources is a crucial step for multi-messenger counterpart associations. As the astronomical community prepares for the coming of a number of new facilities able to survey the non-thermal sky at unprecedented depths, from radio to gamma-rays, machine learning techniques for fast and reliable source identification are ever more relevant. The purpose of this work was to develop a deep learning architecture to identify blazar within a population of AGN based solely on non-contemporaneous spectral energy distribution information, collected from publicly available multi-frequency catalogues. This study uses an unprecedented amount of data, with SEDs for $approx 14,000$ sources collected with the Open Universe VOU-Blazars tool. It uses a convolutional long-short term memory neural network purposefully built for the problem of SED classification, which we describe in detail and validate. The network was able to distinguish blazars from other types of AGNs to a satisfying degree (achieving a ROC area under curve of $0.98$), even when trained on a reduced subset of the whole sample. This initial study does not attempt to classify blazars among their different sub-classes, or quantify the likelihood of any multi-frequency or multi-messenger association, but is presented as a step towards these more practically-oriented applications.
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Blazars are usually classified following their synchrotron peak frequency ($ u F( u)$ scale) as high, intermediate, low frequency peaked BL Lacs (HBLs, IBLs, LBLs), and flat spectrum radio quasars (FSRQs), or, according to their radio morphology at large scale, FR~I or FR~II. However, the diversity of blazars is such that these classes seem insufficient to chart the specific properties of each source. We propose to classify a wide sample of blazars following the kinematic features of their radio jets seen in very long baseline interferometry (VLBI). For this purpose we use public data from the MOJAVE collaboration in which we select a sample of blazars with known redshift and sufficient monitoring to constrain apparent velocities. We selected 161 blazars from a sample of 200 sources. We identify three distinct classes of VLBI jets depending on radio knot kinematics: class I with quasi-stationary knots, class II with knots in relativistic motion from the radio core, and class I/II, intermediate, showing quasi-stationary knots at the jet base and relativistic motions downstream. A notable result is the good overlap of this kinematic classification with the usual spectral classification; class I corresponds to HBLs, class II to FSRQs, and class I/II to IBLs/LBLs. We deepen this study by characterizing the physical parameters of jets from VLBI radio data. Hence we focus on the singular case of the class I/II by the study of the blazar BL Lac itself. Finally we show how the interpretation that radio knots are recollimation shocks is fully appropriate to describe the characteristics of these three classes.
We report on X-ray measurements constraining the spectral energy distribution (SED) of the high-redshift $z=5.18$ blazar SDSS J013127.34$-$032100.1 with new XMM-Newton and NuSTAR exposures. The blazars X-ray spectrum is well fit by a power law with $Gamma=1.9$ and $N_{rm H}=1.1times10^{21}rm cm^{-2}$, or a broken power law with $Gamma_l=0.5$, $Gamma_h=1.8$, and a break energy $E_b=0.7$ keV for an expected absorbing column density of $N_{rm H}=3.6times 10^{20}rm cm^{-2}$, supported by spectral fitting of a nearby bright source. No additional spectral break is found at higher X-ray energies (1-30 keV). We supplement the X-ray data with lower-energy radio-to-optical measurements and Fermi-LAT gamma-ray upper limits, construct broadband SEDs of the source, and model the SEDs using a synchro-Compton scenario. This modeling constrains the bulk Doppler factor of the jets to $ge$7 and $ge$6 (90%) for the low- and high-$N_{rm H}$ SEDs, respectively. The corresponding beaming implies $ge$130 (low $N_{rm H}$) or $ge$100 (high $N_{rm H}$) high-spin supermassive black holes similar to J0131 exist at similar redshifts.
Fermi-LAT analyses show that the gamma-ray photon spectral indices Gamma_gamma of a large sample of blazars correlate with the vFv peak synchrotron frequency v_s according to the relation Gamma_gamma = d - k log v_s. The same function, with different constants d and k, also describes the relationship between Gamma_gamma and peak Compton frequency v_C. This behavior is derived analytically using an equipartition blazar model with a log-parabola description of the electron energy distribution (EED). In the Thomson regime, k = k_EC = 3b/4 for external Compton processes and k = k_SSC = 9b/16 for synchrotron self-Compton (SSC) processes, where b is the log-parabola width parameter of the EED. The BL Lac object Mrk 501 is fit with a synchrotron/SSC model given by the log-parabola EED, and is best fit away from equipartition. Corrections are made to the spectral-index diagrams for a low-energy power-law EED and departures from equipartition, as constrained by absolute jet power. Analytic expressions are compared with numerical values derived from self-Compton and external Compton scattered gamma-ray spectra from Ly alpha broad-line region and IR target photons. The Gamma_gamma vs. v_s behavior in the model depends strongly on b, with progressively and predictably weaker dependences on gamma-ray detection range, variability time, and isotropic gamma-ray luminosity. Implications for blazar unification and blazars as ultra-high energy cosmic-ray sources are presented. Arguments by Ghisellini et al. (2014) that the jet power exceeds the accretion luminosity depend on the doubtful assumption that we are viewing at the Doppler angle.
Blazars are highly variable, radio-loud active galactic nuclei with jets oriented at a small angle to the line of sight. The observed emission of these sources covers the whole electromagnetic spectrum from radio frequencies up to the high or even very high energy gamma-ray range. To understand the complex physics of these objects, multi-wavelength observations and studies on the variability and correlations between different wavelengths are therefore essential. The long-term multi-frequency observations of PKS 0048-097 are analysed here to investigate its spectral and temporal features. The studies includes nine years of observations of the blazar, which is well studied in the optical and radio domain, but not in the other frequencies. Multi-wavelength data collected with OVRO, KAIT, Catalina, Swift/UVOT, Swift/XRT and Fermi/LAT were studied. The performed analysis revealed strong variability in all wavelengths that is most clearly manifested in the X-ray range. The correlation studies do not exhibit any relation between different wavelengths, except for the very strong positive correlation between the optical emission in V and R bands.
A study of the gravitationally lensed blazar PKS 1830-211 was carried out using multi waveband data collected by Fermi-LAT, Swift-XRT and Swift-UVOT telescopes between MJD 58400 to MJD 58800 (9 Oct 2018 to 13 Nov 2019). Flaring states were identified by analysing the gamma-ray light curve. Simultaneous multi-waveband SED were obtained for those flaring periods. A cross-correlation analysis of the multi-waveband data was carried out, which suggested a common origin of the gamma-ray and X-ray emission. The broadband emission mechanism was studied by modelling the SED using a leptonic model. Physical parameters of the blazar were estimated from the broadband SED modelling. The blazar PKS 1830-211 is gravitationally lensed by at least two galaxies and has been extensively studied in the literature because of this property. The self-correlation of the gamma-ray light curve was studied to identify the signature of lensing, but no conclusive evidence of correlation was found at the expected time delay of 26 days.
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