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In this paper, multi-wavelength data are compiled for a sample of 1425 Fermi blazars to calculate their spectral energy distributions (SEDs). A parabolic function, log(νF ν ) = P 1 (logν − P 2 ) 2 + P 3 , is used for SED fitting. Synchrotron peak frequency (logν p ), spectral curvature (P 1 ), peak flux (ν p F νp ), and integrated flux (νF ν ) are successfully obtained for 1392 blazars (461 flat spectrum radio quasarsFSRQs, 620 BL Lacs-BLs and 311 blazars of uncertain type-BCUs, 999 sources have known redshifts). Monochromatic luminosity at radio 1.4 GHz, optical R band, X-ray at 1 keV and γ-ray at 1 GeV, peak luminosity, integrated luminosity and effective spectral indexes of radio to optical (α RO ), and optical to X-ray (α OX ) are calculated. The "Bayesian classification" is employed to logν p in the rest frame for 999 blazars with available redshift and the results show that 3 components are enough to fit the logν p distribution, there is no ultra high peaked subclass. Based on the 3 components, the subclasses of blazars using the acronyms of Abdo et al. (2010a) are classified, and some mutual correlations are also studied. Conclusions are finally drawn as follows: (1) SEDs are successfully obtained for 1392 blazars. The fitted peak frequencies are compared with common sources from samples available ( Sambruna et al. 1996, Nieppola et al. 2006, 2008, Abdo et al. 2010a. (2) -2 -peak sources (ISPs) if 14.0 < log ν p (Hz) ≤ 15.3, and high synchrotron peak sources (HSPs) if log ν p (Hz) > 15.3. (3) γ-ray emissions are strongly correlated with radio emissions. γ-ray luminosity is also correlated with synchrotron peak luminosity and integrated luminosity. (4) There is an anti-correlation between peak frequency and peak luminosity within the whole blazar sample. However, there is a marginally positive correlation for HBLs, and no correlations for FSRQs or LBLs. (5) There are anti-correlations between the monochromatic luminosities (γ-ray and radio bands) and the peak frequency within the whole sample and BL Lacs. (6) The optical to X-ray (α OX ) and radio to optical (α RO ) spectral indexes are strongly anti-correlated with peak frequency (log ν p ) within the whole sample, but the correlations for subclasses of FSRQs, LBLs, and HBLs are different.

The Crab Nebula is a bright source of gamma-rays powered by the Crab Pulsar's rotational energy, through the formation and termination of a relativistic electron-positron wind. We report the detection of γ-rays from this source with energies from 5 × 10−4 to 1.1 petaelectronvolts (PeV), with a spectrum showing gradual steepening over three energy decades. The ultra-high-energy photons imply the presence of a PeV electron accelerator (a pevatron) in the nebula, with an acceleration rate exceeding 15% of the theoretical limit. We constrain the pevatron's size between 0.025 and 0.1 pc, and magnetic field ≈110 μG. The production rate of PeV electrons, 2.5 × 1036 erg s−1, constitutes 0.5% of the pulsar spin-down luminosity, although we cannot exclude a contribution of PeV protons to the production of the highest energy γ-rays.

A sub-array of the Large High Altitude Air Shower Observatory (LHAASO), KM2A is mainly designed to observe a large fraction of the northern sky to hunt for γ-ray sources at energies above 10 TeV. Even though the detector construction is still underway, half of the KM2A array has been operating stably since the end of 2019. In this paper, we present the KM2A data analysis pipeline and the first observation of the Crab Nebula, a standard candle in very high energy γ-ray astronomy. We detect γ-ray signals from the Crab Nebula in both energy ranges of 10 100 TeV and 100 TeV with high significance, by analyzing the KM2A data of 136 live days between December 2019 and May 2020. With the observations, we test the detector performance, including angular resolution, pointing accuracy and cosmic-ray background rejection power. The energy spectrum of the Crab Nebula in the energy range 10-250 TeV fits well with a single power-law function dN/dE = (1.13 0.05 0.08 ) 10 (E/20 TeV) cm s TeV . It is consistent with previous measurements by other experiments. This opens a new window of γ-ray astronomy above 0.1 PeV through which new ultrahigh-energy γ-ray phenomena, such as cosmic PeVatrons, might be discovered.

In this work, we exploit the luminosity of the broad-line region (BLR) for 50 Fermi blazars through matching the coordinates between the Sloan Digital Sky Survey catalog, Fermi Large Area Telescope Third Source Catalog, and the Fermi Large Area Telescope Fourth Source Catalog and fitting their emission lines. In total, we collected a sample of 350 blazars with broad-line emissions including 50 new objects and 300 blazars from published works to revisit the correlation between the γ-ray luminosity and the broad-line luminosity, and proposed a new method to estimate the Doppler factors based on the correlation between the radiative power of the jet and luminosity of the BLR. We come to following conclusions. (1) For the 50 Fermi blazars, their broad-line luminosity (log L BLR) ranges from 41.82–45.2 erg s−1 with a mean value of 44.39 erg s−1. (2) The Doppler factor (δ) ranges from δ = 0.35 to δ = 85.66 with a mean value of 12.54, which is consistent with the results in the literature. (3) There are positive correlations between γ-ray luminosity and broad-line luminosity, and between γ-ray luminosity and the Doppler factor.