We revisit the blazar sequence exploiting the complete, flux limited sample of blazars with known redshift detected by the Fermi satellite after 4 years of operations (the 3LAC sample). We divide the sources into gamma-ray luminosity bins, collect all the archival data for all blazars, and construct their spectral energy distribution (SED). We describe the average SED of blazars in the same luminosity bin through a very simple, completely phenomenological function consisting of two broken power laws connecting with a power law of fixed slope describing the radio emission. We do that separately for BL Lacs and for flat spectrum radio quasars (FSRQs) and also for all blazars together. The main results are: i) FSRQs display approximately the same SED as the luminosity increases, except for the fact that the relative importance of the high energy peak increases; ii) as a consequence, X-ray spectra of FSRQs become harder for larger luminosities; iii) BL Lacs form indeed a sequence: they become redder (i.e. the peak frequencies becomes smaller) for increasing luminosities, with a steeper gamma-ray slope and a larger dominance of the high energy peak; iv) for all blazars (BL Lacs+FSRQs) these properties becomes more prominent, as the highest luminosity bin is populated mostly by FSRQs and the lowest luminosity bin mostly by BL Lacs. This agrees with the original blazar sequence, although BL Lacs never have an average gamma-ray slope as hard as found in the original sequence. v) At high luminosities, a large fraction of FSRQs shows signs of thermal emission from the accretion disc, contributing in the optical-UV.

The Fermi blazar sequence

Science with e-ASTROGAM: A space mission for MeV–GeV gamma-ray astrophysics

High-energy γ-rays of interstellar origin are produced by the interaction of cosmic-ray (CR) particles with the diffuse gas and radiation fields in the Galaxy. The main features of this emission are well understood and are reproduced by existing CR propagation models employing 2D galactocentric cylindrically symmetrical geometry. However, the high-quality data from instruments like the Fermi Large Area Telescope reveal significant deviations from the model predictions on few to tens of degrees scales, indicating the need to include the details of the Galactic spiral structure and thus requiring 3D spatial modeling. In this paper, the high-energy interstellar emissions from the Galaxy are calculated using the new release of the GALPROP code employing 3D spatial models for the CR source and interstellar radiation field (ISRF) densities. Three models for the spatial distribution of CR sources are used that are differentiated by their relative proportion of input luminosity attributed to the smooth disk or spiral arms. Two ISRF models are developed based on stellar and dust spatial density distributions taken from the literature that reproduce local near- to far-infrared observations. The interstellar emission models that include arms and bulges for the CR source and ISRF densities provide plausible physical interpretations for features found in the residual maps from high-energy γ-ray data analysis. The 3D models for CR and ISRF densities provide a more realistic basis that can be used for the interpretation of the nonthermal interstellar emissions from the Galaxy.

High-energy Gamma Rays from the Milky Way: Three-dimensional Spatial Models for the Cosmic-Ray and Radiation Field Densities in the Interstellar Medium.

A critical review of the standard paradigm for the origin of Galactic cosmic rays is presented. Recent measurements of local and far-away cosmic rays reveal unexpected behaviours, which challenge the commonly accepted scenario. These recent findings are discussed, together with long-standing open issues. Despite the progress made thanks to ever-improving observational techniques and theoretical investigations, at present our understanding of the origin and of the behaviour of cosmic rays remains incomplete. We believe it is still unclear whether a modification of the standard paradigm, or rather a radical change of the paradigm itself is needed in order to interpret all the available data on cosmic rays within a self-consistent scenario.

The origin of Galactic cosmic rays: challenges to the standard paradigm

An anomalous emission component at energies of a few gigaelectronvolts and located towards the inner Galaxy is present in the Fermi-LAT data. At present, the two most promising explanations are the annihilation of dark matter particles or the presence of a large population of unresolved point sources, most probably millisecond pulsars, at the Galactic Centre. Here, we report an analysis of the excess characteristics using almost eight years of Pass 8 ULTRACLEAN Fermi-LAT data with SkyFACT—a tool that combines image reconstruction with template-fitting techniques. We find that an emission profile that traces stellar mass in the boxy and nuclear bulge is preferred over conventional dark matter profiles. A model including the bulge is favoured over a model with dark matter at 16σ.

The Fermi-LAT GeV excess as a tracer of stellar mass in the Galactic bulge

e-ASTROGAM (‘enhanced ASTROGAM’) is a breakthrough Observatory space mission, with a detector composed by a Silicon tracker, a calorimeter, and an anticoincidence system, dedicated to the study of the non-thermal Universe in the photon energy range from 0.3 MeV to 3 GeV – the lower energy limit can be pushed to energies as low as 150 keV, albeit with rapidly degrading angular resolution, for the tracker, and to 30 keV for calorimetric detection. The mission is based on an advanced space-proven detector technology, with unprecedented sensitivity, angular and energy resolution, combined with polarimetric capability. Thanks to its performance in the MeV-GeV domain, substantially improving its predecessors, e-ASTROGAM will open a new window on the non-thermal Universe, making pioneering observations of the most powerful Galactic and extragalactic sources, elucidating the nature of their relativistic outflows and their effects on the surroundings. With a line sensitivity in the MeV energy range one to two orders of magnitude better than previous generation instruments, e-ASTROGAM will determine the origin of key isotopes fundamental for the understanding of supernova explosion and the chemical evolution of our Galaxy. The mission will provide unique data of significant interest to a broad astronomical community, complementary to powerful observatories such as LIGO-Virgo-GEO600-KAGRA, SKA, ALMA, E-ELT, TMT, LSST, JWST, Athena, CTA, IceCube, KM3NeT, and the promise of eLISA.

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P. Laurent

Papers

The e-ASTROGAM mission: Exploring the extreme Universe with gamma rays in the MeV – GeV range