MeV Blazars Research Articles

BASS. XXXIII. Swift-BAT Blazars and Their Jets through Cosmic Time

We derive the most up-to-date Swift-Burst Alert Telescope (BAT) blazar luminosity function in the 14–195 keV range, making use of a clean sample of 118 blazars detected in the BAT 105 month survey catalog, with newly obtained redshifts from the BAT Active Galatic Nucleus Spectroscopic Survey. We determine the best-fit X-ray luminosity function for the whole blazar population, as well as for flat-spectrum radio quasars (FSRQs) alone. The main results are: (1) at any redshift, BAT detects the most luminous blazars, above any possible break in their luminosity distribution, which means we cannot differentiate between density and luminosity evolution; (2) the whole blazar population, dominated by FSRQs, evolves positively up to redshift z ∼ 4.3, confirming earlier results and implying lower number densities of blazars at higher redshifts than previously estimated. The contribution of this source class to the cosmic X-ray background at 14–195 keV can range from 5%–18%, while possibly accounting for 100% of the MeV background. We also derived the average 14 keV–10 GeV spectral energy distribution for BAT blazars, which allows us to predict the number counts of sources in the MeV range, as well as the expected number of high-energy (>100 TeV) neutrinos. A mission like COSI will detect 40 MeV blazars, of which two may have coincident neutrino detections. Finally, taking into account beaming selection effects, the distribution and properties of the parent population of these extragalactic jets are derived. We find that the distribution of viewing angles is quite narrow, with most sources aligned within <5° of the line of sight. Moreover, the average Lorentz factor, 〈Γ〉 = 8–12, is lower than previously suggested for these powerful sources.

The Blazar Sequence and Its Physical Understanding

Introduced in 1998 to attempt a first unified view of the broad-band emission properties of blazars, the blazar sequence has been extensively used in the past 25 years to guide observations as well as the physical interpretation of the overall emission from these galaxies. In this review, we describe the evolution of the sequence along with the tremendous advances in the observational field, in particular in the gamma-ray band. A new version of the sequence built on TeV-detected objects is also presented. Two extreme classes of objects (MeV and hard-TeV blazars) are included in the discussion, given their relevance for future observatories. Finally, the current physical understanding at the base of the sequence is presented along with the major criticisms to the blazar sequence.

NuSTAR Perspective on High-redshift MeV Blazars

Abstract With bolometric luminosities exceeding 1048 erg s−1, powerful jets, and supermassive black holes at their center, MeV blazars are some of the most extreme sources in the universe. Recently, the Fermi-Large Area Telescope detected five new γ-ray emitting MeV blazars beyond redshift z = 3.1. With the goal of precisely characterizing the jet properties of these extreme sources, we started a multiwavelength campaign to follow them up with joint Nuclear Spectroscopic Telescope Array, Swift, and the Southeastern Association for Research in Astronomy’s optical telescopes. We observe six high-redshift quasars, four of them belonging to the new γ-ray emitting MeV blazars. Thorough X-ray analysis reveals spectral flattening at soft X-ray for three of these objects. The source NVSS J151002+570243 also shows a peculiar rehardening of the X-ray spectrum at energies E &gt; 6 keV. Adopting a one-zone leptonic emission model, this combination of hard X-rays and γ-rays enables us to determine the location of the Inverse Compton peak and to accurately constrain the jet characteristics. In the context of the jet-accretion disk connection, we find that all six sources have jet powers exceeding accretion disk luminosity, seemingly validating this positive correlation even beyond z &gt; 3. Our six sources are found to have black holes, further raising the space density of supermassive black holes in the redshift bin z = [3, 4].

High-redshift Blazars through NuSTAR Eyes

Abstract The most powerful sources among the blazar family are MeV blazars. Often detected at z &gt; 2, they usually display high X- and γ-ray luminosities, larger-than-average jet powers, and black hole masses ≳109 M ☉. In the present work, we perform a multiwavelength study of three high-redshift blazars: 3FGL J0325.5+2223 (z = 2.06), 3FGL J0449.0+1121 (z = 2.15), and 3FGL J0453.2−2808 (z = 2.56), analyzing quasi-simultaneous data from GROND, Swift-UVOT and XRT, Nuclear Spectroscopic Telescope Array (NuSTAR), and Fermi-LAT. Our main focus is on the hard X-ray band recently unveiled by NuSTAR (3–79 keV) where these objects show a hard spectrum that enables us to constrain the inverse Compton (IC) peak and the jet power. We found that all three targets resemble the most powerful blazars, with the synchrotron peak located in the submillimeter range and the IC peak in the MeV range, and therefore belong to the MeV blazar class. Using a simple one-zone leptonic emission model to reproduce the spectral energy distributions, we conclude that a simple combination of synchrotron and accretion disk emission reproduces the infrared–optical spectra, while the X-ray to γ-ray part is well reproduced by the IC scattering of low-energy photons supplied by the broad-line region. The black hole masses for each of the three sources are calculated to be ≳4 × 108 M ☉. The three studied sources have jet power at the level of, or beyond, the accretion luminosity.

THE ORIGIN OF THE COSMIC GAMMA-RAY BACKGROUND IN THE MeV RANGE

ABSTRACT There has been much debate about the origin of the diffuse γ-ray background in the MeV range. At lower energies, AGNs and Seyfert galaxies can explain the background, but not above ≃0.3 MeV. Beyond ∼10 MeV blazars appear to account for the flux observed. That leaves an unexplained gap for which different candidates have been proposed, including annihilations of WIMPS. One candidate is Type Ia supernovae (SNe Ia). Early studies concluded that they were able to account for the γ-ray background in the gap, while later work attributed a significantly lower contribution to them. All those estimates were based on SN Ia explosion models that did not reflect the full 3D hydrodynamics of SN Ia explosions. In addition, new measurements obtained since 2010 have provided new, direct estimates of high-z SN Ia rates beyond z ∼ 2. We take into account these new advances to see the predicted contribution to the gamma-ray background. We use here a wide variety of explosion models and a plethora of new measurements of SN Ia rates. SNe Ia still fall short of the observed background. Only for a fit, which would imply ∼150% systematic error in detecting SN Ia events, do the theoretical predictions approach the observed fluxes. This fit is, however, at odds at the highest redshifts with recent SN Ia rate estimates. Other astrophysical sources such as flat-spectrum radio quasars do match the observed flux levels in the MeV regime, while SNe Ia make up to 30%–50% of the observed flux.

PROBING THE COSMIC X-RAY AND MeV GAMMA-RAY BACKGROUND RADIATION THROUGH THE ANISOTROPY

While the cosmic soft X-ray background is very likely to originate from individual Seyfert galaxies, the origin of the cosmic hard X-ray and MeV gamma-ray background is not fully understood. It is expected that Seyferts including Compton thick population may explain the cosmic hard X-ray background. At MeV energy range, Seyferts having non-thermal electrons in coronae above accretion disks or MeV blazars may explain the background radiation. We propose that future measurements of the angular power spectra of anisotropy of the cosmic X-ray and MeV gamma-ray backgrounds will be key to deciphering these backgrounds and the evolution of active galactic nuclei (AGNs). As AGNs trace the cosmic large-scale structure, spatial clustering of AGNs exists. We show that e-ROSITA will clearly detect the correlation signal of unresolved Seyferts at 0.5-2 keV and 2-10 keV bands and will be able to measure the bias parameter of AGNs at both bands. Once the future hard X-ray all sky satellites achieve the sensitivity better than 10^{-12} erg/cm^2/s at 10-30 keV or 30-50 keV - although this is beyond the sensitivities of current hard X-ray all sky monitors - angular power spectra will allow us to independently investigate the fraction of Compton-thick AGNs in all Seyferts. We also find that the expected angular power spectra of Seyferts and blazars in the MeV range are different by about an order of magnitude, where the Poisson term, so-called shot noise, is dominant. Current and future MeV instruments will clearly disentangle the origin of the MeV gamma-ray background through the angular power spectrum.

The broad-band X-ray spectrum of the blazar PKS B1830-211 by Chandra and INTEGRAL

In this paper we present a broad-band study of the X-ray emission of the blazar PKS 1830−211 based on Chandra and INTEGRAL observations. Notwithstanding the high redshift (z = 2.507), it is a bright X-ray source (F(2-10 keV) � 10 −11 erg cm −2 s −1 ), due to gravitational lensing by an intervening galaxy at z = 0.89. Previous X-ray ob- servations attribute the observed absorption at E < 2 keV to the lensing galaxy. Our analysis, although not in contrast with this hypothesis, suggests also the possibility of an intrinsic (ionized) absorption, taking place at the front side of the jet. This scenario is also supported by some evidence, in the same data, of a feature observed at 2.15 keV which can be interpreted as a blueshifted iron line (v/c � 0.18). The SED of PKS 1830−211 can be well modelled by combining a Synchrotron Self-Compton component and an external source of photons to be scattered up to γ-ray energies by relativistic electrons moving outward in the jet. The main source of low energy photons is a dust torus at the temperature of 10 3 K as expected in MeV blazars.

X-Ray Emission from the Quasar PKS 1127-145: Comptonized Infrared Photons on Parsec Scales

We model the broadband spectral energy distribution of the innermost core-jet region of the redshift z = 1.187 quasar PKS 1127-145. We propose a scenario in which the high-energy photons are produced via the Compton scattering of thermal IR radiation by the relativistic particles in a parsec-scale jet. The high-energy spectrum, together with the observed radio variability and superluminal expansion, suggest that PKS 1127-145 may be a blazar, despite the fact that its optical/UV component is likely dominated by thermal radiation from an accretion disk. The relation of PKS 1127-145 to MeV blazars is discussed.

On the Nature of MeV Blazars

Broadband spectra of the flat-spectrum radio quasars (FSRQs) detected in the high-energy γ-ray band imply that there may be two types of such objects: those with steep γ-ray spectra, hereafter called MeV blazars, and those with flat γ-ray spectra, GeV blazars. We demonstrate that this difference can be explained in the context of the external radiation Compton (ERC) model using the same electron injection function. A satisfactory unification is reachable, provided that (1) spectra of GeV blazars are produced by internal shocks formed at the distances where cooling of relativistic electrons in a jet is dominated by Comptonization of broad emission lines, whereas spectra of MeV blazars are produced at the distances where cooling of relativistic electrons is dominated by Comptonization of near-IR radiation from hot dust, and (2) electrons are accelerated via a two-step process and their injection function takes the form of a double power law, with the break corresponding to the threshold energy for the diffusive shock acceleration. Direct predictions of our model are that, on average, variability timescales of the MeV blazars should be longer than variability timescales of the GeV blazars, and that both types of the blazar phenomenon can appear in the same object.

Transrelativistic pair plasmas in AGN jets

Models of relativistic jets filled with ultrarelativistic pair plasma are very successful in explaining the broadband radiation of γ-ray blazars. Assuming that the initial injection and cooling of ultrarelativistic pair plasma in an AGN jet has occurred, producing the observed high-energy γ-ray radiation, we investigate the further evolution of the pair plasma as it continues to move out from the central engine. The effects of thermalization and reacceleration, the emission of pair bremsstrahlung and annihilation radiation and the bulk Compton process, and the possible application to MeV blazars are discussed. A model calculation to the special case of PKS 0208-512 is presented.

Inverse Compton scattering model for gamma-ray production in MeV blazars

It is proposed that the spectra of so called 'MeV blazars' can be explained in terms of previously developed models of the external comptonization of accretion disk radiation provided that the structure of the inner and outer parts of the accretion disk is different. The electron acceleration is saturated by the inverse Compton losses in the inner geometrically thick disk and the outer geometrically thin disk at different maximum energies which causes the appearance of two spectral components, one strongly peaked in the MeV energy range and the second one of a power law type extending through the GeV energy range. The spectra, computed in terms of such a simple geometrical model, are in a good agreement with observations of the MeV blazar PKS0208-512. They are consistent with the transient appearance of a strong MeV peak, the power law spectrum in the EGRET energy range, and a possible cut-off at high energies.

Inverse Compton scattering model for gamma-ray production in MeV blazars

It is proposed that the spectra of so-called ‘MeV blazars’ can be explained in terms of previously developed models of the external Comptonization of accretion disc radiation, provided that the structure of the inner and outer parts of the accretion disc is different. The electron acceleration is saturated by the inverse Compton losses in the inner geometrically thick disc and the outer geometrically thin disc at different maximum energies; this causes the appearance of two spectral components, one strongly peaked in the MeV energy range and the other of a power-law type extending through the GeV energy range. The spectra, computed in terms of such a simple geometrical model, are in good agreement with observations of the MeV blazar PKS 0208−512. They are consistent with the transient appearance of a strong MeV peak, the power-law spectrum in the EGRET energy range, and a possible cut-off at high energies.

TheASCAspectrum of thez = 4.72 blazar GB 1428+4217

The X-ray-luminous quasar GB 1428+4217 at redshift 4.72 has been observed with ASCA. The observed 0.5–10 keV flux is 3.2 × 10−12 erg cm−2 s−1. We report here on the intrinsic 4 − 57 keV X-ray spectrum, which is very flat (photon index 1.29). We find no evidence for flux variability within the ASCA data set or between it and ROSAT data. We show that the overall spectral energy distribution of GB 1428+4217 is similar to that of lower redshift MeV blazars, and present models that fit the available data. The Doppler beaming factor is likely to be at least 8. We speculate on the number density of such high-redshift blazars, which must contain rapidly formed massive black holes.

The Beaming Pattern of Doppler‐boosted Thermal Annihilation Radiation: Application to MeV Blazars

The beaming pattern of thermal annihilation radiation is broader than the beaming pattern produced by isotropic nonthermal electrons and positrons in the jets of radio-emitting active galactic nuclei which Compton scatter photons from an external isotropic radiation field. Thus blueshifted thermal annihilation radiation can provide the dominant contribution to the high-energy radiation spectrum at observing angles theta > 1/Gamma, where Gamma is the bulk Lorentz factor of the outflowing plasma. This effect may account for the spectral features of MeV blazars discovered with the Compton Telescope on the Compton Gamma Ray Observatory. Coordinated gamma-ray observations of annihilation line radiation to infer Doppler factors and VLBI radio observations to measure transverse angular speeds of outflowing plasma blobs can be used to determine the Hubble constant.

FSRQ and the Gamma-Ray Background

The contribution of Flat Spectrum Radio Quasars (FSRQ) to the γ-ray background (GRB) is modeled. FSRQ are known to make a substantial contribution to the hard (E&gt;100 MeV) background. The so called “MeV bump”, however, cannot be accounted for in terms of this class of sources even taking into account the newly discovered class of FSRQ with a broad band spectrum sharply peaked in the MeV range (“MeV blazars”). This is in agreement with the recent results obtained by Kappadath et al. (1995) using COMPTEL data.