Inverse Compton Peak Research Articles

A Study of Particle Acceleration in Blazar Jets

The particle acceleration of blazar jets is crucial to high-energy astrophysics, yet the acceleration mechanism division in blazar subclasses and the underlying nature of these mechanisms remain elusive. In this work, we utilized the synchrotron spectral information (synchrotron peak frequency, logνsy , and corresponding curvature, b sy) of 2705 blazars from the literature and studied the subject of particle acceleration in blazar jets by analyzing the correlation between logνsy and 1/b sy. Our results suggested that the entire sample follows an energy-dependent probability acceleration (EDPA). Specifically, the low inverse Compton peak sources (LCPs) follow the mechanism of fluctuations of fractional gain acceleration (FFGA), while the high inverse Compton peak sources (HCPs) follow an acceleration mechanism of EDPA. Our results indicated that the separation between LCPs and HCPs results from the electron peak Lorentz factor (γ p), and the differentiation should originate from different acceleration mechanisms. Moreover, our study revealed a transition in the acceleration mechanism from FFGA to EDPA around logνsy∼15 through a detailed analysis of binned- logνsy . The mechanism of FFGA dominates the particle acceleration in LCP jets because of stronger jets and the EDPA dominates the particle energy gain in the HCPs due to a more efficient acceleration process.

An intermittent extreme BL Lac: MWL study of 1ES 2344+514 in an enhanced state

ABSTRACT Extreme high-frequency BL Lacs (EHBL) feature their synchrotron peak of the broad-band spectral energy distribution (SED) at νs ≥ 1017 Hz. The BL Lac object 1ES 2344+514 was included in the EHBL family because of its impressive shift of the synchrotron peak in 1996. During the following years, the source appeared to be in a low state without showing any extreme behaviours. In 2016 August, 1ES 2344+514 was detected with the ground-based γ-ray telescope FACT during a high γ-ray state, triggering multiwavelength (MWL) observations. We studied the MWL light curves of 1ES 2344+514 during the 2016 flaring state, using data from radio to very-high-energy (VHE) γ-rays taken with OVRO, KAIT, KVA, NOT, some telescopes of the GASP-WEBT collaboration at the Teide, Crimean, and St. Petersburg observatories, Swift-UVOT, Swift-XRT, Fermi-LAT, FACT, and MAGIC. With simultaneous observations of the flare, we built the broad-band SED and studied it in the framework of a leptonic and a hadronic model. The VHE γ-ray observations show a flux level of 55 per cent of the Crab Nebula flux above 300 GeV, similar to the historical maximum of 1995. The combination of MAGIC and Fermi-LAT spectra provides an unprecedented characterization of the inverse-Compton peak for this object during a flaring episode. The Γ index of the intrinsic spectrum in the VHE γ-ray band is 2.04 ± 0.12stat ± 0.15sys. We find the source in an extreme state with a shift of the position of the synchrotron peak to frequencies above or equal to 1018 Hz.

Observational signatures of gamma-rays from bright blazars and wakefield theory

ABSTRACTGamma-ray observations have revealed strong variability in blazar luminosities in the gamma-ray band over time-scales as short as minutes. We show, for the first time, that the correlation of the spectrum with intensity is consistent with the behaviour of the luminosity variation of blazar spectral energy distributions (SEDs) along a blazar sequence for low synchrotron peak blazars. We show that the observational signatures of variability with flux are consistent with wakefield acceleration of electrons initiated by instabilities in the blazar accretion disc. This mechanism reproduces the observed time variations as short as 100 s. The wakefield mechanism also predicts a reduction of the electron spectral index with increased gamma-ray luminosity, which could be detected in higher energy observations well above the inverse Compton peak.

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 > 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 > 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].

Fermi-LAT Stacking Analysis Technique: An Application to Extreme Blazars and Prospects for their CTA Detection

Abstract We present a likelihood profile stacking technique based on the Fermi-Large Area Telescope (LAT) data to explore the γ-ray characteristics of Fermi-LAT undetected astrophysical populations. The pipeline is applied to a sample of γ-ray unresolved extreme blazars, i.e., sources with the highest synchrotron peak frequencies ( ), and we report a cumulative γ-ray detection with more than 32σ confidence for 2 degrees of freedom. Comparing the generated stacked γ-ray spectrum with the sensitivity limits of the upcoming Cherenkov Telescope Array (CTA), we find that the Fermi-LAT undetected population of such extreme blazars, on average, may remain well below the CTA detection threshold due to their faintness and extragalactic background light (EBL) absorption. However, γ-ray detected blazars belonging to the same class are promising candidates for CTA observations. The EBL-corrected stacked spectra of these sources do not show any softening up to 1 TeV. This finding suggests the inverse Compton peak of extreme blazars lies above 1 TeV, thus indicating a hard intrinsic TeV spectrum. Our analysis also predicts that at 100 GeV, at least ∼10% of the diffuse extragalactic γ-ray background originates from the γ-ray undetected extreme blazars. These results highlight the effectiveness of the developed stacking technique to explore the uncharted territory of γ-ray undetected astrophysical objects.

The Physical Conditions of the Afterglow Implied by MAGIC’s Sub-TeV Observations of GRB 190114C

Abstract MAGIC’s observations of late sub-TeV photons from GRB 190114C enable us, for the first time, to determine the details of the emission process in a GRB afterglow and to pin down the physical parameters, such as the bulk Lorentz factor and the Lorentz factor of the emitting electrons as well as some of the microphysical parameters. We find that the sub-TeV emission is synchrotron–self-Compton radiation produced at the early afterglow stage. Combining the sub-TeV and X-ray observations we narrow uncertainties in the conditions inside the emitting zone, almost eliminating them for some parameters. Seventy seconds after the trigger the external shock had a Lorentz factor ≃100, and the electrons producing the observed sub-TeV radiation had a Lorentz factor ≃104, so that the sub-TeV radiation originates from Comptonization of X-ray photons at the border between the Thomson and Klein–Nishina regimes. The inferred conditions within the emitting zone are at odds with theoretical expectations unless one assumes moderate (with τ ≃ 2) absorption of sub-TeV photons inside the source. With this correction the conditions are in good agreement with predictions of the pair-balance model, but are also acceptable for generic afterglow model as one of many possibilities. The different temporal evolution of the inverse Compton peak energy of these two models opens a way to discriminate between them once late-time detection in the TeV range become available.

Detection of a Gamma-Ray Flare from the High-redshift Blazar DA 193

Abstract High-redshift (z > 2) blazars are the most powerful members of the blazar family. Yet, only a handful of them have both X-ray and γ-ray detection, thereby making it difficult to characterize the energetics of the most luminous jets. Here, we report, for the first time, the Fermi-Large Area Telescope detection of the significant γ-ray emission from the high-redshift blazar DA 193 (z = 2.363). Its time-averaged γ-ray spectrum is soft (γ-ray photon index = 2.9 ± 0.1), and together with a relatively flat hard X-ray spectrum (14–195 keV photon index = 1.5 ± 0.4), DA 193 presents a case to study a typical high-redshift blazar with inverse Compton peak being located at MeV energies. An intense GeV flare was observed from this object in the first week of 2018 January, a phenomenon rarely observed from high-redshift sources. What makes this event a rare one is the observation of an extremely hard γ-ray spectrum (photon index = 1.7 ± 0.2), which is somewhat unexpected because high-redshift blazars typically exhibit a steep falling spectrum at GeV energies. The results of our multifrequency campaign, including both space- (Fermi, NuSTAR, and Swift) and ground-based (Steward and Nordic Optical Telescope) observatories are presented, and this peculiar γ-ray flare is studied within the framework of a single-zone leptonic emission scenario.

Spectral energy distributions for TeV blazars

In this work, we collected a sample of 69 TeV blazars from TeVCat, obtained their multi-wavelength observations, and fitted their spectral energy distributions by using a second degree polynomial function. The structure parameters of synchrotron bumps for 68 blazars and those of inverse-Compton bumps for 56 blazars were derived. Then, we conducted statistical analysis on the parameters (curvature, peak frequency, peak luminosity, bolometric luminosity and X/γ-ray spectral indexes). From our analysis and discussions, we can conclude the following: (1) There is a clear positive correlation between the synchrotron peak frequency, , and the inverse-Compton peak frequency, , and between the synchrotron peak luminosity, , and the inverse-Compton peak luminosity, . (2) The correlation between the peak frequency and the curvature of synchrotron bump is clearly different from that of the inverse-Compton bump, which further indicates that there are different emission mechanisms between them. (3) There is a correlation between and γ-ray spectral index, αγ, for the TeV blazars: , which is consistent with previous work. (4) An “L-shape” relation is found between and αX for both TeV blazars and Fermi blazars. A significant correlation between and X-ray photon index (αX) is found for the TeV blazars with high synchrotron peak frequency: , while the correlation is positive for low synchrotron peaked TeV sources. (5) In the αX − αγ diagram, there is also an “L-shape.” The anti-correlation is consistent with results available in the literature, and we also find a positive correlation between them. (6) Inverse-Compton dominant sources have luminous bolometric luminosities.

A complete sample of LSP blazars fully described in γ-rays

Context. We study the γ-ray and broadband spectral energy distribution (SED) properties of a complete sample of 104 bright, radio-selected low-synchrotron peaked (LSP) blazars, which have well-characterized SEDs from radio to X-rays. Most of the sources have already been detected in the γ-ray band by Fermi-LAT, however almost 20% of these blazars have no counterpart in any of the Fermi catalogs published so far. Aims. Using the Fermi Science Tools, we look for γ-ray emission for those objects not yet reported in any Fermi-LAT catalog, finding new detections and associations. We then study the multifrequency SED for all sources in our sample, fitting their synchrotron (Syn) and inverse Compton (IC) components. A complete sample of LSP blazars with a full description in γ-ray is unique. We use this sample to derive the distribution of the Compton dominance (CD) along with population properties such as Syn and IC peak power, and frequency distributions. Methods. We performed a binned likelihood analysis in the 0.3–500 GeV energy band with Fermi-LAT Pass 8 data, integrating over 7.5 yr of observations. We studied γ-ray light curves and test statistic (TS) maps to validate new detections and associations, thereby building a better picture of the high-energy activity in radio-selected LSP blazars. We fit the IC component for the new detections using all data at our disposal from X-rays to GeV γ-rays, enhancing the amount of information available to study the Syn to IC peak-power correlations. Results. We deliver a unique characterization in γ-rays for a complete sample of LSP blazars. We show that three previously unidentified 3FGL sources can be associated with blazars when using improved γ-ray positions obtained from TS maps. Six previously unreported γ-ray sources are detected at TS > 20 level, while another three show TS values between 10–20. We evaluate two cases in which source confusion is likely present. In four cases there is no significant γ-ray signature when integrating over 7.5 yr. Short-lived flares at ~1 month scale, however, have been detected in these sources. Finally, we measure the log(CD) for the sample, which has a Gaussian-like distribution with median log(CD) ≈ 0.1, implying that on average the peak-power for the Syn and IC components in LSP blazars is similar.

Theγ-ray emitting region in low synchrotron peak blazars

Aims.From the early days inγ-ray astronomy, locating the origin of GeV emission within the core of an active galactic nucleus (AGN) persisted as an open question; the problem is to discern between near- and far-site scenarios with respect to the distance from the super massive central engine. We investigate this question under the light of a complete sample of low synchrotron peak (LSP) blazars which is fully characterized along many decades in the electromagnetic spectrum, from radio up to tens of GeV. We consider the high-energy emission from bright radio blazars and test for synchrotron self-Compton (SSC) and external Compton (EC) scenarios in the framework of localizing theγ-ray emission sites. Given that the inverse Compton (IC) process under the EC regime is driven by the abundance of external seed photons, these photons could be mainly ultraviolet (UV) to X-rays coming from the accretion disk region and the broad-line region (BLR), therefore close to the jet launch base; or infrared (IR) seed photons from the dust torus and molecular cloud spine-sheath, therefore far from jet launch base. We investigate both scenarios, and try to reveal the physics behind the production ofγ-ray radiation in AGNs which is crucial in order to locate the production site.Methods.Based on a complete sample of 104 radio-selected LSP blazars, with 37 GHz flux density higher than 1 Jy, we study broadband population properties associated with the nonthermal jet emission process, and test the capability of SSC and EC scenarios to explain the overall spectral energy distribution (SED) features. We use SEDs well characterized from radio toγrays, considering all currently available data. The enhanced available information from recent works allows us to refine the study of Syn to IC peak correlations, which points to a particularγ-ray emission site.Results.We show that SSC alone is not enough to account for the observed SEDs. Our analysis favors an EC scenario under the Thomson scattering regime, with a dominant IR external photon field. Therefore, the far-site (i.e., far from the jet launch) is probably the most reasonable scenario to account for the population properties of bright LSP blazars in cases modeled with a pure leptonic component. We calculate the photon energy density associated with the external field at the jet comoving frame to beU′ext= 1.69 × 10−2erg cm−3, finding good agreement to other correlated works.

Relativistic turbulence with strong synchrotron and synchrotron self-Compton cooling

Many relativistic plasma environments in high-energy astrophysics, including pulsar wind nebulae, hot accretion flows onto black holes, relativistic jets in active galactic nuclei and gamma-ray bursts, and giant radio lobes, are naturally turbulent. The plasma in these environments is often so hot that synchrotron and inverse-Compton (IC) radiative cooling becomes important. In this paper we investigate the general thermodynamic and radiative properties (and hence the observational appearance) of an optically thin relativistically hot plasma stirred by driven magnetohydrodynamic (MHD) turbulence and cooled by radiation. We find that if the system reaches a statistical equilibrium where turbulent heating is balanced by radiative cooling, the effective electron temperature tends to attain a universal value $\theta = kT_e/m_e c^2 \sim 1/\sqrt{\tau_T}$, where $\tau_T=n_e\sigma_T L \ll 1$ is the system's Thomson optical depth, essentially independent of the strength of turbulent driving or magnetic field. This is because both MHD turbulent dissipation and synchrotron cooling are proportional to the magnetic energy density. We also find that synchrotron self-Compton (SSC) cooling and perhaps a few higher-order IC components are automatically comparable to synchrotron in this regime. The overall broadband radiation spectrum then consists of several distinct components (synchrotron, SSC, etc.), well separated in photon energy (by a factor $\sim \tau_T^{-1}$) and roughly equal in power. The number of IC peaks is checked by Klein-Nishina effects and depends logarithmically on $\tau_T$ and the magnetic field. We also examine the limitations due to synchrotron self-absorption, explore applications to Crab PWN and blazar jets, and discuss links to radiative magnetic reconnection.

High-redshift Blazars through NuSTAR Eyes

Abstract The most powerful sources among the blazar family are MeV blazars. Often detected at z > 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.

MULTIWAVELENGTH STUDY OF QUIESCENT STATES OF Mrk 421 WITH UNPRECEDENTED HARD X-RAY COVERAGE PROVIDED BY NuSTAR IN 2013

ABSTRACT We present coordinated multiwavelength observations of the bright, nearby BL Lacertae object Mrk 421 taken in 2013 January–March, involving GASP-WEBT, Swift, NuSTAR, Fermi-LAT, MAGIC, VERITAS, and other collaborations and instruments, providing data from radio to very high energy (VHE) γ-ray bands. NuSTAR yielded previously unattainable sensitivity in the 3–79 keV range, revealing that the spectrum softens when the source is dimmer until the X-ray spectral shape saturates into a steep power law, with no evidence for an exponential cutoff or additional hard components up to ∼80 keV. For the first time, we observed both the synchrotron and the inverse-Compton peaks of the spectral energy distribution (SED) simultaneously shifted to frequencies below the typical quiescent state by an order of magnitude. The fractional variability as a function of photon energy shows a double-bump structure that relates to the two bumps of the broadband SED. In each bump, the variability increases with energy, which, in the framework of the synchrotron self-Compton model, implies that the electrons with higher energies are more variable. The measured multi band variability, the significant X-ray-to-VHE correlation down to some of the lowest fluxes ever observed in both bands, the lack of correlation between optical/UV and X-ray flux, the low degree of polarization and its significant (random) variations, the short estimated electron cooling time, and the significantly longer variability timescale observed in the NuSTAR light curves point toward in situ electron acceleration and suggest that there are multiple compact regions contributing to the broadband emission of Mrk 421 during low-activity states.

Measurement of the Crab Nebula spectrum over three decades in energy with the MAGIC telescopes

The MAGIC stereoscopic system collected 69 hours of Crab Nebula data between October 2009 and April 2011. Analysis of this data sample using the latest improvements in the MAGIC stereoscopic software provided an unprecedented precision of spectral and night-by-night light curve determination at gamma rays. We derived a differential spectrum with a single instrument from 50 GeV up to almost 30 TeV with 5 bins per energy decade. At low energies, MAGIC results, combined with Fermi-LAT data, show a flat and broad Inverse Compton peak. The overall fit to the data between 1 GeV and 30 TeV is not well described by a log-parabola function. We find that a modified log-parabola function with an exponent of 2.5 instead of 2 provides a good description of the data (χred2=35/26). Using systematic uncertainties of the MAGIC and Fermi-LAT measurements we determine the position of the Inverse Compton peak to be at (53±3stat+31syst−13syst) GeV, which is the most precise estimation up to date and is dominated by the systematic effects. There is no hint of the integral flux variability on daily scales at energies above 300 GeV when systematic uncertainties are included in the flux measurement. We consider three state-of-the-art theoretical models to describe the overall spectral energy distribution of the Crab Nebula. The constant B-field model cannot satisfactorily reproduce the VHE spectral measurements presented in this work, having particular difficulty reproducing the broadness of the observed IC peak. Most probably this implies that the assumption of the homogeneity of the magnetic field inside the nebula is incorrect. On the other hand, the time-dependent 1D spectral model provides a good fit of the new VHE results when considering a 80 μG magnetic field. However, it fails to match the data when including the morphology of the nebula at lower wavelengths.

CURVATURE OF THE SPECTRAL ENERGY DISTRIBUTIONS OF BLAZARS

The SED of blazars show significant curvature. In this paper, we study the curvature properties for a large sample of Fermi/LAT bright blazars based on quasi-simultaneous SED. Both SEDs of synchrotron and inverse Compton (IC) components are fitted by a log-parabolic law in log_v-log_vfv diagram. The second-degree term of log-parabola measures the curvature of SED. We find a statistically significant correlation between synchrotron peak frequency and its curvature. This result is in agreement with the theoretical prediction, and confirms previous studies, which dealt with single source with various epoch observations or a small sample. If a broken power-law is employed to fit the SED, the difference between the two spectral indexes (alpha2-alpha1) can be considered as a "surrogate" of the SED curvature. We collect spectral parameters of a sample blazars from literature, and find a correlation between the synchrotron peak frequency and the spectral difference. We do not find a significant correlation between the IC peak frequency and its curvature, which may be caused by complicated seed photon field. It is also found that the synchrotron curvatures are on average larger than that of IC curvatures, and there is no correlation between these two parameters. As suggested by previous works in literature, both the log-parabolic law of SED and above correlation can be explained by statistical and/or stochastic particle accelerations. Stochastic particle acceleration predicts a different slope of the correlation from that of statistical one, and our result seems favor stochastic acceleration mechanisms and emission processes. Some of other evidences also seem to support that the electron energy distribution (and/or synchrotron SED) may be log-parabolic, which include SED modeling, particle acceleration simulation, and comparisons between some predictions and empirical relations/correlations.

An investigation into the spectral properties of bright Fermi blazars

We investigate the spectral properties of blazars detected with the Fermi-Large Area Telescope (LAT) in the high energy regime 100 MeV - 100 GeV. We find that over long timescales a log-parabola provides an adequate description of the spectrum in almost all objects and in most cases is significantly better than a simple power law or broken power law description. Broken power law descriptions appear to arise from two causes: confusion with nearby sources and as an artifact of older LAT instrument response functions. We create a light curve for 2FGLJ2253.9+1609 (3C 454.3), the brightest of the objects investigated. During the quiescent state we find the spectrum to be fairly stable and well-described by a log-parabola. There is some evidence that, on average, the peak energy of the inverse Compton emission is lower in the quiescent state than in the time-averaged state, suggesting that increases in flux are due to changing parameters within the jet as opposed to changes in an external photon field. However, no correlation between inverse Compton peak energy and flux is apparent. During high flux states, deviation of the spectral shape from a simple power law continues. In some cases a log-parabola provides a significantly better fit than a broken power law but in others the reverse is true.

Overview of galactic results obtained by MAGIC

MAGIC is a system of two atmospheric Cherenkov telescopes which explores the very-high-energy sky, from some tens of GeV up to tens of TeV. Located in the Canary island of La Palma, MAGIC has the lowest energy threshold among the instruments of its kind, well suited to study the still poorly explored energy band below 100 GeV. Although the space-borne gamma-ray telescope Fermi/LAT is sensitive up to 300 GeV, gamma-ray rates drop fast with increasing energy, so γ-ray collection areas larger than 104m2, as those provided by grounds-based instruments, are crucial above a few GeV. The combination of MAGIC and Fermi/LAT observations have provided the first astrophysical spectra sampled in the inverse Compton peak region, resulting in a complete coverage from MeV up to TeV energies, as well as the discovery of a pulsed emission in the very-high-energy band. This paper focuses on the latest results on Galactic sources obtained by MAGIC which are highlighted by the detection of the pulsed gamma-ray emission from the Crab pulsar up to 400 GeV. In addition, we will present the morphological study on the W51 complex which allowed to pinpoint the location of the majority of the emission around the interaction point between the supernova remnant W51C and the star forming region W51B, but also to find a possible contribution from the associated pulsar wind nebula. Other important scientific achievements involve the Crab Nebula with an unprecedented spectrum covering three decades in energy starting from 50 GeV and a morphological study of the unidentified source HESS J1857+026 which supports the pulsar wind nebula scenario. Finally we will report on the searches of very-high-energy signals from gamma-ray binaries, mainly LS I 303+ and HESS J0632+057.

SimultaneousPlanck,Swift, andFermiobservations of X-ray andγ-ray selected blazars

We present simultaneous Planck, Swift, Fermi, and ground-based data for 105 blazars belonging to three samples with flux limits in the soft X-ray, hard X-ray, and gamma-ray bands, with additional 5 GHz flux-density limits to ensure a good probability of a Planck detection. We compare our results to those of a companion paper presenting simultaneous Planck and multi-frequency observations of 104 radio-loud northern active galactic nuclei selected at radio frequencies. While we confirm several previous results, our unique data set allows us to demonstrate that the selection method strongly influences the results, producing biases that cannot be ignored. Almost all the BL Lac objects have been detected by the Fermi Large Area Telescope (LAT), whereas 30% to 40% of the flat-spectrum radio quasars (FSRQs) in the radio, soft X-ray, and hard X-ray selected samples are still below the gamma-ray detection limit even after integrating 27 months of Fermi-LAT data. The radio to sub-millimetre spectral slope of blazars is quite flat, with (alpha) approx 0 up to about 70GHz, above which it steepens to (alpha) approx -0.65. The BL Lacs have significantly flatter spectra than FSRQs at higher frequencies. The distribution of the rest-frame synchrotron peak frequency (nu(sup s)(sub peak)) in the spectral energy distribution (SED) of FSRQs is the same in all the blazar samples with (nu(sup s)(sub peak)) = 10(exp 13.1 +/- 0.1) Hz, while the mean inverse Compton peak frequency, (nu(sup IC)(sub peak)), ranges from 10(exp 21) to 10(exp 22) Hz. The distributions of nu(sup s)(sub peak) and nu(sup IC)(sub peak) of BL Lacs are much broader and are shifted to higher energies than those of FSRQs; their shapes strongly depend on the selection method. The Compton dominance of blazars. defined as the ratio of the inverse Compton to synchrotron peak luminosities, ranges from less than 0.2 to nearly 100, with only FSRQs reaching values larger than about 3. Its distribution is broad and depends strongly on the selection method, with gamma-ray selected blazars peaking at approx 7 or more, and radio-selected blazars at values close to 1, thus implying that the common assumption that the blazar power budget is largely dominated by high-energy emission is a selection effect. A comparison of our multi-frequency data with theoretical predictions shows that simple homogeneous SSC models cannot explain the simultaneous SEDs of most of the gamma-ray detected blazars in all samples. The SED of the blazars that were not detected by Fermi~LAT may instead be consistent with SSC emission. Our data challenge the correlation between bolometric luminosity and nu(sup s)(sub peak) predicted by the blazar sequence.

Planckearly results. XV. Spectral energy distributions and radio continuum spectra of northern extragalactic radio sources

Spectral energy distributions (SEDs) and radio continuum spectra are presented for a northern sample of 104 extragalactic radio sources, based on the Planck Early Release Compact Source Catalogue (ERCSC) and simultaneous multifrequency data. The nine Planck frequencies, from 30 to 857 GHz, are complemented by a set of simultaneous observations ranging from radio to gamma-rays. This is the first extensive frequency coverage in the radio and millimetre domains for an essentially complete sample of extragalactic radio sources, and it shows how the individual shocks, each in their own phase of development, shape the radio spectra as they move in the relativistic jet. The SEDs presented in this paper were fitted with second and third degree polynomials to estimate the frequencies of the synchrotron and inverse Compton (IC) peaks, and the spectral indices of low and high frequency radio data, including the Planck ERCSC data, were calculated. SED modelling methods are discussed, with an emphasis on proper, physical modelling of the synchrotron bump using multiplecomponents. Planck ERCSC data also suggest that the original accelerated electron energy spectrum could be much harder than commonly thought, with power-law index around 1.5 instead of the canonical 2.5. The implications of this are discussed for the acceleration mechanisms effective in blazar shock. Furthermore in many cases the Planck data indicate that gamma-ray emission must originate in the same shocks that produce the radio emission.

A new ordering parameter of spectral energy distributions from synchrotron self-Compton emitting blazars

The broadband SEDs of blazars exhibit two broad spectral components, which in leptonic emission models are attributed to synchrotron radiation and synchrotron self-Compton (SSC) radiation of relativistic electrons. During high state phases, the high-frequency SSC component often dominates the low-frequency synchrotron component, implying that the inverse Compton SSC losses of electrons are at least equal to or greater than the synchrotron losses of electrons. We calculate from the analytical solution of the kinetic equation of relativistic electrons, subject to the combined linear synchrotron and nonlinear synchrotron self-Compton cooling, for monoenergetic injection the time-integrated total synchrotron and SSC radiation fluences and spectral energy distributions (SED). Depending on the ratio of the initial cooling terms, displayed by the injection parameter $\alpha$, we find for $\alpha\ll 1$, implying complete linear cooling, that the synchrotron peak dominates the inverse Compton peak and the usual results of the spectra are recovered. For $\alpha\gg 1$ the SSC peak dominates the synchrotron peak, proving our assumption that in such a case the cooling becomes initially non-linear. The spectra also show some unique features, which can be attributed directly to the non-linear cooling. To show the potential of the model, we apply it to outbursts of 3C 279 and 3C 454.3, successfully reproducing the SEDs. The results of our analysis are promising, and we argue that this non-equilibrium model should be considered in future modeling attempts for blazar flares.