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Register a new userFrom Radio to TeV: The surprising Spectral Energy Distribution of AP Librae
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Following the discovery of high-energy (HE; $E>10,{rm MeV}$) and very-high-energy (VHE; $E>100,{rm GeV}$) $gamma$-ray emission from the low-frequency-peaked BL~Lac (LBL) object AP Librae, its electromagnetic spectrum is studied over 60 octaves in energy. Contemporaneous data in radio, optical and UV together with the $gamma$-ray data are used to construct the most precise spectral energy distribution of this source. The data have been found to be modeled with difficulties with single zone homogeneous leptonic synchrotron self-Compton (SSC) radiative scenarios due to the unprecedented width of the high-energy component when compared to the lower-energy component. The two other LBL objects also detected at VHE appear to have similar modeling difficulties. Nevertheless, VHE $gamma$ rays produced in the extended jet could account for the VHE flux observed by H.E.S.S.
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Ap Lib is one of the rare Low Synchrotron Peaked blazars detected so far at TeV energies. This type of source is not properly modelled by standard one-zone leptonic Synchrotron-self-Compton (SSC) emission scenarios. The aim of this paper is to study the relevance of additional components which should naturally occur in a SSC scenario for a better understanding of the emission mechanisms, especially at very high energies (VHE). Methods. We use simultaneous data from a multi-wavelength campaign of Planck, Swift-UVOT and Swift-XRT telescopes carried out in February 2010, as well as quasi-simultaneous data of WISE, Fermi and H.E.S.S. taken in 2010. The multi-lambda emission of Ap Lib is modelled by a blob-in-jet SSC scenario including the contribution of the base of the VLBI extended jet, the radiative blob-jet interaction, the accretion disk and its associated external photon field. We show that signatures of a strong parsec-scale jet and of an accretion disk emission are present in the SED. We can link the observationnal VLBI jet features from MOJAVE to parameters expected for a VHE emitting blob accelerated near the jet base. The VHE emission appears to be dominated by the inverse-Compton effect of the blob relativistic electrons interacting with the jet synchrotron radiation. In such scenario Ap Lib appears as an intermediate source between BL Lac objects and Flat Spectrum Radio Quasars. Ap Lib could be a bright representative of a specific class of blazars, in which the parsec-scale jet luminosity is no more negligible compared to the blob and contributes to the high energy emission via inverse Compton processes.
Chandra observations of the low-energy peaked BL Lac object AP Librae revealed the clear discovery of a non-thermal X-ray jet. AP Lib is the first low energy peaked BL Lac object with an extended non-thermal X-ray jet that shows emission into the VHE range. The X-ray jet has an extension of ~15 (~ 14 kpc). The X-ray jet morphology is similar to the radio jet observed with VLA at 1.36 GHz emerging in south-east direction and bends by 50 degrees at a distance of 12 towards north-east. The intensity profiles of the X-ray emission are studied consistent with those found in the radio range. The spectral analysis reveals that the X-ray spectra of the core and jet region are both inverse Compton dominated. This adds to a still small sample of BL Lac objects whose X-ray jets are IC dominated and thus more similar to the high luminosity FRII sources than to the low luminosity FRI objects, which are usually considered to be the parent population of the BL Lac objects.
The radio galaxy IC310 located in the Perseus Cluster is one of the brightest objects in the radio and X-ray bands, and one of the closest active galactic nuclei observed in very-high energies. In GeV - TeV $gamma$-rays, IC310 was detected in low and high flux states by the MAGIC telescopes from October 2009 to February 2010. Taking into account that the spectral energy distribution (SED) up to a few GeV seems to exhibit a double-peak feature and that a single-zone synchrotron self-Compton (SSC) model can explain all of the multiwavelength emission except for the non-simultaneous MAGIC emission, we interpret, in this work, the multifrequency data set of the radio galaxy IC310 in the context of homogeneous hadronic and leptonic models. In the leptonic framework, we present a multi-zone SSC model with two electron populations to explain the whole SED whereas for the hadronic model, we propose that a single-zone SSC model describes the SED up to a few GeVs and neutral pion decay products resulting from p$gamma$ interactions could describe the TeV - GeV $gamma$-ray spectra. These interactions occur when Fermi-accelerated protons interact with the seed photons around the SSC peaks. We show that, in the leptonic model the minimum Lorentz factor of second electron population is exceedingly high $gamma_esim10^5$ disfavoring this model, and in the hadronic model the required proton luminosity is not extremely high $sim 10^{44}$ erg/s, provided that charge neutrality between the number of electrons and protons is given. Correlating the TeV $gamma$-ray and neutrino spectra through photo-hadronic interactions, we find that the contribution of the emitting region of IC310 to the observed neutrino and ultra-high-energy cosmic ray fluxes are negligible.
[Abridged] Context. To construct and interpret the spectral energy distribution (SED) of BL Lacertae objects, simultaneous broad-band observations are mandatory. Aims. We present the results of a dedicated multi-wavelength study of the high-frequency peaked BL Lacertae (HBL) object and known TeV emitter 1ES 2344+514 by means of a pre-organised campaign. Methods. The observations were conducted during simultaneous visibility windows of MAGIC and AGILE in late 2008. The measurements were complemented by Metsahovi, RATAN-600, KVA+Tuorla, Swift and VLBA pointings. Additional coverage was provided by the ongoing long-term F-GAMMA and MOJAVE programs, the OVRO 40-m and CrAO telescopes as well as the Fermi satellite. The obtained SEDs are modelled using a one-zone as well as a self-consistent two-zone synchrotron self-Compton model. Results. 1ES 2344+514 was found at very low flux states in both X-rays and very high energy gamma rays. Variability was detected in the low frequency radio and X-ray bands only, where for the latter a small flare was observed. The X-ray flare was possibly caused by shock acceleration characterised by similar cooling and acceleration time scales. MOJAVE VLBA monitoring reveals a static jet whose components are stable over time scales of eleven years, contrary to previous findings. There appears to be no significant correlation between the 15 GHz and R-band monitoring light curves. The observations presented here constitute the first multi-wavelength campaign on 1ES 2344+514 from radio to VHE energies and one of the few simultaneous SEDs during low activity states. The quasi-simultaneous Fermi-LAT data poses some challenges for SED modelling, but in general the SEDs are described well by both applied models. The resulting parameters are typical for TeV emitting HBLs. Consequently it remains unclear whether a so-called quiescent state was found in this campaign.
Infrared-faint radio sources (IFRS) are a class of radio-loud (RL) active galactic nuclei (AGN) at high redshifts (z > 1.7) that are characterised by their relative infrared faintness, resulting in enormous radio-to-infrared flux density ratios of up to several thousand. We aim to test the hypothesis that IFRS are young AGN, particularly GHz peaked-spectrum (GPS) and compact steep-spectrum (CSS) sources that have a low frequency turnover. We use the rich radio data set available for the Australia Telescope Large Area Survey fields, covering the frequency range between 150 MHz and 34 GHz with up to 19 wavebands from different telescopes, and build radio spectral energy distributions (SEDs) for 34 IFRS. We then study the radio properties of this class of object with respect to turnover, spectral index, and behaviour towards higher frequencies. We also present the highest-frequency radio observations of an IFRS, observed with the Plateau de Bure Interferometer at 105 GHz, and model the multi-wavelength and radio-far-infrared SED of this source. We find IFRS usually follow single power laws down to observed frequencies of around 150 MHz. Mostly, the radio SEDs are steep, but we also find ultra-steep SEDs. In particular, IFRS show statistically significantly steeper radio SEDs than the broader RL AGN population. Our analysis reveals that the fractions of GPS and CSS sources in the population of IFRS are consistent with the fractions in the broader RL AGN population. We find that at least 18% of IFRS contain young AGN, although the fraction might be significantly higher as suggested by the steep SEDs and the compact morphology of IFRS. The detailed multi-wavelength SED modelling of one IFRS shows that it is different from ordinary AGN, although it is consistent with a composite starburst-AGN model with a star formation rate of 170 solar masses per year.
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