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Register a new userRadio and gamma-ray emission in faint BL Lacs
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The advent of Fermi is changing our understanding on the radio and gamma-ray emission in Active Galactic Nuclei. In fact, contrary to previous campaigns, Fermi mission reveals that BL Lac objects are the most abundant emitters in gamma-ray band. However, since they are relatively weak sources, most of their parsec scale structure as their multifrequency properties are poorly understood and/or not systematically investigated. Our main goal is to analyse, using a multiwavelength approach, the nuclear properties of an homogeneous sample of 42 faint BL Lacs, selected, for the first time in literature, with no constraint on their radio and gamma-ray flux densities/emission. We began asking and obtaining new VLBA observations at 8 and 15 GHz for the whole sample. We derived fundamental parameters as radio flux densities, spectral index information, and parsec scale structure. Moreover, we investigated their gamma-ray emission properties using the 2LAT Fermi results. Here, we report our preliminary results on the radio and gamma-ray properties of this sample of faint BL Lacs. In the next future, we will complete the multiwavelength analysis.
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Blazars represent the most abundant class of high-energy extragalactic $gamma$-ray sources. The subset of blazars known as BL Lac objects is on average closer to Earth and characterized by harder spectra at high energy than the whole sample. The fraction of BL Lacs that is too dim to be detected and resolved by current $gamma$-ray telescopes is therefore expected to contribute to the high-energy isotropic diffuse $gamma$-ray background (IGRB). The IGRB has been recently measured over a wide energy range by the Large Area Telescope (LAT) on board the Gamma-ray Space Telescope ({it Fermi}). We present a new prediction of the diffuse $gamma$-ray flux due to the unresolved BL Lac blazar population. The model is built upon the spectral energy distribution and the luminosity function derived from the fraction of BL Lacs detected (and spectrally characterized) in the $gamma$-ray energy range. We focus our attention on the ${cal O}(100)$ GeV energy range, predicting the emission up to the TeV scale and taking into account the absorption on the extragalactic background light. In order to better shape the BL Lac spectral energy distribution, we combine the {it Fermi}-LAT data with Imaging Atmospheric Cerenkov Telescopes measurements of the most energetic sources. Our analysis is carried on separately for low- and intermediate-synchrotron-peaked BL Lacs on one hand, and high-synchrotron-peaked BL Lacs on the other one: we find in fact statistically different features for the two. The diffuse emission from the sum of both BL Lac classes increases from about 10$%$ of the measured IGRB at 100 MeV to $sim$100$%$ of the data level at 100 GeV. At energies greater than 100 GeV, our predictions naturally explain the IGRB data, accommodating their softening with increasing energy. Uncertainties are estimated to be within of a factor of two of the best-fit flux up to 500 GeV.
We present the spectral signatures of the Bethe-Heitler pair production ($pe$) process on the spectral energy distribution (SED) of blazars, in scenarios where the hard $gamma$-ray emission is of photohadronic origin. If relativistic protons interact with the synchrotron blazar photons producing $gamma$ rays through photopion processes, we show that, besides the $2-20$ PeV neutrino emission, the typical blazar SED should have an emission feature due to the synchrotron emission of $pe$ secondaries that bridges the gap betweeen the low-and high-energy humps of the SED, namely in the energy range 40 keV-40 MeV. We first present analytical expressions for the photopion and $pe$ loss rates in terms of observable quantities of blazar emission. For the $pe$ loss rate in particular, we derive a new approximate analytical expression for the case of a power-law photon distribution, which has an excellent accuracy with the numerically calculated exact one, especially at energies above the threshold for pair production. We show that for typical blazar parameters, the photopair synchrotron emission emerges in the hard X-ray/soft $gamma$-ray energy range with a characteristic spectral shape and non negligible flux, which may be even comparable to the hard $gamma$-ray flux produced through photopion processes. We argue that the expected $pe$ bumps are a natural consequence of leptohadronic models, and as such, they may indicate that blazars with a three-hump SED are possible emitters of high-energy neutrinos.
We consider some general implications of bright gamma-ray counterparts to fast radio bursts (FRBs). We show that even if these manifest in only a fraction of FRBs, gamma-ray detections with current satellites (including Swift) can provide stringent constraints on cosmological FRB models. If the energy is drawn from the magnetic energy of a compact object such as a magnetized neutron star, the sources should be nearby and be very rare. If the intergalactic medium is responsible for the observed dispersion measure, the required gamma-ray energy is comparable to that of the early afterglow or extended emission of short gamma-ray bursts. While this can be reconciled with the rotation energy of compact objects, as expected in many merger scenarios, the prompt outflow that yields the gamma-rays is too dense for radio waves to escape. Highly relativistic winds launched in a precursor phase, and forming a wind bubble, may avoid the scattering and absorption limits and could yield FRB emission. Largely independent of source models, we show that detectable radio afterglow emission from gamma-ray bright FRBs can reasonably be anticipated. Gravitational wave searches can also be expected to provide useful tests.
We have compared the parsec-scale jet linear polarization properties of the Fermi LAT-detected and non-detected sources in the complete flux-density-limited (MOJAVE-1) sample of highly beamed AGN. Of the 123 MOJAVE sources, 30 were detected by the LAT during its first three months of operation. We find that during the era since the launch of Fermi, the unresolved core components of the LAT-detected jets have significantly higher median fractional polarization at 15 GHz. This complements our previous findings that these LAT sources have higher apparent jet speeds, brightness temperatures and Doppler factors, and are preferentially found in higher activity states.
The spectral energy distribution of blazars around the synchrotron peak can be well described by the log-parabolic model that has three parameters: peak energy ($E_textrm{p}$), peak luminosity ($L_textrm{p}$) and the curvature parameter ($b$). It has been suggested that $E_textrm{p}$ shows relations with $L_textrm{p}$ and $b$ in several sources, which can be used to constrain the physical properties of the emitting region and/or acceleration processes of the emitting particles. We systematically study the $E_textrm{p}$-$L_textrm{p}$ and $E_textrm{p}$-(1$/b$) relations for 14 BL Lac objects using the 3-25~keV $RXTE$/PCA and 0.3-10~keV $Swift$/XRT data. Most objects (9/14) exhibit positive $E_textrm{p}$-$L_textrm{p}$ correlations, three sources show no correlation, and two sources display negative correlations. In addition, most targets (7/14) present no correlation between $E_textrm{p}$ and 1$/b$, five sources pose negative correlations, and two sources demonstrate positive correlations. 1ES~1959+650 displays two different $E_textrm{p}$-$L_textrm{p}$ relations in 2002 and 2016. We also analyze $E_textrm{p}$-$L_textrm{p}$ and $E_textrm{p}$-(1$/b$) relations during flares lasting for several days. The $E_textrm{p}$-$L_textrm{p}$ relation does not exhibit significant differences between flares, while the $E_textrm{p}$-(1$/b$) relation varies from flare to flare. For the total sample, when $L_textrm{p}$ < $textrm{10}^textrm{45} textrm{erg} textrm{s}^textrm{-1}$, there seems to be a positive $E_textrm{p}$-$L_textrm{p}$ correlation. $L_textrm{p}$ and the slope of $E_textrm{p}$-$L_textrm{p}$ relation present an anti-correlation, which indicates that the causes of spectral variations might be different between luminous and faint sources. $E_textrm{p}$ shows a positive correlation with the black hole mass. We discuss the implications of these results.
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