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Register a new userUnderstanding Blazar Jets Through Their Multifrequency Emission
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Rita M. Sambruna
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Being dominated by non-thermal (synchrotron and inverse Compton) emission from a relativistic jet, blazars offer important clues to the structure and radiative processes in extragalactic jets. Crucial information is provided by blazars spectral energy distributions from radio to gamma-rays (GeV and TeV energies), their trends with bolometric luminosity, and their correlated variability properties. This review is focussed on recent multiwavelength monitorings of confirmed and candidate TeV blazars and the constraints they provide for the radiative properties of the emitting particles. I also present recent observations of the newly discovered class of ``blue quasars and the implications for current blazars unification schemes.
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Supermassive black holes launch highly relativistic jets with velocities reaching Lorentz factors as high as $Gamma>50$. How the jets accelerate to such high velocities and where along the jet do they reach terminal velocity are open questions that are tightly linked to their structure, launching and dissipation mechanisms. Changes in the beaming factor along the jets could potentially reveal jet acceleration, deceleration, or bending. We aim to (1) quantify the relativistic effects in multiple radio frequencies and (2) study possible jet velocity--viewing angle variations at parsec scales. We used the state-of-the-art code Magnetron to model light curves from the University of Michigan Radio Observatory and the Mets{a}hovi Radio Observatorys monitoring programs in five frequencies covering about 25 years of observations in the 4.8-37~GHz range for 61 sources. We supplement our data set with high-frequency radio observations in the 100-340~GHz range from ALMA, CARMA, and SMA. For each frequency we estimate the Doppler factor which we use to quantify possible changes in the relativistic effects along the jets. The majority of our sources do not show any statistically significant difference in their Doppler factor across frequencies. This is consistent with constant velocity in a conical jet, as expected at parsec scales. However, our analysis reveals 17 sources where relativistic beaming changes as a function of frequency. In the majority of cases the Doppler factor increases towards lower frequencies. Only 1253-053 shows the opposite behavior. By exploring their jet properties we find that the jet of 0420-014 is likely bent across the 4.8-340~GHz range. For 0212+735 the jet is likely parabolic, and still accelerating in the 4.8-37~GHz range. We discuss possible interpretations for the trends found in the remaining sources.
The most extreme active galactic nuclei (AGN) are the radio active ones whose relativistic jet propagates close to our line of sight. These objects were first classified according to their emission line features into flat-spectrum radio quasars (FSRQs) and BL Lacertae objects (BL Lacs). More recently, observations revealed a trend between these objects known as the emph{blazar sequence}, along with an anti-correlation between the observed power and the frequency of the synchrotron peak. In the present work, we propose a fairly simple idea that could account for the whole blazar population: all jets are launched with similar energy per baryon, independently of their power. In the case of FSRQs, the most powerful jets, manage to accelerate to high bulk Lorentz factors, as observed in the radio. As a result, they have a rather modest magnetization in the emission region, resulting in magnetic reconnection injecting a steep particle energy distribution and, consequently, steep emission spectra in the $gamma$-rays. For the weaker jets, namely BL Lacs, the opposite holds true; i.e., the jet does not achieve a very high bulk Lorentz factor, leading to more magnetic energy available for non-thermal particle acceleration, and harder emission spectra at frequencies $gtrsim$ GeV. In this scenario, we recover all observable properties of blazars with our simulations, including the emph{blazar sequence} for models with mild baryon loading ($50 lesssim mu lesssim 80$). This interpretation of the blazar population, therefore, tightly constrains the energy per baryon of blazar jets regardless of their accretion rate.
The concept of highly relativistic electrons confined to blobs that are moving out with modestly relativistic speeds is often invoked to explain high energy blazar observations. The important parameters in this model such as the bulk Lorentz factor of the blob ($Gamma$), the random Lorentz factor of the electrons ($gamma$) and the blob size are typically observationally constrained, but its not clear how and why the energetic electrons are held together as a blob. Here we present some preliminary ideas based on scenarios for cosmic ray electron self-confinement that could lead to a coherent picture.
Blazars - active galaxies with the jet pointing at Earth - emit across all electromagnetic wavelengths. The so-called one-zone model has described well both quiescent and flaring states, however it cannot explain the radio emission. In order to self-consistently describe the entire electromagnetic spectrum, extended jet models are necessary. Notably, kinetic descriptions of extended jets can provide the temporal and spatial evolution of the particle species and the full electromagnetic output. Here, we present the initial results of a recently developed hadronic extended-jet code. As protons take much longer than electrons to lose their energy, they can transport energy over much larger distances than electrons and are therefore essential for the energy transport in the jet. Furthermore, protons can inject additional leptons through pion and Bethe-Heitler pair production, which can explain a dominant leptonic radiation signal while still producing neutrinos. We will present a detailed parameter study and provide insights into the different blazar sub-classes.
Context. Observations of Type Ia supernovae (SNe Ia) can be used to derive accurate cosmological distances through empirical standardization techniques. Despite this success neither the progenitors of SNe Ia nor the explosion process are fully understood. The U-band region has been less well observed for nearby SNe, due to technical challenges, but is the most readily accessible band for high-redshift SNe. Aims. Using spectrophotometry from the Nearby Supernova Factory, we study the origin and extent of U-band spectroscopic variations in SNe Ia and explore consequences for their standardization and the potential for providing new insights into the explosion process. Methods. We divide the U-band spectrum into four wavelength regions {lambda}(uNi), {lambda}(uTi), {lambda}(uSi) and {lambda}(uCa). Two of these span the Ca H&K {lambda}{lambda} 3934, 3969 complex. We employ spectral synthesis using SYNAPPS to associate the two bluer regions with Ni/Co and Ti. Results. (1) The flux of the uTi feature is an extremely sensitive temperature/luminosity indicator, standardizing the SN peak luminosity to 0.116 $pm$ 0.011 mag RMS. A traditional SALT2.4 fit on the same sample yields a 0.135 mag RMS. Standardization using uTi also reduces the difference in corrected magnitude between SNe originating from different host galaxy environments. (2) Early U-band spectra can be used to probe the Ni+Co distribution in the ejecta, thus offering a rare window into the source of lightcurve power. (3) The uCa flux further improves standardization, yielding a 0.086 $pm$ 0.010 mag RMS without the need to include an additional intrinsic dispersion to reach {chi}$^2$/dof $sim$ 1. This reduction in RMS is partially driven by an improved standardization of Shallow Silicon and 91T-like SNe.
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