MOJAVE. XIX. Brightness Temperatures and Intrinsic Properties of Blazar Jets

We present multiepoch, parsec-scale core brightness temperature observations of 447 active galactic nucleus (AGN) jets from the MOJAVE and 2 cm Survey programs at 15 GHz from 1994 to 2019. The brightness temperature of each jet over time is characterized by its median value and variability. We find that the range of median brightness temperatures for AGN jets in our sample is much larger than the variations within individual jets, consistent with Doppler boosting being the primary difference between the brightness temperatures of jets in their median state. We combine the observed median brightness temperatures with apparent jet speed measurements to find the typical intrinsic Gaussian brightness temperature of 4.1( ± 0.6) × 1010 K, suggesting that jet cores are at or below equipartition between particle and magnetic field energy in their median state. We use this value to derive estimates for the Doppler factor for every source in our sample. For the 309 jets with both apparent speed and brightness temperature data, we estimate their Lorentz factors and viewing angles to the line of sight. Within the BL Lac optical class, we find that high-synchrotron-peaked BL Lacs have smaller Doppler factors, lower Lorentz factors, and larger angles to the line of sight than intermediate and low-synchrotron-peaked BL Lacs. We confirm that AGN jets with larger Doppler factors measured in their parsec-scale radio cores are more likely to be detected in γ rays, and we find a strong correlation between γ-ray luminosity and Doppler factor for the detected sources.

Optical Variability of a Newly Discovered Blazar Sample from the BZCAT Catalog

In an optical monitoring program to characterize the variability properties of blazars, we observed 10 sources from the Roma-BZCAT catalog for 26 nights in V and R bands during 2014 October to 2015 June with two telescopes located in India. The sample includes mainly newly discovered BL Lacertae objects (BL Lacs) for which the redshift of some sources is not yet known. We present the results of flux and color variations of the sample on intraday and short timescales obtained by using the power-enhanced F-test and the nested-ANOVA tests, along with their spectral behavior. We find significant intraday variability in the single flat-spectrum radio quasar in our sample, having an amplitude of variation ∼12%. Although a few of the BL Lacs showed probable variation in some nights, none of them passed the variability tests at 99.9% significance level. We find that 78% of the sample showed significant negative color–magnitude correlations, i.e., a redder-when-brighter spectral evolution. Those that do not show strong or clear chromatism predominantly exhibit a redder-when-brighter trend. Unlike on hourly timescales, the high-synchrotron-peaked blazars in the sample (BZGJ0656+4237, BZGJ0152+0147, and BZBJ1728+5013) show strong flux variation on timescales of days to months, where again we detect a decreasing trend of the spectral slope with brightness. We observe a global steepening of the optical spectrum with increasing flux on the intranight timescale for the entire blazar sample. The nonvariability in the BL Lacs in our sample might be caused by the distinct contribution from the disk as well as from other components in the studied energy range.

A BL Lacertae object at a cosmic age of 800 Myr

BL Lacertae objects (BL Lacs) and flat-spectrum radio quasars (FSRQs), known as blazars, are low- and high-luminosity radio-loud Active Galactic Nuclei (AGNs) with relativistic jets pointed towards Earth (1). Evolving from FSRQs (2,3), BL Lac objects host ~109 Msun supermassive black holes (SMBHs, where Msun is the mass of the Sun) and reside preferentially in giant elliptical galaxies of stellar masses 1011-1012Msun (4-7). The known BL Lacs are relatively nearby objects found below redshift 3.6 (3,8,9). Here, we report the discovery of a BL Lac object, FIRST J233153.20+112952.11 (hereafter J2331+11), at a redshift of 6.57 corresponding to an age of the Universe of ~800 Myr. As the typical BL Lac, J2331+11 is a compact radio source with the flat power-law radio continuum, no emission lines in its near-infrared spectrum, and significant variability. The optical-to-radio continuum of J2331+11 is entirely dominated by the synchrotron emission of a relativistic jet. J2331+11 provides evidence for the shorter formation timescale of massive SMBHs with jets and bulge-dominated galaxies than that expected from the Eddington-limited growth of SMBHs and hierarchical galaxy formation. The rapid formation of BL Lacs at early cosmic epochs should have taken place in the densest regions of the early Universe.

Blazar jets launched with similar energy per baryon, independently of their power

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 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 γ-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 ≳ GeV. In this scenario, we recover all observable properties of blazars with our simulations, including the blazar sequence for models with mild baryon loading (50 ≲ μ ≲ 80). This interpretation of the blazar population, therefore, tightly constrains the energy per baryon of blazar jets regardless of their accretion rate.