Blazar Variability with the Vera C. Rubin Legacy Survey of Space and Time

With their emission mainly coming from a relativistic jet pointing toward us, blazars are fundamental sources for studying extragalactic jets and their central engines, consisting of supermassive black holes fed by accretion disks. They are also candidate sources of high-energy neutrinos and cosmic rays. Because of the jet orientation, the nonthermal blazar emission is Doppler beamed; its variability is unpredictable, and it occurs on timescales from less than 1 hr to years. Comprehension of the diverse mechanisms producing the flux and spectral changes requires well-sampled multiband light curves over long time periods. In particular, outbursts are the best test bench for shedding light on the underlying physics, especially when studied in a multiwavelength context. The Vera C. Rubin Legacy Survey of Space and Time (Rubin-LSST) will monitor the southern sky for 10 yr in six photometric bands, offering a formidable tool for studying blazar variability features in a statistical way. The alert system will allow us to trigger follow-up observations of outstanding events, especially at high (keV-to-GeV) and very high (TeV) energies. We here examine the simulated Rubin-LSST survey strategies with the aim of understanding which cadences are more suitable for blazar variability science. Our metrics include light curve and color sampling. We also investigate the problem of saturation, which will affect the brightest and many flaring sources, and will have a detrimental impact on follow-up observations.

Constraining the shear acceleration model for the X-ray emission of large-scale extragalactic jets

The nature of the intense X-ray emission from powerful extragalactic jets at large (>1 kpc) scale is still debated. The scenario that invokes the inverse Compton scattering of the CMB by electrons is challenged by the lack of gamma-ray emission in the GeV band. An alternative assumes synchrotron emission by a distinct population of ultra-high energy electrons. Here we present a concrete attempt to apply this scenario, exploring the specific model in which the ultra-high energy electrons are accelerated in a shear layer surrounding the jet. We limit the study to non-relativistic flows and particle acceleration is treated by a Fokker-Planck equation. The observed relation between low energy (radio, optical) and X-ray emission prompts us to assume that the required population of pre-accelerated particles is provided by a shock responsible for the acceleration of the electrons emitting at low frequencies. We apply the model to the emission of the principal knots of the jets of PKS 0637-752 and PKS 1136-135, two of the best studied objects. For the set of fiducial parameters adopted, the condition that the jet power does not exceeds a limiting value of 1048 erg s−1 constrains the magnetic field above 10 μG and indicates moderate beaming (δ ≃ 2) for PKS 0637-752. For both sources, the requirement that acceleration of the electrons proceeds faster than radiative cooling can be met if the magnetic turbulence in the shear layer follows a Kolmogorov spectrum, I(k)∝k−q with q = 5/3, but cannot satisfied in the Bohm-like case (q = 1).

Powerful extragalactic jets dissipate their kinetic energy far from the central black hole

Accretion onto the supermassive black hole in some active galactic nuclei (AGN) drives relativistic jets of plasma, which dissipate a significant fraction of their kinetic energy into gamma-ray radiation. The location of energy dissipation in powerful extragalactic jets is currently unknown, with implications for particle acceleration, jet formation, jet collimation, and energy dissipation. Previous studies have been unable to constrain the location between possibilities ranging from the sub-parsec-scale broad-line region to the parsec-scale molecular torus, and beyond. Here we show using a simple diagnostic that the more distant molecular torus is the dominant location for powerful jets. This diagnostic, called the seed factor, is dependent only on observable quantities, and is unique to the seed photon population at the location of gamma-ray emission. Using 62 multiwavelength, quasi-simultaneous spectral energy distributions of gamma-ray quasars, we find a seed factor distribution which peaks at a value corresponding to the molecular torus, demonstrating that energy dissipation occurs ~1 parsec from the black hole (or ~104 Schwarzchild radii for a 109M⊙ black hole).

The different flavors of extragalactic jets: The role of relativistic flow deceleration

We perform three-dimensional numerical simulations of relativistic (with a Lorentz factor of 10), non-magnetized jets propagating in a uniform density environment in order to study the effect of the entrainment and the consequent deceleration. Our simulations investigate the jet propagation inside the galaxy core, where the deceleration most likely occurs more efficiently. We compare cases with different density and pressure ratios with respect to the ambient medium and find that low density jets are efficiently decelerated and reach a quasi-steady state in which, over a length of 600 jet radii, they slow down from highly relativistic to sub-relativistic velocities. Conversely, denser jets keep highly relativistic velocities over the same length. We discuss these results in relation to the Faranoff Riley (FR) radio source classification. We infer that lower density jets can give rise to FR 0 and FR I radio sources, while higher density jets may be connected to FR II radio sources.

Recollimation shocks in relativistic jets

Recollimation shocks (RS) appear associated with relativistic flows propagating through pressure mismatched atmospheres. Astrophysical scenarios invoking the presence of such shocks include jets from AGNs and X-ray binaries and GRBs. We shall start reviewing the theoretical background behind the structure of RS in overpressured jets. Next, basing on numerical simulations, we will focus on the properties of RS in relativistic steady jets threaded by helical magnetic fields depending on the dominant type of energy. Synthetic radio maps from the simulation of the synchrotron emission for a selection of models in the context of parsec-scale extragalactic jets will also be discussed.