Relativistic Jets from AGNs
What properties of relativistic jets can we infer through broadband SED modeling?
Relativistic jets are collimated outflows of plasma launched from the vicinity of accreting compact objects. These jets can extend over distances of Mpc and are among the most powerful persistent phenomena in the Universe. In active galactic nuclei (AGN), jets are responsible for a significant fraction of the observed emission across the entire electromagnetic spectrum, from radio waves to gamma rays.
Understanding the composition, structure, and emission mechanisms of these jets is fundamental to unraveling the physics of accretion and outflow in extreme environments. The broadband spectral energy distribution (SED) of relativistic jets encodes information about the particle populations, magnetic field strength, and geometry of the emitting regions. Through detailed modeling of the observed SEDs, we can constrain key physical parameters such as the jet power, bulk Lorentz factor, and the properties of the accelerated particle distributions. I have worked on understanding both the radiative processes in jets and the fundamental question of how jets are loaded with matter in the first place.
Papers
The accretion flow and jet of M87 across spatial scales
In (Gutiérrez et al., 2026), together with my master's student Tomás Mazzei and Santiago del Palacio, we modeled the broadband emission of M87 across spatial scales. The Event Horizon Telescope multi-wavelength campaigns showed that a single emission zone cannot reproduce the source, so we combined a thermal radiatively inefficient accretion flow (RIAF) with winds and a new multizone jet model that starts magnetically dominated, with continuous injection, advection, and cooling of nonthermal electrons and synchrotron self-absorption along the jet.
The model. A RIAF with winds feeds the black hole, and a jet with a helical magnetic field carries a power Lj = ηj ṀBH c2, with nonthermal electrons injected between z0 and zmax,e. Each observed flux is compared with the RIAF emission plus the jet emission inside the effective aperture R* of that instrument, from the EHT (60 µas) to the EVN (5 mas).
Instead of fitting the total jet emission, we integrated it only up to the height that matches the effective aperture of each instrument, from 7.3 to 3.5×105 gravitational radii, and compared it with the flux measured at that angular scale in the 2017 EHT multi-wavelength campaign. We also computed synthetic 43 GHz maps and spectral-index maps between 22, 43 and 86 GHz.
Synthetic 43 GHz intensity map of the jet. The grey circle shows the beam.
We found that the compact 230 GHz emission seen by the EHT is produced by the accretion flow rather than the jet, with an accretion rate at the horizon of about 10-5 of the Eddington rate, consistent with EHT polarimetry. The jet dominates the lower-frequency radio, optical/UV, and X-ray emission, but cannot account for the observed γ-rays. The two jets carry roughly a quarter of the accretion power, about 1043 erg s-1 each.
Broadband spectral energy distribution of M87 (left) and radio flux densities within each effective aperture, from R < 7.3 Rg to R < 3.5×105 Rg (right).
The model codes, data, and scripts that reproduce every figure and number of the paper are available in a public reproduction package.
The origin of matter at the base of relativistic jets
In (Romero & Gutiérrez, 2020), I investigated the origin of matter at the base of relativistic jets in AGN. We analyzed various mechanisms through which jets can be loaded with leptons and baryons, including processes occurring in the hot accretion flow surrounding the black hole. Understanding jet composition is crucial because it determines the jet's dynamics, stability, and ultimately its radiative properties.