Research

Astrophysical black holes are as complex as they are fascinating. They can be very large, weighing millions to billions of solar masses while being located at the very center of galaxies – where they are known as Supermassive Black Holes (SMBHs). They may also be relatively small, weighing “only” a few solar masses, but these are no less important or interesting than their heavier counterparts.

SMBHs can be seen as powerful engines that convert the energy of accreting plasmas into extremely luminous radiation and produce outflows that affect the entire galaxy evolution. A similar behavior has also been observed in smaller-sized, stellar-mass black hole X-Ray binaries (BHXBs). Therefore, observations suggest that the study of these two classes of systems may give us complementary information about the general physical processes related to the accretion and outflows around black holes. In turn, theoretical studies may give us hints to actually explain many observational features of these systems.

My work aims precisely to bridge the gap between theory and observation. Despite the existence of analytical models to describe many aspects of these systems, they are too complex to be understood without extensive use of numerical simulations. Thus, if we want to understand the underlying physics of black holes and explain what the observations are showing us, we must make heavy use of powerful numerical tools.

Jet efficiencies and spins of jetted quasars

Using a sample of 152 flat-spectrum radio quasars (FSRQs) -- a class of blazars --, we estimated their jet production efficiency, and we found a mean value η ~ 0.1. We used these values as an input to calculate the black hole spins using a theoretical model taken from general relativistic radiative MHD simulations (GRRMHD) of thin accretion disks, and found an average lower limit of a = 0.59. Our results show compatibility with spins obtained from models of supermassive black hole evolution through mergers. We also found a moderate correlation between the BH mass and the gamma-ray luminosity, suggesting that Lγ can be used as an estimator of the BH mass in FSRQs.

This work provides a proof of concept for future methods aiming at obtaining the spins of jetted black holes by means of their gamma-ray luminosity. It is currently being extended to BL Lacs which, unlike FSRQs, are better described by geometrically thicker, radiatively inefficient accretion flows.

Looking for current sheets in black hole jets

Black hole jets emit radiation, but we don't know the full details of how some of this radiation is produced and what produces it. Non-thermal electrons are strong candidates to explain some observational features, with both Fermi acceleration and magnetic reconnection proposed as acceleration mechanisms. We ran GRMHD simulations of accretion flows to study the formation of jets in such systems. We found the presence of current sheets in some of these jets, and we are now working to better characterize them so as to study the radiation that can be emitted in such places. This is important to understand the radiation features of blazars, but since jets are found in different types of active galactic nuclei (AGNs) and stellar-mass black holes, the applicability of such study is much broader.

Accelerating radiative transfer using GPUs

<-->