We study black holes with geometrically thin accretion disks in order to understand the accretion process around these objects, measure black hole spins, and test Einstein's theory of General Relativity in the strong field regime. 

In the disk-corona model, a black hole is accreting from a thin and cold accretion disk (see figure below). The thermal spectrum of the disk is normally peaked in the soft X-ray band (0.1-10 keV) in the case of stellar-mass black holes in X-ray binary systems and in the UV band (1-100 eV) in the case of supermassive black holes in active galactic nuclei. The corona is some hot plasma (~100 keV) near the black hole and the inner part of the accretion disk. The corona may be the atmosphere above the accretion disk, the base of a jet, the material in the plunging region between the inner edge of the disk and the black hole, etc. Since the disk is "cold" and the corona is "hot", thermal photons from the disk can inverse Compton scatter off free electrons in the corona. A fraction of the Comptonized photons can illuminate the accretion disk: here we have Compton scattering and absorption followed by fluorescent emission, and they produce the reflection spectrum.

The reflection spectrum in the rest-frame of the material of the disk is characterized by narrow fluorescent emission lines in the soft X-ray band and a Compton hump with a peak around 20-40 keV. The reflection spectrum of the whole disk as detected far from the source is blurred due to relativistic effects (gravitational redshift and Doppler boosting). In the presence of high-quality data, the analysis of these relativistically blurred reflection features is a powerful tool to study the morphology of the accretion flow around black holes, measure black hole spins, and test Einstein's theory of General Relativity in the strong field regime.

We are mainly working on the following lines of research:

1) Development of astrophysical models of accreting black holes for X-ray data analysis

2) Analysis of X-ray observations of black holes

If you are interested in our projects and you are considering the possibility of working with us, you may check possible opportunities on the page "Join Us".

Developments of astrophysical models

We are developing a new generation of X-ray models for precision measurements of accreting black holes. We want to develop a single model to predict the thermal spectrum of the disk, the Comptonized spectrum of the corona, and the reflection spectrum of the disk. We are developing more accurate reflection models by implementing more advanced models for the disk and the corona, more advanced calculations of the reflection spectrum in the rest-frame of the material of the disk, and by including relativistic effects ignored in current reflection models. 

Requirements for students that want to join us: interest/experience in programming (C/C++ and Python) and machine learning techniques. 

Analysis of black hole X-ray data

We analyze X-ray data of black holes from NuSTAR, NICER, Insight-HXMT, XMM-Newton, RXTE, IXPE, etc. with public theoretical models as well as with new theoretical models developed by our group. 

Requirements for students that want to join us: interest/experience in observational astrophysics.