Accretion Disk Outbursts

Black-hole X-ray binaries brighten by many orders of magnitude as they move from a dim, hot quiescent flow through a hard-intermediate state toward a luminous soft state. Our radiative two-temperature GRMHD simulations are the first to follow the quiescent-to-hard-intermediate transition across eight decades in accretion rate. We demonstrated (paper I and paper II) that magnetic-flux saturation controls the outcome: a weakly magnetized SANE flow collapses into a thin, cold disk above about 5 × 10⁻³ of the Eddington accretion rate, whereas a magnetically arrested disk truncated and develops cold clumps near 10⁻² Eddington. This links the changing spectrum, corona, and jet directly to the magnetic state. Ray tracing of SANE and MAD outbursts is now underway led by Khushi Taori.

Plasmoid-driven Reconnection

Magnetic reconnection at current-sheet X-points offers a physical route for building the corona in the hard-intermediate state. It releases magnetic energy and evaporates cold disk gas into an ultra-hot phase; ongoing work with Christopher Bambic follows this cold-to-hot interface and its imprint on emission, scattering, and reflection. The X-point region connects the evolving density structure, radiation field, and ion temperature during a flare. Reconnection heats and lifts cold disk gas into the corona, providing a concrete mechanism for the rapid spectral changes seen during an outburst.

Formation of Vertical Magnetic Flux

There exists a scientific consensus that large scale poloidal magnetic field loops are necessary to launch astrophysical jets. Understanding the formation of such jets is crucial for understanding structure formation in the universe. However, the origin of these poloidal field loops is a mystery and previous simulations that did not include poloidal magnetic fields failed to produce any jets. In my recent work I, however, showed that an accretion disk threaded with a toroidal magnetic field is able to generate large scale poloidal magnetic flux after a relatively short time of 20.000 rg/c (black hole time units). When this poloidal magnetic flux accretes on the black hole, a powerful astrophysical jet is launched along the poles (blue regions). This is the first demonstration of a large scale poloidal magnetic flux dynamo in (GR)MHD. The colors in the movie below indicate the logarithm of the rest mass density, while the black lines show the generated poloidal flux (solid lines show positive flux, dashed lines show negative flux).

Publications

1) Large-Scale Poloidal Magnetic Field Dynamo Leads to Powerful Jets in GRMHD Simulations of Black Hole Accretion with Toroidal Field

2) Formation of Magnetically Truncated Accretion Disks in 3D Radiation-Transport Two-Temperature GRMHD Simulations

3) Millimeter Observational Signatures of Flares in Magnetically Arrested Black Hole Accretion Models

4) Black Hole Flares: Ejection of Accreted Magnetic Flux through 3D Plasmoid-mediated Reconnection

5) Magnetic Flux Plays an Important Role during a Black Hole X-Ray Binary Outburst in Radiative Two-temperature General Relativistic Magnetohydrodynamic Simulations

6) Two-temperature treatments in magnetically arrested disc GRMHD simulations more accurately predict light curves of Sagittarius A*