Relativistic AGN Jets

Accretion onto black holes can launch powerful, magnetized jets. Our GRMHD simulations follow their formation near the event horizon and their acceleration and interaction with surrounding gas across much larger scales. The video below shows a simulation of a jet like in M87. Here the jet shows various kink and pinch instability modes. This introduces variability in the jet which our group is trying to link to multiwavelength variability in AGN.

Neutron Star Mergers and Short Gamma-Ray Bursts

We simulate the aftermath of neutron-star mergers, following the central engine, its outflows, and the emerging gamma-ray burst. In a black hole–neutron star merger, disrupted stellar matter forms an accretion disk around the remnant black hole. Magnetic fields can launch a relativistic jet, while disk winds carry neutron-rich matter outward. As the jet pushes through the surrounding ejecta, it inflates a cocoon and shapes the burst that an observer sees. Their three-dimensional general relativistic magnetohydrodynamic simulations follow this evolution from the merger to much larger distances, connecting the engine to short gamma-ray bursts and their afterglows. Neutrino transport helps determine the composition and opacity of the outflows, which in turn affect the kilonova and early ultraviolet and optical emission. Together, these calculations link the violent merger dynamics to the range of light signals that accompany gravitational waves. The image below shows a short gamma-ray burst jet from a neutron-star merger from one of Danat Issa’s simulations.

Tidal Disruption Events

A rare glimpse into the properties of a normally quiescent supermassive black hole is afforded when a star passes sufficiently close that it is torn apart by the black hole’s tidal gravitational field. Most of such tidal disruption events (TDEs) result in a `flare' at optical, UV, and X-ray wavelengths that lasts from months to years and is thought to be powered by the gas falling into the supermassive black hole (SMBH). Recently me and Zack Andalman for the first time simulated in GRHD a solar mass star on a realistic parabolic orbit that got tidally disrupted by a million solar mass SMBH. The extreme mass ratio between star and SMBH makes the stream very thin and challenging to resolve numerically. We showed (see video below) that a pericenter shock is the dominant mechanism in forming an accretion disk from a TDE stream and for the first time demonstrated that periodic self-intersections of the accretion stream can be so violent that they totally disrupt the stream and may explain flaring detected by NASA's Swift satellite in the light curve of TDE J1644+57.

The video below shows a similar simulation, but where the TDE stream and accretion disk are misaligned. The similarities and differences between aligned and misaligned models are described in our paper.

In the near future our collaboration is moving towards radiative GRMHD simulations that will evolve the TDEs for much longer timescales. This will allow us to quantify the effects of radiative outflows and to make more accurate observational predictions for TDEs.

Publications

1) Accelerating AGN jets to parsec scales using general relativistic MHD simulations

2) Black Hole to Photosphere: 3D GRMHD Simulations of Collapsars Reveal Wobbling and Hybrid Composition Jets

3) On the Jet-Ejecta Interaction in 3D GRMHD Simulations of a Binary Neutron Star Merger Aftermath

4) Black hole to breakout: 3D GRMHD simulations of collapsar jets reveal a wide range of transients

5) Bridging the Bondi and Event Horizon Scales: 3D GRMHD Simulations Reveal X-shaped Radio Galaxy Morphology

6) Jet Formation in 3D GRMHD Simulations of Bondi-Hoyle-Lyttleton Accretion

7) Tidal Disruption Disks Formed and Fed by Stream-Stream and Stream-Disk Interactions in Global GRHD Simulations

8) Universal radial scaling of large-scale black hole accretion for magnetically arrested and rocking accretion disks