Yajie Yuan - Research

Heating of the Compact X-ray Corona in Active Galactic Nuclei

Pair Producing Gaps in Black Hole Magnetospheres

Fast Radio Bursts from Magnetars

Fast radio bursts (FRB) are mysterious millisecond duration radio bursts of cosmological origin. This is a newly discovered phenomenon, and we still don't know what sources produce FRB. In 2020, a galactic magnetar (a neutron star with very strong magnetic field) was detected to produce FRB-like radio bursts, and the radio bursts were coincident with an X-ray burst from the magnetar. This is giving us good hints!

My collaborators and I developed the following scenario for the FRB from the magnetar: low-amplitude Alfvén waves from a magnetar quake may propagate to the outer magnetosphere and convert to “plasmoids” (closed magnetic loops) that accelerate from the star, driving blast waves into the magnetar wind. The blast wave can produce the observed radio bursts through the synchrotron maser mechanism, and the magnetic reconnection behind the Alfven wave ejecta may produce the simultaneous X-ray bursts. The movie on the right shows the magnetic field and current density during the plasmoid ejection process, obtained from a 2D force-free simulation.

Heating of the Compact X-ray Corona in Active Galactic Nuclei

Seyfert galaxies are a kind of active galactic nuclei (AGN) where the central supermassive black hole accretes at slightly sub-Eddington rate. Some of them are radio quiet, namely, they do not have powerful jets, but they produce prominent X-ray emission, reaching up to 50% of the total AGN power. The X-ray is believed to be produced in a hot corona around the accretion disk. Recent X-ray reverberation mapping and microlensing measurements have shown that the corona is quite compact, not larger than a few tens of the black hole gravitational radii in size.

Since these galaxies do not have jets, we consider the possibility that the compact X-ray corona may be powered by small scale flux tubes near the black hole that are attached to the accretion disk. Due to the continuous shear provided by the disk motion or the black hole-disk relative motion, the flux tubes may get inflated and tangled up especially near the axis, leading to significant dissipation relatively close to the black hole, instead of carrying the energy away in an outflow.

The movie on the right is from a 3D force-free simulation of the magnetic flux tubes attached to the accretion disk and black hole. It shows the flux tube inflation, kink instability and subsequent dissipation. From left to right are: magnetic field on the y=0 plane, magnetic field lines, current density isosurfaces.

Pair Producing Gaps in Black Hole Magnetospheres

In another type of low luminosity AGN like M87, powerful jets can be launched by the supermassive black hole through Blandford-Znajek (BZ) process. This requires sufficient plasma around the black hole to conduct the BZ current. When plasma runs low, regions with unscreened electric field will develop in the jet funnel, accelerating electrons to high energies that initiate electron-positron pair cascades on the background soft photons. The discharge dissipates energy extracted from the black hole and may produce γ-ray emission varying on time scales shorter than the event horizon light crossing time.

The movie on the left shows the dynamics of pair producing gaps in black hole magnetospheres from our 1D general relativistic particle-in-cell simulation. Blue, orange and black dots represent electrons, positrons and photons, respectively. The green line shows the non-ideal electric field in the radial direction. We see quasi-periodic opening and screening of the gap. The process may power the TeV gamma-ray flares from M87.

Magnetoluminescence

A wide range of high energy astrophysical sources show dramatically variable gamma-ray emission, e.g., the 0.1 − 1 GeV gamma-ray flares from the Crab Nebula, the GeV or TeV flares with minute-timescale variability from Blazars and other AGN. This requires efficient particle acceleration over very short time scales and challenges traditional acceleration mechanisms.

A possible scenario we envision is that in the highly magnetized outflow from the central engine, the magnetic configuration might become strongly tangled. When the flow slows down or the surrounding environment changes, the tangled structure becomes unstable and rapidly converts the large scale magnetic energy into particle kinetic energy and radiation. We term such a process “magnetoluminescence”.

The movie on the right shows one example from our 2D particle-in-cell simulations where the large scale magnetic flux tube configuration is unstable and releases the free energy over a dynamic time scale. The process forces current sheet formation and fast reconnection, where particles are accelerated efficiently. Meanwhile, the reconnection region spontaneously creates small scale structures through tearing and bunching, which results in highly variable emission.