I work in theoretical high-energy astrophysics, mainly using high-performance computing to understand the physics of relativistic outflows, pulsar magnetospheres and collisionless shocks. I am also interested in the phenomena that happen on accreting neutron stars during X-ray bursts.
Structure of pulsar magnetospheres: aligned rotators
Pulsars are strongly magnetized spinning neutron stars. They are truly amazing objects. We observe them mainly through pulsed radio emission that comes from their magnetospheres. The structure of these magnetospheres and the physics of pulsar emission is a long standing problem in astrophysics. I have worked on understanding the magnetospheric properties of pulsars by solving for the behavior of plasma in a strong magnetic field around a rotating star. For this I developed a new numerical method for solving the equations of Force-Free Relativistic Magnetohydrodynamics (MHD). With it I am able to simulate the time-dependent evolution of a magnetosphere as the star is spinning up and calculate the magnetospheric shape and energy loss.
Evolution of the magnetosphere of a spinning-up neutron star with aligned magnetic and rotational axes. Poloidal fieldlines are black lines and the torioidal field is shown in color. Notice the development of the equatorial current sheet.
Structure of pulsar magnetospheres: oblique rotators
Real pulsars are certainly oblique rotators. This means that the magnetic axis and the axis of rotation are misaligned (otherwise they wouldn't pulse!). The shape of their magnetosphere is much harder to find because the problem is no longer axially symmetric. To address this I have developed a 3D version of the force-free MHD code and applied it to the time-dependent evolution of a spinning oblique pulsar. This allowed the first measurement of the rate of pulsar spindown as a function of the inclination angle. Current research is aimed at applying the self-consistent 3D magnetospheric models to observations of radio and high-energy (gamma-ray) emission from pulsars, particularly as will be observed by GLAST.
Simulated structure of a pulsar magnetosphere with 60 degree inclination between rotational and magnetic axes. The current sheet still exists but is now flapping around the equator as the star rotates. Color marks the sign of the toroidal field component.
A recent presentation at Elba conference on pulsars: PDF (6Mb). Movies of 3D pulsar magnetosphere: small (1Mb) and large (40Mb) . More pulsar movies: shape of last closed field lines for plasma-filled magnetosphere and for vacuum magnetosphere .
Shape of the current sheet for oblique rotator. Movie (Quicktime, 20Mb)
Physics of relativistic collisionless shocks
Relativistic collisionless shocks are commonly encountered in astrophysics where relativistic flows collide with their environment and are converted into observable radiation. They are thought to occur at the termination of pulsar winds in supernova remnants, in AGN jets and gamma-ray bursts. Such shocks are expected to produce nonthermal particle distributions and generate magnetic fields. Yet, how such shocks really work has remained a puzzle. I investigated the internal structure of such shocks using an explicit plasma simulation using particle-in-cell (PIC) method. This allows to self-consistently simulate the plasma physics of interaction between colliding shells of relativistic material in 3D. The shock structure turned out to be very sensitive to the background magnetization of the plasma. Current work concentrates on understanding partcile acceleration properties of such shocks, the ultimate fate of generated magnetic fields, and the extension to relalistic mass ratios between ions and electrons.
Internal structure of the relativistic collisionless shocks. Color represents the density of plasma which is flowing from left to right through the shock. Left figure has no magnetization upstream and the shock is mediated by the Weibel instability. Right figure has finite magnetization in the upstream (white arrows). Shock is much thinner, mediated by magnetic reflections.
Propagation of nuclear burning on accreting neutron stars
Neutron stars in Low Mass X-ray Binaries (LMXBs) are some of the brightest X-ray sources in the sky. They accrete hydrogen and helium from a companion, and periodically undergo rapid thermonuclear runaways known as Type-I X-ray bursts. Although these stars are expected to rotate rapidly, nearly coherent oscillations during bursts were detected only recently in 13 sources with frequencies between 100 and 600 Hz. These oscillations are interpreted as the rotational modulation of burning asymmetries on the stellar surface, and provide a direct indication of the neutron star spin. Together with Yuri Levin and Greg Ushomirsky, we investigated the propagation of nuclear burning in the atmospheres of neutron stars during X-ray bursts. We included two key ingredients that were missing in previous models: lateral pressure gradients due to hydrostatic expansion of burning material and the rapid rotation of the star. This combination leads to large-scale hydrodynamical shearing flows both along and perpendicular to the burning front, somewhat similar to flows observed in hurricanes on Earth. Recently, I have extended this 1D model to a numerical simulation of the burning atmosphere on a surface of a sphere, and produced a dynamical simulation of the growth of a burning hotspot. Current research is on applications of the geophysically-inspired dynamics to the observations of X-ray oscillations, and on understanding the effects of magnetic fields on the burning.
Spreading of a thermonuclear burning hotspot on the surface of a rotating neutron star. The arrows show the direction of the flow, and the colors mark the temperature. The hotspot has an anticyclonic circulation around it, which leads to the westward drift. The front spreads faster towards the equator, and once equator is ignited the burning spreads to the poles as "walls of fire."