Spider Pulsars
Fig. 1: A schematic of a redback pulsar showing the pulsar, companion star, and intrabinary shock. The zones from which different types of electromagnetic emission originate as well as the different viewing geometries are labeled.
Spider pulsars are a class of binary pulsars with low mass companion stars. The orbital periods of these sources are < 1 day, putting the companion very close to the pulsar. Consequently, the high energy emission from the pulsar irradiates and ablates the companion star, driving off a massive stellar wind. The stellar wind collides with the pulsar wind, producing an intrabinary shock. High energy particle acceleration of electrons and positrons in the intrabinary shock leads to orbital phase-modulated X-ray emission. I have worked on X-ray observations and modeling of the intrabinary shocks to understand the particle acceleration and properties of the shocked pulsar wind.
The pulsar J2215+5135 is one of the more interesting sources as it is among the most massive at ∼2.0 M⊙. This source also has a rather flat intrabinary shock, leading to unique particle acceleration effects. I have modeled the optical and X-ray emission from this source to understand the companion star and shocked pulsar wind properties.
The recent capabilities and observations by the Imaging X-ray Polarimetry Explorer (IXPE) have demonstrated the high polarization and magnetic field structure of pulsar winds in pulsar wind nebulae. To understand how these polarization capabilities can be applied to understanding spider pulsars, I have modeled the polarized X-ray emission from the intrabinary shocks.
Radio Jets in Active Galactic Nuclei
Fig. 2: A diagram of the evolution of a CSO through its three stages of life from left to right.
Active galactic nuclei (AGN) sometimes host massive jets, produced by the black holes at their center. These jets tend to emit detectable radio emission, allowing us to probe their physics. Many jetted sources have been tracked by the Owens Valley Radio Observatory, including a class of sources known as Compact Symmetric Objects (CSOs), double radio sources with sizes < 1 kpc in size. Analyses have suggested that these sources, rather than simply being young radio sources, are also short-lived. We have proposed a model for the evolution of CSOs in which they are connected to tidal disruption events of massive stars, allowing jet launching on timescales ∼ 1000 yr before the jets die out.
Beyond CSOs, I am also studying potential binary supermassive black hole candidates identified by radio observations. Two sources PKS2131-021 and J0805-0111 show periodicity consistent with jet aberration due to binary orbital motion in their radio light curves. We are using this model to study properties of the jets and gain a window into binary supermassive black holes.
Precursor Emission in short Gamma-ray Bursts
Short gamma-ray bursts (sGRBs) are believed to originate from the merging of binary neutron stars. With some sGRBs showing precursor eleoctromagnetic emission prior to the main sGRB, I have worked on a model in which energy is injected into the oceans on the surface of the neutron star via tidal resonances. The deposited energy can then source a flare potentially visible at Earth. This model, if true, allows us to constrain properties of neutron star structure.