Jake's Homepage

Research Interests

My current research primarily focuses on X-ray observations and analysis of a variety of objects, especially millisecond pulsars in globular clusters.

Millisecond pulsars (MSPs) are fast rotating, highly magnetized neutron stars. Imagine compressing the Sun to a radius of only 10 kilometers, with it rotating hundreds of times per second and boasting a magnetic field over 100 million times stronger than the Sun's! These remarkable objects emit beams of radiation, which sweep through space like lighthouse beams, and can be detected as periodic pulses of radio waves (primarily) when they point toward Earth. MSPs are the offspring of neutron star low-mass X-ray binaries, in which the companion star has a mass similar to or less than the Sun. Through accreting material from the companion star, the neutron star is spun up to a period of a few milliseconds.

Optical and X-ray images of the globular cluster Omega Centauri — Figure 1. (a) Optical image of the globular cluster Omega Centauri (NGC 5139) (Credits: ESO). (b) Zoomed-in Chandra X-ray true color image (blue for high-energy, red low-energy photons) of the core of Omega Centauri. (c) Zoomed-in X-ray image of an MSP. The white cross indicates its radio timing position.

MSPs are found to be overabundant in the Galactic globular clusters -- dense, old, spherical collections of stars bound by gravity. Due to the high stellar densities in globular cluster cores, a large number of neutron star low-mass X-ray binaries and therefore MSPs are created. Globular clusters are therefore ideal places for observing and studying MSPs (see Pulsars in Globular Clusters for an up-to-date catalog). So far, we have studied X-ray counterparts to MSPs in M5, M13, Omega Centauri, etc.

Besides identifications of X-ray counterparts to MSPs, I am also interested in phenomena that might be related to pulsars. For example:

Figure 2. Fermi reveals an excess of gamma-ray emission in the Galactic Center. Credit: T. Linden (University of Chicago)

The Galactic Center excess (GCE) is a GeV-scale gamma-ray excess observed toward the Galactic Center, discovered by the Fermi Gamma-ray telescope (see e.g., Hooper & Goodenough 2011). The origin of this excess has puzzled astronomers for over a decade. Even with the increasing amount of data collected by Fermi, the morphology of the GCE (whether spherical or boxy) is still under debate, though its basic spectral features remain consistent. Based on these characteristics, some groups of astronomers propose that the GCE originates from dark matter annihilation, while others believe it is produced by a large number of unresolved MSPs in the Galactic bulge. Therefore, constraining the number of such MSPs and discovering these MSPs are critical steps toward determining the true origin of the GCE. (See Murgia 2020 for a review.)
Magnetars are a type of neutron stars with an ultra-strong magnetic field at ~10¹⁵ Gauss, making them the most magnetic objects known. (For comparison, the magnetic field strength of a typical MSP is about 10⁸ Gauss.) It is believed that magnetars are formed via core-collapse supernova explosions, the same way as most of the neutron stars. However, it is not entirely clear about the conditions for a magnetar to be created instead of a normal neutron star (or pulsar). Magnetars are likely powered by the decay of their ultra-strong magnetic field, producing even high energy emission in X-rays and gamma rays, such as anomalous X-ray pulsars and soft gamma repeaters. (See Kaspi & Beloborodov 2017 for a review.)

More interestingly, a few ultra-long-period magnetars (ULPMs), also known as long-period transients (LPTs), have been discovered recently.