Research

I’m fascinated by super-strong gravitational fields and the extreme physical limits of matter. Compact objects, like neutron stars and stellar black holes, are host to some dizzyingly extreme environments. In a binary system with a low-mass star, a compact object will strip the regular star of its matter in a process called accretion, and this accreted material forms an accretion disk around the compact object. Some material from the accretion disk will accrete onto the compact object itself. Accretion is a powerful process that produces very bright X-ray emission, and so these systems are called X-ray binaries.

Spectroscopy and Timing

There are two ways we can study emission from the accretion disk: energy spectra and photon timing. Spectral observations (i.e., the energy of the detected photons) indicate what process produced the photons and therefore where in the system the emission is coming from. Timing observations (i.e., when the photons are detected) can tell us if the emission is changing due to physical properties on very short (sub-millisecond) timescales. By analyzing X-ray emission from the innermost regions of X-ray binaries with spectral and timing observations, we can learn more about how matter behaves in the strong gravity regime.

Variability

X-ray emission from X-ray binaries is generally not constant. Depending on the source, we can see periodic pulsations and/or quasi-periodic oscillations (QPOs) in the amount of photons detected at different frequencies. The idea is that some physical process is causing the variability in signal, and this process is affected by the geometry of the emitting region. Understanding the variability can help us make sense of the underlying physical processes and the geometry of spacetime close to the compact object.

Why spectral-timing?

We’re not able to directly image these systems because they’re so small in size, and so far away. For example, spatially resolving a 10 solar mass black hole that’s 2.5 kiloparsecs (~8000 lightyears) away is akin to resolving a single strand of hair on the surface of Mars! Read this post for more details on that. So, since we can’t just take a picture to see what’s happening in the strong-gravity regime close to compact objects, we need to deduce it with spectral-timing observations.

I’m working with a new spectral-timing technique to do phase-resolved spectroscopy (i.e., studying how the energy spectra of the X-ray emission changes on sub-second timescales) on rapid periodic and quasi-periodic signals from X-ray binaries (published here). A large part of my research involves developing software to reduce and analyze data from X-ray telescopes like RXTE. I also plan to work with data from XMM-Newton, NuSTAR, ASTROSAT, and NICER (scheduled to launch in March 2017). You can follow me on GitHub to keep an eye on my latest public software developments.

An up-to-date list of my publications is available via ADS.

Reproducible science

I’m also a coordinator for Stingray, an open-source Python library to do timing and spectral-timing analysis of astronomical data. If you have code you might want to contribute, we’d love to hear from you, either via a pull request or an issue! We have a Slack channel that you’re welcome to join if you’d like to contribute to the project, just get in touch.

Summary of research interests:

neutron stars, black holes, accreting X-ray binaries, quasi-periodic oscillations (QPOs), high energy variability, spectral timing, pulsars, thermonuclear X-ray bursts, general relativity, Python programming, optimization algorithms, open-source software