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.

An artist’s impression of a stellar-mass black hole in orbit with a normal star companion, together called an X-ray binary. Credit: NASA/CXC/M. Weiss.

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 or color 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.


X-ray emission from X-ray binaries is variable on timescales from microseconds to years! Depending on the source, we can see sub-second variability in the form of periodic pulsations and/or quasi-periodic oscillations (QPOs). The idea is that some physical process is causing the variability in signal (though we don’t conclusively know what it is), 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 how matter behaves in the curved spacetime close to compact objects.

The black hole at the center is surrounded by matter in an accretion disk. This disk forms as the dust and gas fall onto the black hole, attracted by its gravity. Also shown is an outflowing jet of energetic particles, which is a compact source of high-energy X-ray radiation. Credit: NASA/JPL-Caltech.

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. 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 NICER, NuSTAR, and RXTE. I’m a member of the Observatory Science and Burst & Accretion working groups for NICER. 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.

Open-source science with Stingray

Stingray logo

I’m a coordinator and a lead developer for Stingray, an Astropy-affiliated Python library for timing and spectral-timing analysis of astronomical data. Along with Daniela Huppenkothen, Matteo Bachetti, and the many other people who’ve contributed to Stingray so far, we are making powerful modern statistical tools openly accessible to the fields of X-ray timing and spectral-timing. Read the Stingray paper for more details! If you want to get involved with Stingray, we’d love to hear from you, via a pull request, an issue, Slack, or email.

Next-generation spectral-timing with STROBE-X

STROBE-X initial diagram

I’m a member of the steering committee and science working group for STROBE-X, a proposed Probe-class mission that received NASA funding for a concept study and has been submitted to the Astro2020 Decadal Survey (read the white paper here). I’m in charge of the social media outreach (check out our Facebook, Twitter, Youtube, and interviews on Soundcloud) and part of the working group on strong gravity of stellar-mass compact objects.