Research

I’m fascinated by super-strong gravitational fields and the strange effects they have on matter. Black holes and neutron stars (together called ‘compact objects’ because scientists are clever) are host to extreme environments that cannot be re-created on Earth. In a binary orbit with a low-mass star like our Sun, a compact object will slowly drain the regular star of its matter over millions of years in a process called accretion. This accreted material forms an accretion disk around the compact object like a tutu on a ballerina. Some material from the accretion disk will accrete onto the compact object itself, but compact objects are messy eaters so there are winds blowing material off the disk and strong jets like firehoses of highly energetic particles and photons that often shoot out the poles of the compact object. Accretion is a powerful process that produces bright heat-light in the X-ray range, so these systems are called X-ray binaries.

As an observational astronomer/astrophysicist, I work with data taken by telescopes. Since X-ray binaries are just a single point of light in the sky, I don’t look at pretty pictures like those taken by Hubble — instead, I do a lot of signal processing and data analysis of digital tables of X-ray photons. It’s kind of like a spreadsheet, but so much more fun and interesting than what you’re probably imagining, I promise! If you want to become an astronomer, computer programming, data analysis, and statistics skills will be very helpful.

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.

Variability

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 (roughly 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 X-ray colors 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 (see below for more), NuSTAR, and RXTE. 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.

NICER on the space station

The telescope I use the most in my research is NICER, the Neutron Star Interior Composition ExploreR. It’s a one-meter-cube X-ray telescope built at NASA Goddard that’s attached to the International Space Station! X-rays from space can’t get through Earth’s atmosphere, which is great for humans but inconvenient for X-ray astronomers, so we have to put X-ray telescopes up on satellites. NICER was launched in June 2017 in the ‘trunk’ of a SpaceX resupply mission, and is fully remotely operated without any input from the astronauts. Its detectors are sensitive to low-energy (‘soft’) X-ray photons in the 0.2 – 12 keV range, and it has roughly 100 nanosecond time resolution when recording photon arrival time!! I’m a NICER Affiliated Scientist and I’ve been awarded observation time and grant money as both a Principal Investigator and Co-Investigator on different proposals (see my academic CV for details).

A picture of the International Space Station taken by a remote camera on a resupply mission,
with NICER indicated with an arrow. Credit: NASA

Next-generation spectral-timing with STROBE-X

STROBE-X initial diagram
Mockup of STROBE-X fully deployed in low-Earth orbit. Collecting area is really important to get as many X-ray photons as possible.
Credit: NASA/NRL.

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’ve been in charge of the social media outreach (check out our Facebook, Twitter, Youtube, and interviews on Soundcloud) as we announced the mission concept and generated the collaboration, and I’m part of the working group on strong gravity of stellar-mass black holes and neutron stars.