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 from near the north and south poles of the compact object. Accretion is a powerful process and accretion disks reach temperatures of about 20 million degrees Fahrenheit (100x hotter than the surface of the Sun), which produces bright heat-light in the form of X-rays. 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. If you want to become an astronomer, computer programming, data analysis, and statistics skills will be very helpful.
How do we know this?
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 light) indicate what process produced the light and therefore where in the system the emission is coming from. Timing observations (i.e., when the individual light 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.
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.
Open-source science with Stingray
I’m one of three lead developers 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, or on our Slack! See all my coding resources here.
NICER: X-rays 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 Space Flight Center 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, which is the fastest yet in an X-ray telescope. 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).
JWST: an infrared view of jets
JWST is a joint NASA–ESA flagship space telescope for the highest quality infrared observations of the Universe. I’m a member of two teams that were selected to get data of bright black holes in our galaxy. We’ll use this to help improve the timing calibration of some of the scientific instruments, and to learn how the jets are launched out of the top and bottom of a stellar black hole. The principal investigator of both proposals is Prof. Poshak Gandhi at the University of Southampton, and the co-principal investigators for both are Dr. Aarran Shaw at the University of Nevada Reno and Prof. Tom Maccarone at Texas Tech University. Astronomy is a highly collaborative science, and working on a team with great people makes it lots of fun.
This November I’ll be organizing and hosting a JWST launch event at the Abrams Planetarium with Dr. Shannon Schmoll. We’ll feature Michigan scientists using JWST for their science research, and have hands-on demonstrations of astronomy, physics, and space science.
STROBE-X: next-generation space telescope
STROBE-X is a proposed X-ray space telescope that would provide an unprecedented view of the X-ray sky on timescales from microseconds to years. We received NASA funding for a concept study and submitted a mission proposal to the Astro2020 Decadal Survey (read the white paper here). If selected, STROBE-X would be the high-energy astrophysics telescope with the largest surface area and biggest data capabilities, so that we can stare at the brightest X-ray sources in the night sky without blinking. The amount of surface area on the detectors for collecting light is really important, to get as many X-ray photons as possible. STROBE-X’s core science goals include measuring how fast black holes in our galaxy are spinning, mathematically understanding the ultra-dense cores of neutron stars, and catching X-ray counterparts to LIGO-Virgo gravitational wave source. Medium-sized “Probe-class” missions like STROBE-X likely will have a $1B budget for development and 5-year mission lifetime. Our fearless team leader and principal investigator is Dr. Paul Ray at the Naval Research Lab. As a member of the steering committee, I was 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. I’m also part of the working group on strong gravity of stellar-mass black holes and neutron stars.
If you’d like me to give a talk or write something about my research, either for an academic or general public audience, please get in touch.