In this series of posts, we sit down with a few of the keynote speakers of the 243rd AAS meeting to learn more about them and their research. You can see a full schedule of their talks here, and read our other interviews here!
Hold on tight as we dive into the relativistic world around some of the densest and most extreme objects in the universe – neutron stars! Today, we’re delighted to highlight the work of Prof. Renee Ludlam, a leading expert on accreting neutron stars, which are in a binary and drawing in material from their companion. Now a professor at her alma mater, Wayne State University, Ludlam has been awarded the 2023 Newton Lacy Pierce Prize for novel explorations of the relativistic universe that have revealed fundamental properties of neutron stars. She will deliver her plenary talk, titled “Characterizing the Properties of Accreting Neutron Stars through X-ray Observations” at AAS 243 this week!
Reflection Studies of Neutron Stars
Although she is known for her neutron star expertise, Prof. Ludlam actually started out working on black holes as an undergraduate student. During an undergraduate research experience at the University of Michigan, where Prof. Ludlam would ultimately go to graduate school, she worked on reflection modeling of stellar mass black holes, a technique that she applied to neutron stars during her graduate school career. In neutron stars, things are a bit more complicated than black holes, but for Prof. Ludlam “it was almost a natural progression” transitioning to working on neutron stars. Prof. Ludlam was excited by the complexities surrounding neutron stars. “They differ from black holes in that they have a surface, and that adds a whole new layer of physics and a gap in our understanding,” which she aims to fill with her work.
Prof. Ludlam’s work uses a technique called reflection modeling, which can be thought of just like light bouncing off of a mirror. Essentially, there is a primary source of light, coming from very close to the compact object, that interacts with surrounding material, often the accretion disk. Unlike a mirror, however, the reflected light is changed through processes of absorption, scattering, and emission, meaning that the reflected light we see is not exactly like the primary source of light. This fact is actually crucial to using reflection to learn about how matter accretes from the companion onto the neutron star. For example, Prof. Ludlam’s work commonly uses the emission lines that are produced in the surrounding material, which are often distorted by relativistic effects such as Doppler beaming and gravitational redshift, to study the geometry of these complex systems.
Using state-of-the-art models for this reflected light, and fitting these models to X-ray spectra taken by telescopes like NICER and NuSTAR, Prof. Ludlam has vastly improved our understanding of the fundamental properties of neutron stars and their accretion disks. Her work has shown that indeed accretion is quite complicated in neutron stars, and that nearly “every source you look at behaves differently.” For example, in black holes there seems to be a relatively simple scaling between the accretion disk geometry and how much mass is being accreted, but Prof. Ludlam has shown that this is not the case for neutron stars, primarily because “every neutron star has a different intrinsic magnetic field strength.”
Beyond her work on neutron stars, Prof. Ludlam has also been involved in the development of StrayCats, a new way of using data from NuSTAR. Unlike many telescopes, NuSTAR has open air between the focusing mirrors and the detectors, meaning any stray light from nearby bright sources can also hit the detectors. StrayCats is a catalog of all of the sources with usable stray light data and has produced exciting science, like what is highlighted in this Astrobite! Prof. Ludlam, who got involved in the project during her time as a postdoc at Caltech, has enjoyed repurposing this stray light into usable science data, and she hopes more people can make use of the StrayCats data.
Reflecting on the Past & Glimpsing into the Future
Although Prof. Ludlam was interested in space and other worlds since she was just a kid watching the sci-fi show Stargate SG-1, she didn’t think she could actually make a living doing astronomy. By the time she was starting her undergraduate studies at Wayne State University, she was set on becoming a physical therapist. All of that changed when she saw that Wayne State had just started an undergraduate degree in astronomy the year she started there; she saw this as a sign and immediately signed up. However, as a first generation college student, Prof. Ludlam didn’t have anyone telling her about higher education, and it wasn’t until she attended a Conference for Undergraduate Women in Physics (CUWiP) that she learned about graduate school as an option for her future.
In grad school, Prof. Ludlam fell in love with the research process, and “fast-tracked” her PhD, completing it in just 4 years. But it didn’t start out easy – a major fear of public speaking almost kept Prof. Ludlam from pursuing a career in research. Looking back, the advice she would give her younger self is that “things are going to be uncomfortable, but it’s supposed to be uncomfortable. That’s how you grow.” She also recounted doing poorly on the physics GRE and says that she was motivated by nay-sayers who doubted her ability to pursue astronomy research. She recalls telling people, “I don’t have [a contingency plan] because this is what I’m going to do,” and said that “proving them wrong drove my career.”
What’s next for Prof. Ludlam? Scientifically, she’s excited by the idea of combining X-ray polarization measurements with reflection modeling and hopes that these two techniques together will help us to better understand key parameters in accreting neutron stars. Prof. Ludlam is also a guest scientist for the recently-launched XRISM mission, which is currently in an initial commissioning phase and is set to revolutionize the field of X-ray spectroscopy with extremely high-resolution X-ray spectra. In the classroom, Prof. Ludlam has the personal experience of being an undergraduate student at Wayne State, which has driven her to be more intentional in her teaching. Prof. Ludlam also wants to ensure that everyone that wants to can learn physics and astronomy, saying, “there is no innate ability to do physics or research. It’s all some skill that you learn, and anyone can learn if they’re driven and want to put in the effort.”
To hear more about the study of accreting neutron stars, tune into Prof. Ludlam’s Plenary Lecture at 4:40 PM – 5:30 PM CT on Wednesday, January 10 at #AAS243!
Astrobite edited by: Jessie Thwaites
Featured image credit: American Astronomical Society