An X-ray binary with an accretion disc.

A typical student research project might build on information found in various online or print resources. Hongyu Zhang ’24 is working with data from a slightly more extraterrestrial source: NASA’s Neutron Star Interior Composition Explorer (NICER) telescope. 

Hongyu Zhang '24
Hongyu Zhang '24

Zhang and Caltech postdoc McKinley Brumback ’14 will use this data to analyze Hercules X-1 (Her X-1), an X-ray binary. X-ray binaries are star systems composed of a normal star and either a neutron star or black hole. With Her X-1, it’s a neutron star: the incredibly dense collapsed core of a massive star. In an X-ray binary, the neutron star functions as the “accretor,” pulling matter from the normal star, or “donor,” into a sheet of gas and dust — the “accretion disc” — that emits X-rays.

About Hongyu Zhang ’24

Major: Undeclared

Hometown: Taiyuan, China

High School: International School of Indiana

read about other student research 

In the case of Her X-1, the accretor is also a pulsar: a rotating neutron star that emits pulses of radiation at regular intervals. As a pulsar rotates, its strong magnetic fields expel particles out of the star’s magnetic poles. This process generates beams of light, which can, at times, be observed blinking on and off from Earth. Observing the pulses of a pulsar is often likened to watching a lighthouse, in that the beam of light is only visible when its rotation collides with one’s line-of-sight. 

It is the accretion disc of Her X-1’s pulsar that is the focus of Zhang’s project. “What I’m interested in is studying the geometry and the pulse profile of the accretion disc … the gases surrounding this neutron star and how it is changing in the long period of time,” he said. To accomplish this, he and Brumback will be downloading Her X-1 data from NICER, cleaning it, and then loading it into a computer program. 

With this program, they will be able to simulate how the accretion disc might develop over time. “We are comparing what the computer generates to what we actually saw [from NICER],” Zhang said. NICER “can look into this physical feature [the disc] in more detail,” he explained, which will help them assess the accuracy of the computer model. Ultimately, Zhang said they hope to produce a paper for publication in an astrophysics journal. 

Though he only recently completed his first year at Hamilton, Zhang is no stranger to doing research at the college level. “I started reaching out to my advisors and physics professors as soon as I got to Hamilton,” he recalled. All told, Zhang estimates he has already done around “a semester and a half of research in physics.” 

He was first introduced to this particular project earlier this year after hearing Brumback give a talk on accretion physics. Interested, Zhang reached out to her about working together. Initially, he recalled, Brumback wasn’t sure what the summer would offer in the way of collaborative research opportunities. But in March, she got back to him with some potential avenues to explore. From there, Zhang said, all they had to do was write out the research proposal and get to work.

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