A massive star locked in orbit with an unknown object became a source of fascination for Dr. Pragati Pradhan, assistant professor of Physics and Astronomy at ²ÝÝ®ÊÓƵ.

The stellar system, known as IGR J16318-4848, has been shrouded in mystery since its discovery in 2003, due to the thick clouds of gas and dust surrounding it. Astronomers know one member of the pair is a giant star. But the identity of its companion — a compact object that is feeding on the star’s material — is unclear.

“We think it’s a neutron star or a black hole. We don’t know for sure,” Pradhan said.

Making the stellar system unusually difficult to study is a dense, dusty environment that obscures key signals. To better understand what was happening beneath that cloud, Pradhan began investigating the star and its companion using X-ray data collected by NASA’s Chandra X-ray Observatory.

“If I see variable X-ray emission from a particular star, I try to figure out what is causing it,” Pradhan said. “X-rays can only be produced when you have something really, really hot.”

Pradhan assembled a series of 11 observations from February to August 2021 to compare how the system changed over time.

By analyzing the system's X-ray emissions, researchers found a distinctive signal produced by iron atoms. The iron atoms’ emissions also shifted in a repeating pattern over time.

A newly in uses those shifting patterns to provide some of the clearest evidence yet of the stellar system’s motion. The study led by Pradhan also found evidence that the compact object is surrounded by dense gas carried outward by the massive star's stellar wind.

A better understanding of the system's orbit could help astronomers estimate the masses of the objects involved — a critical step toward identifying the hidden companion.

“This result provides an important missing piece in the puzzle,” Pradhan wrote in her summary.

But even though researchers now have a better understanding of the system, Pradhan said whether the compact object is a neutron star or a black hole “is still an open question.”

That answer may come from the X-ray Imaging and Spectroscopy Mission (XRISM), a next-generation X-ray observatory developed in collaboration with NASA and the Japan Aerospace Exploration Agency.

XRISM will allow Pradhan and other researchers to study the system with even greater precision.

“We already have new data from XRISM,” said Pradhan. “We are planning to team up with scientists who study other wavelengths to determine the mass of the star. With these two pieces together, we can finally solve the mystery of the compact object in the system.”

Pradhan said the answer could help researchers better understand how some of the universe’s most extreme binary systems form and evolve and could also help scientists studying atomic physics and matter under extreme conditions.

“I’m also trying to advertise opportunities for students to help with the data,” Pradhan said. “High-energy astrophysics may not have pretty pictures, but we have spectra where the actual physics lie.”