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NASA’s Chandra Finds Source of Star’s Mysterious Outbursts
Using NASA’s Chandra X-ray Observatory, astronomers have discovered what is triggering repeated outbursts from a giant, unstable star.
A new study, published Monday in the latest issue of The Astrophysical Journal, details the discovery of a previously hidden companion object that keeps pulling material away from the massive star every 113 days as they orbit one another. This partner is either a stellar-mass black hole, or the dense collapsed core of a massive star called a neutron star.
This is the first X-ray detection of one of this class of erupting stars. It is also the first of these objects found with a black hole or neutron star companion.
The volatile star, known as AT 2016blu, is in the spiral galaxy NGC 4559, located about 30 million light-years from Earth. Astronomers estimate the mass of this star is at least 33 times that of the Sun.
“For 15 years, astronomers have been watching this star get really bright and then dim again on a regular basis,” said Mojgan Aghakhanloo of the University of Virginia in Charlottesville, who led this study. “Until we were able to get Chandra data at the right time, we couldn’t figure out what was making this happen.”
The star AT 2016blu is a type of luminous blue variable, which are hot, massive stars characterized by changes in brightness. Eta Carinae, for example, is a particularly well-known luminous blue variable.) AT 2016blu is a special type of luminous blue variable that undergoes rapid and repeated increases in brightness. The spikes in brightness are so extreme that they resemble supernova explosions, where a massive star collapses onto itself and material then hurtles outward into space. However, unlike in a supernova explosion, the star is not destroyed, so AT 2016blu is also referred to as a “supernova impostor.”
Between 2012 and 2026, astronomers detected at least 27 bursts of optical light, or light in wavelengths that are visible to our eyes, from AT 2016blu roughly every 113 days.
“This star’s behavior was confusing at first because it kept acting like a supernova, but clearly it’s not because it keeps surviving these outbursts about three times a year,” said co-author Núria Torres-Albà, also of the University of Virginia. “To understand the cause of these eruptions, we realized we needed X-ray data, timed so that we’d observe the star when it was in an outburst.”
The team used Chandra because it is the only X-ray telescope with sharp enough vision to distinguish AT 2016blu from another nearby bright X-ray source.
“We suspected that the bright repeating flashes at optical wavelengths might actually be powered by periodic blasts of high-energy X-rays,” said co-author Nathan Smith of the University of Arizona in Tucson. “The dense gas surrounding the system is heated by the X-rays and then glows in the optical. So, we needed to plan the Chandra X-ray observations carefully to be simultaneous with an optical outburst.”