Supernovae in the Sky

Credit: Chandra X-ray Observatory Center

Gravitational wave astronomy started with a bang back in 2015, when an international collaboration called LIGO detected the first rumblings from the collision of two black holes. Shifted into the audible range, it sounded like a bird’s chirp. Since then, 90 such events have swelled into a cosmic chorus. Among the most intriguing discoveries are two mysterious objects that, until recently, scientists thought shouldn’t exist.

All the black holes discovered using electromagnetic observations have had masses more than five times the size of the sun. Meanwhile, theorists have determined that the most massive a neutron star can be before collapsing to form a black hole is around 2.5 times the sun’s mass. And yet gravitational wave events recorded by LIGO and its European counterpart VIRGO announced last year revealed one object of about 2.6 solar masses and another of about 2.8 solar masses. So the question is, are these unusually small black holes or exceptionally large neutron stars?

Aaron Zimmerman, assistant professor of physics and a member of the LIGO-VIRGO-KAGRA Scientific Collaboration, said if these are small black holes, they probably formed when stars exploded in so-called supernovae. These events are very difficult to model and understand.

“If it is common to get black holes right at the edge of [the mass] where neutron stars collapse under their own weight, that will tell us something new about what is going on inside stars when they explode.”

“If it is common to get black holes right at the edge of [the mass] where neutron stars collapse under their own weight, that will tell us something new about what is going on inside stars when they explode,” he said. “We need to see a few more of these objects to know more, but having a second example of these mysterious systems gives me confidence that there are many more out there to be discovered.”