In astronomy, one of our more annoying struggles has come from trying to find stellar-mass black holes. While we know they should exist, saying with certainty that any one particular object is a black hole and not a neutron star has been a challenge. Both these objects represent the end stage of a massive star’s life. Stars that end with less than about 3.5 solar masses after experiencing both mass loss as a star and mass ejection as a supernova end up as a neutron star – an extremely dense object about the diameter of Manhattan Island that is made largely of neutrons in a matrix. These objects have a surface and, in some cases, will undergo dramatic novae activity as the material building up on their surfaces or in a disk around them periodically undergoes nuclear explosions.

Objects that end up larger than that 3.5 solar masses end up as black holes. This happens because neutrons can’t repel one another under that much gravity, and they crush together into some state we can’t explain. No one really knows what goes on within a black hole’s event horizon. What we do know is that a black hole is so small that any surface that does exist is within a radius from which you have to go faster than the speed of light to escape. Yes, folks, the defining factor of a black hole is that its escape velocity is greater than the speed of light, and since nothing can go faster than the speed of light, whatever is going on at that possible surface is beyond our ability to see or presently understand. 

While the physics of neutron stars and the smallest black holes are vastly different, they are nevertheless super hard to see and distinguish. In binary star systems, we detect both objects from their pulls on a companion star and the interaction between them and their surroundings. In many cases, we can’t directly see either object. What we see instead are X-rays from the material they are consuming. 

New research published in Monthly Notices of the Royal Astronomical Society (MNRAS) with first author Srimanta Banerjee, looks at the X-rays from a large selection of sources they think they can identify as neutron stars or black holes to see if there are any defining characteristics that may separate the neutron stars from the blackholes. They found that black holes shift the spectrum with their gravity, causing two different populations to stand out. This means that we should be able to look at other systems where the geometry makes it hard or impossible to accurately measure the dead object’s mass, and say that “because it looks like this in the X-rays” it should be a black hole (or a neutron star). This is the first diagnostic we’ve had other than mass.

More Information

Tata Institute of Fundamental Research press release (Eureka Alert)