The universe is full of bizarre objects, and now astronomers have discovered a doozy – superheavy neutron stars that existed for only fractions of a second, before they collapsed into black holes.
When stars of a certain mass range explode as a supernova, they leave behind a dense core known as a neutron star. These strange stars cram more than the mass of the Sun into a ball the size of a city. They often end up in binary systems, where eventually two neutron stars will spiral inwards until they collide to form one object.
What that object is depends on the total mass. A neutron star can have a maximum mass of just over two Suns, before it will collapse under its own gravity and form a black hole – so if the total of the two neutron stars comes in under that limit, they form a new neutron star. If the mass is higher, the collision will create a black hole instead.
In the new study, astronomers detected two mergers between neutron stars that resulted in black holes. However, they also discovered the signals of an intriguing intermediate stage – superheavy neutron stars that only exist for mere milliseconds.
According to computer simulations of neutron star mergers, if a superheavy neutron star is formed a specific pattern known as quasiperiodic oscillations (QPOs) should arise in the gravitational waves thrown off during the event. While current observatories aren’t sensitive enough to detect these in gravitational waves, the team on the new study determined that their fingerprints should show up in gamma rays as well.
To test the idea, the astronomers scanned archival data of 700 short gamma-ray bursts (GRBs) captured by three observatories over the last few decades. And sure enough, gamma-ray QPOs showed up in two events captured by the Compton Gamma Ray Observatory – one that occurred in July 1991 and the other in November 1993.
The team calculated that the superheavy neutron stars detected would have had masses of over 2.5 times that of the Sun, and would have lasted no longer than 300 milliseconds before collapsing into black holes. They also would have been spinning extremely fast – almost 78,000 revolutions per minute, if they’d lasted that long. By comparison, the fastest-spinning pulsar clocks under 43,000 rpm.
The team says that future gravitational wave detectors should become sensitive enough to directly spot the signatures of superheavy neutron stars, which could help provide new info on these short-lived objects.
The research was published in the journal Nature. A simulation of two neutron stars merging can be seen in the video below.
Source: NASA