In June 2018, astronomers spotted a bright flash in space from galaxy CGCG 137-068, located about 200 million light-years away. Subsequent spectroscopic observations confirmed that the flash is much faster and far brighter than any stellar explosion.
Named AT2018cow, the transient was soon given the nickname ‘Cow’. Astronomers also cataloged it as a fast blue optical transient, or FBOT — a bright, short-lived event of unknown origin.
Now an MIT-led team has found strong evidence for the signal’s source. Their study suggests that the signal was originated from a dying star that, in collapsing, gave birth to a compact object in the form of a black hole or neutron star. That compact object absorbs surrounding material, eating the star from inside. This process generates an enormous burst of energy.
Scientists looked to X-ray data collected by NASA’s Neutron Star Interior Composition Explorer (NICER). They studied the data to identify X-ray signals emanating near AT2018cow.
They detected a strobe-like pulse of high-energy X-rays. After tracing hundreds of millions of X-ray pulses back to the Cow, scientists found that these pulses occurred like clockwork every 4.4 milliseconds over a span of 60 days.
Scientists then determined the frequency of these pulses. Their frequency suggests that the x-rays must have originated from a massive object.
According to scientists, that object must be no more than 1,000 kilometers wide, with a mass smaller than 800 suns. By astrophysical standards, such an object would be considered compact, like a small black hole or a neutron star.
Lead author Dheeraj “DJ” Pasham, a research scientist in MIT’s Kavli Institute for Astrophysics and Space Research, said, “We have likely discovered the birth of a compact object in a supernova. This happens in normal supernovae, but we haven’t seen it before because it’s such a messy process. We think this new evidence opens possibilities for finding baby black holes or baby neutron stars.”
The x-ray observation also suggests that the Cow emits bursts at a frequency of 225 hertz, or once every 4.4 milliseconds. The pulse frequency could be used to calculate the size of whatever was pulsing directly.
In this case, the size of the pulsing object cannot be larger than the distance that the speed of light can cover in 4.4 milliseconds. By this reasoning, he calculated that the object’s size must be no larger than 1.3×108 centimeters, or roughly 1,000 kilometers wide.
Pasham said, “The only thing that can be that small is a compact object — either a neutron star or black hole.”
Based on the energy emitted by the signal ‘Cow,’ scientists calculated that the object must have no more than 800 solar masses.
Pasham said, “This rules out the idea that the signal is from an intermediate black hole.”
“Apart from pinning down the source for this particular signal, the study demonstrates that X-ray analyses of FBOTs and other ultrabright phenomena could be a new tool for studying infant black holes.”
“Whenever there’s a new phenomenon, there’s an excitement that it could tell something new about the universe. For FBOs, we have shown we can study their pulsations in detail in a way that’s not possible in the optical. So, this is a new way to understand these newborn compact objects.”
- Pasham, D.R., Ho, W.C.G., Alston, W. et al. Evidence for a compact object in the aftermath of the extragalactic transient AT2018cow. Nat Astron (2021). DOI: 10.1038/s41550-021-01524-8