Scientists may have solved mystery of 'the Cow,' a superbright space explosion

In June 2018, astronomers detected an explosion so bright and so strange that it defied easy explanation. The event, catalogued as AT2018cow and quickly nicknamed "the Cow" by the astronomical community, erupted in a galaxy called CGCG 137-068 roughly 200 million light-years from Earth. What made it remarkable was not just its luminosity—it blazed at least ten times brighter than a typical supernova—but the speed at which it appeared and then faded. Within months, the initial burst had dimmed considerably, leaving researchers with more questions than answers about what had actually occurred.

For years, the mystery deepened. Scientists proposed competing theories. Perhaps the explosion had been triggered by an intermediate-mass black hole, weighing between 10,000 and 100,000 times the mass of our sun, tearing a star to shreds. Or maybe it was a standard stellar explosion whose shock waves had collided with dense material surrounding the dying star. A third possibility held that the blast itself had given birth to a compact object—either a black hole or a neutron star—that was powering the observed radiation. None of these ideas could be definitively ruled in or out based on the available evidence.

That changed when Dheeraj Pasham, a researcher at the Massachusetts Institute of Technology, led a team to examine the X-ray data from the Cow with fresh analytical tools. Rather than looking at the overall brightness of the X-rays, the researchers focused on how that brightness changed over time. They constructed what is called a power density spectrum—essentially a mathematical fingerprint of any regular patterns hidden in the data. If the X-rays were flickering in a consistent rhythm, that pattern would reveal itself as a peak in the spectrum.

The analysis found exactly what they were looking for. The X-ray brightness from the Cow fluctuated with remarkable regularity every 4.4 milliseconds. That extraordinarily short timescale was the key. Such rapid oscillations could only be produced by matter orbiting or accreting onto a compact object—something so dense and so small that material near it moves at tremendous speeds. A black hole or neutron star fit the bill perfectly. A larger object, or a more diffuse source of radiation, could not produce such tight, regular variations.

With this evidence in hand, Pasham and his team could narrow the field considerably. The presence of these X-ray oscillations strongly suggested that a compact object lay at the heart of the explosion. They bolstered this conclusion by examining the location of the Cow within its host galaxy. The explosion had occurred in a star-forming region—an area populated by massive stars nearing the end of their lives. This context made sense: a dying star had undergone a catastrophic collapse and, in doing so, had likely given birth to a black hole or neutron star.

The findings, published in Nature Astronomy on December 13, 2021, represented a significant step forward in understanding one of astronomy's recent puzzles. Yet the work also highlighted what remained unknown. The analysis could confirm that a compact object had formed, but it could not yet distinguish between the two possibilities. Whether the Cow had created a black hole or a neutron star—each with profoundly different properties and implications—remained an open question. Pasham suggested that existing optical and other observational data might eventually provide the answer, but that determination would require further study. For now, the Cow had yielded one of its secrets, even as it guarded another.