Texas Tech University researchers discovered peculiar X-ray characteristics of AT2018cow, an unusual transient source, and suggest it was likely an uncommon core collapse supernova.
Liliana Rivera Sandoval, a postdoctoral research fellow in the Texas Tech University Department of Physics & Astronomy, has had something of a mystery on her hands for the last few months.
In mid-June 2018, astronomers using the Asteroid Terrestrial-impact Last Alert System (ATLAS) noticed a sudden burst of bright light in the darkness of space, in the direction of a dwarf spiral galaxy 200 million light-years away. Initially, the ATLAS team thought it could be a foreground white dwarf, the ultra-dense core remnant of a star that was once like our sun, stripping away the outer layers of a companion star, and that matter was then being pulled toward the surface of the white dwarf. When such material lands on the surface, it causes the white dwarf to increase several times in brightness. It's a not-uncommon scenario astronomers call a cataclysmic variable (CV) in outburst.
Rivera Sandoval and several collaborators obtained the first X-ray and ultraviolet (UV)/optical measurements of the event – officially named AT2018cow and unofficially "the Cow" – only three days after its discovery.
"With our optical observations using the Swift Observatory, we constrained the distance of the transient under the CV scenario and, with that distance, we determined it was too bright in X-rays to be a normal CV," Rivera Sandoval explained. "We also noted with the Swift data that the object was much brighter in UV than in optical light, so it was very blue. CVs also are brighter in UV, but due to the combination of our findings, the CV scenario did not seem convincing."
The researchers monitored the Cow in X-rays during the first 27 days after its discovery and identified an unusually variable behavior in its luminosity: its light spiked every few days. They also analyzed the Cow's X-ray spectrum and found it did not change, even during the luminosity spikes. Those X-ray characteristics have no precedence. The team published their findings in the journal Monthly Notices of the Royal Astronomical Society and subsequently continued to follow the event until it became too faint to observe, about 70 days after its discovery.
As Rivera Sandoval explained today (Jan. 10) during a news conference at the 233rd meeting of the American Astronomical Society, the team suspects the event was actually an uncommon core collapse supernova in which a massive star rapidly collapsed and violently exploded.
"The Cow is remarkably important because it was a transient with a very interesting and unexpected behavior in all wavelengths: it showed high variability in its X-ray luminosity, surprising spectral evolution, a very fast rise time, a very blue color, a high peak luminosity, and it occurred at a relatively short distance," Rivera Sandoval said. "I am extremely glad that we started to follow this object in X-rays and UV/optical at a very early stage, which is rarely possible.
"It was very exciting to detect that the Cow was very blue and bright in X-rays, and when we discovered its very peculiar luminosity variability, I was astonished, as it was totally unexpected."
Rivera Sandoval's collaborators on the project were Texas Tech professor Tom Maccarone and associate professor Alessandra Corsi, as well as Peter Brown at Texas A&M University, David Pooley at Trinity University and J. Craig Wheeler at the University of Texas-Austin. Maccarone's research focuses on X-ray binary systems, especially those involving black holes and neutron stars. Corsi, a member of the LIGO team that contributed to the first discovery of gravitational waves in 2015, specializes in supernovae and gamma-ray bursts in addition to gravitational waves.
