Observations using NASA's Hubble and Spitzer space telescopes and the Large Binocular Telescope hint that a massive, dying star was being reborn into a black hole without a bang.
Dying Star Just Fizzled Out
Scientists were looking for the remnants of the dead star and realized that it just disappeared out of sight. Instead of exploding into a very bright supernova, the star, which was 25 times as massive as the solar system's sun, just fizzled out and then left behind a black hole.
Stellar black holes, the most common type of medium-sized black holes, are known to form when the center of a very massive star collapses in upon itself. The collapse causes a supernova that blasts part of the star into space.
Scientists said that the black hole candidate they investigated and described in the Monthly Notices of the Royal Astronomical Society formed through a process called "failed supernova."
Christopher Kochanek, from Ohio State University, said that massive fails such as this in a nearby galaxy may offer an explanation as to why scientists rarely witness supernovae from the most massive stars.
"The typical view is that a star can form a black hole only after it goes supernova," Kochanek said. "If a star can fall short of a supernova and still make a black hole, that would help to explain why we don't see supernovae from the most massive stars."
Although it is still too early to determine the frequency of stars going through massive fails, experts estimate that between 10 and 30 percent of massive stars possibly collapse into black holes without requiring a supernova.
Kochanek and colleagues have been watching the spiral galaxy NGC 6946 located about 22 million light- years away. The galaxy is also known as "Fireworks Galaxy" because supernovae often occur there.
No Sign Of N6946-BH1
Beginning in 2009, one star in the galaxy called N6946-BH1 started to brighten weakly but by 2015, it seems to have gone out of existence.
Astronomers used the LBT as well as the Hubble and Spitzer to see if the star was still there and merely dimmed. They also attempted to find any infrared radiation from the spot where the star was using the Spitzer telescope, which could be a sign that could indicate the star was still there although just hidden by a dust cloud.
The tests all had negative results, which means that the star was no longer there. The researchers eventually concluded by a process of elimination that the massive star has become a black hole albeit they noted that future observations are still needed to confirm their interpretation.
"The event is consistent with the ejection of the envelope of a red supergiant in a failed SN and the late-time emission could be powered by fallback accretion on to a newly formed black hole. Future IR and X-ray observations are needed to confirm this interpretation of the fate for the star," the researchers wrote in their report, which was published in April this year.