On May 21, 2019, astronomers using the Virgo interferometer and the National Science Foundation's Laser Interferometer Gravitational-wave Observatory (LIGO) detected a gravitational wave signature consistent with a black hole merger.
The Advanced Virgo detector at the European Gravitational Observatory (EGO) in Italy and two wave observatories in the United States discovered the object previous year and calculated it to weigh around 2.6 times our own Sun.
The gravitational wave that washed over earth previous year left scientists all the world over puzzled about its source.
The mysterious object is said to be 2.6 times the mass of our sun, while the black hole was that was 23 times the mass of our sun.
Before the two objects merged, their masses differed by a factor of 9, making this the most extreme mass ratio known for a gravitational-wave event.
If it is a light black hole then there is no established theory for how such an object could develop. Other co-authors include: K. Burdge, S.G. Djorgovski, A.J. Drake, D. Duev, A.A. Mahabal, J. Belecki, R. Burruss, G. Helou, S.R. Kulkarni, F.J. Masci, T. Prince, D. Reiley, H. Rodriguez, B. Rusholme, R.M. Smith, all from Caltech; N.P. Ross of the University of Edinburgh; Daniel Stern of the Jet Propulsion Laboratory, managed by Caltech for NASA; M. Coughlin of the University of Minnesota; S. van Velzen of University of Maryland, College Park and New York University; E.C. Bellm of the University of Washington; S.B. Cenko of NASA Goddard Space Flight Center; V. Cunningham of University of Maryland, College Park; and M.T. Soumagnac of the Lawrence Berkeley National Laboratory and the Weizmann Institute of Science. The Virgo detector is located in Cascina, Italy.
How will researchers ever know if the mystery object was a neutron star or a black hole?
Colliding neutron stars, the super-dense remnants of exploded stars, produce all sorts of emission spectra, including infrared, ultraviolet, visible light, x-rays, gamma rays, and radio waves.
Astronomers have cataloged black holes that are as light as five solar masses, and neutron stars that are as heavy as 2.5 solar masses. The question remained: does anything lie in this so-called mass gap?
"Mergers of a mixed nature - black holes and neutron stars - have been predicted for decades, but this compact object in the mass gap is a complete surprise. We are really pushing our knowledge of low-mass compact objects", Vicky Kalogera, study coauthor was quoted as saying.
No such flash was detected, and astronomers said there could be a range of explanations for that: One possibility is that the event was so far away that any flash would be too dim to be seen, even if the mystery object was a neutron star. "Either way, it breaks a record", she said.
When the LIGO and Virgo scientists spotted this merger, they immediately sent out an alert to the astronomical community. "The flare occurred on the right timescale, and in the right location, to be coincident with the gravitational-wave event".
According to the LIGO and Virgo scientists, the August 2019 event was not seen by light-based telescopes for a few possible reasons. In the latter case, this would mean it is the lightest black hole ever detected. Scientists believed this is due to the distance of the event, which was six times farther away than the 2017 merger. Secondly, if the collision involved two black holes, it likely would have not shone with any light. "As the detectors get more and more sensitive, we will observe even more of these signals, and we will be able to pinpoint the populations of neutron stars and black holes in the universe".
"I think of Pac-Man eating a little dot", Kalogera said.
"That can not be explained without defying our understanding of extremely dense matter or what we know about the evolution of stars", Marronetti said. "This observation is yet another example of the transformative potential of the field of gravitational-wave astronomy, which brings novel insights to light with every new detection".
When the most massive stars die, they collapse under their own gravity and leave behind black holes. But there's a chance that future observations of other objects in the mass gap will reveal enough additional clues to solve the mystery.
Charlie Hoy, a PhD student from Cardiff University, UK, involved in the study, said the new discovery would transform our understanding.