Special observatory near Richland finds ‘mystery object’ in space

LIGO Hanford, along with other gravitational wave observatories, is reporting finding a “mystery object” in space.

In the past they’ve detected evidence of black holes, created when the most massive stars die and collapse under their own gravity.

And they have detected evidence of neutron stars, smaller stars that explode in supernovas and leave behind dense, dead remnants of stars.

But this object is larger than any known neutron star and smaller than any known black hole.

The heaviest known neutron star is no more than 2.5 times the mass of the Earth’s sun. And the lightest known black hole is about 5 times the mass.

But scientists have been puzzled about what lies between those masses, in what they call the “mass gap.”

On Aug. 14, the Hanford Laser Interferometer Gravitational-wave Observatory just north of Richland, its twin observatory in Louisiana and the Virgo observatory in Italy detected something different.

It was 2.6 times the mass of the sun as it merged with a black hole 23 times the mass of the sun.

Together they formed a new black hole about 800 million light-years from Earth.

The merger sent gravitational waves rippling toward Earth, where they were detected by the three observatories.

“We’ve been waiting decades to solve this mystery,” said Vicky Kalogera, a Northwestern University professor who coauthored a paper about the detection published in “The Astrophysical Journal of Letters” this week.

“We don’t know if this object is the heaviest neutron star, or the lightest known black hole, but either way it breaks a record,” she said.

It is possible that there is no “mass gap,” just a lack of observations, said Patrick Brady, professor at the University of Wisconsin, Milwaukee, and a coauthor.

“Time and more observations will tell,” he said.

Black hole or neutron star?

Observatories that search the night sky for light were alerted to the event detected with gravitational waves on Aug. 14, but they were not able to spot any light from it.

That might be because if it was a collision of two black holes, no light would be expected.

Or it might have been too far away for observatories looking for light to see.

But if the mystery object were a neutron star, the much larger black hole might have swallowed it whole and it would not have given off any light.

“I think of Pac-Man eating a little dot,” said Kalogera. When one mass is much larger than the other, “the smaller neutron star can be eaten in one bite.”

More detections of objects that fall in the mass gap may help reveal to scientists whether additional objects exist in the mass gap.

LIGO Hanford and the second LIGO observatory in Louisiana, both funded by the U.S. National Science foundation, made scientific history when they detected the existence of gravitational waves from space for the first time in 2015.

Albert Einstein had predicted their existence almost 100 years earlier.

Gravitational wave observatories search for minute changes in the Earth that indicate gravitational waves from a violent event in space is passing through the planet.

How LIGO works

The LIGO near Richland has two vacuum tubes that extend for 2.5 miles across the Hanford shrub steppe at right angles. At the end of each, a mirror is suspended on fine wires.

A high-power laser beam is split to go down each tube, bounding off the mirrors at each end. If the beam is undisturbed, it will bounce back and recombine perfectly.

But a gravity wave pulsing through the Earth stretches objects lengthwise and causes them to compress sideways. A circle would become an ellipse.

At LIGO Hanford, one arm would become longer and the other shorter. The laser beams would no longer perfectly combine.

Data at the LIGO and Virgo observatories are compared to confirm gravitational waves and help determine where the waves originated.

This story was originally published June 24, 2020 at 10:17 AM with the headline "Special observatory near Richland finds ‘mystery object’ in space."