From Symmetry: “Chasing gravitational waves”

Symmetry Mag
From Symmetry<

Diana Kwon

A. Tonita, L. Rezzolla/Goethe University of Frankfurt​, &​ F. Pannarale/Sapienza University of Rome

When LIGO and Virgo detected the echoes that likely came from a collision between a black hole and a neutron star, dozens of physicists began a hunt for the signal’s electromagnetic counterpart.

Around mid-afternoon on August 14, a pulse passed simultaneously through the laser beams of three enormous gravitational-wave detectors, the twin locations of the Laser Interferometer Gravitational-Wave Observatory, or LIGO, in Louisiana and Washington, and the Virgo detector in Italy.

MIT /Caltech Advanced aLigo

VIRGO Gravitational Wave interferometer, near Pisa, Italy

Caltech/MIT Advanced aLigo Hanford, WA, USA installation

Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

VIRGO Gravitational Wave interferometer, near Pisa, Italy

Almost immediately, alerts beeped smartphones, tablets and laptops to life around the globe. Many of the physicists who saw this notification immediately dropped what they were doing and dashed to their computers to investigate.

What they soon hoped to find was the first optical evidence that they were detecting a collision between a black hole and a neutron star.

Scientists may have detected violent collision between neutron star, black hole

An ear on the universe

All objects with mass—including stars, planets and even humans—emit gravitational waves when they change speed or direction. But most gravitational waves are much too weak to detect. Even highly sensitive instruments like LIGO and Virgo, which are akin to microphones that hear signals from all directions, can only identify very “loud” gravitational waves caused by exceptionally massive objects that are accelerating rapidly.

So far, the gravitational waves detected have all come from compact binaries, or two massive objects spiraling around and smashing into one another. There are three different types of pairings in a compact binary—two black holes, two neutron stars, or a neutron star and a black hole.

Since black holes are more massive than neutron stars, their clashes generate the loudest gravitational waves. Because of this, they are the easiest to find. After their first two observing runs, the LIGO-Virgo collaboration announced official detections of 10 colliding black hole pairs.

Neutron star-neutron star mergers, called kilonovae, are the quietest of the three. In October 2017, scientists announced the first observation of such a collision.

It was the first successful use of LIGO-Virgo in multi-messenger astronomy, in which cosmological phenomena are examined through multiple types of signals, such as gravitational waves and electromagnetic sources like X-rays, radio waves and light.

LIGO and Virgo send out an alert each time they hear an interesting signal to allow scientists at other observatories to immediately point their instruments at the spot in the sky where the signal most likely originated to collect as much data as possible about the source.

The alert that went out after the neutron star merger mobilized scientists at telescopes around the world, including the DECam, a US Department of Energy-funded instrument mounted onto the National Science Foundation-funded 4-meter Blanco Telescope in Chile; the Very Large Array in New Mexico; and the Fermi Gamma-Ray Space Telescope orbiting our planet. Together, their observations helped provide evidence for a long-standing hypothesis that heavy elements such as gold and uranium were formed in the cosmic explosions that occur when two neutron stars crash.

Dark Energy Survey

Dark Energy Camera [DECam], built at FNAL

NOAO/CTIO Victor M Blanco 4m Telescope which houses the DECam at Cerro Tololo, Chile, housing DECam at an altitude of 7200 feet

Timeline of the Inflationary Universe WMAP

The Dark Energy Survey (DES) is an international, collaborative effort to map hundreds of millions of galaxies, detect thousands of supernovae, and find patterns of cosmic structure that will reveal the nature of the mysterious dark energy that is accelerating the expansion of our Universe. DES began searching the Southern skies on August 31, 2013.

According to Einstein’s theory of General Relativity, gravity should lead to a slowing of the cosmic expansion. Yet, in 1998, two teams of astronomers studying distant supernovae made the remarkable discovery that the expansion of the universe is speeding up. To explain cosmic acceleration, cosmologists are faced with two possibilities: either 70% of the universe exists in an exotic form, now called dark energy, that exhibits a gravitational force opposite to the attractive gravity of ordinary matter, or General Relativity must be replaced by a new theory of gravity on cosmic scales.

DES is designed to probe the origin of the accelerating universe and help uncover the nature of dark energy by measuring the 14-billion-year history of cosmic expansion with high precision. More than 400 scientists from over 25 institutions in the United States, Spain, the United Kingdom, Brazil, Germany, Switzerland, and Australia are working on the project. The collaboration built and is using an extremely sensitive 570-Megapixel digital camera, DECam, mounted on the Blanco 4-meter telescope at Cerro Tololo Inter-American Observatory, high in the Chilean Andes, to carry out the project.

Over six years (2013-2019), the DES collaboration used 758 nights of observation to carry out a deep, wide-area survey to record information from 300 million galaxies that are billions of light-years from Earth. The survey imaged 5000 square degrees of the southern sky in five optical filters to obtain detailed information about each galaxy. A fraction of the survey time is used to observe smaller patches of sky roughly once a week to discover and study thousands of supernovae and other astrophysical transients.

NRAO/Karl V Jansky Expanded Very Large Array, on the Plains of San Agustin fifty miles west of Socorro, NM, USA, at an elevation of 6970 ft (2124 m)


NASA/Fermi Gamma Ray Space Telescope

“That event was unbelievably exciting and scientifically rich,” says Daniel Holz, an astrophysicist at the University of Chicago and a member of both LIGO and DES-GW. “The thrill of being part of that was incredible.”

LIGO and Virgo have released dozens of public alerts about potential detections since the beginning of their third observing run this April. To date, these include potential observations of 20 binary black holes, 4 binary neutron stars, and 2 neutron star-black holes—but none of the these has been seen with a separate electromagnetic signal.

Localizations of gravitational-wave signals detected by LIGO in 2015 (GW150914, LVT151012, GW151226, GW170104), more recently, by the LIGO-Virgo network (GW170814, GW170817). After Virgo came online in August 2018

Masses in the Stellar Graveyard LIGO Virgo| Frank Elavsky | Northwestern

The alerts are sent out almost immediately after the detectors pick up on a promising signal. In addition to identifying the potential candidate and its location on the sky, the notifications include a false alarm rate that portrays the likelihood that the proposed event was real. As researchers conduct further analyses on the data, the collaboration publishes updated estimates. If the signal turns out to be mere noise, they release a retraction.

A needle in a haystack

After the alert this August, the Slack channel for DES-GW, a group of scientists who use DECam to search for the optical counterparts of gravitational waves, was abuzz with chatter.

At first, the gravitational wave was only classified as a “mass gap” detection, a signal from a pair of objects that was between the mass of the lightest black hole and the heaviest neutron star. This suggested the gravitational wave came from a new type of source, says Antonella Palmese, a postdoc at Fermilab and a member of DES-GW. But it could have been a merger between two unusually small black holes, in which case, it would be invisible to an instrument like DECam.

After further analysis of the gravitational wave data, LIGO and Virgo scientists were able to categorize the signal as most likely a clash between a black hole and a neutron star. Although the collaboration hasn’t yet announced an official detection, their alert classified the event with greater than 99% confidence. It was their clearest gravitational-wave signal from a black hole-neutron star merger yet.

“The moment when we got really excited and were all over our laptops was when we received the classification that it was a black hole-neutron star merger,” Palmese says. “In that case, you might expect the material from the neutron star to emit some electromagnetic counterpart that we can observe from our telescopes.”

Once they received the classification, the group immediately got to work analyzing the information provided from the gravitational wave observatories and making plans to take data of their own with DECam as soon as possible. “We had to jump into action right away to try to do the observation,” says Marcelle Soares-Santos, an astrophysicist at Brandeis University who was at home when she received the first LIGO-Virgo message on her phone. “We managed to be on the sky in less than 24 hours.”

According to the alert, the slice of sky that the signal could have originated from was tiny, providing astronomers with a clear target at which to point their instruments.

“It was clear from the beginning that this event was special,” Soares-Santos says. “Several of us ended up staying up multiple nights because we were observing the same area of the sky multiple times.”

Continuing the chase

To identify optical counterparts of the source, DES-GW scientists look for transients: short-lived bursts of electromagnetic energy. Over the course of seven observing nights, DES-GW identified approximately 23 potential sources for the gravitational waves. Over the last few weeks, the group has been examining each of these to try to rule out the possibility that they are other celestial objects.

One of the most common contaminants in these candidates are supernovae. Physicists can identify whether an object is a supernova by observing how long it takes to disappear. Unlike a black hole-neutron star merger, which would fade quickly, the aftermath of these stellar explosions usually remains in the sky for around a month or more.

Finding the optical counterpart of the black hole-neutron star collision would be exciting for several reasons. First of all, this would be the first detection of such an event, so it could help reveal how such a process happens—whether the neutron star is ripped apart by a black hole or swallowed in one fell swoop. By observing this process, physicists could learn about the material that neutron stars are made of, which is the densest matter in the universe. These mergers may also help researchers better understand which elements are generated during this process.

At this point, enough time has passed that it’s unlikely that DES-GW will identify the optical counterpart to the gravitational waves produced from this likely neutron star-black hole collision. Still, even without the detection, the data gathered at electromagnetic instruments can be helpful. For example, it suggests that instead of colliding, the neutron star may simply have been swallowed by the black hole instead, leaving no visible signs.

More observations of neutron star-black hole mergers will be necessary to determine how, exactly, this process is happening. Scientists don’t yet know for sure when LIGO-Virgo will hear another gravitational wave coming from this type of event. But there are still several months to go during the current observing run, so physicists are anticipating another detection that will be worth pursuing.

“This was the most exciting event this season so far,” Soares-Santos says. “But I wouldn’t bet it’s the last.”

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Symmetry is a joint Fermilab/SLAC publication.