Season 17, Episode 33: Gravitational Waves from Colliding Neutron Stars

Hey StarTalkians! Episode 33 of Season 17 was another Cosmic Queries edition, with Neil and Negin working through a grab-bag of questions covering everything from LIGO to lycanthropy. One interesting question asked about visible sources of gravitational waves:

Cosmic Queries – LIGO, Light, & Lycanthropy - StarTalk Radio

(from 34:00)

Neil addressed this question very well, but after the excitement of the famous first gravitational wave observation, this “visible” result got comparatively little attention.  

The GW170817 Observation: Seeing the Source

Neil’s answer is based on the GW170817 observation in August 2017. The LIGO and Virgo gravitational wave detectors picked up a signal consistent with two in-spiralling neutron stars.

This is kind of fitting, because the first indirect evidence of gravitational waves came from a binary neutron star system.

Neutron stars aren’t particularly massive – around 1.4-times the mass of the sun – but they are incredibly compact, crammed into a radius of just 10 km or so. If the Earth was shrunk until it was as dense a neutron star, it would end up just 305 meters in diameter.

Observations of two neutron stars orbiting closely showed a decrease in orbital energy, which physicists assumed was a sign of gravitational wave emission.

The GW170817 observation corroborates this. Two US-based LIGO detectors made the observation, and the Virgo detector in Italy helped localize the source. Such binary neutron star mergers also create gamma-ray bursts, and right around that time, NASA’s Fermi space telescope spotted a matching burst.

It’s like hearing a siren from inside your apartment. You know there’s an emergency somewhere, but you don’t know the exact source until you follow the sound and locate the flashing light.  

How Pulsars Can Become the Detector

While checking into Neil’s comment, something else incredibly cool came up. Other researchers used pulsar signals to detect gravitational waves in a different way. Pulsars are neutron stars that give off regular flashes, like a lighthouse whipping around and periodically pointing at you. They’re like cosmic clocks.

Tracking 25 pulsars, researchers looked for slight variations in the timing of the pulses, a sign of disturbance by a gravitational wave. Like a spider waiting for vibrations along its web, they were able to detect a gravitational wave background. These were probably created by binary black hole systems.