Gravitational waves, an oscillatory space warp –an oscillating stretch and squeeze of space, according to Stephen Hawking, have revealed a neutron star binary, called GW190425, more massive than any ever seen in the Milky Way that has mystified scientists—until now. A team of astrophysicists from Australia’s ARC Center of Excellence for Gravitational Wave Discovery (OzGrav) think they might have the answer.
Earlier this year, an international team of scientists using gravitational waves –electromagnetic waves consist of oscillating electric and magnetic forces that travel at light speed–announced the second detection of a gravitational-wave signal from the collision of two neutron stars. The event, called GW190425, is puzzling: The combined mass of the two neutron stars is greater than any other observed binary neutron star system. The combined mass is 3.4 times the mass of our sun.
The gravitational waves from GW190425 tell of a neutron star binary more massive than any neutron star binary previously observed, either through radio-wave or gravitational-wave astronomy. A recent study led by OzGrav Ph.D. candidate Isobel Romero-Shaw from Monash University proposes a formation channel that explains both the high mass of this binary and the fact that similar systems aren’t observed with traditional radio astronomy techniques.
The next generation of gravitational-wave astronomers will use these waves to detect and monitor gravitational waves from the singular birth of our universe to test ideas about how our universe came to be.
“We propose that GW190425 formed through a process called ‘unstable case BB mass transfer,” says Romero-Shaw says, a procedure that was originally defined in 1981. It starts with a neutron star that has a stellar partner: a helium (He) star with a carbon-oxygen (CO) core. If the helium part of the star expands far enough to engulf the neutron star, this helium cloud ends up pushing the binary closer together before it dissipates. The carbon-oxygen core of the star then explodes in a supernova and collapses to a neutron star.”
Binary neutron stars that form in this way can be significantly more massive than those observed through radio waves. They also merge very fast following the supernova explosion, making them unlikely to be captured in radio astronomy surveys. “Our study points out that the process of unstable case BB mass transfer could be how the massive star system formed,” says Romero-Shaw.
The OzGrav researchers also used a recently-developed technique to measure the eccentricity of the binary—how much the star system’s orbital shape deviates from a circle. Their findings are consistent with unstable case BB mass transfer.
Current ground-based gravitational-wave detectors aren’t sensitive enough to precisely measure the eccentricity; however, future detectors—like space-based detector LISA, due for launch in 2034—will allow scientists to make more accurate conclusions.
Source: Isobel M Romero-Shaw et al. Searching for eccentricity: signatures of dynamical formation in the first gravitational-wave transient catalogue of LIGO and Virgo, Monthly Notices of the Royal Astronomical Society (2019). DOI: 10.1093/mnras/stz2996
Daily Galaxy, Max Goldberg, via ARC Center of Excellence for Gravitational Wave Discovery and Space Australia
Image credit: NASA’s Goddard Space Flight Center/CI Lab