Simulated Map of Milky Way Gives Glimpse into Future Space-Based Gravitational Wave Observations
Scientists have created a simulated map of the Milky Way that showcases what future space-based detectors will be able to observe in terms of gravitational waves. While over 90 gravitational wave events have been detected thus far by ground-based detectors, such as the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States, Virgo in Italy, and KAGRA in Japan, none of these events have been found originating from our own galaxy.
However, the Milky Way is rich with “ultracompact binaries,” which were once binary stars but have now evolved into stellar remnants. These binaries are expected to contain compact objects like neutron stars, white dwarfs, and black holes in close orbits. The challenge lies in the fact that their gravitational waves produce frequencies too low for ground-based detectors to pick up, making a space observatory necessary to detect them.
LIGO and similar observatories can identify gravitational waves with frequencies ranging from 5 to 20,000 Hertz. Nevertheless, ultracompact binaries within our galaxy spiral around each other and eventually merge at frequencies in the milliHertz range.
Several space-based gravitational wave detectors are currently being developed. The European Space Agency’s Laser Interferometer Space Antenna (LISA) is at the forefront, scheduled to launch in the 2030s. Chinese scientists are also working on two mission concepts called TianQin and Taiji.
A team of researchers led by Kaitlyn Szekerczes from the Gravitational Astrophysics Laboratory at NASA Goddard, including Cecilia Chirenti from the University of Maryland and NASA’s Goddard Space Flight Center, have now simulated the intensity and frequency of gravitational waves emitted by ultracompact binaries in our Milky Way galaxy. The resulting image provides a glimpse into how observatories like LISA will be able to study our galaxy using gravitational waves, much like astronomers study it using other forms of light such as X-rays and gamma rays. The simulated image depicts ultracompact binaries concentrated in the Milky Way’s spiral disc plane and extending into the galactic halo.
James Ira Thorpe, another member of the team based at NASA Goddard, describes the image as an “all-sky view of the sky in a particular type of light,” emphasizing the promise of gravitational waves to observe the universe in a completely new way.
Currently, astronomers are aware of only a few ultracompact binaries with orbital periods shorter than an hour, which would place the compact objects close enough to each other to emit detectable gravitational waves. These binaries are challenging to locate due to the limited amount of light emitted by neutron stars and black holes. This is where LISA comes into play, as ultracompact binaries are expected to radiate gravitational waves brightly, enabling LISA to discover tens of thousands of such binaries.
The shorter the orbital period of an ultracompact binary, the higher the frequency and the lower the amplitude of the gravitational waves it produces. If the objects are in close proximity, scientists may even observe some mass transfer between them, which can be further explored using optical, X-ray, and gamma-ray telescopes. This combination of electromagnetic and gravitational-wave observations is referred to as “multi-messenger astronomy” by scientists.
The details of the simulated image were published in a paper in The Astronomical Journal in June. As space-based detectors like LISA prepare to launch, researchers are excited about the potential for a new era in gravitational wave astronomy, as they anticipate uncovering the secrets of our galaxy through the faint hum of these extraordinary cosmic events.