Chapter 1: A New Cosmic Discovery

Recent observations in gravitational waves are shedding light on an extraordinary cosmic event. The ability to detect these waves may not have the same dramatic flair as when it was first achieved three years ago, but it continues to unveil hidden phenomena in the universe. Scientists have previously documented neutron stars colliding and pairs of black holes merging. Now, they may have evidence of a black hole colliding with a neutron star—a scenario that was once considered uncertain.

“It represents the discovery of a new astrophysical system whose existence was previously in question,” explains Katerina Chatziioannou, a member of the LIGO team from the Perimeter Institute in Waterloo, Canada, and an upcoming professor at Caltech. “These systems have been hypothesized to exist in various cosmic environments, and confirming their existence will enhance our understanding of these settings.”

To summarize, two new gravitational wave signals were detected on April 25 and 26 by the Laser Interferometer Gravitational-Wave Observatory (LIGO) facilities in Louisiana and Washington, along with the Virgo observatory in Italy. The first signals seem to indicate a collision between two neutron stars, while the second signals point toward a unique merger involving a black hole and a neutron star.

The latest findings follow significant upgrades to the LIGO and Virgo observatories. The enhancements have doubled the power of their lasers, minimizing noise and boosting the detectors' sensitivity by nearly 40 percent. “These detections may have been possible before, but the enhanced sensitivity gives us a clearer understanding,” remarks Rana Adhikari, a LIGO team member and professor of physics at Caltech. “It’s akin to conversing in a quiet room instead of a bustling café.”

The April 25 detection, labeled S190425z, is believed to have occurred around 500 million light-years from Earth, which is two to three times farther than the first observed neutron star merger. Only the LIGO Livingston and Virgo facilities detected the gravitational waves from this event, as the Hanford observatory was offline. This incomplete detection leaves some questions about the event's precise origin, which spans about one-fourth of the sky.

Conversely, the April 26 event, known as S190426c, likely transpired approximately 1.2 billion light-years away. All three observatories captured its weaker signal, allowing scientists to narrow down its location to a 3 percent area of the sky.

What indicates this event is a merger of a neutron star and a black hole instead of just pairs of neutron stars? According to Chatziioannou, the answer lies in their masses. Neutron stars are generally less massive than black holes, and the mass estimates derived from the gravitational wave signals fall into a range that suggests a merger.

However, that’s largely what is known about the April 26 signals. Confirmation of their origins as a neutron star colliding with a black hole is necessary before scientists can determine the characteristics of these celestial bodies and the resulting cosmic entity. Chatziioannou mentions that her team will need time to analyze the gravitational wave data alongside other measurements, such as gamma rays and x-rays. Additional events like this would also provide more assurance that it’s not a false alarm. Currently, the likelihood of it being a neutron star-black hole merger is four times greater than it being simply a pair of neutron stars.

There are several theories regarding how this event may have unfolded. One theory, proposed by Shaon Ghosh, a postdoctoral research associate at the University of Wisconsin Milwaukee and a member of the LIGO team, posits a “co-evolving system.” In this scenario, two massive stars exist in a binary system, evolve, and eventually form a neutron star and a black hole. These compact objects emit gravitational waves, lose energy and angular momentum, and eventually coalesce. Another theory suggests a phenomenon called dynamical capture, where an unrelated neutron star and black hole drift too close together and begin to interact gravitationally until they merge.

Ghosh notes that since black holes lack a surface, the merger of a neutron star with a black hole is not a violent collision that ejects matter in all directions, but rather a gentle merging of the two bodies. “If the event indeed resulted from such a merger,” Ghosh states, “the black hole’s gravity may distort the neutron star, tearing it apart and creating a cosmic halo of matter around the black hole’s event horizon.”

Should confirmation be obtained, it would signify a groundbreaking discovery. The detection of gravitational waves has already been hailed as evidence supporting a crucial aspect of Einstein's theory of general relativity. Many scientists hope that such discoveries could illuminate a new realm of astrophysics. This event could serve as one of the first examples of that anticipated breakthrough.

“Any system involving a neutron star contains information about matter at extreme densities,” Chatziioannou observes. “By analyzing the gravitational wave data, we might glean insights into the properties of neutron star matter.” Ghosh adds that further observations will aid in our understanding of the physical effects occurring under the intense gravitational pull of black holes.

While it will take time for the LIGO and Virgo teams to analyze these findings thoroughly, if the initial hypotheses prove accurate, we may be on the verge of a significant shift in our comprehension of astrophysics in the universe.

The first video, We Just Found Neutron Stars Crashing Into Black Holes, discusses the recent findings in gravitational wave detections and their implications for astrophysics.

The second video, NASA | Colliding Neutron Stars Create Black Hole and Gamma-ray Burst, provides insights into the phenomena of neutron star collisions and their role in forming black holes and gamma-ray bursts.