Almost seven years ago (September 14, 2015), researchers at the Gravitational wave observatory with laser interferometer (LIGO) detected gravitational waves (GW) for the first time. Their results were shared with the world. six months later and earned the discovery team the Nobel Prize in Physics Next year. Since then, a total of 90 signals created by binary systems of two black holes, two neutron stars, or one of each have been observed. This last scenario presents some very interesting opportunities for astronomers.
If a merger involves a black hole and a neutron star, the event will produce GW and a serious light display! Using data collected from the three neutron star-black hole mergers we’ve detected so far, a team of astrophysicists from Japan and Germany was able to model the entire process of a black hole colliding with a neutron star, including everything from the final orbits of the binary to the merger and post-merger phase. The results of it could help inform future surveys that are sensitive enough to study GW mergers and events in much more detail.
The research team was led by Kota Hayashi, a researcher at Kyoto University. Yukawa Institute for Theoretical Physics (YITP). He was joined by several colleagues from YITP and Toho University in Japan and the Albert Einstein Institute in the Max Planck Institute for Gravitational Physics (MPIGP) in Potsdam, Germany. The article describing his findings was led by YITP Prof. Koto Hayashi and recently appeared in the scientific journal Physical Review D.
In short, GWs are mysterious waves in space-time originally predicted by Einstein’s General Theory of Relativity. They are created whenever massive objects merge and create tidal disruptions in the very fabric of the Universe, which can be detected thousands of light-years away. To date, only three mergers involving a binary system consisting of a black hole and a neutron star have been observed. During one of these, GW170817, detected on August 17, 2017, astronomers detected an electromagnetic counterpart to the GWs it produced.
In the coming years, the most sensitive telescopes and interferometers are expected to see much more of these events. Based on the mechanics involved, scientists anticipate that neutron star-black hole mergers will include matter ejected from the system and a tremendous release of radiation (which could include short bursts of gamma rays). For their study, the team modeled what neutron star-black hole mergers might look like to test these predictions.
They selected two different model systems consisting of a rotating black hole and a neutron star, with the black hole set at 5.4 and 8.1 solar masses and the neutron star at 1.35 solar masses. These parameters were selected so that the neutron star would likely be ripped apart by tidal forces. The fusion process was simulated using the “Sakura” computer cluster at the MPIGP facilities. Department of Computational Relativistic Astrophysics. in a MPIGP Press releasedepartment director and co-author Masaru Shibata explained:
“We get information about a process that lasts one or two seconds; that sounds short, but in fact a lot happens during that time: from the final orbits and the disruption of the neutron star by tidal forces, the ejection of matter, to the formation of an accretion disk around the nascent black hole and increased ejection of matter in a jet. This high-energy jet is probably also the reason for the brief gamma-ray bursts, the origin of which remains a mystery. The simulation results also indicate that the ejected matter should synthesize heavy elements such as gold and platinum.”
The team also shared the details of their simulation in an animation (shown above) via the Max Planck Institute for Gravitational Physics. Youtube channel. On the left side, the simulation shows the density profile as blue and green outlines, magnetic field lines penetrating the black hole as pink curves, and matter ejected from the system as cloudy white masses. On the right side, the magnetic field strength of the merger is depicted in magenta, while the field lines appear as light blue curves.
In the end, their simulations showed that during the merger process, the neutron star is ripped apart by tidal forces within seconds. About 80% of the neutron star’s matter was consumed by the black hole in the first few milliseconds, increasing the black hole’s mass by an additional solar mass. In the next ten milliseconds, the neutron star formed a single-arm spiral structure, some of the matter was expelled from the system while the rest (02.-0.3 solar masses) formed an accretion disk around the black hole.
After the merger was complete, the accretion disk fell into the black hole, causing a focused jet-like stream of electromagnetic radiation and matter. This jet emanates from the poles, similar to what is often seen with Active Galactic Nuclei (AGN), and could result in a brief gamma-ray burst. What was especially surprising was that, while the simulations took two months to generate, the simulated merger took about two seconds! Said Dr. Kenta Kiuchi, the leader of the group in Shibata’s department that developed the simulation code:
“Such general relativistic simulations are very time consuming. That is why research groups around the world have so far focused only on short simulations. In contrast, an end-to-end simulation, such as the one we have now performed for the first time, provides a self-consistent picture of the entire process for given binary initial conditions that are defined once at the beginning.”
Long-term simulations also allow astronomers to explore the mechanism behind short-lived gamma-ray bursts (GRBs). In addition to being a transient phenomenon, like fast radio bursts (FRBs) that also last only seconds or milliseconds, GRBs are the most energetic phenomenon in the Universe and astronomers are eager to investigate them further. Looking ahead, Shibata and his colleagues are working on more complex numerical simulations to model neutron star mergers and their outcomes.
Neutron star mergers are also expected to include an electromagnetic contribution and short-lived gamma-ray bursts. This study serves to illustrate how the GW study has advanced by leaps and bounds in recent years and how observations are more sensitive and keep pace with improvements in computing and simulations. The result is advances in our understanding of the Universe occurring at an ever-increasing rate! Who knows what discoveries could be just around the corner?
Other readings: MPIGP, Physical Review D