Almost seven years ago (September 14th, 2015), researchers at the Laser Interferometer Gravitational-wave Observatory (LIGO) detected gravitational waves (GWs) for the first time. Their results were shared with the world six months later and earned the discovery team the Noble Prize in Physics the following year. Since then, a total of 90 signals have been observed that were created by binary systems of two black holes, two neutron stars, or one of each. This latter scenario presents some very interesting opportunities for astronomers.
If a merger involves a black hole and neutron star, the event will produce GWs and a serious light display! Using data collected from the three black hole-neutron star mergers we’ve detected so far, a team of astrophysicists from Japan and Germany was able to model the complete process of the collision of a black hole with a neutron star, which included everything from the final orbits of the binary to the merger and post-merger phase. Their results could help inform future surveys that are sensitive enough to study mergers and GW events in much greater detail.
The research team was led by Kota Hayashi, a researcher with Kyoto University’s Yukawa Institute for Theoretical Physics (YITP). He was joined by multiple colleagues from YITP and Toho University in Japan and the Albert Einstein Institute at the Max Planck Institute for Gravitational Physics (MPIGP) in Postdam, Germany. The paper that describes their findings was led by YITP Prof. Koto Hayashi and recently appeared in the scientific journal Physical Review D.
The mergers of compact objects discovered so far by LIGO and Virgo (in O1, O2, and O3a). Credit: LIGO Virgo Collaboration / Frank Elavsky, Aaron Geller / Northwestern
To recap, GWs are mysterious ripples in spacetime originally predicted by Einstein’s General Theory of Relativity. They are created whenever massive objects merge and create tidal disruptions to the very fabric of the Universe, which can be detected thousands of light-years away. To date, only three mergers have been observed involving a binary system consisting of a black hole and a neutron star. During one of these – GW170817, detected on August 17th, 2017 – astronomers detected an electromagnetic counterpart to the GWs it produced.
In the coming years, telescopes and interferometers of greater sensitivity are expected to see much more from these events. Based on the mechanics involved, scientists anticipate that black hole-neutron star mergers will include matter ejected from the system and a tremendous release of radiation (which might include short gamma-ray bursts). For their study, the team modeled what black hole-neutron star mergers would 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 was likely to be torn apart by tidal forces. The merger process was simulated using the computer cluster “Sakura” at the MPIGP’s Department of Computational Relativistic Astrophysics. In an MPIGP press release, Department director and co-author Masaru Shibata explained:
“We get insights into a process that lasts one to 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 the tidal forces, the ejection of matter, to the formation of an accretion disk around the nascent black hole, and further ejection of matter in a jet. This high-energy jet is probably also a reason for short gamma-ray bursts, whose origin is still mysterious. 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 contours, the magnetic field lines that penetrate the black hole are shown as pink curves, and the 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 torn apart by tidal forces in a matter of 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 following ten milliseconds, the neutron star formed a one-armed spiral structure, part of the matter was ejected 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 (AGNs), and could result in a short gamma-ray burst. What was especially astounding was that while the simulations took two months to generate, the simulated merger lasted about two seconds! Said Dr. Kenta Kiuchi, the group leader in Shibata’s department who developed the simulation code:
“Such general relativistic simulations are very time-consuming. That’s 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.”
Artist’s illustration of two merging neutron stars. The narrow beams represent the gamma-ray burst, while the rippling spacetime grid indicates gravitational waves. Credit: National Science Foundation/LIGO/Sonoma State University/A. Simonnet
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 for only seconds or milliseconds, GRBs are the most energetic phenomenon in the Universe, and astronomers are keen to investigate them further. Looking ahead, Shibata and his colleagues are working on more complex numerical simulations to model the merger of neutron stars and what results.
The merger of neutron stars is also expected to include an electromagnetic contribution and short-lived gamma-ray bursts. This study serves to illustrate how the study of GW has advanced by leaps and bounds in recent years and how more sensitive observations and keeping pace with improvements in computing and simulations. The result is breakthroughs in our understanding of the Universe that occur at an ever-increasing rate! Who knows what discoveries might be right around the next corner?
Further Reading: MPIGP, Physical Review D