Modeling The Merger Of A Black Hole With A Neutron Star And The Subsequent Process In A Single Simulation

Modeling the merger of a black hole with a neutron star and the subsequent process in a single simulation

Using supercomputer calculations, scientists at the Max Planck Institute for Gravitational Physics in Potsdam and from Japan show a consistent picture for the first time: They modeled the complete process of the collision of a black hole with a neutron star. In their studies, they calculated the process from the final orbits through the merger to the post-merger phase in which, according to their calculations, high-energy gamma-ray bursts may occur. The results of their studies have now been published in the journal Physical Review D. Almost seven years have passed since the first detection of gravitational waves. On September 14, 2015, the LIGO detectors in the U.S. recorded the signal of two merging black holes from the depths of space.

Since then, a total of 90 signals have been observed: from binary systems of two black holes or neutron stars, and also from mixed binaries. If at least one neutron star is involved in the merger, there is a chance that not only gravitational-wave detectors will observe the event, but also telescopes in the electromagnetic spectrum.

When two neutron stars merged in the event detected on August 17, 2017 (GW170817), about 70 telescopes on Earth and in space observed the electromagnetic signals. In the two mergers of neutron stars with black holes observed so far (GW200105 and GW200115), no electromagnetic counterparts to the gravitational waves were detected. But when more such events are measured with the increasingly sensitive detectors, the researchers expect electromagnetic observations here as well. During and after the merger, matter is ejected from the system and electromagnetic radiation is generated. This probably also produces short gamma-ray bursts, as observed by space telescopes.

For their study, the scientists chose two different model systems consisting of a rotating black hole and a neutron star. The masses of the black hole were set at 5.4 and 8.1 solar masses, respectively, and the mass of the neutron star was set at 1.35 solar masses. These parameters were chosen so that the neutron star could be expected to be torn apart by tidal forces.

“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,” says Masaru Shibata, director of the Department of Computational Relativistic Astrophysics at the Max Planck Institute for Gravitational Physics in Potsdam. “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.”

What happens during and after the merger?

The simulations show that during the merger process the neutron star is torn apart by tidal forces. About 80% of the neutron star matter falls into the black hole within a few milliseconds, increasing its mass by about one solar mass. In the subsequent about 10 milliseconds, the neutron star matter forms a one-armed spiral structure. Part of the matter in the spiral arm is ejected from the system, while the rest (0.2–0.3 solar masses) forms an accretion disk around the black hole.

When the accretion disk falls into the black hole after the merger, this causes a focused jet-like stream of electromagnetic radiation, which could ultimately produce a short gamma-ray burst.

Seconds-long simulations

It took the department’s cluster computer “Sakura” about 2 months to solve Einstein’s equations for the process that takes about two seconds. “Such general relativistic simulations are very time-consuming.

That’s why research groups around the world have so far focused only on short simulations,” explains Dr. Kenta Kiuchi, group leader in Shibata’s department, who developed the code. “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.”

Moreover, only with such long simulations the researchers can explore the generation mechanism of short gamma-ray bursts, which typically last one to two seconds.

Shibata and the scientists in his department are already working on similar but even more complex numerical simulations to consistently model the collision of two neutron stars and the phase after the merger.

The Very Beginnings of the History of Astronomy

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Astronomy in Ancient History

The Very Beginning

Since the beginning of time, humans have looked up at the stars and wondered... But the first documented instance of actual astronomical observations dates all the way back to the Assyro-Babylonians in 1000 BCE. These clever ancient people collected data on celestial bodies and recorded their periodic motions--quite impressive when you consider that the ancient Assyro-Babylonians did not have telescopes or really anything besides their eyes to observe the night sky.

Ancient Greece

Many ancient civilizations would continue to observe the stars, but it would be the Ancient Greeks who first attempted to use astrometry to estimate the location of celestial bodies in the sky. Copernicus is most well-known for his theory of heliocentrism, but as far back as the third century BCE, some Greek astronomers believed in the heliocentric system. Aristarchus of Samos was one such supporter, and he managed to use trigonometry to assess the relative distance of the Sun and the Moon from Earth. His measurement was not very precise, with him claiming the Sun was 18-20 times the distance of the Moon from Earth (current data puts that number at about 400 times more), but he definitely was on the right track.

A century later Greek astronomer Hipparchus of Nicaea created the first stellar catalogue using the ancient Babylonian practice of dividing a circle into 360 degrees and each degree into 60 arc minutes. This original catalog listed the positions of 850 stars to the accuracy of one degree--this might not seem so impressive today, but if you consider he was able to do this based on naked-eye observations and rudimentary gnomons, astrolabes, and armillary spheres. It's also thanks to Hipparchus that we have a magnitude system for describing the brightness of stars.

The Rest is Ancient History

It would be impossible to list every ancient astronomer who observed something important to astronomy, but needless to say, astronomers from ancient civilizations were all extremely intelligent individuals who collected data and created systems that are still in wide use today.

How To Prepare To Go Stargazing

Preparing To Go Stargazing

So you want to go stargazing...and you have the perfect night and location picked out...the question is: What do you bring? What do you wear? Should you bring food? Drinks? Chairs? A backpack?

Well, you've come to the right place. Prepare to have all your questions answered here!

Clothing

Generally, the best nights for stargazing are colder ones or you'll be up at higher altitudes in the middle of the night, so dress cozily! Check the weather forecast before heading out and dress appropriately, with a nice, warm jacket, pants (shorts are probably a no-no), a beanie, and gloves depending on where you're going. It never hurts to have backup extra layers stored in the car as well.

Stargazing Materials

Obviously, bring your telescope! If you don't have one, no worries, you can bring binoculars, borrow a telescope from your nearby observatory, or just go watch the stars with your naked eye--I promise it won't be any less breathtaking.

But if you're planning on bringing your telescope, make sure to bring a beach towel or something else to place your telescope on--a plastic tub as a base works well too for telescopes that don't have tripods. Also, make sure to have something handy to clean your lens with, just in case it gets dusty or windy.

Lights

It's best to avoid looking at your phone or any white lights to help your eyes adjust to the darkness and see the stars better, so pack a red light torch and activate red light on your phone screen so if you need to check your phone for any reason, or to access an astronomy app, you don't blind your eyes with the white light.

Food and Drinks

This is all based on preference, but it's always fun to have a small campfire and roast s'mores while drinking hot chocolate. Depending on how long you plan to be stargazing, prepare drinks (have a few water bottles on hand just in case) and some snacks and have a good time talking, watching the stars, and snacking with others.

Other Essentials

Make sure to have extra power chargers--portable batteries, power tanks, a pack of batteries, etc.--just in case anything runs out of power, especially if you're in a remote location.

A first-aid kit is important because you never know what might happen or when someone will need a band-aid. Keep a small first-aid kit in your car, stocked with (at the very least) band-aids (large and small size), Neosporin, gauze, and clean anti-bacterial wipes.

If it's summertime, it's probably also a smart idea to invest in some mosquito or bug spray, or get bug-repelling bracelets to keep the bugs from spoiling your night.