Using all senses: Listening to gravitational waves and watching the light from binary neutron star mergers

Published in Astronomy
Using all senses: Listening to gravitational waves and watching the light from binary neutron star mergers
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A breakthrough in multi-messenger astronomy happened on the 17th of August 2017, namely the event GW170817: the detection of gravitational waves, tiny ripples in the fabric of spacetime, and the observation of light from the same merger of two neutron stars. Although more than six years ago, the wealth of information provided by this observation still allows scientists all around the world to understand how nuclear matter behaves under conditions unreachable on the Earth using multi-messenger studies. But what does the word "multi-messenger" mean?

Let's say we want to know more about the properties of any object, e.g., snooker balls; how should we proceed? We can investigate it using different senses. When two snooker balls bump into each other, we can see the collision and hear its distinct sound. And instead of looking at one snooker ball, we can study a few of them: the one on the shelf, the one sitting on the table, and the one in the pocket. We have a more complete picture by gathering all these pieces of information. For us, the snooker ball becomes a neutron star, and we "look" at them with telescopes and "hear" them with gravitational-wave detectors to understand how nuclear matter behaves under conditions unreachable on the Earth.

Before our software was developed, people had been conducting multi-messenger studies "piece-by-piece". First, you listen to the snooker balls colliding with your eyes being closed and then look at them colliding with earmuffs, only to combine the two signals afterward. But now, with our newly developed software, Nuclear Physics and Multi-messenger Astrophysics (NMMA), we can look into collisions with our eyes and ears! NMMA allows us to analyze the binary neutron-star mergers' light and gravitational-wave "sound" simultaneously. In addition, we can add other pieces of information from radio and X-ray observations of neutron stars, e.g., from the Neutron Star Interior Composition ExploreR, NICER, theoretical calculations of neutron-star matter, and from heavy-ion collisions experiments conducted in laboratories on Earth.

By simultaneously analyzing all these puzzle pieces at once, we can avoid making assumptions and approximations when looking  "piece-by-piece". As a result, we gain one of the most robust pictures of the neutron star. In fact, our results on the radius of a neutron star with 1.4 solar masses have less than 4% uncertainty. This is a factor of two improvement over previous results with the “piece-by-piece” process.

To enable this ambitious approach, we have used numerous computational techniques. One good example is calculating the light generated by a binary neutron-star merger, a.k.a., a kilonova. To simulate the light created, scientists previously have used the so-called “radiative transfer” technique, which simulates the motion of millions of photons bouncing around the matter ejected by the merger. Such simulations are time-consuming and can take 500 hours on a single computer to generate a single prediction, while we need millions of them for the analysis! To speed things up, we have simulated several predictions with varying input parameters and placed them on a grid. But of course, there is a lot of unknown space between the grid points, and we do not have the predictions for those! To fix this, we have used a machine learning technique, a neural network, to connect the dots and fill up the space. We can then calculate the predicted light for arbitrary input parameters in less than a second!

With the ongoing LIGO-Virgo-KAGRA collaboration's observation run, we are impatiently waiting for the next "GW170817". Once that is detected, we will "unleash" NMMA on it to further unlock the mystery of the neutron stars and the secret of the cosmos.

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Compact Astrophysical Objects
Physical Sciences > Physics and Astronomy > Astronomy, Cosmology and Space Sciences > Astrophysics > Compact Astrophysical Objects
Multimessenger Astronomy
Physical Sciences > Physics and Astronomy > Astronomy, Cosmology and Space Sciences > Astronomy, Observations and Techniques > Multimessenger Astronomy
Astronomy, Observations and Techniques
Physical Sciences > Physics and Astronomy > Astronomy, Cosmology and Space Sciences > Astronomy, Observations and Techniques

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