• The four "messengers" available to observe the universe are photons, neutrinos, cosmic rays and gravitational waves, according to our partner The Conversation.

  • However, observing the same astrophysical source via at least two messengers provides a more complete view of the physical mechanisms involved.

  • The analysis of this phenomenon was conducted by Pierre-Alexandre Duverne, doctoral student in multi-messenger astronomy at the University of Paris-Saclay.

Historically, visible light made it possible to observe the sky, first with the naked eye and then with a telescope.

More recently, radio telescopes and X-ray and gamma-ray detectors installed on satellites have enriched our understanding of the universe.

The instruments have evolved a lot and have become widely diversified in order to collect the most varied information possible.

But

ultimately

, it is the same physical object that is observed: the photon.

It is the particle that transports the light that we see, but also the X-rays from radios, when they are of higher energy, or even the microwaves with which we reheat dishes from the day before.

This is the big change that astronomy has been experiencing in recent years.

Indeed, it is now possible to observe astrophysical sources through other signals, providing new information, inaccessible by electromagnetic waves alone.

​4 "messengers" to study the universe

To date, four "messengers" are available to study the universe:

  • Photons

  • Neutrinos: neutral particles produced during nuclear reactions, particularly difficult to observe because of their very weak interaction with matter.

    The ANTARES and IceCube experiments search for astrophysical emissions of very energetic neutrinos

  • Cosmic rays: charged particles accelerated by “cosmic accelerators” to energies well above the best particle accelerator: CERN's LHC.

    They are detected on the ground, among others, by the HESS, Pierre Auger and soon CTA experiments.

  • Gravitational waves: space-time vibrations produced when a massive object is accelerated.

    Three instruments have been detecting these signals for some years: the two Ligo detectors built in the United States and Virgo in Italy.

Observing the same astrophysical source via at least two messengers makes it possible to have a more complete view of the physical mechanisms involved. This perspective constitutes the primary motivation of what is called multi-messenger astronomy.

The beginnings of multi-messenger astronomy

The very first multi-messenger observation took place in 1987, when a star located in a small satellite galaxy of the Milky Way exploded in a very bright supernova, observed by both conventional optical telescopes and neutrino detectors. .

These particles were emitted in very large numbers, a few hours before the explosion, when the star at the end of its life, having become unable to resist the gravity induced by its own mass, collapsed on itself.

But if the event marked the history of astrophysics, it is thanks to the proximity of the explosion (on the scale of the universe).

While supernovae are fairly common in the universe – several are discovered every week – the exploding star must be in our galaxy or its close vicinity for the neutrinos to be detectable.

It is estimated that a supernova only occurs once or twice a century in the Milky Way, which drastically limits the number of observations.

This is the major difficulty of multi-messenger astronomy: each of the messengers must be detectable on Earth, which requires particularly sensitive detectors.

This is why studies in this area have emerged only recently.

​Gravitational wave, gamma-ray burst and kilonova

It was in the summer of 2017 that everything changed for multi-messenger astronomy.

A little over a hundred million years ago, after a very long dance, the remains of two massive stars, called neutron stars, merged, emitting a gravitational wave.

It then spread to us, to be finally detected on August 17, 2017 by Ligo and Virgo.

During the collision, two other phenomena occurred.

On the one hand the emission of a narrow and symmetrical jet of gamma rays, photons of very high energy, called "gamma burst".

It arrived on Earth two seconds after the gravitational waves and was detected by the Fermi and Integral satellites.

LIGO-Virgo collaboration measurements of signal GW170817 © LIGO Scientific Collaboration and Virgo Collaboration / Wikimedia CC BY-SA 4.0

On the other hand, a fraction of the matter constituting the neutron stars was ejected in the form of heavy atomic nuclei.

These, after having been rendered unstable by the capture of the neutrons projected by the fusion, disintegrate by radioactivity, which heats the surrounding environment.

Causing the emission of a blue light during the first two to three days following the collision, then red when the environment cools.

This second phenomenon, called kilonova, was detected about ten hours after the gravitational wave by the Swope terrestrial telescope, triggering the largest astronomical monitoring campaign in history, involving nearly 70 observatories.

For several weeks, the community of astronomers was in turmoil and the harvest of results was colossal.

​Grandma, international network of telescopes

At the end of summer 2017, the Ligo and Virgo collaborations stop their observations in order to improve their instruments and make them even more sensitive.

Time-frequency representation of the gravitational wave signal detected on August 17, 2017 by Ligo and Virgo.

The signal, called “chirp”, is visible in the Ligo-Handford (top), Livingstone (middle) detectors, but not in Virgo (bottom).

Its shape is characteristic of a very dense fusion of stars © LIGO Scientific Collaboration and Virgo Collaboration / Wikimedia CC BY-SA 4.0

In doing so, gravitational waves become excellent candidates for regular multi-messenger observations, since for the next data run named "O3", scheduled for the period from April 2019 to March 2020, between one and ten new fusions of neutron stars are expected.

However, to maximize the chances of a new sighting, astronomers had to prepare well ahead of O3.

Detecting a new kilonova in fact poses two major problems: on the one hand, for reasons linked both to the detectors and to the way in which the data they produce are analyzed, it is difficult to obtain precisely the location of the sky from where the source emits the wave, which greatly complicates the discovery of a kilonova.

On the other hand, it must be found in the hours following the detection of the gravitational wave in order to understand the physical processes at work and refine as much as possible the models describing the collisions of neutron stars.

That's why, in 2018, telescopes around the world pooled some of their resources to create the Grandma Network, which can observe large portions of the sky at all times, thus addressing both issues.

The kilonova associated with the GW170817 event observed by the Hubble telescope.

The bottom left image is the kilonova observed on August 22, 2017, five days after its discovery, the following images were taken a few days later, and it is clearly visible that the phenomenon is weakening © NASA and ESA: A. Levan ( U. Warwick), N. Tanvir (U. Leicester), A. Fruchter and O. Fox (STScI) - CC BY-NC-ND (via The Conversation)

​Fusion of a neutron star and a black hole

The O3 campaign was very rich from the point of view of gravitational waves: at least one new neutron star merger, unfortunately too far from Earth and too badly located in the sky to associate a kilonova or a gamma-ray burst with certainty .

There was also the first observation of the merger of a neutron star and a black hole, for which we can also hope to observe a kilonova.

During this year of observations, a gravitational wave signal was detected on average every week, and Grandma tracked most of them with her telescopes.

Despite this, no electromagnetic counterpart has been found, either by Grandma or other teams.

Grandma, a global telescope network.

Each of the dots on the map indicates a telescope used during O3 © Collaboration Grandma, CC BY-NC-ND (via The Conversation)

​Prospects for the coming years

Currently, the Ligo and Virgo collaborations are improving their detectors and preparing the O4 gravitational wave observation campaign, which promises to be particularly intense since current estimates are around one gravitational wave detection per day, against only one per week during O3.

One of the big challenges will therefore be to select and monitor only the events most likely to produce a kilonova or a gamma-ray burst.

Also, many telescopes, including Grandma's, have been upgraded in light of O3's results to increase the chance of detecting kilonova.

The start of the O4 campaign is scheduled for the second half of 2022.

Our "ASTROPHYSICS" file

Later that same year, the Vera Rubin Observatory, a telescope more than 8 meters in diameter, will make its first observations of the sky in search of transient astrophysical objects and phenomena.

And among its discoveries, astronomers hope to find a few kilonovae.

The era of multi-messenger astronomy has only just begun… and it promises to be radiant!

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This analysis was written by Pierre-Alexandre Duverne, doctoral student in multi-messenger astronomy at the University of Paris-Saclay.


The original article was published on The Conversation website.

​Declaration


of interests

As part of his doctorate, Pierre-Alexandre Duverne is working with the GRANDMA, LIGO and Virgo collaborations mentioned in this article.

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