The first pulsar was discovered in 1967 when Jocelyn Bell detected a celestial pulse signal at a frequency 81.5 MHz radio at Mullard Radio Astronomy Observatory near Cambridge, UK. Quickly, in the year that followed, the astrophysicists Thomas Gold and Franco Pacini understand and make it known that these are the neutron stars theorized as early as the 1930s. These stellar corpses left by the explosion of a supernova are indeed neutron stars, stars able to gather in a sphere of a few tens of kilometers in diameter the mass of the Sun.
L’collapse gravity of a massive star which leads to the birth of a neutron star produces it in a state of rotation at one speed high angular, inherited from the initial star with a period that can be of the order of a few tens of milliseconds (the bulk of the pulsar population has a rotation period centered on one second). For such a dense object, the laws of mechanics imply that it must possess a cinematic moment colossal which can only be changed with great difficulty by an external couple of forces. However, it turns out that the neutron star also inherits the magnetic field of its parent star which is also amplified, so that by a relativistic effect this magnetic field also behaves like a electric field on the surface of the neutron star, which can accelerate charges. It can be shown that in the end, electrodynamic processes lead to the existence of theepisode of a bundle ofelectromagnetic waves collimated.
A short presentation of pulsars. To obtain a fairly accurate French translation, click on the white rectangle at the bottom right. The English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Translate automatically”. Choose “French”. © NASA Goddard
From pulsars to supermassive black hole collisions
A pulsar therefore behaves like a kind of beacon which manifests itself in the form of a series of very stable radio pulses in a radio telescope if the beam passes through the Earth. For a time, some pulsars were even more stable natural clocks than the first ones. atomic clocks, so that one could think of using them as sort of beacons well characterized by their periodicity. These tags allow a priori to make precise interstellar or interplanetary navigation. Better, from the 1970s, we knew that we could use it to locate ourselves in the Milky Wayand this is why the position of the Earth has been indicated in relation to certain pulsars on the famous gold plate of the Pioneer probes. We proceeded in the same way for the famous Golden Record Voyager probes. We thus know in our Galaxy several thousand pulsars distant from the Sun from a few thousand to a few tens of thousands oflight years at most.
However, since the 1970s also, we had understood that we could use the extraordinary stability of the pulses of pulsars to highlight the close passage of the Solar system ofgravitational waves very low frequency and therefore very large wavelengthsgreatly exceeding the size of the Solar System.
Indeed, during this passage, a set of pulsars observable on Earth will see the arrival times of the pulses vary due to the passage of these waves, so that we can, in theory, highlight their existence and their characteristics. This is what the members of the NANOGrav collaboration (North American Nanohertz Observatory for Gravitational Waves), as Futura already explained in previous articles to which we refer you for more details and also to the excellent PBS Space Time video, below.
In fact and if we want to be more explicit, the waves detectable in this way are the gravitational waves of supermassive black holes binaries that can form on the occasion of merger of galaxies and more generally of the background stochastic gravitational waves of the observable cosmos.
Many strategies exist to detect gravitational waves of various frequencies which are as many windows open on different astrophysical and cosmological phenomena. To obtain a fairly accurate French translation, click on the white rectangle at the bottom right. The English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Translate automatically”. Choose “French”. © PBS SpaceTime
From radio pulsars to gamma pulsars
Just recently, an international team of astrophysicists including Aditya Parthasarathy and Michael Kramer from the Max Planck Institute for Radio Astronomy in Bonn, Germany, reported that the same game could be played not only with radio waves so-called millisecond pulsarsbut also with their emissions in the field of gamma rays, also periodicals. The thesis is developed in an article published in Science and of which an open-access version is available on arXiv.
These are nothing less than gravitational waves with wavelengths of several hundreds of thousands of billions of kilometers that the researchers believe they can highlight by reviewing the data already collected for a little over a decade by the Fermi gamma space telescope. In fact, it would take five more years, but the signals already recorded concerning a hundred pulsars already inspire optimism in astrophysicists.
Ultimately, the two detection methods, with radio and gamma waves, will therefore complement each other, giving much more robust results. This is all the more true as the photons gamma pass much more easily through the plasma of the interstellar medium and that they can therefore provide a cleaner signal and less disturbed by the turbulence plasma and its transmission properties of electromagnetic waves at a given frequency.
Researchers using NASA’s Fermi Gamma-Ray Space Telescope have discovered the first gamma-ray pulsar in a galaxy other than our own. The object sets a new record for the brightest known gamma pulsar. The pulsar sits on the outskirts of the Tarantula Nebula in the Large Magellanic Cloud, a small galaxy orbiting our Milky Way and located 163,000 light-years away. The gamma ray pulses from J0540-6919 are 20 times the intensity of the previous record holder, the famous Crab Nebula pulsar. Yet they have roughly similar levels of radio, optical, and X-ray emission. Considering these differences will guide astronomers toward a better understanding of the extreme physics at work in young pulsars. To obtain a fairly accurate French translation, click on the white rectangle at the bottom right. The English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Translate automatically”. Choose “French”. © NASA Goddard
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