What's Geminga- things behind naming

 In this post, I would like to briefly talk about a long-standing mysterious object, and then I will also say a few words about why the blog was named this way.

Geminga was discovered by the SAS-2 satellite in 1973 as a significant gamma-ray source in the direction of the Gemini constellation, near the Crab Nebula (see CGRO image, Figure 4). Since the detectors were still inaccurate at the time, it was not possible to determine its exact celestial position for a long time, so its identity could only be guessed at. It is named after its discoverer, the Italian Giovanni F. Bignami (1944-2017), which is an abbreviation of Gemini Gamma-ray source. On the other hand, "ghè minga" in the Milanese Lombard dialect means: it is not there, it does not exist (an interesting addition is that Bignami himself was born near Milan, studied in the city and even played a role in local political life, so he was closely connected to the region). The name was made official by the International Astronomical Union (IAU) on April 4, 2022. The naming is also apt because Geminga could not be identified for almost twenty years: in 1975, further observations were made with the COS-B, and then the Einstein and Uhuru X-ray astronomy satellites. 

In 1983, Bignami and his colleagues in Milan examined X-ray images from the Einstein Observatory, which they determined to be a previously unknown object. A few years later, images were also taken of it with large ground-based telescopes (Palomar, CFHT, NTT), on which several features were visible. The most interesting of them was a faint bluish "star" - its brightness is 25.5 magnitude, which is about 100 million times fainter than stars that are still visible to the naked eye (the spot marked on the images, Figure 2). Plus, it was noticeable that the latter shape was moving quickly compared to the other stars.   

Finally, ROSAT broke the ice: they showed that Geminga emits signals in X-ray light that repeat about every 2 milliseconds (this "pulsation" was subsequently detected in the SAS-2 and COS-B data). It turned out that the object is a young stellar corpse, a rapidly rotating neutron star, or pulsar. It may have been formed about 300,000 years ago by the violent explosion of a massive star (supernova), and with a distance of about 800 light years, it is the closest such celestial body. Later, in more detailed X-ray images, a U-shaped formation is visible around the pulsar (e.g. in the image of the XMM-Newton space telescope, Figure 3). 

Observations in the gamma-ray range (by the HAWC observatory in Mexico and the AMS-2 and PAMELA instruments in space) have revealed that we are dealing with a shock wave front: the magnetic field of the fast-moving pulsar makes its way through the interstellar material, and the front is formed where the two meet. The Fermi Gamma-ray Space Observatory discovered a large, glowing spherical region (so-called halo) around Geminga: this is created when electrons and positrons (the antiparticles of the former) accelerated by the pulsar's magnetic field collide with particles (photons) of starlight, which gives the latter more energy, so that - at least some of them - they will be detected as gamma rays, of course at the expense of the "other half" (this interaction is called inverse Compton scattering/effect by physicists). At the same time, AMS-2 also revealed that Geminga alone is responsible for 20% of the positrons in cosmic rays, the fast particle shower that constantly bombards our Earth from all over space (by the way, there is another strong positron-emitting pulsar nearby, but more on that later...).

Geminga is (apparently) located near the star Alhena (gamma Geminorum). But before anyone dares to look for this mysterious celestial body from the end of their garden on a clear winter night, I have some bad news: with its aforementioned magnitude of 25.5, it is beyond the reach of amateur telescopes. It appears only as a faint dot in the attached image from the Hubble Space Telescope (bottom left in Figure 1), so it probably wouldn't be noticeable in long-exposure amateur photographs either. However, for informational purposes, I will provide a map with which we can roughly delimit its location.

Although the object itself is special, it is not unique: since Fermi, many pulsars have been identified that have similar properties: for example, unlike most rotating neutron stars, Geminga is a so-called radio-silent pulsar, i.e. it shows no or only a slight fluctuation in radio waves (the same is true for its visible light). That is why Geminga remains an important experimental subject in high-energy astrophysics, since we do not know enough about either neutron stars or cosmic rays. And since positrons are the simplest components of antimatter, they can also contribute to the study of the latter.

Finally: why did Geminga become the name of the blog? The answer: the uniqueness of the expression, its melodiousness (as is so typical of the Italian language), and the fact that it is easy to remember. I hope that the aspects I have indicated will help my readers to find it again and again and follow the blog after the first quick "passage". And I will try to share content that is interesting, useful, and understandable. If you have any questions, I will try to answer them. As for Geminga, we may well see his name in some posts. 

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