For thousands of years, inhabitants of the southern hemisphere have gazed at two mysterious glows in the night sky. These cosmic objects, which we now know as the Magellanic Clouds, have fired the imagination and shaped the mythologies of local cultures for centuries. However, they remained unknown to Europeans until the time of the great geographical discoveries, when Portuguese sailors brought back the first accounts of these unusual ‘clouds’ in the southern sky. They owe their current name to Ferdinand Magellan, although the great sailor himself was one of many European explorers who spotted them.
Today we know that these bright spots in the southern sky are not fragments of the Milky Way, but two independent dwarf galaxies – our closest galactic neighbours. The larger of the two, the Large Magellanic Cloud, stretches between the constellations of Table Mountain and Golden Fish. Its smaller companion is located in the constellation Toucan. Scientists have discovered that the two objects are connected by a spectacular bridge of interstellar gas, known as the Magellanic Stream. The latest research confirms that both galaxies orbit our Milky Way, bound to it by invisible gravitational forces.
And it is in these cosmic neighbours that an international team of astronomers, led by scientists from the Astronomical Observatory of the University of Warsaw, has made a groundbreaking discovery. After more than 20 years of systematic observations as part of the OGLE project, the researchers have identified an entirely new class of X-ray emitting astronomical objects.
Although we associate X-rays mainly with medical diagnostics, the cosmos can also be a source of X-rays. As Dr Przemysław Mróz, lead author of the paper published in the prestigious Astrophysical Journal Letters, explains, this radiation can come from very hot objects, such as gas falling on a white dwarf, neutron star or black hole, or be created by the accelerated motion of charged particles [1].
Mróz P., Król K., Szegedi H., Charles P., Page K. L., Udalski A., Buckley D. A. H., Dewangan G., Meintjes P., Szymański M. K. Millinovae: A New Class of Transient Supersoft X-Ray Sources without a Classical Nova Eruption, The Astrophysical Journal Letters, Vol. 977, No. 2, 2024, L37. DOI: 10.3847/2041-8213/ad969b
In the Magellanic Clouds, the team discovered 29 unusual objects that can increase their brightness by up to 20 times during prolonged flares. Of particular interest was the object OGLE-mNOVA-11, which flared in November 2023. Observations with the South African Large Telescope (SALT) revealed the presence of partially ionised helium, carbon and nitrogen atoms, indicative of extremely high temperatures. Further investigations with the Swift X-ray Telescope confirmed that the object’s temperature reaches a dizzying 600,000 degrees Celsius and that its X-ray power is more than a hundred times that of the Sun’s total radiation power.
These fascinating objects, dubbed ‘milliarcseconds’ (because they are about a thousand times fainter than classical new ones), turned out to be binary systems consisting of a white dwarf and a subgiant – a star whose supply of thermonuclear fuel has run out. These stars are so close together that matter from the sub-giant flows to the surface of the white dwarf.
Scientists propose two possible explanations for the observed X-rays [1]: they may be produced directly as matter falls to the surface of the white dwarf or as a result of a thermonuclear explosion of the accumulated hydrogen layer. If the second hypothesis turns out to be true, millinities could be the key to understanding the origin of Type Ia supernovae – the cosmic ‘standard candles’ that allowed astronomers to discover the accelerating expansion of the Universe, for which the 2011 Nobel Prize in Physics was awarded.
Bibliography:[1] New sources of X-rays, Warsaw University https://www.uw.edu.pl/nowe-zrodla-promieniowania-rentgenowskiego/
