The Magellanic Clouds are two satellite galaxies that orbit the Milky Way. Between them, there’s a bridge of materials and stars, and among those, back in 1993, a pulsar was detected flaring up. For the next 26 years, the pulsar apparently went back to sleep but last November it produced another incredible flare, one that shone with a light of over 1 million Suns.
The degenerate star is known as RX J0209.6-7427. The extreme flare it released a few months ago revealed its true nature as an ultra-luminous X-ray pulsar (ULXP). Less than 10 of these objects are known in the universe. This is also the second closest ever discovered after one found a few years ago in our own Milky Way. The findings are reported in the Monthly Notices of the Royal Astronomical Society.
A pulsar is a special type of neutron star, one of the possible remnants of supernovae. Neutron stars are extremely dense, packing more than the mass of the Sun in a sphere with a radius of just 10 kilometers (6 miles). They are made by degenerate states of matter with incredible properties.
Some of these neutron stars can pulsate, hence the name pulsar, and among pulsars, there are those that rotate on their axis every few milliseconds. RX J0209.6-7427 is one of these, spinning 100 times per second. The pulsar is also surrounded by material that can spiral inward and hit the surface. When that happens a powerful flare is released.
The flare is a violation of the Eddington limit, the maximum luminosity a body (like a star) can achieve when there is a balance between gravity pulling in material and the radiant energy pushing it out. When this is surpassed incredible luminosities can be achieved. In fact, ULXP and the wider category of ultra-luminous X-ray sources can single-handedly outshine a galaxy with their flares.
“This is only the eighth ULXP detected so far, and the first one near the Magellanic Clouds,” lead author Amar Deo Chandra, from the Center of Excellence in Space Sciences India, said in a statement. “It raises the interesting possibility that a significant fraction of ultra-luminous X-ray sources may really be neutron stars accreting at super Eddington rates, rather than black holes as previously thought.”
The observations were conducted with AstroSat, India’s first UV/X-ray observatory launched in 2015.