Andromeda galaxy’s first spinning neutron star discovered
The Andromeda (or M31) galaxy is the closest galaxy to our own Milky Way and is a popular focus of study amongst astronomers. On a clear night and away from urban light pollution, it is possible to see Andromeda with the naked eye, and with powerful observatories, it is possible to look deep inside of Andromeda. As such, it has been extensively studied for decades by telescopes covering the whole electromagnetic spectrum. Despite it being a major focus of astronomers’ attention, one particular class of object has never been detected, until now that is: spinning neutron stars. A spinning neutron star is the small and extremely dense remains of a once-enormous star that exploded as a powerful supernova at the end of its natural life. They often spin very rapidly, are the smallest stellar objects known to science, and can sweep regular pulses of radiation towards Earth. They are a common occurrence in the Milky Way, with astronomers estimating that there are over 100 million of them alone, but have remained elusive in Andromeda. The EXTRAS (Exploring the X-ray Transient and Variable Sky) project was formed to undertake an unprecedented programme to search and characterise variable sources in the soft X-ray range of data collected by the XXM-Newton telescope. This includes a search for fast transients, missed by standard image analysis, as well as a search and characterisation of variability in tens of thousands of sources on a broad range of timescales. Through this process of systematically searching through the XXM-Newton archives, the EXTRAS astronomers have uncovered the signal of an unusual source from Andromeda fitting the profile of a fast-spinning neutron star. The neutron star in question, newly named 3XMM J004301.4+413017 spins every 1.2 seconds and appears to be feeding on a neighbouring star that orbits it every 1.3 days. Many neutron stars exist in such binary systems, which makes easier for astronomers to detect. They cannibalise their companion star, drawing gas from the companion into their magnetic fields. ‘We were expecting to detect periodic signals among the brightest X-ray objects in Andromeda, in line with what we already found during the 1960s and 1970s in our own Galaxy,’ commented Gian Luca Israel, one of the EXTRAS project astronomers, based at the Italian National Institute for Astrophysics. ‘But persistent, bright X-ray pulsars like this are still somewhat peculiar, so it was not completely a sure thing we would find one in Andromeda.’ The project team looked through archival data of Andromeda from 2000–13, but it wasn't until 2015 that they were finally able to identify this object in Andromeda’s outer spiral in just two of the 35 measurements studied. Whilst the precise nature of the system remains unclear, the data does imply that it is both an unusual and exotic find. European astronomers are now in a better position to uncover more objects like 3XMM J004301.4+413017 both with the XMM-Newton and with future missions such as the ESA’s next-generation high-energy observatory, ATHENA. The EXTRAS project began in September 2013 and will continue until January 2017. For more information please see: project website
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