A pulsing neutron star system has potentially been spotted in Andromeda, the nearest galaxy to our own Milky Way. Even though the galaxy has been studied closely over many decades, this type of system has never before been seen there.
"The well-known Andromeda galaxy has long been a source of exciting discoveries, and now an intriguing periodic signal has been detected by our flagship X-ray mission," said European Space Agency scientist Norbert Schartel, who works on the XMM-Newton space telescope team, and whose data revealed the object.
Neutron stars are the small, extremely dense remains of a once-massive star that exploded as a powerful supernova.
They often spin very rapidly pulsing out radiation as they turn and are viewed from afar in the way a lighthouse beacon appears to flash on and off as it rotates.
The precise nature of the Andromeda system remains unclear, but it could be binary, in which one of these "pulsars" cannibalises a neighbouring neutron star. There are many such binary neutron star systems in the Milky Way, but this would be the first seen in Andromeda.
A review of previous data from the XMM-Newton X-ray telescope shows the star spins every 1.2 seconds, and appears to be feeding on a neighbouring star that orbits it every 1.3 days.
"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," says Gian Luca Israel, from INAF-Osservatorio Astronomica di Roma, Italy, and one of the authors of the paper describing the results.
"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 scientists looked through archival data of Andromeda from 2000, but it wasn't until 2015 that they were finally able to identify the object in the galaxy's outer spiral in just two of the 35 measurements.
"It could be what we call a 'peculiar low-mass X-ray binary pulsar' – in which the companion star is less massive than our Sun – or alternatively an intermediate-mass binary system, with a companion of about two solar masses," says Paolo Esposito of INAF-Istituto di Astrofisica Spaziale e Fisica Cosmica, Milan, Italy.
He said more observations of the pulsar and its companion were needed to help determine the likely scenario.
Bill Condie is a science journalist based in Adelaide, Australia.
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