Researchers have, for the first time, used data from flashing neutron stars to constrain what form of matter these dense objects, known as pulsars, might take. Although what we call neutron stars are made mainly of neutrons, mystery surrounds the true nature of the matter contained deep within them.

But in an upcoming paper in Physical Review Letters, Mark Alford and Kai Schwenzer of Washington University in the US say that the trick is to use data from millisecond pulsars.

Previous studies of the interior of neutron stars relied on pulsars' precursors – low-mass X-ray binaries (LMXBs). LMXBs exist in binary systems, accreting matter from their companion star until the partner gets entirely eaten up to create a pulsar spinning incredibly fast – once every millisecond or so. Unfortunately, LMXBs can only provide limited information about what kind of matter exists inside neutron stars.

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