A space object called Terzan 5 CX1 acts as one thing and then as something completely different.
In 2003, it looked exactly like a binary system consisting of a neutron star accreting material from a companion, a so-called low-mass X-ray binary dwarf.
But then, from 2009 to 2014, it dimmed to resemble a millisecond pulsar, a type of star that blinks bright radio waves in millisecond pulses as the star rotates.
Then, in 2016, the object's profile reverted to a low-mass X-ray double beam.
So what does this mean?
Terzan 5 CX1 lies about 19,000 light years away in a cluster of stars called Terzan 5 in the constellation Sagittarius. It was first discovered by the Chandra X-ray Observatory in 2003 when the object flashed brightly in X-rays.
This strange switch is believed to be related to the age of the stars. As pulsars age, they lose angular momentum; after all, they are 'dead' stars that no longer merge with their cores, so they gradually lose heat and slow down.
But when they are in a binary system, they can throw matter from their companion star from time to time. This heats up the pulsar again, restoring its angular momentum – giving the rotation a push back into a higher gear.
And studying another system in Terzan 5 can shed light on this process. IGR J17480-2446 is a low-mass X-ray double beam that was observed to cool after an accretion burst in 2010. Interestingly, the star's crust was still cooling 5.5 years after the outburst – much more slowly than other periodically accreting neutron stars.
This may be due to an unusually strong magnetic field; or perhaps this object is younger than other pulsars.
The study will be published in the Royal Astronomical Society's Monthly Notices and available on the arXiv website.
Sources: Photo: NASA / ESA / Hubble / F. Ferraro
