Andromeda's spinning neutron star | European Space Agency

Decades of searching in the Milky Way’s nearby ‘twin’ galaxy
Andromeda have finally paid off, with the discovery of an elusive breed ofstellar corpse, a neutron star, by ESA’s XMM-Newton space telescope.


Andromeda, or M31, is a popular target among astronomers.
Under clear, dark skies it is even visible to the naked eye. Its proximity and
similarity in structure to our own spiral galaxy, the Milky Way, make it an
important natural laboratory for astronomers. It has been extensively studied
for decades by telescopes covering the whole electromagnetic spectrum.

Despite being extremely well studied, one particular class of
object had never been detected: spinning neutron stars.

Neutron stars are the small and extraordinarily dense remains
of a once-massive star that exploded as a powerful supernova at the end of its
natural life. They often spin very rapidly and can sweep regular pulses of
radiation towards Earth, like a lighthouse beacon appearing to flash on and off
as it rotates.

These ‘pulsars’ can be found in stellar couples, with the
neutron star cannibalizing its neighbor. This can lead to the neutron star
spinning faster, and to pulses of high-energy X-rays from hot gas being
funneled down magnetic fields on to the neutron star.

Binary systems hosting a neutron star like this are quite
common in our own Galaxy, but regular signals from such a pairing had never
before been seen in Andromeda.

Now, astronomers systematically searching through the
archives of data from XMM-Newton X-ray telescope have uncovered the signal of
an unusual source fitting the bill of a fast-spinning neutron star.

It spins every 1.2 seconds, and appears to be feeding on a
neighboring 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, 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.

“We looked through archival data of Andromeda spanning
2000–13, but it wasn’t until 2015 that we were finally able to identify this
object in the galaxy’s outer spiral in just two of the 35 measurements.

” While the precise nature of the system remains unclear, the
data imply that it is unusual and exotic.

“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.

“We need to acquire more observations of the pulsar and its
companion to help determine which scenario is more likely.”

“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,” adds Norbert Schartel, ESA’s XMM-Newton project
scientist.

“We’re in a better position now to uncover more objects like
this in Andromeda, both with XMM-Newton and with future missions such as ESA’s
next-generation high-energy observatory, Athena.”