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Neighbor Galaxy Has a Secret: Andromeda Hosts Massive Pulsar
NASA's Nuclear Spectroscope Telescope Array (NuSTAR) has likely unmasked a mysterious source of X-ray energy in the neighboring Andromeda galaxy — a powerful, fast-rotating pulsar.
Credit: NASA/JPL-Caltech/GSFC/JHU

Researchers have cracked a secret of the Milky Way's neighboring Andromeda galaxy: A mysterious X-ray source is likely a pulsar, the rapidly spinning corpse of a massive star. 

The researchers unmasked the strange source in Andromeda, which sits "only" 2 million light-years away, using data from NASA's Nuclear Spectroscopic Telescope Array (NuSTAR), a space-based telescope sensitive to high-energy X-rays. 

NuSTAR was able to pinpoint the source of the X-ray emission: an object called Swift J0042.6+4112, first cataloged by NASA's Swift satellite. The X-ray source is brighter than anything else in the galaxy at that wavelength, and the spectrum of light it releases looks a lot like pulsars in the Milky Way.

This pulsar is actually brighter in high-energy, shorter wavelength X-rays than all of Andromeda's black holes combined. "We didn't know what it was until we looked at it with NuSTAR," Mihoko Yukita, a postdoctoral researcher at Johns Hopkins and lead author of the study about the pulsar, said in a statement.

Pulsars are very dense — as a type of neutron star, they can pack a mass three times that of the sun into a space only a few miles across. The researchers think the pulsar is part of a binary system, in which a star orbiting the pulsar loses matter to the pulsar, which gets heated as it falls and emits energy as X-rays. 

Galaxies like the Milky Way, and Andromeda, have supermassive black holes at their centers, and matter falling into the galactic core generates X-rays as it gets heated to high temperatures. But that didn't explain the emission from Andromeda. Its powerful X-ray source was originally picked up in the 1970s, by the orbiting Einstein Observatory, though nobody knew what it was. Later, the Chandra X-ray Observatory detected it as well, as did European Space Agency's XMM-Newton. Those satellites, though, saw the lower-energy end of Swift J0042.6+4112's X-ray emissions. It took the observations of NuSTAR and additional data from Swift to show that those emissions all came from the same object — a pulsar. 

The study appeared in the Astrophysical Journal on March 22. 

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