3 days ago
Something Deep in Our Galaxy Is Pulsing Every 44 Minutes. No One Knows Why.
Here's what you'll learn when you read this story:
ASKAP J1832-0911 is a long-period radio transient (LPT) object, which emits radio waves in periods of tens of minutes. But it is also the first LPT known to emit X-rays.
This mysterious object could take many forms, including a pulsar, a white dwarf star in a binary with a low-mass star, or a magnetar.
The object's properties don't exactly fit with any of those proposed options, however.
Deep in the galactic plane of the Milky Way, within a region of stars shrouded in gas and dust, one star is behaving like no other that has ever been observed before.
Stars can be as mysterious as they are mesmerizing. When scientists at NASA's Chandra X-Ray Observatory were making observations with the Advanced CCD Imaging Spectrometer (ACIS) X-ray imaging instrument, something bizarre appeared—a previously unidentified source of X-rays. Intense X-ray and radio pulses were coming from this source, which is known as ASKAP J1832-0911.
This extreme object is a long-period radio transient, or LPT—an astrophysical object whose brightness keeps changing. Few long-period radio transients, whose radio wave emissions vary over tens of minutes, are known. In fact, the first one was only spotted in 2022. The radio wave intensity of ASKAP J1832-0911 cycled every 44 minutes, and was exceptionally bright in radio at the time of observation. Most excitingly for scientists, LPTs had never before been observed emitting both X-rays and radio waves.
Astronomer Ziteng Wang from Curtin University in Australia—who led the team of researchers investigating ASKAP J1832-0911—wanted to find out more. The object was observed six months later with ACIS and the Follow-Up Telescope on board ESA's Einstein Probe. By then, its X-ray luminosity had decreased, and so had its radio emissions. But its existence alone is a breakthrough in itself.
'ASKAP J1832–0911 is currently the only LPT detected with (pulsed) X-ray emission — perhaps unsurprisingly, given its extreme radio brightness and the potential correlation between the radio and X-ray luminosities,' Wang and his team said in a study recently published in Nature.
This object is thought to be some sort of star, but exactly what kind of star remains debatable. The researchers believe ASKAP J1832–0911 to be compact and to have strong magnetic fields, which would align with the properties of either a magnetic white dwarf (the corpse of a star that has lost its outer layers of gas) or a pulsar (a rotating neutron star that is highly magnetized). But then, there is the issue of its unprecedented pulses. Its X-ray pulses alone are too variable to line up with the steady flash of a pulsar, and X-ray emissions from white dwarves also tend to be fainter and more stable than those produced by ASKAP J1832–0911.
So, if it isn't a pulsar or white dwarf, then what is it?
Well, it might be a binary white dwarf system—fast-spinning white dwarfs are sometimes found locked in binary systems with other low-mass stars. The radio emissions from these binaries are usually significantly weaker than those from ASKAP J1832–0911, but Wang suggests that the situation remains possible.
If not a binary white dwarf system, however, ASKAP J1832–0911 could be a magnetar. These isolated neutron stars have powerful magnetic fields, and the mystery object's properties (including varied pulses and a radio spectrum that is constantly changing)are in line with those of magnetars that emit strong radio waves. Magnetars also throw 'tantrums' with similar X-ray outbursts. But ASKAP J1832–0911 also differs from the typical presentation of magnetars—its quiescent X-ray luminosity (the opposite of its peak luminosity) doesn't line up with expected magnetar behavior, and its especially low luminosity when its rotation slows down doesn't match either. Older magentars are thought to act more like this, but cannot reach such bright radio extremes as ASKAP J1832–0911.
'It could host an unusual core-dominated magnetic field […] requiring a revision to models of magnetic field evolution in neutron stars,' Wang said. 'If ASKAP J1832–0911 is an old magnetar, explaining the radio emission challenges existent models.'
Whatever ASKAP J1832–0911 actually is, classifying it will demand consideration of the strength and frequency of both X-ray and radio emissions. For now, it just winks at us.
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