Astronomers revealed this week that they have detected an intense radio emission from a black hole's radio-emitting jet. The emission was moving at more than 85% the speed of light, though it was also "quite variable." The observations, published this week in the journal Nature, are the first detection of radio-emitting jets from a stellar-mass black hole outside the Milky Way galaxy.
The emission is from an X-ray source that was discovered in January, when it flared and rapidly brightened in the Andromeda galaxy (M31). The ultraluminous X-ray source (ULX), as it's currently classified, is only the second ever seen in the Andromeda galaxy. Observatories and telescopes, including NASA's Swift space telescope, have been observing the object for nearly a year.
"There are four black hole binaries within our own galaxy that have been observed accreting at these extreme rates," said Matthew Middleton, an astronomer at the Anton Pannekoek Astronomical Institute. "Gas and dust in our own galaxy interfere with our ability to probe how matter flows into ULXs, so our best glimpse of these processes comes from sources located out of the plane of our galaxy, such as those in M31."
According to astronomers, the gas around black holes becomes compressed, it is heated to temperatures high enough to emit X-rays. At a certain point called an Eddington limit, the X-ray emissions become intense enough to push back gas that is spiraling into the object. Theoretically, this caps the rate at which a black hole can ingest matter.
"Black-hole binaries in our galaxy that show accretion at the Eddington limit also exhibit powerful radio-emitting jets that move near the speed of light," said Middleton.
The jets seen coming from the M31 ULX are special because they could represent a rare middle-of-the-road black hole of hundreds to thousands of solar masses. Black holes that have been detected generally are either "lightweight" and created by stars of up to a few dozen solar masses, or they are "heavyweight" and represent millions to billions of solar masses. The supermassive "heavyweight" black holes are generally found in the centers of galaxies, such as the one spotted last month that is 17 billion times more massive than the sun.
"The discovery of jets tells us that this particular ULX is a typical stellar remnant about 10 times the mass of the sun, swallowing as much material as it possibly can," said Middleton. "We may well find jets in ULXs with similar X-ray properties in other nearby galaxies, which will help us better understand the nature of these incredible outflows."
(Image courtesy Bill Schoening, Vanessa Harvey/REU program/NOAO/AURA/NSF/ESA/M. Middleton et al.)