An international team of researchers using ESA’s XMM-Newton Space Telescope has found the first evidence of heavy atoms in jets emitted by a stellar-mass black hole.
This is a model of the black hole system with jets. Image credit: J. Miller-Jones, ICRAR / R. Hynes.
Stellar-mass black holes are often found feasting on material from a companion star. Matter flows from the star towards the black hole, circling in a disc around it with a temperature so high that it emits X-rays.
The black hole can be a fussy eater: instead of swallowing all of the material, it sometimes pushes a fraction of it away in the form of two powerful jets of particles. Because these jets release mass and energy into the surroundings, the black hole has less material to feed on.
By studying the composition of the jets, scientists can learn more about the feeding habits of black holes.
Observations at radio wavelengths have already found that black hole jets contain electrons moving at close to the speed of light. But, until now, it was not clear whether the negative charge of the electrons is complemented by their anti-particles, positrons, or rather by heavier positively-charged particles in the jets, like protons or atomic nuclei.
In a new study, published online today in the journal Nature, the scientists observed a black hole binary system called 4U1630-47.
The black hole was known to be active, but radio observations did not show any jets, and the X-ray spectrum didn’t reveal anything unusual. However, a few weeks later, the team took another look and this time saw radio emissions corresponding to the sudden appearance of these jets, and even more interestingly, lines had appeared in the X-ray spectrum – the tell-tale signature of ordinary atoms – around the black hole.
“Intriguingly, we found the lines were not where they should be, but rather were shifted significantly,” said study second author Dr James Miller Jones of the International Center for Radio Astronomy Research.
The same effect occurs when a siren from a vehicle changes pitch as it moves towards or away from us, as the sound wave is shortened or lengthened by the movement.
“It led us to conclude the particles were being accelerated to fast speeds in the jets, one directed towards Earth, and the other one in the opposite direction,” said co-author Dr Simone Migliari from the University of Barcelona said.
This is the first strong evidence of such particles in jets from a typical small black hole.
“We’ve known for a long time that jets contain electrons, but haven’t got an overall negative charge, so there must be something positively charged in them too,” Dr Miller Jones said.
Positively charged atoms are much heavier than the positrons scientists thought might make up the jets, and therefore the jets can carry away far more energy from the black hole than previously confirmed.
What’s more, astronomers aren’t sure whether the jets are powered by the spin of the rotating black hole itself, or whether they are instead launched directly from the disk of matter that surrounds the black hole.
“Our results suggest it’s more likely the disk is responsible for channeling the matter into the jets, and we are planning further observations to try and confirm this,” Dr Miller-Jones said.
The scientists also determined the jets were moving at 66 percent of the speed of light, or 198,000 km/s, the most accurate determination to date of the jet speed from a run-of-the-mill black hole that’s a few times the mass of the Sun.
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Bibliographic information: María Díaz Trigo et al. Baryons in the relativistic jets of the stellar-mass black-hole candidate 4U 1630-47. Nature, published online November 13, 2013; doi: 10.1038/nature12672
