The new picture around the central engine of M87 revealed by this observation. The bright radio 'core' of the jet base is located very close to the central black hole no larger than about 10 times the size of the event horizon.
Credit: NAOJ/AND You Inc.
ESSEX: Scientists have pinpointed the location of a black hole at the centre of a nearby galaxy by mapping out the shape of its erupting radio jets.
The findings, published in today's Nature, show that the distance between the 'core' of the radio jets and the black hole at the centre of the galaxy M87 appear to be closer than previously thought - and closer than what has been observed in other galaxies.
"The most remarkable finding is the strong constraint on the location of the central black hole of M87 on a 10 times of the radius of the event horizon scale", said lead author Kazuhiro Hada of the Graduate University for Advanced Studies in Tokyo. "Observations inside this scale are terra incognita for astronomers and we are very excited."
Distance gauged from high-energy eruptions
Radio jets are believed to result from the accretion of material falling onto a black hole. Ionized gas orbits the black hole, which suggestively twists the magnetic fields in the gas resulting in a high-energy eruption of charged particles in opposite directions.
The core of these jets was unexpectedly found to exist within a hundredth of a light year from the black hole, which is roughly 37.8 billion kilometres wide and six billion times the mass of our Sun.
Hada and colleagues used VLBA techniques - the Very Long Baseline Array, a collection of radio antennas from 10 different sites across the Earth - to take multi-frequency radio observations before finding that "the jet was broad between the black hole and the jets brightest part, its core".
"The jet then became tightly focused farther out, agreeing with some theoretical models that explain radio jets as the products of twisted magnetic fields within ionized gas swirling around black holes," explained astrophysicist Alan Marscher, Director of the Institute for Astrophysical Research at Boston University, who did not take part in the study.
Observing radio jets
The VLBA can make images of the jets at their highest frequencies; the microwave range. It was thought that the microwaves wouldn't be visible so close to the black hole because the densities would be too great for the microwaves to escape. Hada's results show that the jets spread out into a wide angle first, lowering the density so that the microwaves can escape before being absorbed.
Hada and colleagues observed the jets on frequencies between two and 43GHz, which allowed the team to achieve a positional accuracy of about 20 microarcseconds.
At 43GHz they found that the jet base is located on 14-23 Schwarzschild radii from the black hole itself. The Schwarzschild radius is the distance between the black hole's centre and its event horizon. This was an important find because estimates of black hole to core distance in quasars, more luminous cousins of M87, have been found at more than 100,000 times the Schawarzschild radius.
Understanding the event horizon
"I think the result of the jet formation occurring closer to the black hole than predicted is interesting in that we have a lot of theories as to the physics of what happens near the event horizon of a black hole, but we are just now achieving the technical capabilities to actually test these predictions," commented Ashley Zuaderer, an astrophysicist at Harvard University near Boston.
Marscher predicts reasons why the radio jet base may be so much closer in M87 than in other quasars, suggesting that bright jets of some quasars are pointing directly towards us and so we see their ultra-fast spines, parts of the jets moving at 99% the speed of light, leaving a sheath, emissions around 90% the speed of light, too weak to notice.
But the jet of M87 is more inclined to our line of sight so the spine may appear dimmer, allowing us to observe the slower sheath. "The images of M87 thereby reveal slower regions of the jet that are close to the black hole, whereas in quasars the jets become bright only where the spines reach their terminal velocity at much greater distances from the black holes," said Marscher.
Continued improvements in array techniques could provide high-resolution direct imaging of accretion flow, say the authors. "This will enable us to succeed direct imaging of the accretion flow onto black holes and the real site of jet production on a scale of the event horizon. It is a dream for astronomers," said Hada.
