By Rob Waugh
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Two gigantic beams blasted out of the centre of the Milky Way a million years ago - intensely powerful blasts of energy that would have required a black hole devouring a mass 10,000 times that of our Sun.
'The faint jets we see today are a ghost or after-image of what existed a million years ago,' said Meng Su, an astronomer at the Harvard-Smithsonian Center for Astrophysics, after the Fermi Space Telescope detected traces of the blasts.
The huge beams might help explain mysterious 'bubbles' that surround the centre of our galaxy.
'The faint jets we see today are a ghost or after-image of what existed a million years ago,' said Meng Su, an astronomer at the Harvard-Smithsonian Center for Astrophysics, after the Fermi Space Telescope detected traces of the blasts
'They strengthen the case for an active galactic nucleus in the Milky Way’s relatively recent past,' he added.
The two beams, or jets, were revealed by NASA’s Fermi space telescope. They extend from the galactic center to a distance of 27,000 light-years above and below the galactic plane.
They are the first such gamma-ray jets ever found, and the only ones close enough to resolve with Fermi.
It would take a tremendous influx of matter for the galactic core to fire up again. Finkbeiner estimates that a molecular cloud weighing about 10,000 times as much as the Sun would be required.
'Shoving 10,000 Suns into the black hole at once would do the trick. Black holes are messy eaters, so some of that material would spew out and power the jets,' said cFA co-author Daniel Finkbeiner.
The newfound jets may be related to mysterious gamma-ray bubbles that Fermi detected in 2010. Those bubbles also stretch 27,000 light-years from the center of the Milky Way. However, where the bubbles are perpendicular to the galactic plane, the gamma-ray jets are tilted at an angle of 15 degrees. This may reflect a tilt of the accretion disk surrounding the supermassive black hole.
Fermi has given astronomers some of the most accurate maps of the universe
The two structures also formed differently. The jets were produced when plasma squirted out from the galactic center, following a corkscrew-like magnetic field that kept it tightly focused. The gamma-ray bubbles likely were created by a 'wind' of hot matter blowing outward from the black hole’s accretion disk. As a result, they are much broader than the narrow jets.
Both the jets and bubbles are powered by inverse Compton scattering. In that process, electrons moving near the speed of light collide with low-energy light, such as radio or infrared photons. The collision increases the energy of the photons into the gamma-ray part of the electromagnetic spectrum.
The discovery leaves open the question of when the Milky Way was last active. A minimum age can be calculated by dividing the jet’s 27,000-light-year length by its approximate speed. However, it may have persisted for much longer.
'These jets probably flickered on and off as the supermassive black hole alternately gulped and sipped material,' said Finkbeiner.
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