25 June 2015

Starving Milky Way Black Hole Gorges on Companion Star

The massive Milky Way black hole has not dined so spectacularly on material streaming off its companion star since 1989, the European Space Agency reports.

The viewing is spectacular for those equipped with X-ray and gamma ray optics -- not so much so for the amateur astronomers forced to gaze out in the visible light band that is blind to the bright flashes.
For more than a week, the professionals have been taking advantage of the opportunity to witness one of the most extreme activities in the universe unfolding just 8,000 light years away in the constellation Cygnus with a collection of deep space, International Space Station and ground-based observatories.
"The behavior of this source is extraordinary at the moment, with repeated bright flashes of light on time scales shorter than an hour, something rarely seen in other black hole systems,'' Erik Kuulkers, project scientist for the European Space Agency's deep space Integral observatory, notes in the ESA summary. "In these moments, it becomes the brightest object in the X-ray sky, up to fifty times brighter than the Crab Nebula, normally one of the brightest sources in the high-energy sky.''
The celestial feeding frenzy was detected June 15 by the Burst Alert Telescope on NASA's 11-year-old Swift satellite. A gamma ray burst automatically triggered the attention of Swift's X-ray optics. An X-ray flare in the region soon caught the attention of the six-year-old  Monitor of All-sky X-ray Image investigation, or MAXI, sensor on the Kibo Japanese Experiment Module of the ISS.
Alerts from the two space X-ray monitors triggered a worldwide observing campaign using telescopes equipped to study V404 Cygni at multiple  wavelengths. ESA's 13-year-old Integral gamma ray observatory joined the effort on June 17.
The fireworks come from material that has been pulled away from the companion star by the black hole's gravity. It accumulates in a disk surrounding the black hole until reaching a critical point that triggers a periodic feeding frenzy.
After the 1989 feasting,  astronomers were able to gauge the black hole's mass at 12 stellar masses, the companion star's at half the mass of the sun. That outburst was detected by Japan's Ginga X-ray satellite and similar sensors aboard Russia's former Mir space station.
At that time, scientists were still struggling to confirm the existence of black holes.
By combing through past observations with optical telescopes, experts believe there is evidence of similar outbursts in 1956 and 1938.
This time, equipped with the latest and largest radio and optical sensors, astronomers plan to monitor the action at V404 Cygni well into July to gather clues as to what triggers the disk material to shine brightly before spiraling into the black hole. 

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