The Milky Way's Black Hole Is Not So Quiescent 152
esocid writes in with a followup to the recent discussion about the possibility that our galaxy's central black hole could reignite. "Using NASA, Japanese, and European X-ray satellites, a team of Japanese astronomers has discovered that Sagittarius A* let loose a powerful flare three centuries before the time at which we are observing it (i.e., 26,000 years in the past). X-ray pulses emanating from just outside the black hole take 300 years to traverse the distance between the central black hole and a large cloud known as Sagittarius B2, so the cloud responds to events that occurred 300 years earlier. 'By observing how this cloud lit up and faded over 10 years, we could trace back the black hole's activity 300 years ago,' says team member Katsuji Koyama of Kyoto University. 'The black hole was a million times brighter three centuries ago.'"
Light echo (Score:3, Insightful)
Remember that the Earth frame is arbitrary. Although relativity stipulates that there is no privileged frame, strictly speaking there is only one intertial frame which is at rest with respect to the cosmic microwave background radiation; if the Earth were at rest in it then we would see a sky with a uniform temperature in all directions. Instead we can observe a dipole moment in the sky's CMB spectrum consistent with motion at 380 km/s toward the direction of Virgo. The inertial frame of the black hole would also be worthy of consideration. But of course this is all just Slashdot nitpicking, you do your calculations in the Earth's frame because you want your result to come out in Earth proper time, and realistically this means you don't do anything different.
Not actually true: they are larger at those relative speeds but are certainly present and noticeable at far lower velocities e.g. atomic clocks on Concord, GR corrections to GPS satellite clocks etc.
Those effects are negligible with this level of approximation. Basically everything can be considered to be at rest; you guys are making this way harder than it is. This is a simple problem of geometric optics. We're seeing this glowing cloud, with a region 10 light years across, brightening and darkening within the space of 5 light years. That's very hard to explain as anything other than a light echo from a source nearby that must have been bright, and small, and rapidly varying in brightness. And look, there's this supermassive black hole sitting here 300 light years away. You don't have to be Einstein to figure this one out.
The star V838 Mon [nasa.gov] is a good example of a light echo. This star emitted a huge flash in 2002 that made it the brightest star in the galaxy for a couple months. Then it dimmed to a normal brightness. Once it did, starting in mid-late 2002, we started to see a huge reflection of the flash begin to expand out from the star as it lit up the gas and dust in the vicinity. At any given time we see a glowing sheet of gas shaped like a paraboloid open towards us with the star at its focus, and every year this paraboloid gets bigger. Now that it's 2008 this thing has become a Firefox logo 12 light years wide [wired.com] that continues to expand outward in all directions at the speed of light.