Astronomers have discovered three flares near our galaxy's central black holes that look like gas surrounding the invisible object. This…
Astronomers have discovered three flares near our galaxy’s central black holes that look like gas surrounding the invisible object.
Welcome to the threshold of a black hole.
An international team of astronomers has taken the motion of magnetized gas right near the event horizon in our galaxy’s central super massive black hole, Sagittarius A * (pronounced “A-Star”). This gas is likely to be part of a puffy disk that Sgr A * dissolves. The disc provides a flashing light source, the constant glow sometimes stinging in spots as observers see wavelengths from X-ray to radio.
While the star S2 whiz at the black hole this summer, astronomers working with the GRAVITY instrument on the Great Telescope Interferometer in Chile happened to see three bright spots – two of them almost as bright as S2 in infrared – from near Sgr A *. Each flare lasted between 30 and 90 minutes and did not stay in place. Instead, they seemed to drive around the black hole at 30% light speed, trace about two-thirds of a clockwise eye, as (data side uncertainties) have the black hole in the middle.
Behavior looks like a hotspot the disc would make. Behind 2005, Avery Broderick (now University of Waterloo, Canada) and Abraham Loeb (Harvard) predicted that hotspots in the charged gas could be detected, their pathways investigating gravity near the innermost stable circle around the black hole.  Changes in flare polarization support that image. The light is polarized due to magnetic fields in the disc, which serve as sheep dogs to the charged particles that emit light. These magnetic fields are threaded through the disc, like large hula hoops around the waist of the black hole, explains Broderick. A hotspot only lights up a small part of the accretion flow, as it circles, the direction of the magnetic field in the illuminated area seems to spin – which is what the GRAVITY team saw.
As the team reports in October Astronomy & Astrophysics the data point to the hotspots circles very close to the black hole’s event horizon and fills a pass every 45 minutes or so.
“This is an incredible measurement,” says Broderick. “If astronomers look more flares do the same,” this presents an additional opportunity to do exact gravity testing in its most extreme environments: right about the horizons of black holes. “
Participants at a galactic center workshop in Germany last week discussed the analysis in the long run, but be careful about the hotspot interpretation. “Everyone thinks there is some kind of movement in a flare near the black hole,” says Sera Markoff (University of Amsterdam, The Netherlands) , which specializes in accretion physics, but not all are sold on the hotspot solution. “Most of us think more complicated things are possible, such as movements associated with magnetic spots or rays.” (Astronomers have looked for rays from Sgr A * but have not seen anything definitely.)
Another concern is the angle where we may see the black hole disc. The GRAVITY team believes the disk is pretty the face in our perspective. Others think the odds are statistically low, and radio observations have favored a more edge-on view. Upcoming radio results can give some clarity about the slope of the disc.
A more edge view can also create a better silhouette for the Event Horizon Telescope, the global project attempts to capture the “Shadow” of Sgr A * and other super-massive black holes against its surrounding gas glow.
GRAVITY Collaboration. “Detection of orbital movements near the last ringway in the massive black hole Sgr A *.” Astronomy and Astrophysics . October 2018.