NuSTAR's eyes are two Wolter-I optic units separated by a 33 foot long boom to increase its field of view, just like human eyes. Each optic unit is made of a series of mirrors that reflect an incoming X-ray twice, first off an upper parabolic mirror then again off a lower hyperbolic mirror. The mirrors are nearly parallel to the direction of the incoming X-ray so they reflect most of the energy rather than absorbing it. The slight angle also means the collection area is quite small, meaning mirrors of varying size are nested together to gather a complete picture. In NuSTAR's case, each eye is made of 133 mirrors.
Even though it's a highly sophisticated orbiting telescope, there was an element of luck in capturing the black star burst. "We got lucky to have captured an outburst from the black hole during our observing campaign," said Fiona Harrison, the mission's principal investigator at Caltech in Pasadena. "These data will help us better understand the gentle giant at the heart of our galaxy and why it sometimes flares up for a few hours and then returns to slumber."
Also like human eyes, sometimes more telescopes viewing an object are better than one. For two days in July, NuSTAR teamed up with NASA's Chandra X-ray Observatory (which sees lower-energy X-ray light) and the W.M. Keck Observatory on Mauna Kea in Hawaii (which took infrared images) to get a better picture of Sagittarius A*.
Typically, Sagittarius A* is on the quiet end of the black hole spectrum. While some black holes devour stars and other sources of fuel in orbit around them, Sagittarius A* just nibbles or lets fuel pass right by. It's unusual behavior scientists can't yet fully explain.
But one thing Sagittarius A* has in common with other black holes is its regular eruption of excess energy when it does consume fuel. That's what NuSTAR saw: X-rays emitted by consumed matter heated to about 180 million degrees Fahrenheit and originating from an area where particles are traveling very close to the speed of light.
"Astronomers have long speculated that the black hole's snacking should produce copious hard X-rays, but NuSTAR is the first telescope with sufficient sensitivity to actually detect them," said NuSTAR team member Chuck Hailey of Columbia University.
Astronomers say this data from NuSTAR in conjunction with simultaneous observations taken at other wavelengths will help shed light (or X-rays?) on the physics of how black holes consume fuel and grow.
Image credit: NASA/JPL-Caltech