Black holes are objects so massive, and the gravity around them so strong, that not even light can escape. Neutron stars are the collapsed remnants of giant stars that have all but annihilated themselves in tremendous explosions called supernovae.

These objects are among the most energetic in the galaxy, according to Richard Rothschild, a research physicist at the Center for Astrophysics and Space Sciences at the University of California at San Diego. Neutron stars contain the mass of the sun but are less than 10 miles in diameter, and they spin extremely rapidly -- some as fast as 1000 times per second.

Rothschild leads the team that built one of the three instruments aboard Rossi. To him, X-ray astronomy is particularly exciting, because it is so dynamic.

"If you were to look at the sky with X-ray eyes it would be changing on all sorts of time scales. Almost every object out there would be flickering, either over fractions of a second, to seconds, to hours, to days, to years. And that's sort of the neat part about X-ray astronomy," he said. "You never know what you're going to see."

Some of the main targets of the Rossi observations are systems called binary neutron star pairs. These are systems in which two stars orbit each other, but where one of the pairs is either a neutron star or, in some cases, a black hole. The scientists are using Rossi to watch these star pairs and observing the way material from stars falls into the black holes.

"We can look at binaries that have neutron stars and binaries that have black holes, and contrast them to try to find out what the differences are," Rothschild said. Attributing those differences to either neutron stars or black holes, scientists can learn how matter reacts differently in the two cases. "We can then start to learn a little bit about black holes and the region around them," he said.

Observing the binary systems in this way is the tool Rossi scientists are using to learn whether space and time might behave very differently in regions where gravity is extremely concentrated.

In some cases the X-ray observations have detected signs of material being sucked away from a star in a star/black-hole binary pair and falling into a black hole, Rothschild said. Scientists make careful measurements of the speed at which the material swirls around the black hole and falls into it, looking for signs of knots or curves in space.

"Normally space, we picture it as flat," Rothschild explained. "But as you get in close (to a black hole) it can actually twist. As the black hole is spinning maybe the space starts to go around the black hole a little bit instead of going straight into it.

"If that's what's going on, matter should take a little longer to get to the black hole instead of going straight in because it's had to take this little more circuitous route."

The interpretation of the data is something that is a hot topic of debate among theorists, but with more data and analysis, scientists might actually be able to prove whether space and time are warped and dragged along by the edges of spinning black holes.