UNIVERSITY PARK – A powerful stellar explosion — a gamma-ray burst — has shattered the record for the most distant object visible to the naked eye and also ranks as the most intrinsically bright object ever observed by humans in the universe. The explosion was detected March 19 by NASA’s Swift satellite, which is controlled by Penn State from its Mission Operations Center at University Park.
“It’s amazing — we’ve been waiting for a flash this bright from a gamma-ray burst ever since Swift began observing the sky three years ago, and now we’ve got one that is so bright that it was visible to the naked eye even though its source is half-way across the universe,” said David Burrows, senior scientist and professor of astronomy and astrophysics at Penn State, who directs the continuing operation of Swift’s X-ray telescope, the “XRT,” and the analysis of the data it collects. The most distant previous object that could be seen by the naked eye is the nearby galaxy M33, a relatively short 2.9 million light-years from Earth.
“Within a 24-hour period, we had an incredible outpouring of activity from the sky — five Gamma-ray bursts and several other kinds of outbursts — and in the middle of it all there was the brightest burst we’ve ever seen. It’s been a zoo here,” said Burrows, referring to the frenzied activity of the scientists and engineers working non-stop at Swift’s Mission Operations Center near Penn State.
Gamma-ray bursts are the most luminous explosions in the universe since the Big Bang. “These optical flashes from gamma-ray bursts are the most extreme such phenomena that we know of,” said Derek Fox, assistant professor of astronomy and astrophysics at Penn State, who confirmed the distance measurement for the burst. “If this burst had happened in our galaxy it would have been shining brighter than the sun for almost a minute — sunglasses would definitely be advised.”
“This burst was a whoppers,” said Swift principal investigator Neil Gehrels of NASA’s Goddard Space Flight Center in Greenbelt, Md. “It blows away every gamma-ray burst we’ve seen so far.” Most gamma-ray bursts occur when massive stars run out of nuclear fuel. Their cores collapse to form black holes or neutron stars, releasing an intense burst of high-energy gamma-rays and ejecting particle jets that rip through space at nearly the speed of light like turbocharged cosmic blowtorches. When the jets plow into surrounding interstellar clouds, they heat the gas, often generating bright afterglows.
According to Peter Meszaros, holder of the Eberly family chair in astronomy and astrophysics and professor of physics at Penn State and leader of the theory team for Swift, an unusual combination of circumstances may have contributed to the exceptionally bright afterglow of the burst in the visible wavelengths of light. “When the jet that formed during the explosion of the star slammed into surrounding gas clouds, shock waves were generated that heated the jet. The exceptional brightness of this burst requires the jet to have just the right combination of magnetic fields and velocity, which occurs very rarely,” said Meszaros.
Analysis of GRB 080319B is just getting under way, so astronomers don’t yet know for sure why this burst and its afterglow were so bright. Possibly the burst was more energetic than others, perhaps because of the mass, spin or magnetic field of the collapsing star or its jet. Or perhaps the burst concentrated its energy in a narrow jet that was aimed directly at Earth.
Swift’s Burst Alert Telescope picked up the GRB at 2:12 Eastern Daylight Time on March 19 and pinpointed the coordinates in the constellation Bootes so telescopes in space and on the ground could quickly point toward the afterglow in order to observe it. The burst itself is named GRB 080319B, since it was the second GRB detected on March 19.
Swift’s other two instruments, the X-ray Telescope and the Ultraviolet/Optical Telescope, observed bright afterglows. Several ground-based telescopes observed the afterglow brighten to visual magnitudes between 5 and 6. In the logarithmic magnitude scale used by astronomers, the brighter the object, the lower the magnitude number. From a dark location in the countryside, people with normal vision can see stars a little fainter than magnitude 6, so the afterglow would have appeared dim, but clearly visible to the naked eye.
Swift’s other two instruments, the X-ray Telescope and the Ultraviolet/Optical Telescope, also observed brilliant afterglows. Several ground-based telescopes saw the afterglow brighten to visual magnitudes between 5 and 6 in the logarithmic magnitude scale used by astronomers. The brighter an object is, the lower its magnitude number. From a dark location in the countryside, people with normal vision can see stars slightly fainter than magnitude 6. That means the afterglow would have been dim, but visible to the naked eye. GRB 080319B’s optical afterglow was 2.5 million times more luminous than the most luminous supernova ever recorded, making it the most intrinsically bright object ever observed by humans in the universe.
Later that evening, the burst’s redshift was measured at 0.94 by the Very Large Telescope in Chile and the Hobby-Eberly Telescope in Texas, which is a joint project of the University of Texas at Austin, Penn State, Stanford University, Ludwig-Maximilians-Universitat Muenchen and Georg-August-Universitat Goettingen. A redshift is a measure of the distance to an object. A redshift of 0.94 translates into a distance of 7.5 billion light years, meaning the explosion took place 7.5 billion years ago, a time when the universe was less than half its current age and Earth had yet to form. This is more than halfway across the visible universe.
“Here at Penn State’s Mission Operations Center, Jamie Kennea is the guy who was scrambling to command the Swift satellite to ‘ping-pong’ back and forth between all the exploding gamma-ray bursts, while Mark Hilliard was leading the frenzied flight operations team,” Burrows said.
GRB 080319B was one of four bursts that Swift detected on March 19, a Swift record for one day. “Coincidentally, the passing of Arthur C. Clarke seems to have set the universe ablaze with gamma-ray bursts,” said Swift science team member Judith Racusin, a Penn State graduate student.