Gamma-Ray Bursts Active Longer than Previously Thought

The X-ray Telescope on NASA's Swift satellite captured the intensity of a bright X-flare from GRB 060714. The elongated blue and gray regions are an artifact of the image processing (left). After the flare, (right) the GRB's X-ray emission faded considerably, revealing a faint afterglow. Credit: NASA / Swift / XRT Science Team.

Using NASA's Swift satellite, which Penn State controls from the Mission Operations Center at University Park, astronomers have discovered that energetic flares seen after gamma-ray bursts are not just hiccups, but appear to be a continuation of the burst itself.

"In the gamma-ray burst that we discovered on July 14th, 2006, we saw for the first time that the flares detected first by Swift's Burst Alert Telescope and then by its X-Ray Telescope appear to form a continuously connected and continuously evolving succession of flares," explains Penn State's Peter Mészáros, Holder of the Eberly Family Chair in Astronomy and Astrophysics and Professor of Physics, and a member of the team that will describe the discovery in a paper scheduled for publication in the 10 August 2007 issue of the Astrophysical Journal.

"Previously, flares had been observed in the prompt gamma-ray emission observed with the high-energy Burst Alert Telescope, and then a bit later the lower-energy X-Ray Telescope started to see flares, but the scarcity of well-observed flares was insufficient to make a clear connection between the two types of flares," Mészáros says. "We see with this burst, however, that the properties of the flares are very similar in the prompt and the subsequent phase, smoothly fading into each other in a series of increasingly fainter and softer flares. This clearly makes the point that the late flaring behavior is likely to be the ring-down of the central engine emission, which thus appears to be longer than it was previously thought." Fully understanding this longer emission time of the central engine is a new challenge to theorists, Mészáros says.

Gamma-ray bursts (GRBs) release in seconds the same amount of energy our Sun will emit over its expected 10-billion-year lifetime. The staggering energy of a long-duration GRB (lasting more than a few seconds) comes from the core of a massive star collapsing to form a black hole or neutron star. In current theory, in-rushing gas forms a disk around the central object. Magnetic fields channel some of that material into two jets moving at near-light speed. Collisions between shells of ejected material within the jet trigger the actual GRB.

This artwork depicts the central engine of a gamma-ray burst. A powerful jet of radiation and fast-moving particles blasts its way out of the central region of a dying star. The jet is presumably powered by material spiraling into a black hole or neutron star. Multiple episodes of infall provides fuel for the engine, leading to the burst and later X-ray flares. Credit: NASA / SkyWorks Digital.

Early in Swift's mission, Swift's X-ray Telescope (XRT) discovered that the initial pulse of gamma-rays, known as prompt emission, often is followed minutes to hours later by short-lived but powerful X-ray flares. The flares suggested — but did not prove — that GRB central engines remain active long after the prompt emission. After analyzing data from the burst detected on 14 July 2006 (GRB 060714), Hans Krimm of Universities Space Research Association, Columbia, Maryland, and NASA's Goddard Space Flight Center in Greenbelt, Maryland, and eight colleagues including Penn State astronomers Peter Mészáros, David Burrows, and David Morris, have demonstrated that X-ray flares are indeed a continuation of the prompt emission, showing that GRB central engines are active much longer than previously thought.

Swift's Burst Alert Telescope (BAT) picked up the initial GRB in the constellation Libra. Then, from about 70 to 200 seconds after the initial burst, the BAT and XRT registered five flares. Each flare exhibited rapid and large-scale variability in intensity, but the overall energy steadily diminished from flare to flare. Moreover, the peak photon energy of each flare "softened" by progressing from gamma-rays to X-rays (from higher to lower energy). The prompt gamma-ray emission and the subsequent X-ray flares appear to form a continuously connected and evolving succession of events. "This pattern points to a continuous injection of energy from the central engine, perhaps fueled by sporadic in-fall of material onto a black hole, Krimm says. "The black hole just keeps gobbling up gas and the engine keeps spewing out energy."

The rapid and large-scale variability of the X-ray flares argues strongly against the idea that they come from jets sweeping up the surrounding gas. Since the observed emission comes from a wide region, the afterglow should vary smoothly with time. Nobody has come up with a viable explanation for why the surrounding material would be so lumpy to lead to such rapid variability.

Swift's Burst Alert Telescope, and then its X-ray Telescope, picked up the prompt emission of GRB 060714, and then several flares. The outbursts gradually decreased in total energy, showing that the XRT flares were related to the prompt emission. The BAT data appears in black, and the XRT data appears in red. Even though the X-ray brightness of the first XRT flare was less than the next two flares, it was more energetic, since its X-ray photons had higher energy. Credit: NASA / Swift / Hans Krimm.

"This particular GRB had a series of flares over a wide range in time that were bright enough that we could study their properties in detail," says study coauthor Jonathan Granot of the Kavli Institute for Particle Astrophysics and Cosmology at Stanford University, Stanford, California. "It clearly shows a gradual evolution with time in the properties of the flares within the same GRB, while in other GRBs there are typically only one or two flares that are bright enough to be studied in detail, making it very hard to reach a similar conclusion."

"This is a very important result," adds Swift principal investigator and study coauthor Neil Gehrels of NASA Goddard. "By chance, if you look at enough bursts you'll find the one that opens the door. GRB 060714 shows that everything happening in the first few minutes is driven by the central engine."

Swift is managed by NASA's Goddard Space Flight Center and was built and is operated in collaboration with Penn State University, the Los Alamos National Laboratory, and General Dynamics in the US; the University of Leicester and Mullard Space Sciences Laboratory in the UK; Brera Observatory and the Italian Space Agency in Italy; plus partners in Germany and Japan.

CONACTS:

Peter Mészáros: 814-865-0418, pmeszaros@astro.psu.edu

David Burrows: 814-865-7707, burrows@astro.psu.edu

Lynn Cominsky, Swift PIO: 707-664-2655, lynnc@universe.sonoma.edu

Barbara K. Kennedy, Penn State PIO: 814-863-4682, science@psu.edu

MORE ABOUT SWIFT

Swift was launched in November 2004 and was fully operational by January 2005. Swift carries three main instruments: the Burst Alert Telescope, the X-ray Telescope, and the Ultraviolet/Optical Telescope. Swift's gamma-ray detector, the Burst Alert Telescope, provides the rapid initial location and was built primarily by the NASA Goddard Space Flight Center in Greenbelt and Los Alamos National Laboratory and constructed at GSFC. Swift's X-Ray Telescope and UV/Optical Telescope were developed and built by international teams led by Penn State and drew heavily on each institution's experience with previous space missions. The X-ray Telescope resulted from Penn State's collaboration with the University of Leicester in England and the Brera Astronomical Observatory in Italy. The Ultraviolet/Optical Telescope resulted from Penn State's collaboration with the Mullard Space Science Laboratory of the University College-London. These three telescopes give Swift the ability to do almost immediate follow-up observations of most gamma-ray bursts because Swift can rotate so quickly to point toward the source of the gamma-ray signal. The spacecraft was built by General Dynamics. Penn State controls Swift's science and flight operations for NASA from the Penn State Mission Operations Center at University Park. More information about Swift, including links to the Swift song and more images, are on the Web at http://www.science.psu.edu/alert/Swift.htm.