Newly discovered subclass of “local” gamma ray bursts may solve a mystery or two

Although scientists have believed for some time that most gamma-ray
bursts are very distant, a Goddard scientist has discovered 100 of them
that are quite "local," within 325 million light years from Earth.

These nearby events — whose sheer number and proximity has taken
scientists by surprise — represent a new subclass of gamma-ray bursts,
the most powerful known explosions in the Universe. They may appear as
frequently as certain star explosions called Type Ib/c supernovae; they
could be a source of detectable gravitational radiation; and their
presence could explain the existence of ultrahigh-energy cosmic rays.

Jay Norris of NASA’s Goddard Space Flight Center in Greenbelt, Md.,
presented these results at the 199th Meeting of the American
Astronomical Society in Washington, D.C.

Norris’ work is based on the analysis of over 1,400 gamma-ray bursts.
These bursts — frequent and random, powerful and mysterious — fade so
quickly that scientists have been unable to determine the source of the
bursts. Most gamma-ray bursts last from only a few seconds to about a
minute. The afterglow can linger in X-ray, optical and radio wavebands
for a few days to weeks.

"Gamma-ray bursts may come from merging black holes or neutron stars,
or from the collapse of theorized massive stars tens to hundreds of
times more massive than the Sun," Norris said.

Norris and his colleagues at Goddard had previously uncovered a
relationship between the distance to a burst, its luminosity, and its
so-called "lag time." In any given burst, the high-energy gamma-ray
photons (particles of light) arrive at Earth-orbiting detectors
slightly faster than the lower-energy gamma-ray photons. This "lag
time" in photon arrival is the result of the physics of the burst.

More luminous bursts seem to have shorter lag times. Comparing the
intrinsic burst luminosity (the actual brightness, determined by photon
lag times) with the measured luminosity (how bright the burst appears
to Earth-orbiting gamma-ray detectors) yields a distance to the source.

By characterizing gamma-ray bursts in terms of lag time and luminosity,
Norris could determine that most of the 1,437 archived burst profiles
he studied came from bursts with high luminosities originating at
cosmological distances, billions of light years from Earth, as
scientists have long suspected.

However, about 100 bursts were of lower luminosity. Norris speculates
that these kinds of bursts are created by the collapse of massive stars,
perhaps 10 to 50 times as massive as the Sun. These bursts seem to
concentrate in an oblate distribution towards the Supergalactic Plane,
an imaginary plane that slices through several galaxy clusters within
about 325 million light-years from Earth.

This plane follows the local matter distribution, and no one had
detected such clustering of gamma-ray bursts along this plane before
this analysis. If the bursts originated from this region, this could
explain the origin of ultrahigh-energy cosmic rays, yet another
longstanding mystery. Thus far, scientists have been hard-pressed to
explain the presence of these cosmic rays, which are atomic particles
moving at near light speed carrying the kinetic energy of a major
league fastball.

Scientists might be able to confirm this new subclass of local gamma-ray
bursts with LIGO, a ground-based gravitational wave detector funded by
the National Science Foundation. LIGO theoretically could detect the
ripples in spacetime caused by collapsing stars within several hundred
million light years from Earth.

NASA’s Swift Gamma Ray Burst Explorer, scheduled for 2003 launch, could
also confirm the existence of these nearby bursts. Swift will have
imaging capability and the sensitivity to see these bursts — which are
lower in energy, less luminous, have longer lags compared to bursts at
cosmological distances, and have been difficult to detect thus far.

Swift can also quickly determine the precise location of the bursts. If
the bursts are associated with Type 1b/c supernovae, they would appear
just before the supernovae, which would provide scientists with
advanced warning to witness the entire supernovae event.