Astronomers using
X-ray, radio, and optical telescopes have announced a big leap in
solving the origin of mysterious objects known as X-ray flashes
(XRFs) by finding that they originate from blue star forming
galaxies. This discovery of the cosmic distant scale effectively
ends the widely-held speculation that XRFs are the death-cries from
stars exploding in infant universe.
X-ray Flashes resemble a lower energy and longer-duration
version of a gamma-ray burst, an energetic explosion thought to
signal the death of a massive star. The properties of XRFs led to
speculation that they were gamma-ray bursts that occurred less than
a few billion years after the Big Bang, and whose light had been
subsequently weakened and time-stretched by the expansion of the
universe.
"Now that the very distant origin has been ruled out, X-ray
flashes could be due to exploding massive stars, just like
gamma-ray bursts" explained Dr. Joshua Bloom at the
Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.,
lead author on the paper announcing the results to be published in
The Astrophysical Journal. Bloom continued: "But the explosion from
an X-ray flash would need to contain less matter or less energy
than a typical gamma-ray burst. Alternatively, X-ray flashes could
be gamma-ray bursts viewed off-axis."
These results are being discussed at the "30th Anniversary of
the Discovery of Gamma-ray Bursts" conference currently being held
in Sante Fe, New Mexico.
The location of the sources studied by Bloom's group required a
careful coordination of NASA's Chandra X-ray Observatory and Hubble
Space Telescope, along with the National Radio Astronomy
Observatory's Very Large Array (VLA) in Socorro, New Mexico.
Chandra and the VLA provided a precise location of the fading X-ray
and radio "afterglow" of two X-ray flashes known as XRF 011030 and
XRF 020427. Hubble was used to identify and study galaxies at these
locations and estimate their distances to between about 6 to 11
billion light years from Earth.
X-ray flashes were discovered by John Heise (Space Research
Organization, the Netherlands) and colleagues in 2001 using the
Dutch-Italian X-ray satellite BeppoSAX. Bloom added the perspective
that "Nearly thirty years of active research was required to
discover the distance scale to gamma-ray bursts, but the distance
scale mystery was solved in only two years for X-ray flashes."
The universe is particularly rich in objects that exhibit bursts
at X-ray wavelengths. Bursts of X-rays are routinely detected from
the Sun, from magnetically active stars, from neutron stars and
black hole systems in the Milky Way, and from active super-massive
black holes near the centers of distant galaxies.
"What sets X-ray flashes apart from all the other X-ray
transients out there are their characteristic duration and
spectrum," said Dr. Derek Fox at the California Institute of
Technology, a coauthor on the paper.
X-ray flashes are relatively rare compared to other bursting
sources -- with a rate of about one per day in the universe. Each
flash comes without warning from a seemingly random position on the
sky and lasts for tens to hundreds of seconds.
An examination of galaxies that hosted the X-ray flashes hints
at a stellar origin for the explosions. "Those two galaxies in
which the flashes occurred are incredibly blue," explained Prof.
Pieter van Dokkum at Yale University. Since a galaxy's blueness is
often taken as a crude measure of the rate of star formation,
"these XRF hosts are churning out stars at an appreciable rate for
their size," van Dokkum said.
The X-ray flash results were obtained through a collaborative
effort with Dr. Bloom, Dr. Fox, Prof. van Dokkum, Prof. Shri
Kulkarni (Caltech), Edo Berger (Caltech), Prof. George Djorgovski
(Caltech), and Dr. Dale Frail (NRAO, Socorro, New Mexico). The XRFs
in this study were originally detected by the now-defunct BeppoSAX
telescope.
NASA's Marshall Space Flight Center, Huntsville, Ala., manages
the Chandra program, and TRW, Inc., Redondo Beach, Calif., is the
prime contractor for the spacecraft. The Smithsonian's Chandra
X-ray Center controls science and flight operations from Cambridge,
Mass., for the Office of Space Science, NASA Headquarters,
Washington.