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 supermassive 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.
The image and additional information are available at:
