|
NASA’s Swift, Hubble Space Telescope and Chandra X-ray Observatory
have teamed up to study one of the most puzzling cosmic blasts yet
observed. More than a week later, high-energy radiation continues to
brighten and fade from its location.
Astronomers
say they have never seen anything this bright, long-lasting and
variable before. Usually, gamma-ray bursts mark the destruction of a
massive star, but flaring emission from these events never lasts more
than a few hours.
Although
research is ongoing, astronomers say that the unusual blast likely
arose when a star wandered too close to its galaxy’s central black hole.
Intense tidal forces tore the star apart, and the infalling gas
continues to stream toward the hole. According to this model, the
spinning black hole formed an outflowing jet along its rotational axis. A
powerful blast of X- and gamma rays is seen if this jet is pointed in
our direction.
On
March 28, Swift’s Burst Alert Telescope discovered the source in the
constellation Draco when it erupted with the first in a series of
powerful X-ray blasts. The satellite determined a position for the
explosion, now cataloged as gamma-ray burst (GRB) 110328A, and informed
astronomers worldwide.
|
As
dozens of telescopes turned to study the spot, astronomers quickly
noticed that a small, distant galaxy appeared very near the Swift
position. A deep image taken by Hubble on April 4 pinpoints the source
of the explosion at the center of this galaxy, which lies 3.8 billion
light-years away.
That
same day, astronomers used NASA’s Chandra X-ray Observatory to make a
four-hour-long exposure of the puzzling source. The image, which locates
the object 10 times more precisely than Swift can, shows that it lies
at the center of the galaxy Hubble imaged.
“We
know of objects in our own galaxy that can produce repeated bursts, but
they are thousands to millions of times less powerful than the bursts
we are seeing now. This is truly extraordinary,” said Andrew Fruchter at
the Space Telescope Science Institute in Baltimore.
“We
have been eagerly awaiting the Hubble observation,” said Neil Gehrels,
the lead scientist for Swift at NASA’s Goddard Space Flight Center in
Greenbelt, Md. “The fact that the explosion occurred in the center of a
galaxy tells us it is most likely associated with a massive black hole.
This solves a key question about the mysterious event.”
|
Most
galaxies, including our own, contain central black holes with millions
of times the sun’s mass; those in the largest galaxies can be a thousand
times larger. The disrupted star probably succumbed to a black hole
less massive than the Milky Way’s, which has a mass four million times
that of our sun
Astronomers
previously have detected stars disrupted by supermassive black holes,
but none have shown the X-ray brightness and variability seen in GRB
110328A. The source has repeatedly flared. Since April 3, for example,
it has brightened by more than five times.
Scientists
think that the X-rays may be coming from matter moving near the speed
of light in a particle jet that forms as the star’s gas falls toward the
black hole.
“The
best explanation at the moment is that we happen to be looking down the
barrel of this jet,” said Andrew Levan at the University of Warwick in
the United Kingdom, who led the Chandra observations. “When we look
straight down these jets, a brightness boost lets us view details we
might otherwise miss.”
This
brightness increase, which is called relativistic beaming, occurs when
matter moving close to the speed of light is viewed nearly head on.
Astronomers plan additional Hubble observations to see if the galaxy’s core changes brightness.
NASA
Goddard manages Swift, and Hubble, and NASA’s Marshall Space Flight
Center in Huntsville, Ala., manages Chandra. The Hubble Space Telescope
was built and is operated in partnership with the European Space Agency.
Science operations for all three missions include contributions from
many national and international partners.
NASA’s Goddard Space Flight Center, Greenbelt, Md.
