June 10, 1997
Photo No.: STScI-PRC97-20
This is a false-color Hubble Space Telescope image of the visible fireball which accompanied the gamma ray burst called GRB 970228. This is the first optical image ever taken which associates a gamma-ray burst source with a potential host galaxy. This observation provides strong supporting evidence that gamma-ray bursts are cosmological- they originate in distant galaxies across the universe.
The burst was detected on February 28, 1997. The Hubble picture is a combination of two images taken on March 26 and April 7, with the Wide Field Planetary Camera 2 (in planetary camera mode).
The arrow points to the fireball, which is a white blob immediately to the upper left of image center. Immediately to the lower right of center is an extended object (roughly resembling an "E") interpreted to be the host galaxy where the gamma-ray burst is embedded. A follow-up image taken on April 7 showed the fireball had faded, but not the extended source, supporting the interpretation it is a host galaxy at great distance.
It may be a significant finding that the fireball is offset from the center of the galaxy. This rules out a supermassive black hole that could be at the galaxy's core, as the source of the gamma-ray burst. A more likely explanation is that the burst came from the titanic collision of two neutron stars, or a neutron star with a stellar-mass black hole, in the disk of the galaxy.
Nature's most powerful explosions, gamma-ray bursts, occur among the normal stellar population inside galaxies scattered across the universe. This means that, on average, a gamma-ray burst goes off once every few million years inside our Milky Way galaxy.
The energy released in such a titanic explosion, which can last from a fraction of a second to a few hundred seconds, is equal to all of the Sun's energy generated over its 10 billion year lifetime.
A team of astronomers, led by Kailash Sahu of the Space Telescope Science Institute (STScI) in Baltimore, MD, is reporting this conclusion at the 190th Meeting of the American Astronomical Society in Winston-Salem, NC. They used the Hubble Space Telescope to study the fading optical counterpart to a burst that happened on February 28, 1997.
The team's results are based on Hubble images taken on March 26 and April 7, 1997 that show the gamma-ray burst is offset from the center of an extended "fuzzy" object that looks like a galaxy. The researchers argue, statistically, that this must be more than a chance alignment. The gamma-ray burst was embedded inside the galaxy.
"Hubble's unmatched resolution was crucial in pinpointing the fact that the gamma-ray burst is away from the center," Sahu says. "This would rule out massive black holes, thought to dwell in the cores of most galaxies, as the source of these incredible explosions."
"These observations definitely represent a huge step forward towards the full understanding of these enigmatic objects," says co-investigator Mario Livio, of STScI. However, he points out that this conclusion assumes the mechanism for creating a gamma-ray burst is basically the same.
Keck telescope spectroscopic measurements of the optical counterpart to another gamma-ray burst, which exploded on May 8, found that its distance from Earth is several billion light-years. The Keck observation establishes that gamma-ray bursts are truly extragalactic in location and origin.
However, a June 2nd Hubble observation of this newer burst source, made with both of the newly installed science instruments, failed to reveal a galaxy adjacent to the optical counterpart.
"If the gamma-ray burster is at the distance indicated by the Keck spectrum, then its host galaxy is far less luminous than is the Milky Way," says Andrew Fruchter, one of the leaders of another STScI team in making the observation. "What is clear is that further observations of new bursters, and of these two bursters at later times, will be required to better understand the nature and location of these astonishing objects."
One possible mechanism for unleashing such a titanic fireball of energy is the collision of a neutron star with another neutron star or a black hole. The Hubble observations support this model because it appears gamma-ray bursts occur in the disk of a galaxy, where there is ongoing stellar formation, and so there should be an abundance of neutron stars from recently exploded supernovae.
The astronomers estimate a gamma-ray burst may explode within a few thousand light-years from Earth every few hundred million years.
Credits: K. Sahu, M. Livio, L. Petro, D. Macchetto, STScI and NASA