Hubble Image of Gamma-Ray Burst Confirms Rice Predictions

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Hubble Image of Gamma-Ray Burst Confirms Rice Predictions

HOUSTON – (April 7, 1997) – It’s ideal for astronomers to have their predictions tested in space — which is just what happened for Rice scientists recently.

When the Hubble Space Telescope tracked the visible-light counterpart of a gamma-ray burst just over a week ago, it did more than give astronomers their first close-up look at its fading light — it confirmed a prediction made by Rice University scientists that the x-ray and visible light afterglows would fade at a certain rate.

The find is leading astronomers closer to solving modern astronomy’s greatest mystery, the origin of gamma-ray bursts — the highest-energy radiation in the universe.

The Rice University scientists predicted that the brightness of the x-ray and visible-light counterparts would fall as the reciprocal of time. Such a slow decay is highly unusual in energetic astrophysical phenomena.

The burst was detected by several space-based, high-energy astrophysics observatories on Feb. 28, and then a fading visible-light source was discovered by ground-based telescopes. On March 26, this burster was finally imaged by NASA’s Hubble Space Telescope with unprecedented precision.

“Researchers have been diligently looking for these fading counterparts for decades, without success, so this is a real breakthrough for the field,” Rice professor Edison Liang says, “and a key confirmation for our model.”

In a series of articles written at the end of last year, and to appear in the April 10 issue of The Astrophysical Journal Letters, the Rice team proposes that a gamma-ray burst fades, not because of explosive expansion as often conjectured, but mainly due to radiation cooling, in which the energy of the escaping radiation, or so-called photons, closely mimics the energy of the cooling plasma. This physical model is proposed by Liang together with research scientist Ian Smith, graduate student Anthony Crider and professor Masaaki Kusunose of Japan.

“Picture the source as a large ball of very energetic particles whizzing around,” explains Liang. “A low energy radio or microwave photon injected into this ionized plasma — most likely created by the plasma itself — bounces off many particles before it can finally escape. But in the scatterings, the particles transfer energy to the photons. During the burst, the escaping photons span almost the entire electromagnetic spectrum, with most of the energy appearing in the gamma-ray region. As the source slowly cools, gamma rays are no longer produced, but the source is still visible across the rest of the spectrum, from x-rays to visible-light, as a slowly fading object.”

In addition to the inverse-time fading law, the Rice model also predicts that the ratio of the burster distance to the size of the source should be around one to ten trillion. “This relation plus other physical considerations will eventually provide powerful limits to the distance of bursters, which is the subject of the hottest debate in astronomy today,” comments Liang.

Discovered accidentally in the late 1960s by United States nuclear test surveillance satellites, the origin of gamma-ray bursts has puzzled astronomers for almost three decades. Results obtained by the Burst and Transient Source Experiment (BATSE) on board the NASA Compton Observatory, launched in 1991, further deepened the mystery. Currently, astronomers are deeply divided over whether the bursts originate from an extended halo around our own Milky Way or if they originate in galaxies billions of light years away.

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Editors: For a graphic illustrating the Rice predictions in GIF and TIFF formats, see:
http://spacsun.rice.edu/~acrider/papers/GRB_DECAY.HTML.

Edison Liang, Rice professor of space physics and astronomy, can be reached at (713) 527-8101, ext. 3524, or liang@spacsun.rice.edu.