GRB 970228
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GRB 970228 was a gamma ray burst (GRB) that occurred on February 28, 1997 which was designated in accordance with the date of the event.
The BeppoSAX satellite located the optical counterpart to the GRB. Some observations seemed to show the object was moving rapidly across the sky. That meant it couldn't be too far away, implying the bursts are a local phenomenon.
On May 8, 1997, BeppoSAX recorded another burst in the constellation Camelopardalis. The spacecraft's science team sent out an alert over the Internet. Seven hours later, an optical source was detected by astronomer Howard Bond, using a 90-centimeter telescope at Kitt Peak National Observatory in Arizona.
On May 11, astronomers used the 10 meter Keck II telescope on the island of Mauna Kea, Hawaii, to obtain a spectrum of the object. The spectrum showed absorption lines, or frequencies where the light was absorbed by gases between the object and Earth. The patterns of absorption lines are specific to different atoms and molecules.
The motion of an object causes a Doppler shift in the wavelengths of these lines, with the expansion of the Universe causing a redshift towards longer wavelengths. The amount of redshift is proportional to the distance of the object, and in this case the spectrum showed a redshift of 0.835, indicating the object was billions of light-years away.
Astronomers were baffled. One observation indicated a local origin, the other a distant origin. Astronomers suspected that at least one of the correlated optical sources may have had nothing to do with a gamma-ray burst, and simply happened to be in the right place at the right time. Some astronomers were also unable to detect any proper motion in the object linked to GRB 970228.
Following these observations, astronomers were able to track down more faint visible-light and radio afterglows of GRBs, hours or days after the occurrence of the burst. A few more redshifts were obtained, and confirmed that the bursts occurred in the distant cosmos. The high proper motion reported for GRB 970228 was clearly erroneous, and in fact observations made by the Hubble Space Telescope in September 1997 showed no proper motion in the faint afterglow that remained from GRB 970228.
[edit] References
- Esin AA, Blandford R (2000). "Dust Echoes from Gamma-Ray Bursts". Astrophysical Journal 534 (2): L151-L154. PMID 10813670.
