GRB 970508 - Observations

Observations

After a rough position of the burst had been determined, Enrico Costa of the BeppoSAX team contacted astronomer Dale Frail at the National Radio Astronomy Observatory's Very Large Array. Frail began making observations at a wavelength of 20 centimeters at 01:30 UTC, less than four hours after the discovery. While preparing for his observations Frail contacted astronomer Stanislav Djorgovski, who was working with the Hale telescope. Djorgovski immediately compared his images of the region with older images from the Digitized Sky Survey, but he found no new sources of light within the error box. Mark Metzger, a colleague of Djorgovski at the Caltech observatory, conducted a more extensive analysis of the data, but was also unable to identify any new light sources.

The following evening Djorgovski again observed the region. He compared the images from both nights but the error box contained no objects that had decreased in luminosity between May 8 and May 9. Metzger noticed one object that had increased in luminosity, but he assumed it was a variable star rather than the GRB afterglow. Titus Galama and Paul Groot, members of a research team in Amsterdam led by Jan van Paradijs, compared images taken by the WIYN Telescope on May 8 and the William Herschel Telescope on May 9. They were also unable to find any light sources which had faded during that time.

After discovering the burst's X-ray afterglow, the BeppoSAX team provided a more accurate localization, and what Metzger had assumed to be a variable star was still present in this smaller error box. Both the Caltech team and the Amsterdam team were hesitant to publish any conclusions on the variable object. On May 10 Howard Bond of the Space Telescope Science Institute published his discovery, which was later confirmed to be the burst's optical afterglow.

On the night between May 10 and May 11, 1997, Metzger's colleague Charles Steidel recorded the spectrum of the variable object at the W. M. Keck Observatory. He then sent the data to Metzger, who after identifying a system of absorption lines associated with magnesium and iron determined a redshift of z = 0.8349 ± 0.0002, indicating that light from the burst had been absorbed by matter roughly 6 billion light-years from Earth. Although the redshift of the burst itself had not been determined, the absorbent matter was necessarily located between the burst and the Earth, implying that the burst itself was at least as far away. The absence of Lyman-alpha forest features in the spectra constrained the redshift to z ≤ 2.3, while further investigation by Daniel E. Reichart of the University of Chicago suggested a redshift of z ≈ 1.09. This was the first instance in which scientists were able to measure the redshift of a GRB. Several optical spectra were also obtained at the Calar Alto Observatory at wavelength ranges of 4,300–7,100 Å (430–710 nm) and 3,500–8,000 Å (350–800 nm), but no emission lines were identified.

On May 13, five days after the first detection of GRB 970508, Frail resumed his observations with the Very Large Array. He made observations of the burst's position at a wavelength of 3.5 cm and immediately detected a strong signal. After 24 hours, the 3.5 cm signal became significantly stronger, and he also detected signals at the 6 and 21 cm wavelengths. This was the first confirmed observation of a radio afterglow of a GRB.

Over the next month, Frail observed that the luminosity of the radio source fluctuated significantly from day to day but increased on average. The fluctuations did not occur simultaneously along all of the observed wavelengths, which Jeremy Goodman of Princeton University explained as being the result of the radio waves being bent by interstellar plasma in the Milky Way. Such radio scintillations (rapid variations in the radio luminosity of an object) occur only when the source has an apparent diameter of less than 3 microarcseconds.