GRB 970508

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GRB 970508
UTC
May 8, 1997
ConstellationCamelopardalis Edit this on Wikidata
Right ascension06h 53m 49s[1]
Declination+79° 16′ 19.6″[1]
Distance6,000,000,000 ly (1.8×109 pc)
Redshift0.835 ≤ z ≤ 2.3
Peak apparent magnitude19.6
Total energy output5 × 1050 erg (5 × 1043 J)
Other designationsGRB 970508
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GRB 970508 was a

UTC; it is historically important as the second GRB (after GRB 970228
) with a detected afterglow at other wavelengths, the first to have a direct redshift measurement of the afterglow, and the first to be detected at radio wavelengths.

A gamma-ray burst is a highly

radio
).

GRB 970508 was detected by the Gamma Ray Burst Monitor on the Italian–Dutch

light years from Earth
; this was the first measurement of the distance to a gamma-ray burst.

Until this burst, astronomers had not reached a consensus regarding how far away GRBs occur from Earth. Some supported the idea that GRBs occur within the Milky Way, but are visibly faint because they are not highly energetic. Others concluded that GRBs occur in other galaxies at cosmological distances and are extremely energetic. Although the possibility of multiple types of GRBs meant that the two theories were not mutually exclusive, the distance measurement unequivocally placed the source of the GRB outside the Milky Way, effectively ending the debate.

GRB 970508 was also the first burst with an observed radio frequency afterglow. By analyzing the fluctuating strength of the radio signals, astronomer Dale Frail calculated that the source of the radio waves had expanded almost at the speed of light. This provided strong evidence that GRBs are relativistically expanding explosions.

Discovery

Artist's conception of BeppoSAX in orbit

A gamma-ray burst (GRB) is a highly

optical, infrared, and radio). The first GRB afterglow to be discovered was the X-ray afterglow of GRB 970228,[3] which was detected by BeppoSAX, an Italian–Dutch satellite originally designed to study X-rays.[4]

On Thursday May 8, 1997, at 21:42 UTC, BeppoSAX's Gamma Ray Burst Monitor registered a gamma-ray burst that lasted approximately 15 seconds.

arcminutes.[6]

Observations

The Very Large Array in New Mexico

After a rough position of the burst had been determined,

Caltech observatory, conducted a more extensive analysis of the data, but was also unable to identify any new light sources.[9]

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.

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.[10]

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,[11] which was later confirmed to be the burst's optical afterglow.[10]

On the night between May 10 and May 11, 1997, Metzger's colleague

Å (430–710 nm) and 3,500–8,000 Å (350–800 nm), but no emission lines were identified.[20]

On May 13, five days after the first detection of GRB 970508, Frail resumed his observations with the Very Large Array.[21] He made observations of the burst's position at a wavelength of 3.5 cm and immediately detected a strong signal.[21] After 24 hours, the 3.5 cm signal became significantly stronger, and he also detected signals at the 6 and 21 cm wavelengths.[21] This was the first confirmed observation of a radio afterglow of a GRB.[21][22][23]

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.[22][24] 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.[24]

Characteristics

BeppoSAX's Gamma-Ray Burst Monitor, operating in the energy range of 40–700 

nJ/m2), and the Wide Field Camera (2–26 keV) recorded a fluence of (0.7 ± 0.1) × 10−6 erg/cm2 (0.7 ± 0.1 nJ/m2).[25] BATSE (20–1000 keV) recorded a fluence of (3.1 ± 0.2) × 10−6 erg/cm2 (3.1 ± 0.2 nJ/m2).[8]

About 5 hours after the burst the apparent magnitude of the object—a logarithmic measure of its brightness with a higher number indicating a fainter object—was 20.3 ± 0.3 in the U-band (the ultraviolet region of the spectrum) and 21.2 ± 0.1 in the R-band (the red region of the spectrum).[20] The afterglow reached its peak luminosity in both bands approximately 2 days after the burst was first detected—19.6 ± 0.3 in the U-band at 02:13 UTC on May 11, and 19.8 ± 0.2 in the R-band at 20:55 UTC on May 10.[20]

James E. Rhoads, an astronomer at the Kitt Peak National Observatory, analyzed the burst and determined that it was not strongly beamed.[26] Further analysis by Frail and his colleagues indicated that the total energy released by the burst was approximately 5×1050 ergs (5×1043 J), and Rhoads determined that the total gamma-ray energy was approximately 3×1050 erg (3×1043 J).[27] This implied that the gamma-ray and kinetic energy of the burst's ejecta were comparable, effectively ruling out those GRB models which are relatively inefficient at producing gamma rays.[27]

Distance scale and emission model

Image of GRB 970508's host galaxy taken in August 1998

Prior to this burst, astronomers had not reached consensus regarding how far away GRBs occur from Earth. Although the

halo, concluding that the bursts are visibly faint because they are not highly energetic. Others concluded that GRBs occur in other galaxies at cosmological distances and that they can be detected because they are extremely energetic. The distance measurement and the calculations of the burst's total energy release unequivocally supported the latter theory, effectively ending the debate.[28]

Throughout the month of May the radio scintillations became less noticeable until they ceased altogether. This implies that the radio source significantly expanded in the time that had passed since the burst was detected. Using the known distance to the source and the elapsed time before the scintillation ended, Frail calculated that the radio source had expanded at almost the speed of light.[29] While various existing models already encompassed the notion of a relativistically expanding fireball, this was the first strong evidence to support such a model.[30][31]

Host galaxy

The afterglow of GRB 970508 reached a peak total luminosity 19.82 days after the burst was detected. It then faded with a

ellipticity of 0.70 ± 0.07.[32] The redshift of GRB 970508's optical afterglow, z = 0.835, agreed with the host galaxy's redshift of z = 0.83, suggesting that, unlike previously observed bursts, GRB 970508 may have been associated with an active galactic nucleus.[32]

See also

Notes

  1. ^ a b Djorgovski 1997
  2. ^ Schilling 2002, pp. 12–16
  3. ^ Costa 1997
  4. ^ Schilling 2002, pp. 58–60
  5. ^ Pedersen 1997
  6. ^ a b Schilling 2002, pp. 115–116
  7. ^ Pian 1998
  8. ^ a b Kouveliotou 1997
  9. ^ a b Schilling 2002, pp. 116–117
  10. ^ a b c Schilling 2002, pp. 118–120
  11. ^ Bond 1997
  12. ^ a b Schilling 2002, pp. 121–123
  13. ^ Varendoff 2001, p. 383
  14. ^ a b Metzger 1997a
  15. ^ a b Metzger 1997b
  16. ^ Katz 2002, p. 148
  17. ^ Katz 2002, p. 149
  18. ^ Schilling 2002, p. 120
  19. ^ Reichart 1998
  20. ^ a b c Castro-Tirado 1998
  21. ^ a b c d Schilling 2002, p. 124
  22. ^ a b Katz 2002, p. 147
  23. ^ NRAO 1997
  24. ^ a b Schilling 2002, p. 125
  25. ^ Galama 1998
  26. ^ Rhoads 1999
  27. ^ a b Paczyński 1999, p. 2
  28. ^ Schilling 2002, p. 123
  29. ^ Waxman 1998
  30. ^ Schilling 2002, p. 126
  31. ^ Piran 1999, p. 23
  32. ^ a b c d Fruchter 2000
  33. ^ Bloom 1998

References

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