GRB 970508
Encyclopedia
GRB 970508 was a gamma-ray burst (GRB) detected on May 8, 1997, at 21:42 UTC. A gamma-ray burst is a highly luminous
flash associated with an explosion in a distant galaxy and producing gamma ray
s, the most energetic form of electromagnetic radiation
, and often followed by a longer-lived "afterglow" emitted at longer wavelengths (X-ray
, ultraviolet
, optical, infrared
, and radio
).
GRB 970508 was detected by the Gamma Ray Burst Monitor on the Italian–Dutch X-ray astronomy
satellite BeppoSAX
. Astronomer Mark Metzger determined that GRB 970508 occurred at least 6 billion 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.
A gamma-ray burst (GRB) is a highly luminous
flash of gamma ray
s—the most energetic form of electromagnetic radiation
. GRBs were first detected in 1967 by the Vela satellites (a series of spacecraft designed to detect nuclear explosions in space). The initial burst is often followed by a longer-lived "afterglow" emitted at longer wavelengths (X-ray
, ultraviolet
, optical, infrared
, and radio
). The first GRB afterglow to be discovered was the X-ray afterglow of GRB 970228
, which was detected by BeppoSAX
, an Italian–Dutch satellite originally designed to study X-rays.
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. It was also detected by Ulysses, a robotic space probe designed to study the Sun
, and by the Burst and Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory
. The burst also occurred within the field of view of one of BeppoSAX's two X-ray
Wide Field Cameras. Within a few hours, the BeppoSAX team localized the burst to an error box—a small area around the specific position to account for the error
in the position—with a diameter of approximately 10 arcminutes.
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-year
s 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 4300–7100 Å (430–710 nm) and 3500–8000 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.
of (1.85 ± 0.3) × 10−6 erg
/cm2 (1.85 ± 0.3 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). BATSE (20–1000 keV) recorded a fluence of (3.1 ± 0.2) × 10−6 erg/cm2 (3.1 ± 0.2 nJ/m2).
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). 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.
James E. Rhoads, an astronomer at the Kitt Peak National Observatory
, analyzed the burst and determined that it was not strongly beamed
. 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). 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.
of bursts suggested that they do not occur within the disk of the Milky Way
, some astronomers supported the idea that they occur within the Milky Way's 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.
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
. While various existing models already encompassed the notion of a relativistically
expanding fireball, this was the first strong evidence to support such a model.
The afterglow of GRB 970508 reached a peak total luminosity 19.82 days after the burst was detected. It then faded with a power law
slope over about 100 days. The afterglow eventually disappeared, revealing the burst's host, an actively star-forming dwarf galaxy with an apparent magnitude
of V = 25.4 ± 0.15. The galaxy was well fitted by an exponential disk with an ellipticity of 0.70 ± 0.07. 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
.
Luminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...
flash associated with an explosion in a distant galaxy and producing gamma ray
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...
s, the most energetic form of electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
, and often followed by a longer-lived "afterglow" emitted at longer wavelengths (X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
, ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
, optical, infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
, and radio
Radio waves
Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies from 300 GHz to as low as 3 kHz, and corresponding wavelengths from 1 millimeter to 100 kilometers. Like all other electromagnetic waves,...
).
GRB 970508 was detected by the Gamma Ray Burst Monitor on the Italian–Dutch X-ray astronomy
X-ray astronomy
X-ray astronomy is an observational branch of astronomy which deals with the study of X-ray observation and detection from astronomical objects. X-radiation is absorbed by the Earth's atmosphere, so instruments to detect X-rays must be taken to high altitude by balloons, sounding rockets, and...
satellite BeppoSAX
BeppoSAX
BeppoSAX was an Italian–Dutch satellite for X-ray astronomy which played a crucial role in resolving the origin of gamma-ray bursts , the most energetic events known in the universe...
. Astronomer Mark Metzger determined that GRB 970508 occurred at least 6 billion 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
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
, but are visibly faint because they are not highly energetic. Others concluded that GRBs occur in other galaxies at cosmological
Physical cosmology
Physical cosmology, as a branch of astronomy, is the study of the largest-scale structures and dynamics of the universe and is concerned with fundamental questions about its formation and evolution. For most of human history, it was a branch of metaphysics and religion...
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
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
, effectively ending the debate.
GRB 970508 was also the first burst with an observed radio frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
afterglow. By analyzing the fluctuating strength of the radio signals, astronomer Dale Frail
Dale Frail
Dale A. Frail is an astronomer working at the National Radio Astronomy Observatory in Socorro, New Mexico. He was born in Canada, spent much of his childhood in Europe, and his professional career has been based in the United States.-Career:...
calculated that the source of the radio waves had expanded almost at the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
. This provided strong evidence that GRBs are relativistically
Theory of relativity
The theory of relativity, or simply relativity, encompasses two theories of Albert Einstein: special relativity and general relativity. However, the word relativity is sometimes used in reference to Galilean invariance....
expanding explosions.
Discovery
Luminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...
flash of gamma ray
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...
s—the most energetic form of electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
. GRBs were first detected in 1967 by the Vela satellites (a series of spacecraft designed to detect nuclear explosions in space). The initial burst is often followed by a longer-lived "afterglow" emitted at longer wavelengths (X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
, ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
, optical, infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
, and radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
). The first GRB afterglow to be discovered was the X-ray afterglow of GRB 970228
GRB 970228
GRB 970228 was the first gamma-ray burst for which an afterglow was observed. It was detected on 28 February 1997 at 02:58 UTC. Since 1993, physicists had predicted GRBs to be followed by a lower-energy afterglow , but until this event, GRBs had only been observed in highly luminous bursts of...
, which was detected by BeppoSAX
BeppoSAX
BeppoSAX was an Italian–Dutch satellite for X-ray astronomy which played a crucial role in resolving the origin of gamma-ray bursts , the most energetic events known in the universe...
, an Italian–Dutch satellite originally designed to study X-rays.
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. It was also detected by Ulysses, a robotic space probe designed to study the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
, and by the Burst and Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory
Compton Gamma Ray Observatory
The Compton Gamma Ray Observatory was a space observatory detecting light from 20 KeV to 30 GeV in Earth orbit from 1991 to 2000. It featured four main telescopes in one spacecraft covering x-rays and gamma-rays, including various specialized sub-instruments and detectors...
. The burst also occurred within the field of view of one of BeppoSAX's two X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
Wide Field Cameras. Within a few hours, the BeppoSAX team localized the burst to an error box—a small area around the specific position to account for the error
Measurement uncertainty
In metrology, measurement uncertainty is a non-negative parameter characterizing the dispersion of the values attributed to a measured quantity. The uncertainty has a probabilistic basis and reflects incomplete knowledge of the quantity. All measurements are subject to uncertainty and a measured...
in the position—with a diameter of approximately 10 arcminutes.
Observations
Enrico Costa
Enrico Costa is an Italian bobsledder who competed from 1994 to the early 2000s. He won a gold medal in the two-man event at the 1999 FIBT World Championships in Cortina d'Ampezzo....
of the BeppoSAX team contacted astronomer Dale Frail
Dale Frail
Dale A. Frail is an astronomer working at the National Radio Astronomy Observatory in Socorro, New Mexico. He was born in Canada, spent much of his childhood in Europe, and his professional career has been based in the United States.-Career:...
at the National Radio Astronomy Observatory
National Radio Astronomy Observatory
The National Radio Astronomy Observatory is a Federally Funded Research and Development Center of the United States National Science Foundation operated under cooperative agreement by Associated Universities, Inc for the purpose of radio astronomy...
's Very Large Array
Very Large Array
The Very Large Array is a radio astronomy observatory located on the Plains of San Agustin, between the towns of Magdalena and Datil, some fifty miles west of Socorro, New Mexico, USA...
. Frail began making observations at a wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
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
Hale telescope
The Hale Telescope is a , 3.3 reflecting telescope at the Palomar Observatory in California, named after astronomer George Ellery Hale. With funding from the Rockefeller Foundation, he orchestrated the planning, design, and construction of the observatory, but did not live to see its commissioning...
. Djorgovski immediately compared his images of the region with older images from the Digitized Sky Survey
Digitized Sky Survey
The Digitized Sky Survey is a digital version of several photographic atlases of the night sky, and an ongoing project to produce more digital versions of photographic astronomical datasets.- Versions and source material :...
, 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
Variable star
A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...
rather than the GRB afterglow. Titus Galama and Paul Groot, members of a research team in Amsterdam
Amsterdam
Amsterdam is the largest city and the capital of the Netherlands. The current position of Amsterdam as capital city of the Kingdom of the Netherlands is governed by the constitution of August 24, 1815 and its successors. Amsterdam has a population of 783,364 within city limits, an urban population...
led by Jan van Paradijs, compared images taken by the WIYN Telescope on May 8 and the William Herschel Telescope
William Herschel Telescope
The William Herschel Telescope is a optical/near-infrared reflecting telescope located at the Observatorio del Roque de los Muchachos on the island of La Palma in the Canary Islands, Spain. The telescope, which is named after William Herschel, is part of the Isaac Newton Group of Telescopes...
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
Space Telescope Science Institute
The Space Telescope Science Institute is the science operations center for the Hubble Space Telescope and for the James Webb Space Telescope...
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
Spectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
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
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
and iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
determined a redshift
Redshift
In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...
of z = 0.8349 ± 0.0002, indicating that light from the burst had been absorbed by matter roughly 6 billion light-year
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
s 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
Lyman-alpha forest
In astronomical spectroscopy, the Lyman-alpha forest is the sum of absorption lines arising from the Lyman-alpha transition of the neutral hydrogen in the spectra of distant galaxies and quasars....
features in the spectra constrained the redshift to z ≤ 2.3, while further investigation by Daniel E. Reichart of the University of Chicago
University of Chicago
The University of Chicago is a private research university in Chicago, Illinois, USA. It was founded by the American Baptist Education Society with a donation from oil magnate and philanthropist John D. Rockefeller and incorporated in 1890...
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
Calar Alto Observatory
The Calar Alto Observatory is an astronomical observatory owned and operated jointly by the Max-Planck-Institut für Astronomie in Heidelberg, Germany, and the Instituto...
at wavelength ranges of 4300–7100 Å (430–710 nm) and 3500–8000 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
Princeton University
Princeton University is a private research university located in Princeton, New Jersey, United States. The school is one of the eight universities of the Ivy League, and is one of the nine Colonial Colleges founded before the American Revolution....
explained as being the result of the radio waves being bent by interstellar plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
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.
Characteristics
BeppoSAX's Gamma-Ray Burst Monitor, operating in the energy range of 40–700 keV, recorded a fluenceFluence
In physics, fluence is the flux integrated over time. For particles, it is defined as the total number of particles that intersect a unit area in a specific time interval of interest, and has units of m–2...
of (1.85 ± 0.3) × 10−6 erg
Erg
An erg is the unit of energy and mechanical work in the centimetre-gram-second system of units, symbol "erg". Its name is derived from the Greek ergon, meaning "work"....
/cm2 (1.85 ± 0.3 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). BATSE (20–1000 keV) recorded a fluence of (3.1 ± 0.2) × 10−6 erg/cm2 (3.1 ± 0.2 nJ/m2).
About 5 hours after the burst the apparent magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
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
Photometric system
In astronomy, a Photometric system is a set of well-defined passbands , with a known sensitivity to incident radiation. The sensitivity usually depends on the optical system, detectors and filters used. For each photometric system a set of primary standard stars is provided.The first known...
(the ultraviolet region of the spectrum) and 21.2 ± 0.1 in the R-band (the red region of the spectrum). 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.
James E. Rhoads, an astronomer at the Kitt Peak National Observatory
Kitt Peak National Observatory
The Kitt Peak National Observatory is a United States astronomical observatory located on 2,096 m Kitt Peak of the Quinlan Mountains in the Arizona-Sonoran Desert on the Tohono O'odham Nation, southwest of Tucson...
, analyzed the burst and determined that it was not strongly beamed
Light beam
A light beam or beam of light is a narrow projection of light energy radiating from a source into a beam. Sunlight is a natural example of a light beam when filtered through various mediums...
. 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). 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.
Distance scale and emission model
Prior to this burst, astronomers had not reached consensus regarding how far away GRBs occur from Earth. Although the isotropic distributionIsotropy
Isotropy is uniformity in all orientations; it is derived from the Greek iso and tropos . Precise definitions depend on the subject area. Exceptions, or inequalities, are frequently indicated by the prefix an, hence anisotropy. Anisotropy is also used to describe situations where properties vary...
of bursts suggested that they do not occur within the disk of the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
, some astronomers supported the idea that they occur within the Milky Way's halo, concluding that the bursts are visibly faint because they are not highly energetic. Others concluded that GRBs occur in other galaxies at cosmological
Physical cosmology
Physical cosmology, as a branch of astronomy, is the study of the largest-scale structures and dynamics of the universe and is concerned with fundamental questions about its formation and evolution. For most of human history, it was a branch of metaphysics and religion...
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.
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
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
. While various existing models already encompassed the notion of a relativistically
Theory of relativity
The theory of relativity, or simply relativity, encompasses two theories of Albert Einstein: special relativity and general relativity. However, the word relativity is sometimes used in reference to Galilean invariance....
expanding fireball, this was the first strong evidence to support such a model.
Host galaxy
Power law
A power law is a special kind of mathematical relationship between two quantities. When the frequency of an event varies as a power of some attribute of that event , the frequency is said to follow a power law. For instance, the number of cities having a certain population size is found to vary...
slope over about 100 days. The afterglow eventually disappeared, revealing the burst's host, an actively star-forming dwarf galaxy with an apparent magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
of V = 25.4 ± 0.15. The galaxy was well fitted by an exponential disk with an ellipticity of 0.70 ± 0.07. 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
Active galactic nucleus
An active galactic nucleus is a compact region at the centre of a galaxy that has a much higher than normal luminosity over at least some portion, and possibly all, of the electromagnetic spectrum. Such excess emission has been observed in the radio, infrared, optical, ultra-violet, X-ray and...
.