|Detection time||07:55 UTC
April 23, 2009
|Right ascension||09h 55m 33.08s|
|Declination||+18° 08′ 58.9″|
|Redshift||8.0 ≤ z ≤ 8.3|
|Distance||13 billion ly (4 billion pc)|
GRB 090423 was a gamma-ray burst (GRB) detected by the Swift Gamma-Ray Burst Mission on April 23, 2009 at 07:55:19 UTC. The afterglow of GRB 090423 was detected in the infrared, and allowed astronomers to determine that the redshift of GRB 090423 is z = 8.2, which makes GRB 090423 the second most distant object detected as of October 2010.
A gamma-ray burst is an extremely luminous event flash of gamma rays that occurs as the result of an explosion, and is thought to be associated with the formation of a black hole. The burst itself typically only lasts for a few seconds, but gamma-ray bursts frequently produce an "afterglow" at longer wavelengths that can be observed for many hours or even days after the burst. Measurements at these wavelengths, which include X-ray, ultraviolet, optical, infrared, and radio, allow follow up study of the event.
The finite speed of light means that, at the time of discovery, GRB 090423 was also the earliest object ever detected. The universe was only 630 million years old when the light from GRB 090423 was emitted, and its detection confirms that massive stars were born and dying even very early on in the life of the universe. GRB 090423 and similar events provide a unique means of studying the early universe, as few other objects of that era are bright enough to be seen with today's telescopes.
 Discovery and observation
Swift localized the field in which GRB 090423 occurred, and 77 seconds after the burst, the Swift UVOT Photometric System took a 150 second exposure of the field, but was unable to detect an optical or ultraviolet afterglow. A few minutes after its discovery, ground based telescopes began observing the field. Within 20 minutes of the burst, Nial Tanvir and his team found an infrared source at the Swift position using the United Kingdom Infrared Telescope on Mauna Kea, Hawaii. They observed a drop off in flux beyond 1.13 micrometres with the VLT. Attributing this drop off to Lyman alpha absorption by neutral hydrogen in the intergalactic medium, they calculated a redshift of 8.2 for GRB 090423. The team of C.C. Thöne and Paolo D'Avanzo observed the afterglow of GRB 090423 using the Italian TNG 3.6m telescope located in the Canary Islands, Spain. They obtained two hours of spectra, which when combined, suggested a very weak signal at the position of the afterglow. They too saw a drop off in flux near 1.1 micrometres, and reported a redshift of 8.1 for GRB 090423, which is consistent, within error, of the redshift reported by Tanvir et al.
The intergovernmental astronomy organisation, European Southern Observatory (ESO) operates the Very Large Telescope that obtained the spectrum reported by Tanvir et al. The GRB was not visible in Chile when Swift first detected the burst at 07:55 UTC, but was the following day at 03:00 UTC, which allowed the Gamma-Ray Burst Optical/Near-Infrared Detector (GROND) at La Silla Observatory to make observations of the burst, and find a photometric redshift consistent with the value reported elsewhere. The last observers to gather data during the event was the Combined Array for Research in Millimeter-wave Astronomy (CARMA) observatory. The observation of GRB 090423 by CARMA was taken at a frequency of 92.5 GHz. While the afterglow was not detected, they were able to place a 3-sigma upper limit of 0.7 mJy on the flux density of the afterglow.
 Observation history
|Time (UTC)||Details of the sequence of GRB 090423 observations|
|April 23, 2009 07:55 UTC||Swift starts detecting burst, GRB 090423 is not yet visible in Chile|
|April 23, 2009 07:58 UTC||Several groups in the United States begin their observations of the GRB|
|April 23, 2009 08:16 UTC||First observations of an infrared afterglow by Tanvir's team using UKIRT in Mauna Kea, Hawaii (http://gcn.gsfc.nasa.gov/gcn3/9202.gcn3)|
|April 23, 2009 15:00 UTC||Using Gemini-North, Cucchiara’s team also in Hawaii, reports a wrong photometry claiming that z=9 (http://gcn.gsfc.nasa.gov/gcn3/9209.gcn3)|
|April 23, 2009 20:30 UTC||Cucchiara retracts report and revises photometry placing a constrain on the redshift between 7 and 9 (http://gcn.gsfc.nasa.gov/gcn3/9213.gcn3)|
|April 23, 2009 22:00 UTC||An Italian team led by Thöne using Telescopio Nazionale Galileo (TNG) starts its observation|
|April 23, 2009 23:00 UTC||The GRB now becomes visible in Chile and the Gamma-Ray Burst Optical/Near-Infrared Detector (GROND) at La Silla Observatory, observes at 7 bands simultaneously|
|April 24, 2009 01:30 UTC||Tanvir's team using the Very Large Telescope (VLT) starts its observations|
|April 24, 2009 03:00 UTC||Olivarez' team in Chile report a photometric redshift of z=8 (with errors +0.5, -1.2)|
|April 24, 2009 03:15 UTC||The Italian team led by Thöne reports a spectroscopic redshift of z=7.6|
|April 24, 2009 07:30 UTC||Tanvir reports a spectroscopic redshift of z=8.2|
|April 24, 2009 14:00 UTC||Italian team revises their spectroscopic redshift to z=8.1|
|April 25, 2009 03:45 UTC||Krimm's team using BAT released a lag analysis where long or short burst was inconclusive|
|April 25, 2009 10:40 UTC||VLA non-detection|
|April 25, 2009 18:30 UTC||Olivarez' team at GROND releases final photometry at z=8.0 (with errors +0.4, -0.8)|
|April 28, 2009 00:30 UTC||PdB observations led by Castro-Tirado on 23-24 Apr allow to detect the mm afterglow at a flux density of 0.2 mJy at 90 GHz|
|April 28, 2009 02:00 UTC||Non-detection by CARMA ( >0.7 mJy) at 92.5 GHz|
At a redshift of z = 8.2, the burst was the most distant known object of any kind at the time of its discovery. GRB 090423 was also the oldest known object in the Universe, as the light from the burst took approximately 13 billion years to reach Earth. It has since been surpassed by the discovery of a galaxy, UDFy-38135539, with a redshift of 8.6. Another gamma-ray burst, GRB 090429B, was subsequently found to have a photometric redshift of 9.4, which surpasses that of GRB 090423, although the comparatively large error bars from the photometric redshift technique mean that the GRB 090429B result is much less certain. The burst occurred when the Universe was approximately one twentieth of its present age. Prior to the observations done on GRB 090423, the previous record holder for age and distance for GRBs was GRB 080913, which was observed in September 2008. That burst had a redshift of 6.7, placing it approximately 190 million light-years closer to Earth than GRB 090423. Derek Fox, who led the observations done by Pennsylvania State University, suggests that the GRB was most likely the result of the explosion of a massive star and its demise, which would probably have signalled the birth of a black hole. The event occurred roughly 630 million years after the Big Bang, confirming that massive stellar births (and deaths) did indeed occur in the very early Universe.
Joshua Bloom of the University of California, Berkeley, who was able to observe the location of the GRB at the Gemini South telescope in Chile, called the discovery of GRB 090423 a "watershed event" as it marked "the beginning of the study of the universe as it was before most of the structure that we know about today came into being." Nial Tanvir, who was part of the VLT team, suggests that gamma-ray bursts provide a unique tool to study the universe at early times because everything else is too faint to be observed. For instance, the first generation of stars have yet to be directly observed, but the progenitor of GRB 090423 may belong to this class. These early stars are expected to contribute to the reionisation of the universe, a process which ended at a redshift of about 6. As more powerful telescopes begin operation, such as the James Webb Space Telescope that is set to launch in 2018, astronomers hope to pinpoint the locations of faint GRB host galaxies by using blasts similar to that of GRB 090423.
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- Tanvir, N. et al. "GRB 090423: UKIRT K-band observations". GCN Circulars (9202).
- Tanvir, N. et al. "GRB 090423: VLT/ISAAC spectroscopy". GCN Circulars (9219).
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- Video of GRB 090423 on Youtube
- Slashdot article "Most Distant Object Yet Detected, Bagged By Galileo Scope"
- Swift Mission at NASA/Goddard Space Flight Center
- GRB 090423 on WikiSky
|Most distant astronomical object
2009 — 2010
|Most distant gamma-ray burst
2009 — 2011