History of events
- Apr 23, 07:55 UT Swift detects burst, slews with its narrow-field
instruments, and find X-ray afterglow (Krimm et al.,
GCN 9198)
At that time, this GRB is not visible anymore from Chile - Apr 23, 07:58 UT Several groups in the US start observing.
- Apr 23, 11:00 UT UKIRT/Hawaii people are the first to report an infrared afterglow (Tanvir et al., GCN 9202)
- Apr 23, 15:00 UT A group observing with Gemini-North reports (wrong) photometry, and claims z=9 (Cucchiara et al., GCN 9209)
- Apr 23, 20:30 UT Cucchiara report revised photometry (which later turns
out still to be wrong), and claim 7
- Apr 23, 22:00 UT an italian group starts observing with the TNG
- Apr 23, 23:00 UT the GRB finally becomes visible from Chile, and GROND starts observing. We observe in 7 bands simultaneously.
- Apr 24, 01:30 UT VLT/Tanvir starts observing
- Apr 24, 03:00 UT GROND reports z=8 (with errors +0.5, -1.2) (Olivares et al.: GCN 9215)
- Apr 24, 03:15 UT the italian group reports z=7.6 (without error) shortly after the GROND report (Thöne et al. GCN 9216)
- Apr 24, 07:30 UT Tanvir reports z=8.2 from VLT data, though no error estimate as well (Tanvir et al., GCN 9219)
- Apr 24, 14:00 UT The italian group revises their z-estimate to 8.1; but still no error given (Fernandez-Soto, GCN 9222)
- Apr 25, 03:45 UT BAT lag analysis and other considerations on classification as long or short burst is inconclusive (Krimm et al., GCN 9241 )
- Apr 25, 10:40 UT VLA non-detection (Chandra et al., GCN 9249)
- Apr 26, 18:30 UT GROND reports final photometry with 8.0+0.4-0.8 (Krühler et al., GCN 9261)
- Apr 28, 00:30 UT Detection of 0.2 mJy (without error estimate) at 3 mm with Plateau de Bure Interferometer (Castro-Tirado et al., GCN 9273)
- Apr 28, 02:00 UT CARMA nondetection ( >0.7 mJy) at 92.5 GHz (Bock et al., GCN 9274)
GROND results
PNG format or TIFF format
False-color image, created from the GROND i', z' and J band images. Since the GRB afterglow is not detected in the i' and z' bands, it appears dark red in the image. Objects which have about the same brightness in all three filter bands appear white. Objects which are brighter in the i' or z' band than in J, appear bluish.
JPEG format or TIFF format
Spectral energy distribution of the afterglow of GRB 090423 as derived from the GROND data. The top row shows the image of the corresponding filter band. The jump from non-detection in the z'-band to a bright source in the J band is obvious. The fact that the J-band data point lies slightly below the power law fit indicates that the Ly-alpha absorption edge is within the J band.
TNG results
2D spectrum of GRB 090423 taken with the TNG and the AMICI prism (from http://www.tng.iac.es/news/2009/04/24/grb/). A faint trace is seen at the position of the GRB afterglow. This was the basis for the GCN reports 9216 and 9222 (see above).
Tanvir: A glimpse of the end of the dark ages: the gamma-ray burst of 23 April 2009 at redshift 8.3
Abstract: It is thought that the first generations of massive stars in the Universe were an important, and quite possibly dominant, source of the ultra-violet radiation that reionized the hydrogen gas in the intergalactic medium (IGM); a state in which it has remained to the present day. Measurements of cosmic microwave background anisotropies suggest that this phase-change largely took place in the redshift range z=10.8 +/- 1.4, while observations of quasars and Lyman-alpha galaxies have shown that the process was essentially completed by z=6. However, the detailed history of reionization, and characteristics of the stars and proto-galaxies that drove it, remain unknown. Further progress in understanding requires direct observations of the sources of ultra-violet radiation in the era of reionization, and mapping the evolution of the neutral hydrogen fraction through time. The detection of galaxies at such redshifts is highly challenging, due to their intrinsic faintness and high luminosity distance, whilst bright quasars appear to be rare beyond z~7. Here we report the discovery of a gamma-ray burst, GRB 090423, at redshift z=8.26 -0.08 +0.07. This is well beyond the redshift of the most distant spectroscopically confirmed galaxy (z=6.96) and quasar (z=6.43). It establishes that massive stars were being produced, and dying as GRBs, ~625 million years after the Big Bang. In addition, the accurate position of the burst pinpoints the location of the most distant galaxy known to date. Larger samples of GRBs beyond z~7 will constrain the evolving rate of star formation in the early universe, while rapid spectroscopy of their afterglows will allow direct exploration of the progress of reionization with cosmic time.
Salvaterra et al: GRB 090423 reveals an exploding star at the epoch of re-ionization
Abstract: The observation of the very early stages of the Universe represents one of the main challenges of modern cosmology. 200-300 million years after the Big Bang stars began to form, thus providing the Universe with the first sources of light and heat after the Big Bang. This event marks the transition between the epoch when the Universe was dark and neutral and the time when the Universe became fully ionized. The direct investigation of this cosmic epoch, usually accomplished by observing distant quasars, has been revolutionized in the last decade through the study of Gamma-ray Bursts (GRBs). GRBs are gamma-ray flashes, detected from space, produced by rare types of massive stellar explosions. Their rapidly fading afterglows are often bright at optical wavelengths, such that GRBs are detectable up to cosmological distances. Here we report on the Swift observation of GRB 090423 and the near-infrared spectroscopic measurement of its redshift z=8.1^{+0.1}_{-0.3} obtained with the Telescopio Nazionale Galileo (TNG). This GRB was produced in a cosmic explosion that occurred before the re-ionization process was completed, when the Universe was only ~4% of its current age. Unexpectedly, this primordial object exhibits properties similar to those of GRBs observed at low/intermediate redshifts, indicating that the mechanisms and progenitors which gave rise to GRBs about 600 million years after the Big Bang are not markedly different from those producing GRBs ~10 billion years later. The detection of this GRB at z=8.1 indicates either that the GRBs are not good tracers of the cosmic star formation or that the number of bright GRBs was greater at high redshift.