Gamma-ray bursts (GRBs) and their afterglows are the most brilliant transient events in the Universe. Both the bursts themselves and their afterglows have been predicted to be visible out to redshifts of z ≈ 20, and therefore to be powerful probes of the early Universe1,2. The burst GRB 000131, at z = 4.50, was hitherto the most distant such event identified3. Here we report the discovery of the bright near-infrared afterglow of GRB 050904 (ref. 4). From our measurements of the near-infrared afterglow, and our failure to detect the optical afterglow, we determine the photometric redshift of the burst to be (refs 5–7). Subsequently, it was measured8 spectroscopically to be z = 6.29 ± 0.01, in agreement with our photometric estimate. These results demonstrate that GRBs can be used to trace the star formation, metallicity, and reionization histories of the early Universe.
At 01:51:44 Universal Time (ut) on 4 September 2005, Swift's Burst Alert Telescope (BAT) detected GRB 050904 and 81 seconds later a 4′-radius localization was distributed to observers on the ground. Swift's X-Ray Telescope (XRT) automatically slewed to the BAT localization and 76 minutes after the burst a 6″-radius XRT localization was distributed.9
Over the next few hours, we observed the XRT localization at both near-infrared (NIR) and visible wavelengths (Table 1). In the NIR, we discovered a bright (J ≈ 17.4 mag at 3.1 hours after the burst) and fading source within the XRT localization using the 4.1-m Southern Observatory for Astrophysical Research (SOAR) telescope on top of Cerro Pachon in Chile (Fig. 1).4
However, at visible wavelengths we did not detect the afterglow to relatively deep limiting magnitudes using one of the six 0.41-m Panchromatic Robotic Optical Monitoring and Polarimetry Telescopes (PROMPT)10 that we are building on top of Cerro Tololo, which is only 10 km away from Cerro Pachon4, the 60-inch telescope at Palomar Observatory in California11, and the 3.5-m telescope at Calar Alto Observatory in Spain. Nor did we detect the afterglow with the 0.30-m Burst Observer and Optical Transient Exploring System (BOOTES)12 1B telescope in El Arenosillo, Spain, which began imaging the field only 2.1 minutes after the burst13. This implies that the GRB either occurred at a very high redshift or was very heavily extinguished by dust4.
Between about 3 hours and about 0.5 days after the burst, the fading of the afterglow appears to be well described by a power law of index (Fig. 2)5,6. However, after this time the fading appears to have slowed to a temporal index of (refs 7, 14, 15). A single power-law description is ruled out at the 3.7σ credible level. One possible explanation is that our initial SOAR observations caught the tail end of a reverse shock that had been stretched in time by a factor of 7.29 owing to cosmological time dilation. Even so, the reverse shock would still be at least a few times longer-lived in the source frame than the reverse shocks of GRBs 990123 and 021211 (refs 16, 17). Another possibility is that we are undersampling a light curve that is undergoing temporal variations, such as in the afterglows of GRBs 021004 and 030329 (refs 18, 19). Indeed, the X-ray afterglow is extremely variable at these times20.
Using these temporal indices to scale all of our measurements to a common time, except for a Z-band (0.84–0.93 µm) measurement from 11.4 hours after the burst (Table 1), we plot the spectral flux distribution of the afterglow in Fig. 3. In the NIR, the spectral index is However, the spectral index between NIR and visible wavelengths is steeper than -5.9. This is too sharp a transition to be explained by dust extinction alone5,21. However, a small amount of extinction cannot be ruled out and is probably present (Fig. 3).
Assuming negligible emission blueward of Lyα, we measure a photometric redshift of (refs 5, 6–7), which is consistent with the spectroscopic redshift of z = 6.29 ± 0.01 (ref. 8). For H0 = 71 km s-1 Mpc-1, Ωm = 0.27, and ΩΛ = 0.73 (ref. 22), this corresponds to about 900 million years after the Big Bang, when the Universe was about 6% of its current age. The next-most-distant GRB that has been identified occurred at z = 4.50 (ref. 3), which was about 500 million years later, when the Universe was about 10% of its current age.
One of the most exciting aspects of this discovery is the brightness of the afterglow: extrapolating back to a few minutes after the burst, the afterglow must have been exceptionally bright redward of Lyα for the robotic 0.25-m TAROT telescope to detect it in unfiltered visible-light observations23. Extrapolating our J-band light curve back to these times yields J ≈ 11–12 mag. This suggests that by pairing visible-light robotic telescopes with NIR robotic telescopes, and these with larger telescopes that are capable of quick-response NIR spectroscopy, all preferably at the same site so that they are subject to the same observing conditions, at least some very-high-redshift afterglows will be discovered, identified, and their NIR spectrum taken while they are still sufficiently bright to serve as a powerful probe of the conditions of the early Universe10.
D.E.R. gratefully acknowledges support from NSF's MRI, CAREER, PREST and REU programmes, NASA's APRA, Swift GI and IDEAS programmes, and especially L. Goodman and H. Cox. D.E.R. also thanks W. Christiansen, B. Carney, and everyone who has worked to make SOAR a reality over the past 19 years. A.L. and N.T. thank B. Cavanagh and A. Adamson of the JAC for their speedy assistance in acquiring and reducing the UKIRT WFCAM data. A.J.C.-T. thanks INTA and CSIC for their support of BOOTES and AYA (including FEDER funds). R.H. and P.K. acknowledge support from GA AV CR and ESA PECS.