Gamma-ray bursts (GRBs) are produced by rare types of massive stellar explosion. Their rapidly fading afterglows are often bright enough at optical wavelengths that they are detectable at cosmological distances. Hitherto, the highest known redshift for a GRB was z = 6.7 (ref. 1), for GRB 080913, and for a galaxy was z = 6.96 (ref. 2). Here we report observations of GRB 090423 and the near-infrared spectroscopic measurement of its redshift, z = 8.1(-0.3)(+0.1). This burst happened when the Universe was only about 4 per cent of its current age. Its properties are similar to those of GRBs observed at low/intermediate redshifts, suggesting that the mechanisms and progenitors that gave rise to this burst about 600,000,000 years after the Big Bang are not markedly different from those producing GRBs about 10,000,000,000 years later.

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We present an updated catalogue of 113 X-ray flares detected by Swift in the ~33 per cent of the X-ray afterglows of gamma-ray burst (GRB). 43 flares have a measured redshift. For the first time the analysis is performed in four different X-ray energy bands, allowing us to constrain the evolution of the flare temporal properties with energy. We find that flares are narrower at higher energies: their width follows a power-law relation w ~ E^(-0.5) reminiscent of the prompt emission. Flares are asymmetric structures, with a decay time which is twice the rise time on average. Both time-scales linearly evolve with time, giving rise to a constant rise-to-decay ratio: this implies that both time-scales are stretched by the same factor. As a consequence, the flare width linearly evolves with time to larger values: this is a key point that clearly distinguishes the flare from the GRB prompt emission. The flare 0.3-10 keV peak luminosity decreases with time, following a power-law behaviour with large scatter: Lpk ~ t^(-2.7+/-0.5). When multiple flares are present, a global softening trend is established: each flare is on average softer than the previous one. The 0.3-10 keV isotropic energy distribution is a lognormal peaked at 10^51 erg, with a possible excess at low energies. The flare average spectral energy distribution is found to be a power law with spectral energy index β ~ 1.1. These results confirmed that the flares are tightly linked to the prompt emission. However, after considering various models we conclude that no model is currently able to account for the entire set of observations

We show that our previously proposed anti-hierarchical baryon collapse scenario for the joint evolution of black holes and host galaxies predicts quasar luminosity functions at redshifts 1.5 z 6 and local properties in nice agreement with observations. In our model the quasar activity marks and originates the transition between an earlier phase of violent and heavily dust-enshrouded starburst activity promoting rapid black hole growth, and a later phase of almost passive evolution; the former is traced by the submillimeter-selected sources, while the latter accounts for the high number density of massive galaxies at substantial redshifts z 1.5, the population of Extremely Red Objects, and the properties of local ellipticals.

We present multiwavelength observations of the afterglow of GRB 130427A, the brightest (in total fluence) gamma-ray burst of the past 29 years. Optical spectroscopy from Gemini-North reveals the redshift of the GRB to be z = 0.340, indicating that its unprecedented brightness is primarily the result of its relatively close proximity to Earth; the intrinsic luminosities of both the GRB and its afterglow are not extreme in comparison to other bright GRBs. We present a large suite of multiwavelength observations spanning from 300 s to 130 d after the burst and demonstrate that the afterglow shows relatively simple, smooth evolution at all frequencies, with no significant latetime flaring or rebrightening activity. The entire dataset from 1 GHz to 10 GeV can be modeled as synchrotron emission from a combination of reverse and forward shocks in good agreement with the standard afterglow model, providing strong support to the applicability of the underlying theory and clarifying the nature of the GeV emission observed to last for minutes to hours following other very bright GRBs. A tenuous, wind-stratified circumburst density profile is required by the observations, suggesting a massive-star progenitor with a low mass-loss rate, perhaps due to low metallicity. GRBs similar in nature to GRB 130427A, inhabiting low-density media and exhibiting strong reverse shocks, are probably not uncommon but may have been difficult to recognize in the past owing to their relatively faint late-time radio emission; more such events should be found in abundance by the new generation of sensitive radio and millimeter instruments. 25 Here and elsewhere we assume a standard ΛCDM cosmological model with Ω Λ = 0.7, Ωm = 0.3, h = 0.7.

The temporal and spectral analysis of nine bright X‐ray flares out of a sample of 113 flares observed by Swift reveals that the flare phenomenology is strictly analogous to the prompt γ‐ray emission: high‐energy flare profiles rise faster, decay faster and peak before the low‐energy emission. However, flares and prompt pulses differ in one crucial aspect: flares evolve with time. As time proceeds, flares become wider, with larger peak lag, lower luminosities and softer emission. The flare spectral peak energy Ep,i evolves to lower values following an exponential decay which tracks the decay of the flare flux. The two flares with best statistics show higher than expected isotropic energy Eiso and peak luminosity Lp,iso when compared to the Ep,i–Eiso and Ep,i–Liso prompt correlations. Ep,i is found to correlate with Liso within single flares, giving rise to a time‐resolved Ep,i(t)–Liso(t). Like prompt pulses, flares define a lag–luminosity relation: L0.3–10 keVp,iso∝t−0.95±0.23lag. The lag–luminosity is proven to be a fundamental law extending ∼5 decades in time and ∼5 decades in energy. Moreover, this is direct evidence that γ‐ray burst (GRB) X‐ray flares and prompt γ‐ray pulses are produced by the same mechanism. Finally we establish a flare–afterglow morphology connection: flares are preferentially detected superimposed to one‐break or canonical X‐ray afterglows.

Aims. In this work the luminosity function of QSOs is measured in the redshift range 3.5 < z < 5.2 for the absolute magnitude interval −21 < M 145 < −28. Determining the faint-end of the luminosity function at these redshifts provides important constraints on models of the joint evolution of galaxies and AGNs. Methods. We have defined suitable criteria to select faint QSOs in the GOODS fields, checking their effectiveness and completeness in detail. A spectroscopic follow-up of the resulting QSO candidates was carried out. The confirmed sample of faint QSOs is compared with a brighter one derived from the SDSS. We used a Monte-Carlo technique to estimate the properties of the luminosity function, checking various parameterizations for its shape and evolution. Results. Models based on pure density evolution show better agreement with observation than do models based on pure luminosity evolution. However, a different break magnitude with respect to z ∼ 2.1 is required at 3.5 < z < 5.2. Models with a steeper faint-end score a higher probability. We do not find any evidence for a bright-end flattening at redshift z > 3.5. Conclusions. The estimated space density evolution of QSOs indicates a suppression of the formation and/or feeding of supermassive black holes at these redshifts. The QSO contribution to the UV background is insufficient for ionizing the IGM at 3.5 < z < 5.2.

We explore the onset of star formation in the early universe, exploiting the observations of high-redshift LBGs and Ly emitters (LAEs), in the framework of the galaxy formation scenario elaborated by Granato and coworkers, already successfully tested against the wealth of data on later evolutionary stages. Complementing the model with a simple, physically plausible recipe for the evolution of dust attenuation in metal-poor galaxies, we reproduce the LFs of LBGs and of LAEs at different redshifts. This recipe yields a much faster increase with galactic age of attenuation in more massive galaxies, endowed with higher SFRs. These objects have therefore shorter lifetimes in the LAE and LBG phases and are more easily detected in the dusty submillimeter-bright (SMB) phase. The short UV-bright lifetimes of massive objects strongly mitigate the effect of the fast increase of the massive halo density with decreasing redshift, thus accounting for the weaker evolution of the LBG LF, compared to that of the halo mass function, and the even weaker evolution between z % 6 and z % 3 of the LAE LF. The much lower fraction of LBGs hosting detectable nuclear activity, compared to SMB galaxies, comes out naturally from the evolutionary sequence yielded by the model, which features the coevolution of galaxies and active nuclei. In this framework LAEs are on the average expected to be younger, with lower stellar masses, more compact, and associated with less massive halos than LBGs. Finally, we show that the IGM can be completely reionized at redshift z % 6Y7 by massive stars shining in protogalactic spheroids with halo masses from a few times 10 10 to a few times 10 11 M , showing up as faint LBGs with magnitude in the range À17 P M 1350 P À20, without resorting to any special stellar IMF.