We present the photometric calibration of the Swift Ultraviolet/Optical Telescope (UVOT) which includes: optimum photometric and background apertures, effective area curves, colour transformations, conversion factors for count rates to flux and the photometric zero-points (which are accurate to better than 4 per cent) for each of the seven UVOT broad-band filters. The calibration was performed with observations of standard stars and standard star fields that represent a wide range of spectral star types. The calibration results include the position-dependent uniformity, and instrument response over the 1600-8000 Å operational range. Because the UVOT is a photon-counting instrument, we also discuss the effect of coincidence loss on the calibration results. We provide practical guidelines for using the calibration in UVOT data analysis. The results presented here supersede previous calibration results.

The Swift mission, scheduled for launch in 2004, is a multiwavelength observatory for gamma-ray burst (GRB) astronomy. It is a first-of-its-kind autonomous rapid-slewing satellite for transient astronomy and pioneers the way for future rapid-reaction and multiwavelength missions. It will be far more powerful than any previous GRB mission, observing more than 100 bursts yr À1 and performing detailed X-ray and UV/optical afterglow observations spanning timescales from 1 minute to several days after the burst. The objectives are to (1) determine the origin of GRBs, (2) classify GRBs and search for new types, (3) study the interaction of the ultrarelativistic outflows of GRBs with their surrounding medium, and (4) use GRBs to study the early universe out to z > 10. The mission is being developed by a NASA-led international collaboration. It will carry three instruments: a newgeneration wide-field gamma-ray (15-150 keV ) detector that will detect bursts, calculate 1 0 -4 0 positions, and trigger autonomous spacecraft slews; a narrow-field X-ray telescope that will give 5 00 positions and perform spectroscopy in the 0.2-10 keV band; and a narrow-field UV/optical telescope that will operate in the 170-600 nm band and provide 0B3 positions and optical finding charts. Redshift determinations will be made for most bursts. In addition to the primary GRB science, the mission will perform a hard X-ray survey to a sensitivity of $1 mcrab ($2 ; 10 À11 ergs cm À2 s À1 in the 15-150 keV band ), more than an order of magnitude better than HEAO 1 A-4. A flexible data and operations system will allow rapid follow-up observations of all types of high-energy transients, with rapid data downlink and uplink available through the NASA TDRSS system. Swift transient data will be rapidly distributed to the astronomical community, and all interested observers are encouraged to participate in follow-up measurements. A Guest Investigator program for the mission will provide funding for community involvement. Innovations from the Swift program applicable to the future include (1) a large-area gamma-ray detector using the new CdZnTe detectors, (2) an autonomous rapid-slewing spacecraft, (3) a multiwavelength payload combining optical, X-ray, and gamma-ray instruments, (4) an observing program coordinated with other ground-based and space-based observatories, and (5) immediate multiwavelength data flow to the community. The mission is currently funded for 2 yr of operations, and the spacecraft will have a lifetime to orbital decay of $8 yr.

No abstract

No abstract

Long gamma-ray bursts (GRBs) are bright flashes of high-energy photons that can last for tens of minutes; they are generally associated with galaxies that have a high rate of star formation and probably arise from the collapsing cores of massive stars, which produce highly relativistic jets (collapsar model 1 ). Here we describe ȍ-and X-ray observations of the most distant GRB ever observed (GRB 050904): its redshift 2,3 (z) of 6.29 means that this explosion happened 12.8 billion years ago, corresponding to a time when the Universe was just 890 million years old, close to the reionization era 4 . This means that not only did stars form in this short period of time after the Big Bang, but also that enough time had elapsed for them to evolve and collapse into black holes.GRB 050904 triggered the Burst Alert Telescope (BAT) on board the Swift 5 satellite on 4 September 2005 at 1:51:44 GMT. The spacecraft quickly slewed to allow observations by the X-ray Telescope (XRT) 6,7 , which measured the burst for ten days after its onset. Figure 1 (top panel) shows the history of the burst. We shall present and discuss the GRB phenomenology from the point of view of the rest frame of its source.The BAT light curve shows three main peaks: two of about 2 s at T+3.8 s and T+7.7 s, and a long-lasting one at about T+13.7 s, where T is the time of the burst onset. It also shows a weak peak at about T+64 s. The early XRT light curve shows a steep power-law decay with an index of ǁ2.07DŽ0.03; two flares are superimposed at T+64 s (coincident with the last peak of the BAT light curve) and T+170 s. Although interrupted by the constraints of low-Earthorbit observation, the X-ray light curve reveals highly irregular intensity variations, probably due to the presence of flares for up to T+1.5 hours. At later times, flaring activity is not detected, leaving only a residual emission that is 10 6 times lower than the initial intensity.The flares in the XRT light curve can be interpreted as late internal shocks related to central engine activity. In this scenario, they would have the same origin as the first ȍ-ray emission [8][9][10] , which would require the central engine to remain active for at least 5,000 seconds, consistent with the collapsar model 1 .Spectral analysis was performed by selecting time intervals corresponding to characteristic phases of the light curve evolution. All spectra were well modelled by a single power law, with both galactic and intrinsic absorption components in the case of the XRT spectra. Figure 1 (bottom) shows the evolution with time of the photon index Ǵ. The BAT spectra have Ǵǃ ǁ1.2. If we exclude the spectrum of the first XRT flare at T+64 s, the XRT photon indices show a clear, decreasing trend from about ǁ1.2 to about ǁ1.8 in the first T+200 s. No further spectral evolution is present in later XRT data.The overall phenomenology of GRB 050904 is not peculiar with respect to other GRBs at lower redshift. This suggests that the mechanisms of GRB explosions in the early Universe and today are similar.Based ...