Although the link between long gamma-ray bursts (GRBs) and supernovae has been established, hitherto there have been no observations of the beginning of a supernova explosion and its intimate link to a GRB. In particular, we do not know how the jet that defines a gamma-ray burst emerges from the star's surface, nor how a GRB progenitor explodes. Here we report observations of the relatively nearby GRB 060218 (ref. 5) and its connection to supernova SN 2006aj (ref. 6). In addition to the classical non-thermal emission, GRB 060218 shows a thermal component in its X-ray spectrum, which cools and shifts into the optical/ultraviolet band as time passes. We interpret these features as arising from the break-out of a shock wave driven by a mildly relativistic shell into the dense wind surrounding the progenitor. We have caught a supernova in the act of exploding, directly observing the shock break-out, which indicates that the GRB progenitor was a Wolf-Rayet star.

Spectral analysis of Swift/XRT dataWe use the xspec v11.3.2 X-ray spectral fitting package to fit both a power law and a blackbody model to the XRT outburst data. In both models we allow for excess neutral hydrogen absorption (N H ) above the Galactic value along the line of sight to NGC 2770, N H,Gal = 1.7 × 10 20 cm −2 . The best-fit power law model (χ 2 = 7.5 for 17 degrees of freedom; probability, P = 0.98) has a photon index, Γ = 2.3 ± 0.3 (or, F ν ∝ ν −1.3±0.3 ) and N H = 6.9 +1.8 −1.5 × 10 21 cm −2 . The best-fit blackbody model is described by kT = 0.71 ± 0.08 keV and N H = 1.3 +1.0 −0.9 × 10 21 cm −2 . However, this model provides a much poorer fit to the data (χ 2 = 26.0 for 17 degrees of freedom; probability, P = 0.074). We therefore adopt the power law model as the best description of the data. The resulting count rate to flux conversion is 1 counts s −1 = 5 × 10 −11 erg cm −2 s −1 . The outburst undergoes a significant hard-to-soft spectral evolution as indicated by the ratio of counts in the 0.3 − 2 keV band and 2 − 10 keV band. The hardness ratio decreases from 1.35 ± 0.15 during the peak of the flare to 0.25 ± 0.10 about 400 s later. In the context of the power law model this spectral softening corresponds to a change from Γ = 1.70 ± 0.25 to 3.20 ± 0.35 during the same time interval. High resolution optical spectroscopyWe obtained the spectrum with the High Resolution Echelle Spectrometer (HIRES) mounted on the Keck I 10-m telescope beginning at Jan 17.46 UT. A total of four 1800-s exposures were obtained with a spectral resolution, R = 48, 000, and a slit width of 0.86 arcsec. The data reach a signal-to-noise ratio of 18 per pixel. We reduced the data with the MAKEE reduction package. We are interested in the Na I D and K I absorption features since they are sensitive to the gas column density, and hence extinction, along the line of the sight to the SN. Rejecting a Relativistic Origin for XRO 080109We investigate the possibility that XRO 080109 is the result of a relativistic outflow similar to that in GRBs. In this context the emission is non-thermal synchrotron radiation. The outburst flux density is 7.5 × 10 2 µJy at 0.3 keV. Simultaneously, we find 3σ limits on the flux density in the UBV bands (∼ 3 eV) of F ν < 9.0 × 10 2 µJy, indicating that the peak of the synchrotron spectrum must be located between the UV and X-ray bands. In the standard synchrotron model this requires the frequencies corresponding to electrons with the minimum and cooling Lorentz factors to obey ν m ≈ ν c ≈ 3 × 10 16 Hz, while the peak of the spectrum is F ν,p ≈ 3 mJy.The inferred values of ν m and ν c allow us to constrain 47 the outflow parameters and thus to check for consistency with the hypothesis of relativistic expansion. The relevant parameters are the bulk Lorentz factor (γ), the magnetic field (B), and the shock radius (R sh ). From the value of ν c we find γB 3 ≈ 8.3 × 10 3 , and since γ > 1 we conclude that B < 20 G. In addition, using ν m we find ǫ 2 e γ 3 B ≈ 3 × 10 4 ; here ǫ e is the fraction of posts...

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.

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