Aims. We have studied the afterglow of the gamma-ray burst (GRB) of February 18, 2006. This is a nearby long GRB, with a very low peak energy, and is therefore classified as an X-ray Flash (XRF). XRF 060218 is clearly associated with a supernova -dubbed SN 2006aj. Methods. We present early spectra for SN 2006aj as well as optical lightcurves reaching out to 50 days past explosion. Results. Our optical lightcurves define the rise times, the lightcurve shapes and the absolute magnitudes in the U, V and R bands, and we compare these data with data for other relevant supernovae. SN 2006aj evolved quite fast, somewhat similarly to SN 2002ap, but not as fast as SN 1994I. Our spectra show the evolution of the supernova over the peak, when the U-band portion of the spectrum rapidly fades due to extensive line blanketing. We compare to similar spectra of very energetic type Ic supernovae. Our first spectra are earlier than spectra for any other GRB-SN. The spectrum taken 12 days after burst in the rest frame is similar to somewhat later spectra of both SN 1998bw and SN 2003dh, implying a rapid early evolution. This is consistent with the fast lightcurve. From the narrow emission lines from the host galaxy we derive a redshift of z = 0.0331 ± 0.0007. This makes XRF 060218 the second closest gamma-ray burst detected. The flux of these emission lines indicate a high-excitation state, and a modest metallicity and star formation rate of the host galaxy.

The optical afterglow spectrum of GRB 050401 (at z ¼ 2:8992 AE 0:0004) shows the presence of a damped Ly absorber (DLA), with log N H i ¼ 22:6 AE 0:3. This is the highest column density ever observed in a DLA and is about 5 times larger than the strongest DLA detected so far in any QSO spectrum. From the optical spectrum, we also find a very large Zn column density, implying an abundance of ½Zn /H ¼ À1:0 AE 0:4. These large columns are supported by the early X-ray spectrum from Swift XRT, which shows a column density (in excess of Galactic) of log N H ¼ 22:21 þ0:06 À0:08 assuming solar abundances (at z ¼ 2:9). The comparison of this X-ray column density, which is dominated by absorption due to -chain elements, and the H i column density derived from the Ly absorption line allows us to derive a metallicity for the absorbing matter of ½ /H ¼ À0:4 AE 0:3. The optical spectrum is reddened and can be well reproduced with a power law with SMC extinction, where A V ¼ 0:62 AE 0:06. But the total optical extinction can also be constrained independent of the shape of the extinction curve: from the optical to X-ray spectral energy distribution, we find 0:5 P A V P 4:5. However, even this upper limit, independent of the shape of the extinction curve, is still well below the dust column that is inferred from the X-ray column density, i.e., A V ¼ 9:1 þ1:4 À1:5 . This discrepancy might be explained by a small dust content with high metallicity ( low dust-to-metals ratio). ''Gray'' extinction cannot explain the discrepancy, since we are comparing the metallicity to a measurement of the total extinction (without reference to the reddening). Little dust with high metallicity may be produced by sublimation of dust grains or may naturally exist in systems younger than a few hundred megayears.

A c c e p t e d m a n u s c r i p t remote sensing and will make measurements on spatial scales of less than 10 km for 57 major elements during solar flares, sufficient to isolate surface landforms, such as craters 58 and their internal structures. The spatial resolution achieved by MIXS-T is made possible 59 by novel, low mass microchannel plate X-ray optics, in a Wolter type I optical geometry. 60 61 MIXS measurements of surface elemental composition will help determine rock types, 62 the evolution of the surface and ultimately a probable formation process for the planet. In 63 this paper we present MIXS and its predicted performance at Mercury as well as 64 discussing the role that MIXS measurements will play in answering the major questions 65 about Mercury. 66 67

Aims. We present early optical spectroscopy of the afterglow of the gamma-ray burst GRB 060206 with the aim of determining the metallicity of the GRB absorber and the physical conditions in the circumburst medium. We also discuss how GRBs may be important complementary probes of cosmic chemical evolution.Methods. Absorption line study of the GRB afterglow spectrum. Results. We determine the redshift of the GRB to be z = 4.04795 ± 0.00020. Based on the measurement of the neutral hydrogen column density from the damped Lyman-α line and the metal content from weak, unsaturated S ii lines we derive a metallicity of [S/H] = −0.84 ± 0.10. This is one of the highest metallicities measured from absorption lines at z ∼ 4. From the very high column densities for the forbidden Si ii*, O i*, and O i** lines we infer very high densities and low temperatures in the system. There is evidence for the presence of H 2 molecules with log N(H 2 ) ∼ 17.0, translating into a molecular fraction of log f ≈ −3.5 with f = 2N(H 2 )/(2N(H 2 ) + N(H i)). Even if GRBs are only formed by single massive stars with metallicities below ∼0.3 Z , they could still be fairly unbiased tracers of the bulk of the star formation at z > 2. Hence, metallicities as derived for GRB 060206 here for a complete sample of GRB afterglows will directly show the distribution of metallicities for representative star-forming galaxies at these redshifts.

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