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We present X-ray grating spectra of the recurrent nova RS Ophiuchi during its 2006 outburst, obtained with XMMNewton and Chandra. For the first month after optical maximum, the X-ray spectrum was hard and dominated by emission lines of H-like and He-like ions. The X-ray luminosity was 2:4 ; 10 36 ergs s À1 in the 0.33Y10 keV range. The spectra indicate a collisionally dominated plasma with a broad range of temperatures and an energy-dependent velocity structure. During an observation obtained in week 4, a soft X-ray flare occurred in which a new system of soft, higher velocity emission lines appeared in the spectrum. Then, during weeks 6Y10, the supersoft continuum of the hot white dwarf atmosphere was the dominant emission component. The X-ray luminosity reached at least 9 ; 10 37 ergs s À1 in the 0.2Y1 keV range, while the intrinsic nebular absorption decreased by a factor of 5 since the first observation. Preliminary model fitting indicates a white dwarf temperature of $800,000 K, and a mass of at least 1.2 M . Therefore, RS Oph may be an important Type Ia supernova progenitor. We show that the data are consistent with mass loss ending before day 54 of the outburst, and nuclear burning ending around day 69. A rapid decay in X-ray luminosity followed after week 10. The X-ray luminosity 5, 7, and 8 months after optical maximum dropped by more than 2 orders of magnitude. The spectra do not appear to be consistent with emission from an accretion disk.

We present densely sampled BVRI light curves of the optical transient associated with the gamma-ray burst (GRB) 030329, the result of a coordinated observing campaign conducted at five observatories. Augmented with published observations of this GRB, the compiled optical data set contains 2687 photometric measurements, obtained between 78 minutes and 79 days after the burst. This data set allows us to follow the photometric evolution of the transient with unprecedented detail. We use the data to constrain the light curve of the underlying supernova (SN) 2003dh and show that it evolved faster than and was probably somewhat fainter than the Type Ic SN 1998bw, associated with GRB 980425. We find that our data can be described by a broken power-law decay perturbed by a complex variable component. The early-and late-time decay slopes are determined to be 1 % 1:1 and 2 % 2. Assuming this single-break power-law model, we constrain the break to lie between $3 and $8 days after the burst. This simple, singly broken power-law model, derived only from the analysis of our optical observations, may also account for available multiband data, provided that the break happened $8 days after the burst. The more complex double-jet model of Berger et al. provides a comparable fit to the optical, X-ray, millimeter, and radio observations of this event. The unique early coverage available for this event allows us to trace the color evolution of the afterglow during the first hours after the burst. We detect a significant change in optical colors during the first day. Our color analysis is consistent with a cooling-break frequency sweeping through the optical band during the first day. The light curves of GRB 030329 reveal a rich array of variations, superposed over the mean power-law decay. We find that the early variations (P8 days after the burst) are asymmetric, with a steep rise followed by a relatively slower (by a factor of about 2) decline. The variations maintain a similar timescale during the first 4 days and then get significantly longer. The structure of these variations is similar to those previously detected in the afterglows of several GRBs.