<i>Swift</i>Observations of the 2006 Outburst of the Recurrent Nova RS Ophiuchi. I. Early X‐Ray Emission from the Shocked Ejecta and Red Giant Wind

Abstract

RS Ophiuchi began its latest outburst on 2006 February 12. Previous outbursts\nhave indicated that high velocity ejecta interact with a pre-existing red giant\nwind, setting up shock systems analogous to those seen in Supernova Remnants.\nHowever, in the previous outburst in 1985, X-ray observations did not commence\nuntil 55 days after the initial explosion. Here we report on Swift observations\ncovering the first month of the 2006 outburst with the Burst Alert (BAT) and\nX-ray Telescope (XRT) instruments. RS Oph was clearly detected in the BAT 14-25\nkeV band from t=0 to $t\\sim6$ days. XRT observationsfrom 0.3-10 keV, started at\n3.17 days after outburst. The rapidly evolving XRT spectra clearly show the\npresence of both line and continuum emission which can be fitted by thermal\nemission from hot gas whose characteristic temperature, overlying absorbing\ncolumn, $[N_H]_W$, and resulting unabsorbed total flux decline monotonically\nafter the first few days. Derived shock velocities are in good agreement with\nthose found from observations at other wavelengths. Similarly, $[N_H]_W$ is in\naccord with that expected from the red giant wind ahead of the forward shock.\nWe confirm the basic models of the 1985 outburst and conclude that standard\nPhase I remnant evolution terminated by $t\\sim10$ days and the remnant then\nrapidly evolved to display behaviour characteristic of Phase III. Around t=26\ndays however, a new, luminous and highly variable soft X-ray source began to\nappear whose origin will be explored in a subsequent paper.\n