X-Ray Emitting Blast Wave from the Recurrent Nova RS Ophiuchi
  Stellar explosions such as novae and supernovae produce most of the heavy elements in the Universe. Although the onset of novae from runaway thermonuclear fusion reactions on the surface of a white dwarf in a binary star system is understood[1], the structure, dynamics, and mass of the ejecta are not well known. In rare cases, the white dwarf is embedded in the wind nebula of a red-giant companion; the explosion products plow through the nebula and produce X-ray emission. Early this year, an eruption of the recurrent nova RS Ophiuchi[2,3] provided the first opportunity to perform comprehensive X-ray observations of such an event and diagnose conditions within the ejecta. Here we show that the hard X-ray emission from RS Ophiuchi early in the eruption emanates from behind a blast wave, or outward-moving shock wave, that expanded freely for less than 2 days and then decelerated due to interaction with the nebula. The X-rays faded rapidly, suggesting that the blast wave deviates from the standard spherical shell structure[4-6]. The early onset of deceleration indicates that the ejected shell had a low mass, the white dwarf has a high mass[7], and that RS Ophiuchi is a progenitor of the type of supernova integral to studies of the expansion of the universe. 
J. L. Sokoloski (1), G. J. M. Luna (1 and 2), K. Mukai (3), and Scott   J. Kenyon (1) ((1) SAO, (2) IAG Brazil, (3) NASA GSFC)