Strong Hard X-Ray Emission from RS Oph in Outburst

ATel #737; J. L. Sokoloski (SAO), K. Mukai (GSFC), G. J. M. Luna (SAO)
on 16 Feb 2006; 17:52 UT
Credential Certification: Jennifer L. Sokoloski (jsokolos@cfa.harvard.edu)

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We report on the first X-ray observation of the recurrent nova RS Oph during the 2006 outburst. The Rossi X-ray Timing Explorer (RXTE) observed RS Oph for 1.5 ks on Feb 15, 2006, beginning at 21:43:00 UT. At that time, RS Oph was less than one week into the eruption. A preliminary analysis shows that it was detected out to approximately 25 keV at about 145 counts/s/PCU, with a 2-10 keV flux of (1.33+0.05/-0.01) e-9 erg/cm/cm/s and a 2-20 keV flux of (1.98+0.07/-0.01) e-9 erg/cm/cm/s. Taking a distance of 0.6 kpc (Snijders 1987, in "RS Oph and the Recurrent Nova Phenomenon, ed. M. F. Bode), the absorbed 2-20 keV luminosity is approximately 8.5-8.8 e34 erg/s. An absorbed thermal bremsstrahlung model and an absorbed shock model both give plasma temperatures near 10 keV and an absorbing column of greater than 4e22 cm^-2. The spectrum also contains a strong line at 6.645+/-0.006 keV according to a single Gaussian fit, which may be a blend of several components that are unresolved with the PCA. The plasma temperature of kT=10 keV is consistent with the X-ray emission arising from nebular material that has been shock-heated by ejecta moving at a velocity of around 3000 km/s. The X-ray emission from RS Oph just a few days into this eruption is significantly harder than the X-ray emission detected by EXOSAT during the previous outburst (in 1985) several months after the start of that event (Mason et al. 1987, in "RS Oph and the Recurrent Nova Phenomenon").