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The decline and rise of the X-ray flux of RS Oph

ATel #764; J Osborne, K Page, A Beardmore (Leicester), M Bode (Liverpool John Moores), T O'Brien (Manchester), G Schwarz (West Chester), S Starrfield, J-U Ness (Arizona State), J Krautter (Heidelberg), J Drake (SAO), A Evans (Keele), S P S Eyres (Central Lancashire)
on 14 Mar 2006; 17:03 UT
Credential Certification: Julian P Osborne (julo@star.le.ac.uk)

Subjects: X-ray, Nova

Referred to by ATel #: 770, 801, 838, 1523, 2025

Swift observations of the current nova outburst of RS Oph have continued. After the two observations reported in IAUC # 8675 , there have been ~1 ksec observations on the following dates: Feb 21.01, 23.82, 26.43, 28.44, Mar 03.00, 10.82, 13.83.

Between Feb 21.01 and Mar 10.82 the count rate in the 0.3-10 keV band has decreased continuously from 19.8 c/s to 6.0 c/s (Mar 10.82). The X-ray spectrum has shown a diminishing contribution from the hotter components, with a very slight rise at sub-keV energies. The Swift-XRT spectrum has been adequately fit with 3 optically thin components, having temperatures on Feb 21 around 0.2, 0.9 & 6.4 keV, and subsequently decreasing.

The most recent Swift observation (Mar 13.83) showed an increased count rate of 17.6 c/s, the increase being due to a massive rise in flux below 0.6 keV. This new spectral component may be due to strong emission lines from hydrogen-like and helium-like oxygen and nitrogen. Alternatively, this component may be optically thick, and so might represent emission from the hot white dwarf. The XRT spectral resolution at these low energies makes the two possibilities hard to distinguish, a grating observation is needed for this purpose.

When fit with a blackbody absorbed by a strong oxygen edge, a temperature of 1.1e6 K and a radius of 1.6e9 cm resulted. We note that a hot white dwarf atmosphere model fit is required for the determination of reliable physical parameter values however.

Swift observations continue. We thank the Swift MOC team at Penn State University for their efforts.