XMM-newton observations of the dwarf nova RU peg in quiescence: Probe of the boundary layer

Abstract

We present an analysis of X-ray and UV data obtained with the XMM-Newton Observatory of the long-period dwarf nova RU Peg. RU Peg contains a massive white dwarf (WD), possibly the hottest WD in a dwarf nova (DN), it has a low inclination, thus optimally exposing its X-ray emitting boundary layer (BL), and has an excellent trigonometric parallax distance. We modeled the X-ray data using XSPEC assuming a multi-temperature plasma emission model built from the MEKAL code (i.e., CEVMKL). We obtained a maximum temperature of 31.7keV, based on the European Photon Imaging Camera MOS1, 2 and pn data, indicating that RU Peg has an X-ray spectrum harder than most DNe, except U Gem. This result is consistent with and indirectly confirms the large mass of the WD in RUPeg. The X-ray luminosity we computed corresponds to a BL luminosity for a mass accretion rate of 2 × 10-11 M ⊙ yr-1 (assuming M wd = 1.3 M ⊙), in agreement with the expected quiescent accretion rate. The modeling of the O VIII emission line at 19 Å as observed by the Reflection Grating Spectrometer implies a projected stellar rotational velocity v rotsin i = 695kms -1, i.e., the line is emitted from material rotating at 936-1245kms-1 (i ∼ 34°-48°) or about 1/6 of the Keplerian speed; this velocity is much larger than the rotation speed of the WD inferred from the Far Ultraviolet Spectroscopic Explorer spectrum. Cross-correletion analysis yielded an undelayed (time lag ∼ 0) component and a delayed component of 116 ± 17 s where the X-ray variations/fluctuations lagged the UV variations. This indicates that the UV fluctuations in the inner disk are propagated into the X-ray emitting region in about 116 s. The undelayed component may be related to irradiation effects. © 2011. The American Astronomical Society. All rights reserved.