XMM-Newton observations of the dwarf nova RU Peg in quiescence: Probe of the boundary layer

TL;DR

This study analyzes X-ray and UV data of the dwarf nova RU Peg, revealing a hot white dwarf, a hard X-ray spectrum, and a lag between UV and X-ray variations, providing insights into the boundary layer and accretion processes.

Contribution

It presents detailed spectral modeling and timing analysis of RU Peg, highlighting its high white dwarf temperature, boundary layer characteristics, and UV-X-ray variability lag.

Findings

RU Peg has a maximum X-ray temperature of 31.7 keV.

The boundary layer luminosity matches the expected quiescent accretion rate.

UV variations lag X-ray variations by approximately 116 seconds.

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, possibly the hottest white dwarf in a dwarf nova, it has a low inclination, thus optimally exposing its X-ray emitting boundary layer, 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. We obtained a maximum temperature of 31.7 keV, based on the EPIC MOS1, 2 and pn data, indicating that RU Peg has an X-ray spectrum harder than most dwarf novae, except U Gem. This result is consistent with and indirectly confirms the large mass of the white dwarf in RU Peg. The X-ray luminosity we computed corresponds to a boundary layer luminosity for a mass accretion rate of 2.E-11 Msun/yr (assuming Mwd=1.3Msun), in agreement with an expected quiescent accretion rate. The modeling of the O VIII emission line at 19A as observed by the RGS implies a projected stellar rotational velocity of 695 km/s, i.e. the line is emitted from material rotating at about 936-1245 km/s (for i about 34-48deg) or about 1/6 of the Keplerian speed; this velocity is much larger than the rotation speed of the white dwarf inferred from the FUSE spectrum. Cross-correlation analysis yielded an undelayed component and a delayed component of 116 +/- 17 sec 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 sec. The undelayed component may be related to irradiation effects.