XMM-Newton and Swift observations of WZ Sagittae: spectral and timing analysis

¹ Department of Mathematics and Physics E. De Giorg, University of Salento, via per Arnesano, CP 193, 73100 Lecce, Italy
e-mail: nucita@le.infn.it
² INFN, Sez. di Lecce, via per Arnesano, CP 193, 73100 Lecce, Italy
³ European Space Astronomy Centre, SRE-O, PO Box 78, 28691 Villanueva de la Cañada ( Madrid), Spain
⁴ CRESST and X-ray Astrophysics Laboratory, NASA Goddard Space Flight Center, Greenbelt MD 20771, USA
⁵ Department of Physics, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore MD 21250, USA

Received: 19 October 2013
Accepted: 19 April 2014

Abstract

Context. WZ Sagittae is the prototype object of a subclass of dwarf novae with rare and long (super)outbursts, in which a white dwarf primary accretes matter from a low mass companion. High-energy observations offer the possibility of a better understanding of the disk-accretion mechanism in WZ Sge-like binaries.

Aims. We used archival XMM-Newton and Swift data to characterize the X-ray spectral and temporal properties of WZ Sge in quiescence.

Methods. We performed a detailed timing analysis of the simultaneous X-ray and UV light curves obtained with the EPIC and OM instruments on board XMM-Newton in 2003. We employed several techniques in this study, including a correlation study between the two curves. We also performed an X-ray spectral analysis using the EPIC data and Swift/XRT data obtained in 2011.

Results. We find that the X-ray intensity is clearly modulated at a period of ≃28.96 s, confirming previously published preliminary results. We find that the X-ray spectral shape of WZ Sge remains practically unchanged between the XMM-Newton and Swift observations. However, after correcting for interstellar absorption, the intrinsic luminosity is estimated to be L_X^Una = (2.65 ± 0.06) × 10³⁰ erg s^-1 and L_X^Una = (1.57 ± 0.03) × 10³⁰ erg s^-1 in 2003 and 2011, respectively. During the Swift/XRT observation, the observed flux is a factor ≃2 lower than that observed by XMM-Newton but is similar to the quiescent levels that are observed various times before the 2001 outburst.