X-ray and UV observations of nova V598 Puppis between 147 and 255 days after outburst

K L Page,J P Osborne,G J Schwarz,A P Beardmore,A M Read,P A Evans,S Starrfield,M Bode,J.-U Ness

doi:10.1051/0004-6361/200912985

Abstract

Aims: The launch of Swift has allowed many more novae to be observed regularly over the X-ray band. Such X-ray observations of novae can reveal ejecta shocks and the nuclear burning white dwarf, allowing estimates to be made of the ejecta velocity. Methods: We analyse XMM-Newton and Swift X-ray and UV observations of the nova V598 Pup, which was initially discovered in the XMM-Newton slew survey. These data were obtained between 147 and 255 days after the nova outburst, and are compared with the earlier, brighter slew detection. Results: The X-ray spectrum consists of a super-soft source, with the soft emission becoming hotter and much fainter between days ~147 and ~172 after the outburst, and a more slowly declining optically thin component, formed by shocks with kT ~ 200-800 eV (corresponding to velocities of 400-800 km s^-1). The main super-soft phase had a duration of less than 130 days. The Reflection Grating Spectrometer data show evidence of emission lines consistent with optically thin emission of kT ~100 eV and place a limit on the density of the surrounding medium of log(n_e/cm^-3) < 10.4 at the 90 % level. The UV emission is variable over short timescales and fades by at least one magnitude (at lambda ~ 2246-2600 angstrom) between days 169 and 255.

Highlights

Novae are cataclysmic explosions which occur in interacting binary systems consisting of a white dwarf (WD) and a lower mass secondary star
The observations of V598 Pup were unusual, in that the nova was first discovered in X-rays rather than optically; observations began more than 100 days after the optical outburst, when the supersoft X-ray phase was already fading
Both BB and local thermal equilibrium (LTE) WD atmosphere models have been used in the past to model the Super-Soft Source (SSS) spectra of novae, there are problems with each method

Summary

Introduction

Novae are cataclysmic explosions which occur in interacting binary systems consisting of a white dwarf (WD) and a lower mass secondary star. Any X-ray emission from the nuclear burning source is initially obscured by the ejected material; as this envelope expands and becomes optically thin, nuclear burning on the surface of the WD becomes visible. This emission peaks in the soft X-ray band and is known as the Super-Soft Source (SSS) state (Krautter 2008). V598 Pup was discovered by Read et al (2007) in the XMM-Newton slew survey (Saxton et al 2008) on 9th October 2007 as a transient X-ray source (designated XMMSL1 J070542.7−381442) and identified as a nova by Torres et al (2007). This paper discusses the later XMM-Newton and Swift X-ray and UV observations

Observations and results

Ultraviolet

Findings

Discussion