Pre-outburst Chandra observations of the recurrent nova T Pyxidis

Abstract

Aims. I study the spectral, temporal, and spatial characteristics of the quiescent X-ray emission (not in outburst) of the recurrent nova T Pyx. Methods. I performed the spectral analysis of the X-ray data obtained using the Chandra Observatory, Advanced CCD Imaging Spectrometer (ACIS-S3) detector. I fit the spectra with several models that describe plasma emission characteristics. In addition, I calculated the light curve of the data and performed power spectral analysis using Fourier transform. Finally, I did high-resolution imaging analysis of the data at the subpixel level and produced radial surface brightness profiles. Results. I present a total of 98.8 ks (∼ 3 × 30 ks) observation of T Pyx obtained with the ACIS-S3 detector onboard the Chandra Observatory obtained during the quiescent phase, about 2-3 months before its outburst in April 2011. The total Chandra spectrum of the source T Pyx gives a maximum temperature kTmax> 37.0 keV (2σ lower limit) with (0.9-1.5) × 10-13-1cm-2 and (1.3-2.2) × 1032-1 (at 3.5 kpc) in the 0.1-50 keV range using a multitemperature plasma emission model with a power-law distribution of temperatures (i.e., CEVMKL in XSPEC). I find a ratio of (Lx/Ldisk) (2-7) × 10-4 and the ratio is smaller if Ldisk is higher than 3 × 1035 erg s-1 indicating considerable inefficiency of emission in the boundary layer. There is no soft X-ray blackbody emission from T Pyx with a 2σ upper limit on the blackbody temperature and the flux/luminosity as kTBB< 25 eV and Lsoft< 2.0 × 1033-1 in the 0.1-10.0 keV band. All fits yield only interstellar NH during quiescence. I suggest that T Pyx has an optically thin boundary layer merged with an advection-dominated accretion flow and/or X-ray corona in the inner disk indicating ongoing quasi-spherical accretion at (very) high rates during quiescent phases. Such a boundary layer structure may be excessively heating the white dwarf, influencing the thermonuclear runaway leading to the recurrent nova events. The orbital period of the system is detected in the power spectrum of the Chandra light curves with no energy dependence over the orbit. The central source (i.e., the binary system) emission and its spectrum is deconvolved from any possible extended emission with a long and detailed procedure at the subpixel level revealing an extended emission with S/N ∼ 6-10. The derived shape looks like an elliptical nebula with a semi-major axis 1.0 arcsec and a semi-minor axis ~0.5 arcsec, also indicating an elongation towards south. The calculated approximate count rate of the extended emission is 0.0013-0.0025 c s-1. The luminosity (within errors) of the nebula is ~(0.6-30.0)×1031-1 (at 3.5 kpc) mostly correct towards the lower end of the range. The nebulosity seems consistent with an interaction of the outflow/ejecta from the 1966 outburst.