X-ray spectroscopy and photometry of the long-period polar AI Trianguli with XMM-Newton

Abstract

Context. The energy balance of cataclysmic variables with strong magnetic fields is a central subject in understanding accretion processes on magnetic white dwarfs. With XMM-Newton, we perform a spectroscopic and photometric study of soft X-ray selected polars during their high states of accretion. Aims. On the basis of X-ray and optical observations of the magnetic cataclysmic variable AI Tri, we derive the properties of the spectral components, their flux contributions, and the physical structure of the accretion region in soft polars.Methods. We use multi-temperature approaches in our xspec modeling of the X-ray spectra to describe the physical conditions and the structures of the post-shock accretion flow and the accretion spot on the white-dwarf surface. In addition, we investigate the accretion geometry of the system by completing a timing analysis of the photometric data. Results. Flaring soft X-ray emission from the heated surface of the white dwarf dominates the X-ray flux during roughly 70% of the binary cycle. This component deviates from a single black body and can be described by a superimposition of mildly absorbed black bodies with a Gaussian temperature distribution between kT bb,low := 2 eV and kT bb,high = 43.9 +3.3 −3.2 eV, and N H,ISM = 1.5 +0.8 −0.7 ×10 20 cm −2 . In addition, weaker hard X-ray emission is visible nearly all the time. The spectrum from the cooling post-shock accretion flow is most closely fitted by a combination of thermal plasma mekal models with temperature profiles adapted from prior stationary two-fluid hydrodynamic calculations. The resulting plasma temperatures lie between kT MEKAL,low = 0.8 +0.4 −0.2 keV and kT MEKAL,high = 20.0 +9.9 −6.1 keV; additional intrinsic, partial-covering absorption is on the order of N H,int = 3.3 +2.5 −1.2 × 10 23 cm −2 . The soft X-ray light curves show a dip during the bright phase, which can be interpreted as self-absorption in the accretion stream. Phase-resolved spectral modeling supports the picture of one-pole accretion and self-eclipse. One of the optical light curves corresponds to an irregular mode of accretion. During a short XMM-Newton observation at the same epoch, the X-ray emission of the system is clearly dominated by the soft component.

Phase-resolved X-ray spectroscopy and spectral energy distribution of the X-ray soft polar RS Caeli

“…The excess in RS Cae is comparable to the XMM-Newton results for polars of similar field strengths: EK UMa (B ∼ 35 MG) with a moderate luminosity ratio of six for single-temperature models during a short 5 ks exposure ; AI Tri (B ∼ 38 MG) with moderate to high flux ratios of 6 to at least 70, depending on the accretion state (Traulsen et al 2010); HU Aqr (B ∼ 35 MG) with low flux ratios during a low state of accretion, balanced fluxes or a slight excess during an intermediate state, and a strong excess during a high state (Schwarz et al 2009). …”

“…Up to now, we have studied three systems with a clear soft X-ray excess during (intermediate) high states of accretion in the ROSAT and in our pointed XMM-Newton observations: AI Tri (Traulsen et al 2010), QS Tel (Traulsen et al 2011), and RS Cae. Their soft X-ray luminosity correlates with their accretion state and can change drastically on time scales as short as days, in agreement with the dependence of the soft-to-hard luminosity ratio on the accretion state as described by .…”

“…It results in a reduced χ 2 red of 1.22 (197 d.o.f.,  plasma component) and an F-test probability of 75% that the fit is improved. In the fits with multiple black bodies, we employed models that have been successfully used for other polars: black-body components whose effective emitting surface areas obey an exponential distribution over temperatures (AM Her, Beuermann et al 2012) or whose individual temperatures obey a Gaussian distribution over emitting radii (AI Tri, Traulsen et al 2010). The model with a Gaussian temperature distribution reproduces the low-energy continuum better, without changing the reduced χ 2 red = 1.22 over the full 0.1−10.0 keV range.…”

“…By exploiting new XMM-Newton data of polars that had not been observed in X-rays since ROSAT, we are studying their system properties and the energy balance, in particular during high states (AI Tri, Traulsen et al 2010, RS Cae, this work) and intermediate high states of accretion (AI Tri, QS Tel, Traulsen et al 2010Traulsen et al , 2011. In this paper, we present our third pointed XMM-Newton observation of a soft X-ray selected polar.…”

XMM-Newtonobservations of the X-ray soft polar QS Telescopii

“…With the optical monitor, we performed ultraviolet fast mode photometry in the 2050−2450 Å band using the UVM2 filter. We employ sas v8.0 standard tasks for the data reduction and apply an effective area correction as described by Traulsen et al (2010) for the EPIC spectra of AI Tri. For the EPIC instruments, we choose a circular source region with a radius of 25 arcsec.…”