Modeling the EUV spectra of optically thick boundary layers of dwarf novae in outburst

Abstract

Context. Disk accretion onto weakly magnetized white dwarfs (WDs) in cataclysmic variables (CVs) leads to the formation of a boundary layer (BL) between the accretion disk and the WD, where the accreted matter loses its excess kinetic energy and angular momentum. It is assumed that angular momentum is effectively transported in the BL, but the transport mechanism is still unknown. Aims. Here we compute detailed model spectra of recently published optically thick one-dimensional radial BL models and qualitatively compare them with observed soft X-ray/extreme ultraviolet (EUV) spectra of dwarf novae in outburst. Methods. Every considered BL model with given effective temperature and surface density radial distribution is divided into a number of rings, and for each ring, a structure model along the vertical direction is computed using the stellar-atmosphere method. The ring spectra are then combined into a BL spectrum taking Doppler broadening and limb darkening into account. Results. Two sets of model BL spectra are computed, the first of them consists of BL models with fixed WD mass (1 M ) and various relative WD angular velocities (0.2, 0.4, 0.6 and 0.8 break-up velocities), while the other deals with a fixed relative angular velocity (0.8 break-up velocity) and various WD masses (0.8, 1, and 1.2 M ). The model spectra show broad absorption features because of blending of numerous absorption lines, and emission-like features at spectral regions with only a few strong absorption lines. The model spectra are very similar to observed soft X-ray/EUV spectra of SS Cyg and U Gem in outburst. The observed SS Cyg spectrum could be fitted by BL model spectra with WD masses 0.8−1 M and relative angular velocities 0.6−0.8 break up velocities. These BL models also reproduce the observed ratio of BL luminosity and disk luminosity. The difference between the observed and the BL model spectra is similar to a hot optically thin plasma spectrum and could be associated with the spectrum of outflowing plasma with a mass loss rate compatible with the BL mass accretion rate. Conclusions. The suggested method of computing BL spectra seems very promising and can be applied to other BL models for comparison with EUV spectra of dwarf novae in outburst.

Dramatic change in the boundary layer in the symbiotic recurrent nova T Coronae Borealis

“…) support this interpretation. Theory predicts that the transition from mostly optically thin to mostly optically thick should occur at accretion rates of about 10 −9 M ⊙ yr −1 for a 1 M ⊙ WD (Popham & Narayan 1995) or even lower values as proposed by Suleimanov et al (2014). The accretion rate before the optical brightening was therefore likely smaller than a few 10 −9 M ⊙ yr −1 .…”

“…A sudden change in the accretion rate can manisfest itself through a brightening in optical/UV and an X-ray fading. Theory predicts a threshold above which the boundary layer will be optical thick to its own radiation and the observed spectrum will be blackbody-like, while below this threshold the spectrum will be that of an optically thin thermal plasma (e.g., Narayan & Popham 1993;Suleimanov et al 2014).…”

X-ray, UV, and optical observations of the accretion disk and boundary layer in the symbiotic star RT Crucis

“…estimated the mass of the WD assuming that the boundary layer was optically thin and therefore the plasma maximum temperature was set by the mass of the WD. Recently,Ducci et al (2016), claimed that the value ofṀ found from the modeling of INTEGRAL + Swift/XRT and Suzaku data would require a mostly optically thick boundary layer in the theoretical framework derived byPopham & Narayan (1995),and Suleimanov et al (2014),; they therefore proposed an alternative scenario in which the X-ray emission is produced in a magnetically channeled flow, analogous to cataclysmic variables of the intermediate polar type and constrained the WD mass to M WD = 0.9-1.1 M ⊙ .…”

Probing disk truncation in Dwarf Novae and CVs with X-ray variations and comparisons with work on other wavelengths

“…The material in the inner disk of nonmagnetic Cataclysmic Variables (CVs) initially moving with Keplerian velocity dissipates its kinetic energy in order to accrete onto the slowly rotating WD creating a boundary layer (BL) (see Warner 1995, Kuulkers et al 2006. Standard accretion disk theory predicts half of the accretion luminosity to originate from the disk in the optical and ultraviolet (UV) wavelengths and the other half to emerge from the boundary layer as X-ray and/or extreme UV (EUV)/soft X-ray emission which may be summarized as L BL ∼L disk =GM W DṀacc /2R W D =L acc /2 (Lynden-Bell & Pringle 1974, Godon et al 1995, Suleimanov et al 2014. During low-mass accretion rates it is expected that,Ṁ acc <10 −(9−9.5) M ⊙ , the boundary layer is optically thin (Narayan & Popham 1993, Popham 1999 emitting mostly in the hard X-rays (kT∼10 (7.5−8.5) K).…”