On the spreading layer emission in luminous accreting neutron stars

Abstract

Emission of the neutron star surface potentially contains information about its size and thus of vital importance for high energy astrophysics. In spite of the wealth of data on the emission of luminous accreting neutron stars, the emission of their surfaces is hard to disentangle from their time averaged spectra. A recent X-ray transient source XTE J1701−462 has provided a unique dataset covering the largest ever observed luminosity range for a single source and showing type I (thermonuclear) X-ray bursts. In this paper, we extract the spectrum of the neutron star surface (more specifically, the spectrum of the boundary layer between the inner part of the accretion disc and the neutron star surface) with the help of maximally spectral model-independent method. We show compelling evidences that the energy spectrum of the boundary layer stays virtually the same over factor of 20 variations of the source luminosity. It is rather wide and cannot be described by a single temperature blackbody spectrum, probably because of the inhomogeneity of the boundary layer and a spread in the colour temperature. The observed maximum colour temperature of the boundary/spreading layer emission of kT ≈2.4-2.6 keV is very close to the maximum observed colour temperature in the photospheric radius expansion X-ray bursts, which is set by the limiting Eddington flux at the neutron star surface. The observed stability of the boundary layer spectrum and its maximum colour temperature strongly supports theoretical models of the boundary/spreading layers on surfaces of luminous accreting neutron stars, which assume the presence of a region emitting at the local Eddington limit. Variations in the luminosity in that case lead to changes in the size of this region, but affect less the spectral shape. Elaboration of this model will provide solid theoretical grounds for measurements of the neutron star sizes using the emission of the boundary/spreading layers of luminous accreting neutron stars.

X-ray burst-induced spectral variability in 4U 1728–34

“…In this case an optically thick boundary layer forms in the disc-star interface, which radiates black body like emission almost as efficiently as the accretion disc (Sunyaev & Shakura 1986). The likely geometry of this boundary is that of a spreading layer (Inogamov & Sunyaev 1999;Suleimanov & Poutanen 2006;Inogamov & Sunyaev 2010), that explains well the spectro-temporal properties of the persistent emission in NSLMXBs (Gilfanov et al 2003;Revnivtsev & Gilfanov 2006;Revnivtsev et al 2013). …”

“…In the soft state, the soft X-ray spectra (and timing properties) of most NS-LMXB can be described by a two-component dual-thermal model, where the cooler component is emitted by the accretion disc and the hotter one by the spreading layer (Gilfanov et al 2003;Revnivtsev & Gilfanov 2006;Revnivtsev et al 2013). A hard X-ray tail can be also detected in the soft state, (see e.g., Asai et al 1994 and discussion below).…”

Anisotropy of X-Ray Bursts From Neutron Stars With Concave Accretion Disks

“…The accretion environment of a neutron star may be more complex. For example, a boundary or spreading layer may be present between the inner disk and the star (Inogamov & Sunyaev 1999;Revnivtsev et al 2013). Depending on the accretion flow, only part of the star may be covered, but during X-ray bursts the entire stellar surface is thought to be covered by the spreading layer (Lapidus & Sunyaev 1985).…”

The influence of accretion geometry on the spectral evolution during thermonuclear (type I) X-ray bursts

“…As the accreted gas enters the SL with roughly Keplerian orbital frequency (always larger than the NS spin frequency), the SL is supported both by centrifugal forces and the radiation pressure force produced by energy dissipation within the SL and by the X-ray burst. This makes the outermost layers of the SL tenuous and, therefore, the resulting radiation spectrum has colour correction factor of about fc ≈ 1.6 − 1.8 for a large range of luminosities from L ∼ L Edd to L ∼ 0.2L Edd (Suleimanov & Poutanen 2006;Revnivtsev et al 2013). In our view, this can explain the irregular cooling behaviour of X-ray bursts in the soft state where the black body radius R bb is constant in the aforementioned luminosity range (i.e.…”