On the stability of the thermal Comptonization index in neutron star low-mass X-ray binaries in their different spectral states

Abstract

Context. Most of the spectra of neutron star low-mass X-ray binaries (NS LMXBs), whether they are persistent or transient, are characterized by the presence of a strong thermal Comptonization bump, which is thought to originate in the transition layer (TL) between the accretion disk and the NS surface. The observable quantities that characterize this component, which is dominating the emission below 30 keV, are the spectral index α and the rollover energy, both related to the electron temperature and optical depth of the plasma. Aims. Starting from observational results on a sample of NS LMXBs in different spectral states, we formulate the problem of X-ray spectral formation in the TL of these sources. We predict a stability of the thermal Comptonization spectral index in different spectral states if the energy release in the TL is much higher than the intercepted flux coming from the accretion disk. Methods. We use an equation for the energy balance and the radiative transfer diffusion equation for a slab geometry in the TL to derive a formula for the thermal Comptonization index α. We show that in this approximation the TL electron temperature kT e and optical depth τ 0 can be written as a function of the energy flux from the disk intercepted by the corona (TL) and that in the corona itself, Q disk /Q cor . Because the spectral index α depends on kT e and τ 0 , this in turn leads to a relation α = f (Q disk /Q cor ), with α ∼ 1 when Q disk /Q cor 1. Results. We show that the observed spectral index α for the sample of sources here considered lies in a belt around 1 ± 0.2 apart for the case of GX 354-0. Comparing our theoretical predictions with observations, we claim that this result, which is consistent with the condition Q disk /Q cor 1, can give us constraints on the accretion geometry of these systems, an issue that seems difficult to be solved with only the spectral analysis method.

STABILITY OF THE PHOTON INDICES INZ-SOURCE GX 340+0 FOR SPECTRAL STATES

“…We explain this stability of the indices Γ of Comptonozed components around a value of 2. The index stability now found for Comptonized spectral components of the Z source GX 340+0 during the state evolution of the source is similar to that previously established in atolls, 4U 1728-34, GX 3+1 and shown for quite a few of other LMXBs (see Farinelli & Titarchuk, 2011).…”

Warm and optically thick dissipative coronae above accretion disks

“…We assume that the atmosphere is optically thick, and therefore adopt the Eddington approximation in the whole medium. We note, that similar approach was made by Farinelli & Titarchuk (2011) in the context of the hot transition layer between the accretion disk and neutron star surface. We allow for an additional dissipation which heats the corona.…”

Black Hole Mass Determination in the X-Ray Binary 4u 1630-47: Scaling of Spectral and Variability Characteristics

“…As one can see from this Figure, HID for 4U 1630-47 exhibits two separate branches related to the hard spectral states (vertical branch) and to the sof t states (horizontal branch), whereas all NS sources (except 4U 1728-34) show only the soft horizontal branch elongated close to the soft branch of 4U 1630-47. A unique stability (or quasi-constancy) of the hardness ratio for NSs versus the 3 − 10 keV flux is an observational demonstration for the stability of the photon index Γ established for a number of NS sources (see Farinelli & Titarchuk 2011, Seifina & Titarchuk 2011and Seifina et al 2013). This hardness ratio vs the 3 − 10 keV flux provides a unique diagnostics for the nature of a given compact object.…”