2011A&A...525A.102F


Query : 2011A&A...525A.102F

2011A&A...525A.102F - Astronomy and Astrophysics, volume 525A, 102-102 (2011/1-1)

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

FARINELLI R. and TITARCHUK L.

Abstract (from CDS):

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. 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. 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 kTe and optical depth τ0 can be written as a function of the energy flux from the disk intercepted by the corona (TL) and that in the corona itself, Qdisk/Qcor. Because the spectral index α depends on kTe and τ0, this in turn leads to a relation α=f(Qdisk/Qcor), with α∼1 when Qdisk/Qcor≪1. 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 Qdisk/Qcor≪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.

Abstract Copyright:

Journal keyword(s): stars: neutron - X-rays: binaries - accretion, accretion disks - radiative transfer

Simbad objects: 17

goto Full paper

goto View the references in ADS

Number of rows : 17
N Identifier Otype ICRS (J2000)
RA
ICRS (J2000)
DEC
Mag U Mag B Mag V Mag R Mag I Sp type #ref
1850 - 2024
#notes
1 V* QX Nor LXB 16 12 43.0 -52 25 23           ~ 911 1
2 V* V818 Sco LXB 16 19 55.0692669024 -15 38 25.017666540 11.60 12.40 11.1     Oev 1653 0
3 4U 1642-45 LXB 16 45 47.7 -45 36 40     14.30     ~ 420 1
4 MXB 1658-298 LXB 17 02 06.54 -29 56 44.1 18.35 18.75 18.3     ~ 361 0
5 V* V1101 Sco LXB 17 05 44.4916006152 -36 25 23.049324480   20.10 18.6     ~ 441 0
6 4U 1722-30 LXB 17 27 33.25 -30 48 07.4     12.50     ~ 265 1
7 V* V2216 Oph LXB 17 31 44.17 -16 57 40.9 16.40 17.10 16.8     ~ 302 1
8 NAME Slow Burster LXB 17 31 57.73 -33 50 02.5     15.50     ~ 828 1
9 X Sgr X-1 LXB 17 47 56.0 -26 33 49           ~ 378 0
10 4U 1758-25 LXB 18 01 09.7308060816 -25 04 44.120221284     11.08     ~ 660 1
11 X Sgr X-3 LXB 18 01 32.3 -20 31 44           ~ 238 1
12 V* V5512 Sgr LXB 18 14 31.55 -17 09 26.7     12.73     K5III 399 0
13 V* NP Ser LXB 18 16 01.389 -14 02 10.62   18.8       K 646 2
14 V* V4634 Sgr LXB 18 29 28.2 -23 47 49 18.90 19.40 19     ~ 357 0
15 V* MM Ser LXB 18 39 57.56 +05 02 09.6   19.2 19.2     ~ 352 0
16 Granat 1915+105 HXB 19 15 11.55576 +10 56 44.9052           ~ 2646 0
17 V* V1341 Cyg LXB 21 44 41.1544345272 +38 19 17.066570988 15.00 15.13 14.68 15.00   A9III 1166 1

To bookmark this query, right click on this link: simbad:objects in 2011A&A...525A.102F and select 'bookmark this link' or equivalent in the popup menu