Measurements of accretion disc corona size in LMXB: consequences for Comptonization and LMXB models

Abstract

We present results of measurements of the radial extent of the accretion disc corona in low mass X-ray binaries, i.e. of the radial extent of the thin, hot ADC above the accretion disc. These results prove conclusively the extended nature of the ADC, with radial extent varying from 20,000 km in the faintest sources to 700,000 km in the brightest, a substantial fraction of the accretion disc radius, typically 15 per cent. This result rules out the Eastern model for LMXB which is extensively used, in which the Comptonizing region is a small central region. The ADC size depends strongly on the 1 -30 keV source luminosity via a simple relationship r ADC = L 0.88±0.16 at 99 per cent confidence, which is close to a simple dependence r ADC ∝ L. We also present limited evidence that the ADC size agrees with the Compton radius r C , or maximum radius for hydrostatic equilibrium. Thus, the results are consistent with models in which an extended ADC is formed by illumination of the disc by the central source. However, the dependence on luminosity may reflect the known decrease of coronal temperature as the source luminosity increases leading to an increase of r C . The extended nature of the ADC means that the seed photons for Comptonization must consist of emission from the disc to the same radial extent as the corona, providing copious supplies of soft seed photons. We thus demonstrate the importance of the size of the ADC to the correct description of Comptonization, and we derive the Comptonized spectrum of a LMXB based on the thermal Comptonization of these seed photons and show that this differs fundamentally from that of the Eastern model, which assumes a cut-off in the spectrum below 1 keV. Finally, we argue that our results are inconsistent with the assumption often made that the X-ray emission of accreting Black Holes and Neutron Stars has a common mechanism depending on the properties of the accretion flow only.

Sco X-1 revisited withKepler, MAXI and HERMES: outflows, time-lags and echoes unveiled

“…Recently, a different model has been proposed based on the idea of an extended accretion disc corona (ADC, Church & Balucinska-Church 1995;Church & Ba lucińska-Church 2004;Church et al 2012). In this modelṀ increases as the source moves upward along the NB track (the opposite of the Psaltis et al 1995 model).…”

Neutral absorber dips in the periodic burster LMXB XB 1323-619 from Suzaku

“…This work establishes that in the dip sources in general, the dominant Comptonized emission is removed slowly showing that the emission is extended, while the point-like blackbody emission which we associate with the neutron star is rapidly removed, and causes rapid variability in dipping. Measurement of dip ingress times allows the size of the extended emitter to be measured, and this has shown the ADC to reach over a large part of the inner accretion disk, have a radial extent R between 20 000 and 700 000 km, and is thin, with height H having H/R 1 (Church & Bałucińska-Church 2004). Strong independent support for the extended ADC comes from the Chandra grating results of Schulz et al (2009) in which emission lines of highly ionizied species were found to originate at radial distances between 20 000 and 110 000 km in an extended ADC.…”

Dipping in CygnusX-2 in a multi-wavelength campaign due to absorption of extended ADC emission

“…These spectra were fitted using the extended ADC emission model so that the non-dip spectrum was fitted by a model of the form abs * (bb + cut), where abs is the Galactic absorption, bb the neutron star blackbody emission and cut the Comptonized emission of an extended ADC (Church & Bałucińska-Church 2004). In fitting, the Xspec model phabs was used as a more accurate form than the generally used wabs model.…”

“…Spectral evolution during dipping can be explained in terms of point-like blackbody emission from the neutron star and the dominant Comptonized emission of an extended ADC (Church & Bałucińska-Church 1997). The extended nature of the ADC is shown by measurement of dip ingress times giving sizes of 20 000−700 000 km depending on luminosity (Church & Bałucińska-Church 2004). Strong support for an extended ADC comes from Chandra high-resolution grating results ) for highly excited H-like lines of Ne, Mg, Si, S and Fe in Cygnus X-2, with Doppler widths corresponding to radii between 18 000 and 110 000 km in good agreement with dip ingress timing.…”

“…However, recent studies have produced models that can explain spectral changes along the Z-track, notably the extended accretion disk corona (ADC) emission model of Church & Bałucińska-Church (2004). This is based on long-term studies of the dipping LMXBs in which absorption in the bulge in the outer disk causes dips in X-ray intensity at the orbital cycle (White & Swank 1982).…”