Physical changes during Z-track movement in Sco X-1 on the flaring branch

¹ Department of Physics and Astronomy, Open University, Milton Keynes, MK7 6AA, UK
² School of Physics and Astronomy, University of Birmingham, Birmingham, B15 2TT, UK
³ Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Cracow, Poland

Corresponding author: R. Barnard, R.Barnard@open.ac.uk

Received: 15 November 2002
Accepted: 6 April 2003

Abstract

We present results of a detailed study of X-ray flaring in the Z-track source Sco X-1 in a highly super-Eddington state made using high quality Rossi-XTE data from the PCA and HEXTE instruments. The emission model successfully used to explain the dipping LMXB, and other classes of LMXB in recent years, was applied to study the physical evolution along the Z-track which remains a major problem. This model consists of blackbody emission from the neutron star plus Comptonized emission from an extended accretion disk corona. As found in earlier work, major changes take place in the neutron star blackbody emission with kT increasing in flaring, and the blackbody radius R_BB increasing substantially to a maximum value of  km, consistent with the radius of the neutron star, after which R_BB decreases. Thus this result is a measurement of neutron star radius. The behaviour of Sco X-1 in flaring is compared with our previous results for the strong flaring that takes place in the bright dipping, flaring LMXB X 1624–490. Remarkably, during movement along the Normal Branch towards the apex with the Flaring Branch, the luminosities of both spectral components decrease, suggesting the possibility that  may decrease on the Normal Branch, contrary to the widely-held view that  increases monotonically along the Z-track. During flaring, we detect for the first time an increase of the Comptonization cut-off energy which may suggest heating of the ADC plasma by the neutron star flare. The energy of a broad Gaussian line at ~6.4 keV does not change, but the intensity of the line increases in flaring suggesting either an increase in ADC size in flaring or the effects of irradiation by the neutron star.