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

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 9.4 +/- 0.6 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 Mdot may decrease on the Normal Branch, contrary to the widely-held view that Mdot 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.

arXiv comment12 pages including 8 figures, Astronomy and Astrophysics in press; reference list corrected

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Cited by in corpus (23)

• Spectral Investigations of the nature of the Sco X-1 like sources
• The Birth of an Ultra-Luminous X-ray Source in M83
• An explanation of the Z-track sources
• Broad-band Spectral Evolution of Scorpius X-1 along its Color-Color Diagram
• The nature of the Island and Banana States in Atoll sources and a Unified Model for Low Mass X-ray Binaries
• X-ray spectral and timing behavior of Scorpius X-1. Spectral hardening during the flaring branch
• Discovery of absorption features of the ADC and systematic acceleration of the X-ray burst rate in XB 1323-619
• A model for the Z-track phenomenon in GX 5-1 and observational evidence for the physical origins of the kHz QPO
• Millisecond dips in the RXTE/PCA light curve of Sco X-1 and TNO occultation
• Tracing a Z-track in the M31 X-ray Binary RX J0042.6+4115
• Demonstrating the likely neutron star nature of five M31 globular cluster sources with Swift-NuSTAR spectroscopy
• New XMM-Newton analysis of three bright X-ray sources in M31 globular clusters, including a new black hole candidate
• Around 200 new X-ray binary IDs from 13 years of Chandra observations of the M31 center
• Discovery of disc precession in the M31 dipping X-ray binary Bo 158
• 50 M31 black hole candidates identified by Chandra and XMM-Newton
• Spectral analysis of LMC X-2 with XMM/Newton: unveiling the emission process in the extragalactic Z-source
• Spectral and Timing evolution of GX 340+0 along its Z-track
• Upper limits to the number of Oort Cloud Objects based on serendipitous occultation events search in X-rays
• Spectro-temporal evolution of 4U 1702-429 using AstroSat-NICER
• XMM-Newton and Chandra observations of the M31 globular cluster black hole candidate XB135: a heavyweight contender cut down to size
• A significant detection of X-ray Polarization in Sco X-1 with PolarLight and constraints on the corona geometry
• Six Years of ScoX-1 Monitoring with BeppoSAX Wide Field Cameras
• The HaloSat and PolarLight CubeSat Missions for X-ray Astrophysics