RXTE Observations of the Low-Mass X-Ray Binary 4U 1608-522 in Upper-Banana State

Abstract

To investigate the physics of mass accretion onto weakly-magnetized neutron stars, 95 archival RXTE datasets of an atoll source 4U 1608-522, acquired over 1996-2004 in so-called upper-banana state, were analyzed. The object meantime exhibited 3-30 keV luminosity in the range of <~ 10^35 - 4 x 10^37 erg s^-1, assuming a distance of 3.6 kpc. The 3-30 keV PCA spectra, produced one from each dataset, were represented successfully with a combination of a soft and a hard component, of which the presence was revealed in a model-independent manner by studying spectral variations among the observations. The soft component is expressed by so-called multi-color disk model with a temperature of ~1.8 keV, and is attributed to the emission from an optically-thick standard accretion disk. The hard component is a blackbody emission with a temperature of ~2.7 keV, thought to be emitted from the neutron-star surface. As the total luminosity increases, a continuous decrease was observed in the ratio of the blackbody luminosity to that of the disk component. This property suggests that the matter flowing through the accretion disk gradually becomes difficult to reach the neutron-star surface, presumably forming outflows driven by the increased radiation pressure. On time scales of hours to days, the overall source variability was found to be controlled by two independent variables; the mass accretion rate, and the innermost disk radius which changes both physically and artificially.

ApJ accepted, 29 pages, 9 figures

References in corpus (2)

• Evaluating Spectral Models and the X-ray States of Neutron-Star X-ray Transients
• On the nature of the first transient Z-source XTE J1701-462: its accretion disk structure, neutron star magnetic field strength, and hard tail

Cited by in corpus (10)

• Soft lags in neutron star kHz Quasi Periodic Oscillations: evidence for reverberation?
• Accretion Geometry of the Low-Mass X-ray Binary Aquila X-1 in the Soft and Hard States
• Suzaku spectroscopy of the neutron star transient 4U 1608-52 during its outburst decay
• Spectral Evolution of a New X-ray Transient MAXI J0556-332 Observed by MAXI, Swift, and RXTE
• Simplified Picture of Low Mass X-ray Binaries based on Data from Aql X-1 and 4U1608-52
• Variable spreading layer in 4U 1608-52 during thermonuclear X-ray bursts in the soft state
• Slow and Fast Transitions in the Rising Phase of Outbursts from NS-LMXB transients, AqlX-1 and 4U1608-52
• Discovery of the new X-ray transient MAXI J1807+132: a Candidate of a Neutron Star Low-mass X-ray binary
• A Hard-to-Soft State Transition of Aquila X-1 Observed with Suzaku
• A study of the accretion mechanisms of the High Mass X-ray Binary IGR J00370+6122