Strong Field Gravity and X-Ray Observations of 4U1820-30

Abstract

The behavior of quasi-periodic oscillations (QPOs) at frequencies near 1 kHz in the x-ray emission from the neutron star x-ray binary 4U1820-30 has been interpreted as evidence for the existence of the marginally stable orbit, a key prediction of strong-field general relativity. The signature of the marginally stable orbit is a saturation in QPO frequency, assumed to track inner disk radius, versus mass accretion rate. Previous studies of 4U1820-30 have used x-ray count rate as an indicator of mass accretion rate. However, x-ray count rate is known to not correlate robustly with mass accretion rate or QPO frequency in other sources. Here, we examine the QPO frequency dependence on two other indicators of mass accretion rate: energy flux and x-ray spectral shape. Using either of these indicators, we find that the QPO frequency saturates at high mass accretion rates. We interpret this as strong evidence for the existence of the marginally stable orbit.

accepted to the Astrophysical Journal Letters, 7 pages

Cited by in corpus (30)

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• Binary and Long-Term (Triple?) Modulations of 4U 1820$-$30 in NGC 6624
• The Amplitude of the Kilohertz Quasi-periodic Oscillations in 4U 1728-34, 4U 1608-52, and Aql X-1, as a Function of X-ray Intensity
• Study of the Temporal Behavior of 4U 1728-34 as a Function of its Position in the Color-Color Diagram
• Changing Frequency Separation of Kilohertz Quasi-Periodic Oscillations in the Sonic-Point Beat-Frequency Model
• Simultaneous Measurements of X-Ray Luminosity and Kilohertz Quasi-Periodic Oscillations in Low-Mass X-Ray Binaries
• RXTE Studies of Long-Term X-ray Spectral Variations in 4U 1820-30
• The kilohertz quasi-periodic oscillations during the Z and atoll phases of the unique transient XTE J1701--462
• Measuring the Black Hole Spin in Sgr A*
• The Island state of the Atoll Source 4U 1820--30
• The crust of rotating strange quark stars
• Small strange stars and marginally stable orbit in Newtonian theory
• ASCA Observations of GX 354-0 and KS 1731-260
• Kilohertz quasi-periodic oscillations in low mass X-ray binary sources and their relation with the neutron star magnetic field
• Spectral state evolution of 4U~1820-30: the stability of the spectral index of Comptonization tail
• Epicyclic frequencies for rotating strange quark stars: the importance of stellar oblateness
• On the Disappearance of Kilohertz Quasi-Periodic Oscillations at a High Mass Accretion Rate in Low-Mass X-ray Binaries
• Accreting Neutron Stars in Low-Mass X-Ray Binary Systems
• Rotating Skyrmion Stars
• An XTE Archival Search for Coherent X-ray Pulsations in LMXB 4U 1820-30
• Modelling the energy dependencies of X-ray quasi-periodic oscillations in accreting compact objects
• Parallel tracks as quasi-steady states for the magnetic boundary layers in neutron-star low-mass X-ray binaries
• Recent Observations of X-Ray Emission from Galactic Binary Neutron Stars
• The mass of the neutron star in 4U 1820-30 revisited