RXTE Studies of Long-Term X-ray Spectral Variations in 4U 1820-30

Abstract

We present the results of detailed spectral studies of the ultra-compact low mass X-ray binary (LMXB) 4U 1820-30 carried out with the Rossi X-ray Timing Explorer (RXTE) during 1996-7. 4U 1820-30 is an ``atoll'' source X-ray burster (XRB) located in the globular cluster NGC 6624. It is known to have an 11 minute binary period and a ~176 day modulation in its 2--12 keV flux. Observations were made with the PCA and HEXTE instruments on RXTE at roughly one-month intervals to sample this long-term period and study flux-related spectral changes. There are clear correlations between our fitted spectral parameters and both the broad-band (2--50 keV) flux and the position in the color-color diagram, as described by the parameter S_a introduced by Mendez et al. (1999). In addition, we find a strong correlation between the position in the color-color diagram and the frequencies of the kilohertz quasi-periodic oscillations (kHz QPOs) reported by Zhang et al. (1998). This lends further support to the notion that evidence for the last stable orbit in the accretion disk of 4U 1820-30 has been observed. For a model consisting of Comptonization of cool photons by hot electrons plus an additional blackbody component, we report an abrupt change in the spectral parameters at the same accretion rate at which the kHz QPOs disappear. For a model consisting of a multicolor disk blackbody plus a cut-off power law, we find that the inner disk radius reaches a minimum at the same accretion rate at which the kHz QPO frequency saturates, as expected if the disk reaches the last stable orbit. Both models face theoretical and observational problems when interpreted physically for this system.

39 pages, 11 figures, accepted to the Astrophysical Journal

References in corpus (1)

• Hard X-ray Emission From Low Mass X-ray Binaries

Cited by in corpus (39)

• Thermonuclear (type-I) X-ray bursts observed by the Rossi X-ray Timing Explorer
• The Mass and Radius of the Neutron Star in 4U 1820-30
• Relativistic iron emission lines in neutron star low-mass X-ray binaries as probes of neutron star radii
• X-ray Point Sources in The Central Region of M31 as seen by Chandra
• An abrupt drop in the coherence of the lower kilohertz QPO in 4U 1636-536
• The broad band X-ray/hard X-ray spectra of accreting neutron stars
• Binary and Long-Term (Triple?) Modulations of 4U 1820$-$30 in NGC 6624
• The Amplitude of the Kilohertz Quasi-periodic Oscillations in 4U 1728-34, 4U 1608-52, and Aql X-1, as a Function of X-ray Intensity
• XMM-Newton observation of 4U 1820-30: Broad band spectrum and the contribution of the cold interstellar medium
• On the maximum amplitude and coherence of the kilohertz quasi-periodic oscillations in low-mass X-ray binaries
• Bright X-Ray Sources in M31 Globular Clusters
• A Chandra Study of the Dense Globular Cluster Terzan 5
• The superorbital variability and triple nature of the X-ray source 4U 1820-303
• The Burst and Transient Source Experiment (BATSE) Earth Occultation Catalog of Low-Energy Gamma-Ray Sources
• Supporting evidence for the signature of the innermost stable circular orbit in Rossi X-ray data from 4U1636-536
• The nature of the Island and Banana States in Atoll sources and a Unified Model for Low Mass X-ray Binaries
• On the multi-periodicities in the X-ray dipper XB 1916-053
• The Island state of the Atoll Source 4U 1820--30
• Dependence of the orbital modulation of X-rays from 4U 1820-303 on the accretion rate
• INTEGRAL spectral variability study of the atoll 4U 1820-30: first detection of hard X-ray emission
• ALMA observations of 4U 1728-34 and 4U 1820-30: first detection of neutron star X-ray binaries at 300 GHz
• Superexpansion as a possible probe of accretion in 4U 1820-30
• Super-Orbital Variability in Hard X-rays
• ASCA Observations of GX 354-0 and KS 1731-260
• Spectral state evolution of 4U~1820-30: the stability of the spectral index of Comptonization tail
• Simultaneous X-ray/optical observations of GX 9+9 (4U 1728-16)
• Propeller driven spectral state transition in LMXB 4U 1608-52
• The MAVERIC Survey: Variable Jet-Accretion Coupling in Luminous Accreting Neutron Stars in Galactic Globular Clusters
• The evolving radio jet from the neutron star X-ray binary 4U 1820$-$30
• Spectral properties of MXB 1658-298 in the low/hard and high/soft state
• A study of accretion discs around rapidly rotating neutron stars in general relativity and its applications to four Low Mass X-ray Binaries
• Accreting Neutron Stars in Low-Mass X-Ray Binary Systems
• BeppoSAX observations of two unclassified LMXBs: X1543-624 and X1556-605
• Broadband X-ray emission and the reality of the broad iron line from the Neutron Star - White Dwarf X-ray binary 4U 1820-30
• The INTEGRAL long monitoring of persistent Ultra Compact X-ray Bursters
• An XTE Archival Search for Coherent X-ray Pulsations in LMXB 4U 1820-30
• X-ray Dip Monitoring of XB 1916-053
• ROSAT/Chandra Observations of a Bright Transient in M81
• X-ray Timing beyond the Rossi X-ray Timing Explorer