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.
Highlights
The broadband spectral sensitivities of two X-ray astronomy spacecraft, the Rossi X-ray T iming Explorer (RXT E ; Bradt, Rothschild, & Swank 1993) and BeppoSAX (Boella et al 1997), combined with the high timing resolution of RXT E, have allowed major advances in recent years in the study of low-mass X-ray binaries (LMXBs)
In this paper we present the results of RXT E observations of 4U 1820[30, an X-ray burster (XRB) with a colorful history. 4U 1820[30 is located in the core of the globular cluster NGC 6624 and was one of the Ðrst globular cluster X-ray sources discovered (Giacconi et al 1974)
We have studied the X-ray spectrum of 4U 1820[30 in all three states displayed by atoll sources, including the Ðrst observation in the island state since the EXOSAT observation described by Stella et al (1987b)
Summary
The broadband spectral sensitivities of two X-ray astronomy spacecraft, the Rossi X-ray T iming Explorer (RXT E ; Bradt, Rothschild, & Swank 1993) and BeppoSAX (Boella et al 1997), combined with the high timing resolution of RXT E, have allowed major advances in recent years in the study of low-mass X-ray binaries (LMXBs). Progress has been rapid in the Ðeld of X-ray bursters (XRBs), binary systems containing a low-magnetic Ðeld neutron star (NS) accreting matter from a late-type companion star via Roche lobe overÑow. It had become apparent by the mid-1990s that NS systems were capable of producing hard X-ray (Z20 keV) emission, as indicated by BATSE and SIGMA detections of several XRB at D100 keV (Barret & Vedrenne 1994 ; Tavani & Barret 1997 and references therein). Many XRB have been observed with these experiments, allowing detailed modeling of the complete X-ray to hard X-ray spectra with a variety of complex models (e.g., Church et al 1998 ; Guainazzi et al 1998 ; in Ït Zand et al 1999 ; Olive et al 1999, Piraino et al 1999a, 1999b ; Barret et al 1999, 2000)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
