RXTEStudies 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. We find that the 176-day period corresponds to normal atoll source motion in the color-color diagram, indicating that the long period is due to mass accretion rate 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 Méndez et al. (1999). The hard X-ray tail becomes more prominent at low flux levels as the source moves into the island state. 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

ASCAObservations of GX 354−0 and KS 1731−260

“…The inner disk temperature of 0.9 keV is also consistent with that observed from 4U 1608[522 and 4U 1820[30 in their high-accretion states (Mitsuda et al 1989 ;Bloser et al 2000). For GX 354[0, the lack of soft emission could indicate that the disk truncates at a large radius.…”

“…At higher accretion rates, the optically thin corona is expected to collapse to an optically thick disk (Narayan & Yi 1995). Correlation of the high-frequency quasi-periodic oscillation to the spectral parameters also gives good indication that the disk moves toward the central source during periods of high accretion Mendez et al 1999 ;Bloser et al 2000). One would then expect to observe more emission from the hot ionized disk at a higher accretion rate.…”

“…The question remains, however, whether the soft and the Comptonized emissions originate in the same regions regardless of the accretion rate Bloser et al 2000). Mitsuda et al (1989) found that a multicolor disk and a Comptonizing boundary layer could explain the emission during a high state of 4U 1608[522, but a number of authors have pointed out that the soft thermal component seen in the low state of other atoll sources could come from either the disk or an optically thick boundary layer (e.g., White et al 1988, Guainazzi et al 1998Barret et al 1999 ;Piraino et al 1999).…”

“…Previously, a saturated Comptonization model had also been used to explain the hard X-ray emission from Cyg X-1 (Sunyaev & Titarchuk 1980). Recent observations of LMXBs with the broadband detectors on Rossi X-Ray T iming Explorer (RXT E) and BeppoSAX have conÐrmed that, in addition to a hard Comptonized emission, an additional soft thermal component is indeed present in many low-luminosity LMXBs (Guainazzi et al 1998 ;Barret et al 1999Barret et al , 2000Piraino et al 1999 ;Bloser et al 2000). It now seems likely that many X-ray binaries possess both a hard and a soft component in their spectrum regardless of the luminosity.…”

Bright X‐Ray Sources in M31 Globular Clusters

“…Indeed, this apparent lack of black holes in MW GC has been commented upon and explained as perhaps due to large kick velocities obtained by black hole binaries upon interactions with other GC members (Sigurdsson & Hernquist 1993;Kulkarni et al 1993). The most luminous MW GC X-ray source (4U 1820À30 in NGC 6624) has L X $ 1 5 ð ÞÂ10 37 ergs s À1 (see Bloser et al 2000 for a review).…”