Identification of black hole power spectral components across all canonical states

From a uniform analysis of a large (8.5 Ms) Rossi X-ray Timing Explorer data set of Low Mass X-ray Binaries, we present a complete identification of all the variability components in the power spectra of black holes in their canonical states. It is based on gradual frequency shifts of the components observed between states, and uses a previous identification in the black hole low hard state as a starting point. It is supported by correlations between the frequencies in agreement with those previously found to hold for black hole and neutron stars. Similar variability components are observed in neutron stars and black holes (only the component observed at the highest frequencies is different) which therefore cannot depend on source-specific characteristics such as the magnetic field or surface of the neutron star or spin of the black hole. As the same variability components are also observed across the jet-line the X-ray variability cannot originate from the outer-jet but is most likely produced in either the disk or the corona. We use the identification to directly compare the difference in strength of the black hole and neutron star variability and find these can be attributed to differences in frequency and strength of high frequency features, and do not require the absence of any components. Black holes attain their highest frequencies (in the hard-intermediate and very-high states) at a level a factor ~6 below the highest frequencies attained by the corresponding neutron star components, which can be related to the mass difference between the compact objects in these systems.Comment: 17 pages, 16 figures, accepted for publication in Ap

A model-independent analysis of the variability of GRS 1915+105

We analyzed 163 observations of the microquasar GRS 1915+105 made with the Rossi X-ray Timing Explorer (RXTE) in the period 1996-1997. For each observation, we produced light curves and color-color diagrams. We classified the observations in 12 separate classes, based on their count rate and color characteristics. From the analysis of these classes, we reduced the variability of the source to transitions between three basic states: a hard state corresponding to the non-observability of the innermost parts of the accretion disk, and two softer states with a fully observable disk. These two soft states represent different temperatures of the accretion disk, related to different local values of the accretion rate. The transitions between these states can be extremely fast. The source moves between these three states following certain patterns and avoiding others, giving rise to a relatively large but limited number of variability classes. These results are the first step towards a linking of the properties of this exceptional source with standard black-hole systems and with accretion disk models.Comment: Accepted for publication in Astronomy & Astrophysics, 2000 January 6t

The X-ray fast-time variability of Sco X-2 (GX 349+2) with RXTE

Sco X-2 (GX 349+2) is a low-mass X-ray binary and Z source. We have analysed 156 ks of Rossi X-ray Timing Explorer data, obtained in 1998 January, on this source. We investigated the fast-time variability as a function of position on the Z track. During these observations, Sco X-2 traced out the most extensive Z track ever reported from this object, making this the most comprehensive study thus far. We found the broad peaked flaring branch noise that is typical of Sco X-2, with a centroid frequency in the range 3.3--5.8 Hz. We also discovered low frequency noise, and a new peaked noise feature, with centroid frequencies in the range 5.4--7.6 Hz and 11--54 Hz, respectively. We discuss the phenomenology of these features, their relationship with the power spectral components found in other low-mass X-ray binaries, and the implications for current models. In particular, the low frequency noise we observed was strongest at intermediate energies, in contrast to the low frequency noise seen in other Z sources. We also detected very low frequency noise, and have calculated complex cross spectra between intensity and hardness. We found that the very low frequency noise is not entirely due to motion along the Z track.Comment: 17 pages, 9 figures, minor improvements, accepted for publication in MNRA

Balanced ternary addition using a gated silicon nanowire

We demonstrate the proof of principle for a ternary adder using silicon metal-on-insulator single electron transistors (SET). Gate dependent rectifying behavior of a single electron transistor results in a robust three-valued output as a function of the potential of the SET island. Mapping logical, ternary inputs to the three gates controlling the potential of the SET island allows us to perform complex, inherently ternary operations, on a single transistor

Rapid X-Ray Variability of Neutron Stars in Low-Mass Binary Systems

The dramatic discovery with the Rossi X-Ray Timing Explorer satellite of remarkably coherent $\sim$300--1200 Hz oscillations in the X-ray brightness of some sixteen neutron stars in low-mass binary systems has spurred theoretical modeling of these oscillations and investigation of their implications for the neutron stars and accretion flows in these systems. High-frequency oscillations are observed both during thermonuclear X-ray bursts and during intervals of accretion-powered emission and appear to be a characteristic feature of disk-accreting neutron stars with weak magnetic fields. In this review we focus on the high-frequency quasi-periodic oscillations (QPOs) seen in the accretion-powered emission. We first summarize the key properties of these kilohertz QPOs and then describe briefly the models that have been proposed to explain them. The existing evidence strongly favors beat-frequency models. We mention several of the difficulties encountered in applying the magnetospheric beat-frequency model to the kilohertz QPOs. The most fully developed and successful model is the sonic-point beat-frequency model. We describe the work on this model in some detail. We then discuss observations that could help to distinguish between models. We conclude by noting some of the ways in which study of the kilohertz QPOs may advance our understanding of dense matter and strong gravitational fields.Comment: 10 pages LaTeX including six figures, uses espcrc2.sty (included), invited talk at "The Active X-Ray Sky", eds. L. Scarsi, H. Bradt, P. Giommi, and F. Fior

Type I X-ray bursts, burst oscillations and kHz quasi-periodic oscillations in the neutron star system IGR J17191-2821

We present a detailed study of the X-ray energy and power spectral properties of the neutron star transient IGR J17191-2821. We discovered four instances of pairs of simultaneous kilohertz quasi-periodic oscillations (kHz QPOs). The frequency difference between these kHz QPOs is between 315 Hz and 362 Hz. We also report on the detection of five thermonuclear type-I X-ray bursts and the discovery of burst oscillations at ~294 Hz during three of them. Finally, we report on a faint and short outburst precursor, which occurred about two months before the main outburst. Our results on the broadband spectral and variability properties allow us to firmly establish the atoll source nature of IGR J17191-2821.Comment: 9 pages, 7 figures - accepted for publication in MNRA