Neutron Stars 1: Equation of State and Structure

We have assembled a sample of 1187 thermonuclear (type I) X-ray bursts from observations of 48 accreting neutron stars by the Rossi X-ray Timing Explorer, spanning more than 10 years. The sample contains examples of two of the three theoretical ignition regimes (confirmed via comparisons with numerical models) and likely examples of the third. We present a detailed analysis of the variation of the burst profiles, energetics, recurrence times, presence of photospheric radius expansion, and presence of burst oscillations, as a function of accretion rate. We estimated the distance for 35 sources exhibiting radius-expansion bursts, and found that the peak flux of such bursts varies typically by 13%. We classified sources into two main groups based on the burst properties: (1) both long and short bursts (indicating mixed H/He accretion), and (2) consistently short bursts (primarily He accretion), and we calculated the mean burst rate as a function of accretion rate for the two groups. The decrease in burst rate observed at > 0.06dot MEdd (~2 × 10^37 ergs s^−1) is associated with a transition in the persistent spectral state and (as has been suggested previously) may be related to the increasing role of steady He burning. We found many examples of bursts with recurrence times <30 minutes, including burst triplets and even quadruplets. We describe the oscillation amplitudes for 13 of the 16 burst oscillation sources, as well as the stages and properties of the bursts in which the oscillations are detected. The burst properties are correlated with the burst oscillation frequency; sources spinning at <400 Hz generally have consistently short bursts, while the more rapidly spinning systems have both long and short bursts. This correlation suggests either that shear-mediated mixing dominates the burst properties, or alternatively that the nature of the mass donor (and hence the evolutionary history) has an influence on the long-term spin evolution.

https://iopscience.iop.org/article/10.1086/592044/pdf

Thermonuclear (Type I) X-Ray Bursts Observed by the Rossi X-Ray Timing Explorer

We present the calibration and background model for the Proportional Counter Array (PCA) aboard the Rossi X-ray Timing Explorer (RXTE). The energy calibration is systematics limited below 10 keV with deviations from a power law fit to the Crab nebula plus pulsar loss than 1%. Unmodelled variations in the instrument background amount to less than 2% of the observed background below 10 keV and less than 1% between 10 and 20 keV. Individual photon arrival times are accurate to 4.4 microseconds at all times during the mission and to 2.5 microseconds after 29 April 1997. The peak pointing direction of the five collimators is known at few arcsecond precision.

/pdf/calibration-of-the-rossi-x-ray-timing-explorer-proportional-36995gqjd8.pdf

Calibration of the Rossi X-Ray Timing Explorer Proportional Counter Array

The hard state of X-ray binaries (XRBs) is characterized by a power-law spectrum in the X-ray band and a flat/inverted radio/IR spectrum associated with occasionally imaged compact jets. It has generally been thought that the hard X-rays result from Compton upscattering of thermal accretion disk photons by a hot, coronal plasma whose properties are inferred via spectral fitting. Interestingly, these properties—especially those from certain magnetized corona models—are very similar to the derived plasma conditions at the jet footpoints. Here we explore the question of whether the "corona" and "jet base" are in fact related, starting by testing the strongest premise that they are synonymous. In such models, the radio through the soft X-rays are dominated by synchrotron emission, while the hard X-rays are dominated by inverse Compton at the jet base—with both disk and synchrotron photons acting as seed photons. The conditions at the jet base fix the conditions along the rest of the jet, thus creating a direct link between the X-ray and radio emission. We also add to this model a simple iron line and convolve the spectrum with neutral reflection. After forward-folding the predicted spectra through the detector response functions, we compare the results to simultaneous radio/X-ray data obtained from the hard states of the Galactic XRBs GX 339-4 and Cygnus X-1. Results from simple Compton corona model fits are also presented for comparison. We demonstrate that the jet model fits are statistically as good as the single-component corona model X-ray fits, yet are also able to address the simultaneous radio data.

https://iopscience.iop.org/article/10.1086/497628/pdf

Going with the flow: can the base of jets subsume the role of compact accretion disk coronae?

We present timing analysis for a Rossi X-Ray Timing Explorer (RXTE) observation of Cygnus X-1 in its hard/low state. This was the first RXTE observation of Cyg X-1 taken after it transited back to this state from its soft/high state. RXTE's large effective area, superior timing capabilities, and ability to obtain long, uninterrupted observations have allowed us to obtain measurements of the power spectral density (PSD), coherence function, and Fourier time lags to a decade lower in frequency and 0.5 decade higher in frequency than typically was achieved with previous instruments. Notable aspects of our observations include a weak 0.005 Hz feature in the PSD coincident with a coherence recovery; a "hardening" of the high-frequency PSD with increasing energy; a broad frequency range measurement of the coherence function, revealing rollovers from unity coherence at both low and high frequency; and an accurate determination of the Fourier time lags over 2.5 decades in frequency. As has been noted in previous similar observations, the time delay is approximately proportional to f−0.7, and at a fixed Fourier frequency the time delay of the hard X-rays compared to the softest energy channel tends to increase logarithmically with energy. Curiously, the 0.01-0.2 Hz coherence between the highest and the lowest energy bands is actually slightly greater than the coherence between the second highest and the lowest energy bands. We carefully describe all of the analysis techniques used in this paper, and wemake comparisons of the data to general theoretical expectations. In a companion paper, we makespecific comparisons to a Compton corona model that we have successfully used to describe the energy spectral data from this observation.

Rossi X‐Ray Timing Explorer Observation of Cygnus X‐1. II. Timing Analysis

The proportional counter array (PCA) is designed to perform microsecond timing of bright galactic sources and broad band, confusion limited, studies of faint extragalactic sources in the 2 - 60 keV x-ray band. The PCA was launched as part of the Rossi X-ray Timing Explorer (RXTE) satellite into a circular orbit of altitude 580 km and 23 degrees inclination on December 30, 1995. The mission contains three experiments: a set of large area xenon proportional counters sensitive from 2 - 60 keV (proportional counter array: PCA), a set of large area sodium iodide scintillators sensitive from 15 - 200 keV (high energy x-ray timing experiment: HEXTE), and three wide field of view scanning detectors which monitor most of the sky each orbit (all sky monitor: ASM). The goals of the mission are summarized by Swank et al. We present performance and calibration data on the measured and predicted in-orbit background, energy response, relative and absolute timing performance, and the operational possibilities made available with the high performance experiment data system (EDS) designed and built by MIT.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In-orbit performance and calibration of the Rossi X-ray Timing Explorer (RXTE) Proportional Counter Array (PCA)

Dead-Time Modifications to Fast Fourier Transform Power Spectra

In this paper we briefly describe the design and initial laboratory performance of the Proportional Counter Array (PCA) for the X-ray Timing Explorer (XTE). The PCA consists of 5 identical, sealed and collimated (1 degree(s) FWHM), xenon/methane multi-anode proportional counters sensitive to x-rays with energies between 2 and 60 keV. Each detector has an effective area of approximately 1,500 cm2. The data system can tag the relative time of arrival of each event with an accuracy of 1 microsecond(s) . The overall absolute time accuracy will be maintained by the spacecraft to better than 1 ms. Following the design principles of the HEAO-1 A2 HED detectors, the PCA adopts the interleaved anode connection scheme with an active propane anti-coincidence layer in the front and a similar anti-coincidence xe/methane layer on three other sides of the detector. The interleaved anode connection scheme and the anti-coincidence layers reject background events caused by charged particles with high efficiency, thus significantly reducing the background event rate at lower energies. As of June, 1993, the fabrication and assembly of all the five detectors had been completed. Their performance evaluation and characterization are currently under way. XTE is scheduled to be launched with a Delta-II rocket in the summer of 1995.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

/pdf/laboratory-performance-of-the-proportional-counter-array-442hltddqf.pdf

Laboratory performance of the proportional counter array experiment for the X-ray Timing Explorer

In this Letter, we provide an introduction to the main features seen in a series of observations of the bursting pulsar GRO J1744-28 The observations were made from 1996 January through May with the proportional counter array on the Rossi X-Ray Timing Explorer spacecraft In the 2-10 keV band, GRO J1744-28 emitted large bursts of ~10 s duration at a rate of about 2 per hour The peak flux during these bursts was ~6-40 times greater than that in the quiescent, or nonbursting, periods For the earliest bursts, the inferred peak luminosity approaches 100 times the Eddington limit, which is suggestive of some kind of beaming mechanism A range of smaller bursts and quasi-periodic oscillation features were also seen All this activity was superposed on an almost perfect sinusoidal modulation at a frequency of 214 Hz with an amplitude of ~10% of the nonbursting flux The source's persistent flux declined in a roughly linear trend from late January until mid-May, by which time its intensity was confused with several other sources

The Main Characteristics of GRO J1744–28 Observed by the Proportional Counter Array Experiment on the Rossi X-Ray Timing Explorer

The X-ray Timing Explorer (XTE) is a NASA satellite designed to perform high-time-resolution studies of known X-ray sources. The two main experiments are a large-area proportional counter array (PCA) from the Goddard Space Flight Center (GSFC) and a high-energy X-ray timing experiment (HEXTE) from the University of California at San Diego (UCSD). The PCA data is processed by an electronic data system (EDS) built by the Massachusetts Institute of Technology (MIT) that performs many parallel processing analysis functions for on-board evaluation and data compression. MIT also provide an all-sky monitor (ASM) experiment so that XTE can be slewed rapidly to new transient sources. The spacecraft provides a mean science telemetry rate for the PCA of ~20 kilobits per second (kbps), with bursts to 256 kbps for durations of 30 minutes. Photons are tagged to 1 μs and absolute timing should be better than 100 μs. XTE is due for launch in late August 1995 and the first NASA Research Announcement (NRA) is due out in January 1995. This paper summarises XTE’s performance and then discusses the interactive and flexible operations of the satellite and some of the science it can do. These features should make XTE a productive spacecraft for coordinated observation programs.

/pdf/prospects-for-coordinated-observations-with-xte-3tow0e1chz.pdf

A. B. Giles

Papers

In-orbit performance and calibration of the Rossi X-ray Timing Explorer (RXTE) Proportional Counter Array (PCA)

Dead-Time Modifications to Fast Fourier Transform Power Spectra

Laboratory performance of the proportional counter array experiment for the X-ray Timing Explorer

The Main Characteristics of GRO J1744–28 Observed by the Proportional Counter Array Experiment on the Rossi X-Ray Timing Explorer

Prospects for Coordinated Observations with XTE