Eric Cassel Ford

Strong-Field General Relativity and Quasi-periodic Oscillations in X-Ray Binaries

Quasi-periodic oscillations (QPOs) at frequencies near 1000 Hz were recently discovered in several X-ray binaries containing neutron stars. Two sources show no correlation between QPO frequency and source count rate. We suggest that the QPO frequency is determined by the Keplerian orbital frequency near the marginally stable orbit predicted by general relativity in strong gravitational fields. The radius of the marginally stable orbit does not depend on the luminosity or mass accretion rate of the source; therefore the QPO frequency does not depend on the source count rate. The QPO frequencies observed from 4U 1636-536 imply that the mass of the neutron star is 2.02 ± 0.12 M☉. Interpretation of the 4.1 keV absorption line observed from 4U 1636-536 as due to Fe XXV ions then implies a neutron star radius of 9.6 ± 0.6 km.

Energy Spectra and High Frequency Oscillations in 4U 0614+091

We investigate the behavior of the high-frequency quasi-periodic oscillations (QPOs) in 4U 0614+091, combining timing and spectral analyses of RXTE observations. The energy spectra of the source can be described by a power law (α ~ 2.8) and a blackbody (kT ~ 1.5 keV), with the blackbody accounting for 10%-20% of the total energy flux. We find a robust correlation of the frequency, ν, of the higher frequency QPO near 1 kHz with the flux of the blackbody, FBB. The slope of this correlation, d log ν/d log FBB, is 0.27-0.37. The source follows the same relation even in observations separated by several months. The QPO frequency does not have a similarly unique correlation with the total flux or the flux of the power-law component. The rms fraction of the higher frequency QPO rises with energy from 6.8% ± 1.5% (3-5 keV) to 21.3% ± 4.0% (10-12 keV). For the lower frequency QPO, however, it is consistent with a constant value of 5.4% ± 0.9%. The results may be interpreted in terms of a beat-frequency model for the production of the high-frequency QPOs.

KiloHertz Quasi-Periodic oscillations in Island State of 4U 1608-52 as observed with RXTE/PCA

We report RXTE/PCA observations of 4U 1608-52 on March 15, 18, and 22 immediately after the outburst in early 1996. The persistent count rates ranged from 190 to 450 counts s-1 (1-60 keV). During this period of time, 4U 1608-52 was in the island state. We detected quasi-periodic oscillation (QPO) features in the power density spectra (PDS) at 567-800 Hz on March 15 and 22, with source fractional root mean square (rms) amplitude of 13%-17% and widths of 78-180 Hz. The average rms amplitude of these QPO features is positively correlated with the energy. Our results imply that the neutron star spin frequency is possibly between 300 and 365 Hz.

Correlation between fast quasi-periodic oscillations and X-ray spectral shape in atoll sources

We present correlations between the frequency of the fast quasi-periodic oscillations (QPOs) and the shape of the X-ray spectrum in the atoll sources 4U 1608-52 and 4U 0614+091. We find that the photon index of a power-law component of the energy spectrum is well correlated with the QPO frequency in both sources. The correlation between the photon index and the QPO frequency most likely represents a common dependence on some physical parameter of the system.

Anticorrelated Hard/Soft X-Ray Emission from the X-Ray Burster 4U 0614+091

We have detected transient X-ray activity from the X-ray burster 4U 0614+091 simultaneously with BATSE on board the Compton Gamma Ray Observatory (20-100 keV) and the all-sky monitor (ASM) on board the Rossi X-Ray Timing Explorer (1-12 keV). The peak fluxes reach approximately 40 mcrab in both instruments over a period of about 20 days. The variable emission shows a clear anticorrelation of the hard X-ray flux with the soft X-ray count rate. The observed anticorrelation is another clear counterexample to the notion that only black hole binaries exhibit such correlations. The individual spectra during this period can be fitted by power laws with photon indices 2.2 ± 0.3 (ASM) and 2.7 ± 0.4 (BATSE), while the combined spectra can be described by a single power law with index 2.09 ± 0.08. BATSE and the Rossi X-Ray Timing Explorer ASM are a good combination for monitoring X-ray sources over a wide energy band.

Identification of the Periodic Hard X-Ray Transient GRO J1849-03 with the X-Ray Pulsar GS 1843-02 = X1845-024: A New Be/X-Ray Binary

We identify the periodic transient hard X-ray source GRO J1849-03 with the transient X-ray pulsar GS 1843-02 = X1845-024 on the basis of the detection of X-ray outbursts from X1845-024 coincident with hard X-ray outbursts of GRO J1849-03. From its spin period of 94.8 s and its orbital period of 241 days, we classify the system as a Be/X-ray binary.

Calibration of the stellar X-ray polarimeter

The stellar x-ray polarimeter (SXRP) will be more than an order of magnitude more sensitive than any previous x-ray polarimeter in the 2 - 15 keV energy band. The SXRP is a focal plane detector for a Danish-Russian SODART telescope, which will be launched on the Russian spectrum-x-gamma (SXG) mission. The SXRP exploits the polarization dependence of Bragg reflection from a graphite crystal, and of Thomson scattering from a target of metallic lithium. The SXRP flight model (FM) was calibrated at a facility at Lawrence Livermore National Laboratory (LLNL) equipped with polarized and unpolarized x-ray sources producing x-rays in the band pass for the graphite and lithium scatterers. By adjusting the orientation of the SXRP with respect to the incident x-ray beam, it was possible to simulate the converging beam from a SODART telescope and to measure the SXRP response to telescope pointing errors. In this paper, we describe the SXRP-FM calibration and present results for the graphite polarimeter.

A Search for Hard X-Ray Emission from Globular Clusters: Constraints from BATSE

We have monitored a sample of 27 nearby globular clusters in the hard X-ray band (20-120 keV) for approximately 1400 days using the BATSE instrument on board the Compton Gamma-Ray Observatory. Globular clusters may contain a large number of compact objects (e.g., pulsars or X-ray binaries containing neutron stars) which can produce hard X-ray emission. Our search provides a sensitive (~50 mCrab) monitor for hard X-ray transient events on time scales of >1 day and a means for observing persistent hard X-ray emission. We have discovered no transient events from any of the clusters and no persistent emission. Our observations include a sensitive search of four nearby clusters containing dim X-ray sources: 47 Tucanae, NGC 5139, NGC 6397, and NGC 6752. The non-detection in these clusters implies a lower limit for the recurrence time of transients of 2 to 6 years for events with luminosities >10^36 erg s^-1 (20-120 keV) and ~20 years if the sources in these clusters are taken collectively. This suggests that the dim X-ray sources in these clusters are not transients similar to Aql~X-1. We also place upper limits on the persistent emission in the range 2-10*10^34 erg s^-1 (2 sigma, 20-120 keV) for these four clusters. For 47 Tuc the upper limit is more sensitive than previous measurements by a factor of 3. We find a model dependent upper limit of 19 isolated millisecond pulsars (MSPs) producing gamma-rays in 47 Tuc, compared to the 11 observed radio MSPs in this cluster.

Kilo-Hertz QPO and X-ray bursts in 4U 1608-52 in low intensity state

We present the results from RXTE/PCA observations of 4U 1608-52 in its island state on March 15, 18 and 22 of 1996. Three type I X-ray bursts were detected in one RXTE orbit on March 22. We observed QPO features peaking at 567–800 Hz on March 15 and 22, with source fractional rms amplitude of 13%–17% and widths of 78–180 Hz in the power density spectra averaged over each spacecraft orbit. The rms amplitudes of these QPOs are positively correlated with the photon energy. The three X-ray bursts, with burst intervals of 16 and 8 minutes, have a duration of 16s. The blackbody emission region of the smallest X-ray burst among the three suggest it was a local nuclear burning. We also discuss a type I X-ray burst candidate in the observation.

Kilohertz oscillations in 4U 0614+091 and other LMXBs

Strong oscillations near 1000 Hz have been discovered in a number of low mass x-ray binaries with NASA’s Rossi X-Ray Timing Explorer. Their behavior should ultimately lead us to a better understanding of the accretion dynamics and the fundamental properties of the neutron star itself. Here we summarize the known properties of fast QPOs, focusing on results from the source 4U 0614+091. We highlight some of the aspects yet to be explored.