Philip Elias Kaaret

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.

An X-Ray Dip in the X-Ray Transient 4U 1630–47

An X-ray dip was observed during a 1996 Rossi X-Ray Timing Explorer observation of the recurrent X-ray transient 4U 1630-47. During the dip, the 2-60 keV X-ray flux drops by a factor of about 3, and, at the lowest point of the dip, the X-ray spectrum is considerably softer than at nondip times. We find that the 4U 1630-47 dip is best explained by absorption of the inner part of an accretion disk, while the outer part of the disk is unaffected. The spectral evolution during the dip is adequately described by the variation of a single parameter, the column density obscuring the inner disk.

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.

Discovery of kilohertz quasi-periodic oscillations in the atoll x-ray binary 4U 1705-44

In observations with the Rossi X-Ray Timing Explorer, we have discovered quasi-periodic oscillations (QPOs) near 1 kHz from 4U 1705-44, a low-mass X-ray binary with a neutron star classified as an atoll source. In six separate observations, we detect one QPO with a frequency ranging between 770 and 870 Hz and a 4% rms fraction in the full detector energy band. There is evidence for a second QPO at 1073 Hz in one interval. The separation in frequency of the two QPOs is 298±11 Hz. The QPOs are present only in observations where the mass accretion rate is inferred to be at an intermediate level, based on the atoll source phenomenology. At the highest accretion rates, the QPOs are not detected with upper limits to the rms fraction of about 2%. At the lowest accretion rates, the upper limits are about 4%. The QPO frequency increases with inferred mass accretion rate. This is expected in models where the QPO frequency is generated by motion at an inner edge of the accretion disk. An increased mass accretion rate causes the disk edge to move in, increasing the orbital frequency. Five type I X-ray bursts are observed with no detectable oscillations.

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.

Do the Unidentified Galactic EGRET Sources Lie in Star-forming Regions?

We find a statistically significant correlation between the positions of unidentified EGRET sources near the Galactic plane and OB associations. Nine of the unidentified EGRET sources have position contours overlapping an OB association, and an additional seven lie within 1° of an OB association. Using the known OB association distances to determine the intrinsic luminosity of the gamma-ray sources, we find a distribution of luminosities consistent with that of known gamma-ray pulsars. These results are consistent with the interpretation that young pulsars constitute the majority of the unidentified gamma-ray point sources near the Galactic plane.

Status of the stellar x-ray polarimeter for the Spectrum-X-Gamma mission

The Stellar X-Ray Polarimeter (SXRP) uses the polarization sensitivity of a graphite Bragg crystal and a lithium Thomsom scattering target to measure the polarization of X-rays from astrophysical sources. The SXRP is a focal plane detector for the Soviet-Danish SODART telescopes which will be launched on the Soviet Spectrum-X-Gamma mission. The SXRP will be the third orbiting stellar X-ray polarimeter, and should provide an order of magnitude increase in polarization sensitivity over its predecessors.

The inverse-compton and extragalactic components of the diffuse gamma-ray emission

We present spectra of the inverse-Compton and extragalactic components of the high-energy gamma-radiation based on an analysis of the emission at high galactic latitudes (

Stellar X-Ray Polarimeter: a focal plane polarimeter for the Spectrum X-Gamma mission

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