X-ray sources in globular clusters of other galaxies

A large number of X-ray sources in globular clusters of galaxies other than the Milky Way has been found with Chandra. We discuss three issues relating to these sources. The X-ray luminosity function (XLF) of the sources in globular clusters of M31 is marginally compatible with the XLF of globular clusters of the Milky Way. The individual XLFs of a dozen elliptical galaxies, after correction for incompleteness, are compatible with one another and show no break; however, the XLF found by adding the individual XLFs of elliptical galaxies has a break at L_x about 5x10(38) ergs/s. For the moment there is no evidence for a difference between the XLFs of sources inside and outside globular clusters of elliptical galaxies. It is not (yet?) possible to decide which fraction of low-mass X-ray binaries in elliptical galaxies outside globular clusters have formed inside globular clusters.Comment: 7 pages, 3 figures, to be published in Interacting Binaries: accretion, evolution and outcomes, AIP, eds. L. Antonelli et. a

Standard Cosmology and the BATSE Number vs. Peak Flux Distribution

The observed 2B BATSE distribution is consistent with the faintest GRBs in our sample originating from a redshift of Zmax ~ 0.8-3.0 (90\%), with the most likely values in the range of 1.0-2.2, and is largely insensitive to Omega for models with no evolution. To constrain the model parameter Omega to the range 0.1-1.0 using only Log N -- Log P distributions, more than 4000 GRBs, with a most likely value of ~ 9,000 GRBs to BATSE sensitivity. This requires a live integration time of >6 years with BATSE. Detectors sensitive to much lower limits (~ 70-400 in sensitivity) require ~ 200 GRBs, with <0.03 year 4pi ster coverage. We place limits on the amount of frequency density and, separately, peak luminosity evolution in the sample of GRBs. We find that frequency density evolution models can place the faintest GRBs at Zmax ~ 10-200, without conflicting with the observations of relative time dilation of ~ 2.Comment: 30 pages, uuencoded gzipped Postscript, 533Kb, also available from http://space.mit.edu/home/rutledge/Welcome.html ; MNRAS in pres

Discovery of KiloHertz Quasi-Periodic Oscillations in 4U 1735-44

We discovered a single kHz quasi-periodic oscillation (QPO) near 1150 Hz in the Rossi X-ray Timing Explorer X-ray light curve of the low mass X-ray binary and atoll source 4U 1735-44. The rms amplitude of this peak was 2-3%, and the FWHM 6-40 Hz. There are indications that the kHz QPO frequency decreased from 1160 Hz to 1145 Hz when the count rate increased, which would be quite different from what is observed in other atoll sources for which kHz QPOs have been discovered. In the X-ray color-color diagram and hardness-intensity diagram the source traced out the curved branch (the so-called banana branch) which has been found by previous instruments. The kHz QPO was only detected when the source was at the lowest count rates during our observations, i.e. on the lower part of the banana branch. When 4U 1735-44 was at higher count rates, i.e. on the upper part of the banana branch and at higher inferred mass accretion rate with respect to that on the lower part of the banana branch, the QPO was not detected. Besides the kHz QPO we discovered a low frequency QPO with a frequency near 67 Hz, together with a complex broad peaked noise component below 30 Hz. This 67 Hz QPO may be related to the magnetospheric beat-frequency QPO, which is observed on the horizontal branch of Z sources. This idea is supported by the (peaked) noise found in both 4U 1735-44 and Z sources at frequencies just below the QPO frequency.Comment: 9 pages, including 2 figures. Accepted for publication in ApJ Letter

Monitoring Chandra observations of the quasi-persistent neutron-star X-ray transient MXB 1659-29 in quiescence: the cooling curve of the heated neutron-star crust

We have observed the quasi-persistent neutron-star X-ray transient and eclipsing binary MXB 1659-29 in quiescence on three occasions with Chandra. The purpose of our observations was to monitor the quiescent behavior of the source after its last prolonged (~2.5 years) outburst which ended in September 2001. The X-ray spectra of the source are consistent with thermal radiation from the neutron-star surface. We found that the bolometric flux of the source decreased by a factor of 7-9 over the time-span of 1.5 years between our first and last Chandra observations. The effective temperature also decreased, by a factor of 1.6-1.7. The decrease in time of the bolometric flux and effective temperature can be described using exponential decay functions, with e-folding times of 0.7 and ~3 years, respectively. Our results are consistent with the hypothesis that we observed a cooling neutron-star crust which was heated considerably during the prolonged accretion event and which is still out of thermal equilibrium with the neutron-star core. We could only determine upper-limits for any luminosity contribution due to the thermal state of the neutron-star core. The rapid cooling of the neutron-star crust implies that it has a large thermal conductivity. Our results also suggest that enhanced cooling processes are present in the neutron-star core.Comment: Accepted for publication in ApJ Letters: March 19, 200

The hard quiescent spectrum of the neutron-star X-ray transient EXO 1745-248 in the globular cluster Terzan 5

We present a Chandra observation of the globular cluster Terzan 5 during times when the neutron-star X-ray transient EXO 1745-248 located in this cluster was in its quiescent state. We detected the quiescent system with a (0.5-10 keV) luminosity of ~2 x 10^{33} ergs/s. This is similar to several other neutron-star transients observed in their quiescent states. However, the quiescent X-ray spectrum of EXO 1745--48 was dominated by a hard power-law component instead of the soft component that usually dominates the quiescent emission of other neutron-star X-ray transients. This soft component could not conclusively be detected in EXO 1745-248 and we conclude that it contributed at most 10% of the quiescent flux in the energy range 0.5-10 keV. EXO 1745-248 is only the second neutron-star transient whose quiescent spectrum is dominated by the hard component (SAX J1808.4-3658 is the other one). We discuss possible explanations for this unusual behavior of EXO 1745-248, its relationship to other quiescent neutron-star systems, and the impact of our results on understanding quiescent X-ray binaries. We also discuss the implications of our results on the way the low-luminosity X-ray sources in globular clusters are classified.Comment: Accepted by ApJ Main Journal, September 22, 2004. Figure 2 is a color figur

Sidebands Due to Quasi-periodic Oscillations in 4U 1626-67

The low-mass X-ray binary pulsar 4U 1626-67 shows 0.048 Hz quasi-periodic oscillations (QPOs) and red noise variability as well as coherent pulsations at the 0.130 Hz neutron star spin frequency. Power density spectra of observations made with the Rossi X-ray Timing Explorer show significant sidebands separated from the pulsar spin frequency (and its harmonics) by the QPO frequency. These show that the instantaneous amplitude of the coherent pulsations is modulated by the amplitude of the QPOs. This phenomenon is expected in models such as the magnetospheric beat frequency model where the QPOs originate near the polar caps of the neutron star. In the 4--8 keV energy range, however, the lower-frequency sidebands are significantly stronger than their higher-frequency complements; this is inconsistent with the magnetospheric beat frequency model. We suggest that the 0.048 Hz QPOs are instead produced by a structure orbiting the neutron star at the QPO frequency. This structure crosses the line of sight once per orbit and attenuates the pulsar beam, producing the symmetric (amplitude modulation) sidebands. It also reprocesses the pulsar beam at the beat frequencies between the neutron star spin frequency and the QPOs, producing the excess variability observed in the lower-frequency sidebands. Quite independently, we find no evidence that the red noise variability modulates the amplitude of the coherent pulsations. This is also in contrast to the expectations of the magnetospheric beat frequency model and differs from the behavior in some high-mass X-ray binary pulsars.Comment: 8 pages, 3 figures, AAS macros v4.0. To appear in ApJ Letter

Chandra observations of the accretion-driven millisecond X-ray pulsars XTE J0929-314 and XTE J1751-305 in quiescence

(Abridge) We observed the accreting millisecond X-ray pulsars XTE J0929-314 and XTE J1751-305 in their quiescent states using Chandra. From XTE J0929-314 we detected 22 photons (0.3-8 keV) in 24.4 ksec, resulting in a count rate of 9 x 10^{-4} c/s. The small number of photons detected did not allow for a detailed spectral analysis, but we can demonstrate that the spectrum is harder than simple thermal emission which is what is usually presumed to arise from a cooling neutron star that has been heated during the outbursts. Assuming a power-law model for the spectrum, we obtain a power-law index of ~1.8 and an unabsorbed flux of 6 x 10^{-15} ergs/s/cm^2 (0.5-10 keV), resulting in a luminosity of 7 x 10^{31} (d/10 kpc)^2 ergs/s, with d in kpc. No thermal component could be detected; such a component contributed at most 30% to the 0.5-10 keV flux. Variability in the count rate of XTE J0929-314 was observed at the 95% confidence level. We did not conclusively detect XTE J1751-305 in our 43 ksec observation, with 0.5-10 keV flux upper limits between 0.2 and 2.7 x 10^{-14} ergs/s/cm^2 depending on assumed spectral shape, resulting in luminosity upper limits of 0.2 - 2 x 10^{32} (d/8 kpc)^2 ergs/s. We compare our results with those obtained for other neutron-star X-ray transients in their quiescent state. Using simple accretion disk physics in combination with our measured quiescent luminosity of XTE J0929-314 and the luminosity upper limits of XTE J1751-305, and the known spin frequency of the neutron stars, we could constrain the magnetic field of the neutron stars in XTE J0929-314 and XTE J1751-305 to be less than 3 x 10^9 (d/10 kpc) and 3 - 7 x 10^8 (d/8 kpc) Gauss (depending on assumed spectral shape of the quiescent spectrum), respectively.Comment: Accepted for publication in ApJ, 29 September 2004. Added spectral variability search for the data of XTE J0929-314 and added the non-detection with Chandra of XTE J1751-30