Gamma-ray astronomy

Abstract:

We report the discovery with the RXTE/PCA of twin kHz QPOs in Cir X-1. Eleven cases of simultaneous double QPOs occurred, with significances of up to 6.3 and 5.5 sigma and centroid frequencies ranging between approximately 56-225 and 230-500 Hz for the two QPO peaks, respectively, i.e., for the most part at frequencies well below those of other sources. The QPO properties clearly indicate that these double peaks are the kHz QPOs known from low magnetic field neutron stars, and not black-hole high-frequency QPOs, confirming that Cir X-1 is a neutron star. The kHz QPO peak separation varies over a wide range, \~175-340 Hz, and increases with QPO frequency. This is contrary to what is seen in other sources but agrees with predictions of the relativistic precession model and Alfven wave models; beat-frequency models require modification to accommodate this. In other observations single kHz QPOs can be seen down to frequencies as low as ~12 Hz, as well as a strong low-frequency (LF) QPO between 1 and 30 Hz. The relations between the frequencies of the kHz QPOs and the LF QPO are in good agreement with those found previously in Z sources, confirming that Cir X-1 may be a peculiar Z source. We suggest that the low frequencies of the kHz QPOs in Cir X-1 and to a lesser extent in (other) Z sources might be due to a relatively stronger radial inflow to the neutron star than in other kHz QPO sources.

Abstract:

We were recently faced with the following problem: The T-shaped Plateau de Bure Interferometer is expanding its tracks to achieve higher spatial resolution in astronomical images at mm wavelengths. Two more stations for positioning the antennas during observations are being built at the ends of the extended tracks. Which of the given stations should the remaining four antennas occupy to accommodate the new stations? What is the optimal set of antenna configurations, given the new extended one, to achieve necessary coverage of the uv-plane at a variety of spatial resolutions? We present in this paper the solutions to the above questions, resulting from a novel method we have recently developed. The method is based on identifying which placement of elements provides the most appropriate uv-plane sampling. © 2006 American Institute of Physics.