MeerKAT

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 use the 6C** sample to investigate the co-moving space density of powerful, steep-spectrum radio sources. This sample, consisting of 68 objects, has virtually complete K-band photometry and spectroscopic redshifts for 32 per cent of the sources. In order to find its complete redshift distribution, we develop a method of redshift estimation based on the K-z diagram of the 3CRR, 6CE, 6C* and 7CRS radio galaxies. Based on this method, we derive redshift probability density functions for all the optically identified sources in the 6C** sample. Using a combination of spectroscopic and estimated redshifts, we select the most radio luminous sources in the sample. Their redshift distribution is then compared with the predictions of the radio luminosity function of Jarvis et al. We find that, within the uncertainties associated with the estimation method, the data are consistent with a constant co-moving space density of steep-spectrum radio sources beyond z > 2.5, and rule out a steep decline.

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.