The Square Kilometre Array (SKA)

Abstract:

In studies of accreting black holes in binary systems, empirical relations have been proposed to quantify the coupling between accretion processes and ejection mechanisms. These processes are probed, respectively, by means of X-ray and radio/optical-infrared observations. The relations predict, given certain accretion conditions, the expected energy output in the form of a jet. We investigated this coupling by studying the black hole candidate Swift J. 1753.5-0127, via multiwavelength-coordinated observations over a period of ∼4 yr. We present the results of our campaign showing that, all along the outburst, the source features a jet that is fainter than expected from the empirical correlation between the radio and the X-ray luminosities in a hard spectral state. Because the jet is so weak in this system the near-infrared emission is, unusually for this state and luminosity, dominated by thermal emission from the accretion disc. We briefly discuss the importance and the implications of a precise determination of both the slope and the normalization of the correlations, listing some possible parameters that broad-band jet models should take into account to explain the population of sources characterized by a dim jet. We also investigate whether our data can give any hint on the nature of the compact object in the system, since its mass has not been dynamically measured. © 2010 The Authors. Journal compilation © 2010 RAS.

Abstract:

One of the most fundamental problems in modern astrophysics concerns the formation of galaxies and clusters of galaxies. The Dutch-European radio telescope LOFAR will open up the last unexplored window of the electromagnetic spectrum for astrophysical studies and make important contributions to our knowledge of the structure formation in the universe. LOFAR's world-class observational capabilities will be used to survey the entire Northern low-frequency sky at a number of key frequencies. Studies of the most distant radio galaxies, clusters of galaxies and the cosmic star formation history and the exploration of new parameter space for serendipitous discovery were the four key topics that drove the areas, depths and frequency coverage of the proposed surveys. In addition to the key topics, the LOFAR surveys will provide a wealth of unique data for a huge number of additional important topics, including: detailed studies of AGN, and AGN physics, AGN evolution and black hole accretion history, nearby galaxies, strong gravitational lenses, cosmological parameters and large-scale structure formation, and Galactic radio sources. In this contribution we will first briefly discuss the scientific topics that have driven the design of the surveys. Subsequently we will present the design of the surveys. We will then briefly report on commissioning work carried out to prepare the instrument and the software pipelines for carrying out these surveys. At the end we will elaborate on LOFAR studies on clusters and show some first LOFAR results related to the nearby rich cluster Abell 2256. With at the time of writing only 15 out of the planned 36 Dutch stations working and several aspects of the calibration pipleline not fully functional, the obtained 135 MHz image already is among the deepest ever produced at low frequencies. The central halo of A2256 is well detected, illustrating the potential of LOFAR to map diffuse steep spectrum radio emission.

Abstract:

The LOFAR-UK station at Chilbolton has recently been completed and significantly increases the angular resolution of the International LOFAR Telescope, as well as providing a unique training site and testbed for british experience with next-generation software telescopes. The station has been funded primarily through the LOFAR-UK, the largest astronomy collaboration in Britain, as well as via the South East Physics Network (SEPNET) and STFC. In this brief paper we discuss the history and organisation of LOFAR-UK, provide a technical description of the Chilbolton site, and discuss how LOFAR stations can be augmented by the addition of extra local processing capabilities such as ARTEMIS.

Abstract:

We present the results of radio observations of the black hole binaries GRO J1655-40 and XTE J1550-564 in quiescence, with the upgraded Australia Telescope Compact Array. Neither system was detected. Radio flux density upper limits (3σ) of 26 μJy (at 5.5 GHz), 47 μJy (at 9 GHz) for GRO J1655-40 and 1.4 mJy (at 1.75 GHz), 27 μJy (at 5.5 GHz), 47 μJy (at 9 GHz) for XTE J1550-564 were measured. In conjunction with quasi-simultaneous Chandra X-ray observations (in the case of GRO J1655-40) and Faulkes Telescope optical observations (XTE J1550-564) we find that these systems provide the first evidence of relatively 'radio-quiet' black hole binaries at low luminosities, indicating that the scatter observed in the hard state X-ray-radio correlation at higher luminosities may also extend towards quiescent levels. © 2010 The Authors. Journal compilation © 2010 RAS.

Abstract:

In this paper, we consider the reported measurements of black hole spin for black hole X-ray binaries and compare them against the measurements of jet power and speed across all accretion states in these systems. We find no evidence for any correlation between the properties of the jets and the reported spin measurements. These constraints are strongest in the hard X-ray state, which is associated with a continuous powerful jet. We are led to conclude that one or more of the following is correct: (i) the calculated jet power and speed measurements are wrong, (ii) the reported spin measurements are wrong and (iii) there is no strong dependence of the jet properties on the black hole spin. In addition to this lack of observational evidence for a relation between the black hole spin and jet properties in stellar mass black holes, we highlight the fact that there appear to be at least three different ways in which the jet power and/or radiative efficiency from a black hole X-ray binary may vary, two of which are certainly independent of spin because they occur in the same source on relatively short time-scales and the third which does not correlate with any reported measurements of black hole spin. We briefly discuss how these findings may impact upon interpretations of populations of active galactic nuclei in the context of black hole spin and merger history. © 2010 The Authors. Journal compilation © 2010 RAS.