Global Jet Watch

Abstract:

We present near-infrared imaging of a sample of the faint, hard X-ray sources discovered in the 2001 Chandra Advanced CCD Imaging Spectrometer (ACIS)-I survey towards the Galactic Centre (GC) by Wang et al. These ∼800 discrete sources represent an important and previously undetected population within the Galaxy. From our Very Large Telescope observations of 77 X-ray sources, we identify candidate K-band counterparts to 75 per cent of the Chandra sources in our sample. The near-infrared magnitudes and colours of the majority of candidate counterparts are consistent with highly reddened stars, indicating that most of the Chandra sources are likely to be accreting binaries at or near the GC. © 2005 RAS.

Abstract:

We review the theory of cosmic ray transport and acceleration with an emphasis on the underlying plasma physics and examine how that theory can be applied to sources such as supernova remnants and giant radio galaxies. Starting with Fermi's original model for scattering off moving magnetized clouds, we discuss quasilinear transport theory and its application to the acceleration of particles at shock fronts. We discuss problems of injection and the excitation of MHD turbulence by the accelerated particles. In the diffusive limit and at strong shocks this mechanism produces a differential energy spectrum of N(E) (proportional to E-2. Recent observations of supernova remnants suggest that their spectra may be steeper than this value. We discuss the transport and acceleration of energetic particles in highly correlated magnetic field structures. In this case particles have an enhanced probability of escape from the shock as they are trapped on field lines and the resulting spectrum is steepened up to a value of 2.5. Fast particle transport also seems to be required by observations of the structures of giant radio galaxy lobes as a function of frequency.

Abstract:

We present results from the first near-IR imaging of the weak X-ray sources discovered in the Chandra/ACIS-I survey (Wang et al. 2002) towards the Galactic Centre (GC). These ~800 discrete sources, which contribute significantly to the GC X-ray emission, represent an important and previously unknown population within the Galaxy. From our VLT observations we will identify likely IR counterparts to a sample of the hardest sources, which are most likely X-ray binaries. With these data we can place constraints on the nature of the discrete weak X-ray source population of the GC.

Abstract:

The low-frequency power spectra of the X-ray and radio emission from four microquasars suggest that two distinct modes of energy output are at work: (i) the `coupled' mode in which the X-ray and radio luminosities are closely coupled and vary only weakly, and (ii) the `flaring' mode, which dramatically boosts the radio luminosity but makes no impact on the X-ray luminosity. The systems are in the flaring mode only a few percent of the time. However, flares completely dominate the power spectrum of radio emission, with the consequence that sources in which the flaring mode occurs, such as GRS 1915+105 and Cyg X-3, have radio power spectra that lie more than an order of magnitude above the corresponding X-ray power spectra. Of the four microquasars for which we have examined data, in only one, Cyg X-1, is the flaring mode seemingly inactive. While Cyg X-1 is a black-hole candidate, one of the three flaring sources, Sco X-1, is a neutron star. Consequently, it is likely that both modes are driven by the accretion disk rather than black-hole spin. Radio imaging strongly suggests that the flaring mode involves relativistic jets. A typical microquasar is in the flaring mode a few percent of the time, which is similar to the fraction of quasars that are radio loud. Thus there may be no essential difference between radio-loud and radio-quiet quasars; radio loudness may simply be a function of the epoch at which the source is observed.