R. J. H. Dunn

The relation between accretion rate and jet power in X-ray luminous elliptical galaxies

Using Chandra X-ray observations of nine nearby, X-ray luminous elliptical galaxies with good optical velocity dispersion measurements, we show that a tight correlation exists between the Bondi accretion rates calculated from the observed gas temperature and density profiles and estimated black hole masses, and the power emerging from these systems in relativistic jets. The jet powers, which are inferred from the energies and timescales required to inflate cavities observed in the surrounding X-ray emitting gas, can be related to the accretion rates using a power law model of the form log(PBondi/10 43 ergs 1 ) = A + B log(Pjet/10 43 ergs 1 ), with A = 0.65± 0.16 and B = 0.77± 0.20. Our results show that a significant fraction of the energy associated with the rest mass of material entering the Bondi accretion radius (2.2 +1.0 0.7 per cent, for Pjet = 10 43 ergs 1 ) eventually emerges in the relativistic jets. The data also hint that this fraction may rise slightly with increasing jet power. Our results have significant implications for studies of accretion, jet formation and galaxy formation. The observed tight correlation suggests that the Bondi formulae provide a reasonable description of the accretion process in these systems, despite the likely presence of magnetic pressure and angular momentum in the accreting gas. The similarity of the PBondi and Pjet values argues that a significant fraction of the matter entering the accretion radius flows down to regions close to the black holes, where the jets are presumably formed. The tight correlation between PBondi and Pjet also suggests that the accretion flows are approximately stable over timescales of a few million years. Our results show that the black hole ‘engines’ at the hearts of large elliptical galaxies and groups can feed back sufficient energy to stem cooling and s formation, leading naturally to the observed exponential cut off at the bright end of the galaxy luminosity function.

Radio bubbles in clusters of galaxies

We extend our earlier work on cluster cores with distinct radio bubbles, adding more active bubbles, i.e. those with GHz radio emission, to our sample, and also investigating “ghost bubbles,” i.e. those without GHz radio emission. We have determined k, which is the ratio of the total particle energy to that of the electrons radiati ng between 10 MHz and 10 GHz. Constraints on the ages of the active bubbles confirm that the ratio of the energy factor, k, to the volume filling factor, f lies within the range 1 . k/f . 1000. In the assumption that there is pressure equilibrium between the radio-emitting plasma and the surrounding thermal X-ray gas, none of the radio lobes has equipartition between the relativistic particles and the magnetic field. A Monte-Carlo simulation of the data led t o the conclusion that there are not enough bubbles present in the current sample to be able to determine the shape of the population. An analysis of the ghost bubbles in our sample showed that on the whole they have higher upper limits on k/f than the active bubbles, especially when compared to those in the same cluster. A study of the Brightest 55 cluster sample shows that 17, possibly 20, clusters required some form of heating as they have a short central cooling time, tcool 6 3 Gyr, and a large central temperature drop, Tcentre/Touter < 1/2. Of these between 12 (70 per cent) and 15 (75 per cent), contain bubbles. This indicates that the du ty cycle of bubbles is large in such clusters and that they can play a major role in the heating process.

Non-thermal X-rays, a high-abundance ridge and fossil bubbles in the core of the Perseus cluster of galaxies

Using a deep Chandra observation of the Perseus cluster of galaxies, we find a high-abundance shell 250 arcsec (93 kpc) from the central nucleus. This ridge lies at the edge of the Perseus radio mini-halo. In addition we identify two Ha filaments pointing towards this shell. We hypothesize that this ridge is the edge of a fossil radio bubble, formed by entrained enriched material lifted from the core of the cluster. There is a temperature jump outside the shell, but the pressure is continuous indicating a cold front. A non-thermal component is mapped over the core of the cluster with a morphology similar to the mini-halo. Its total luminosity is 4.8 × 10 43 erg s -1 , extending in radius to ∼75 kpc. Assuming the non-thermal emission to be the result of inverse Compton scattering of the cosmic microwave background and infrared emission from NGC 1275, we map the magnetic field over the core of the cluster.

On viscosity, conduction and sound waves in the intracluster medium

Recent X-ray and optical observations of the Perseus cluster indicate that the viscous and conductive dissipation of sound waves is the mechanism responsible for heating the intracluster medium and thus balancing radiative cooling of cluster cores. We discuss this mechanism more generally and show how the specific heating and cooling r ates vary with temperature and radius. It appears that the heating mechanism is most effective above 10 7 K, which allows for radiative cooling to proceed within normal galaxy formation but will stifle the growth of very massive galaxies. The scaling of the wavelength of sound waves with cluster temperature and feedback in the system are investigated.

The radio properties of a complete, X-ray selected sample of nearby, massive elliptical galaxies

We investigate the radio properties of a complete sample of nearby, massive, X-ray bright elliptical and S0 galaxies. Our sample contains 18 galaxies with ROSAT All-Sky Survey X-ray fluxes Fx_(0.1-2.4 keV) > 3 x 10^(-12) erg/s/cm^2, within a distance of 100 Mpc. For these galaxies, we have complete (18/18) VLA radio and Chandra X-ray coverage. Nuclear radio emission is detected from 17/18 of the galaxies. Ten of the galaxies exhibit extended radio emission; of these ten, all but one also exhibit clear evidence of interaction of the radio source with the surrounding, X-ray emitting gas. Among the seven galaxies with unresolved radio sources, one has clear, and one has small, cavity-like features in the Chandra X-ray images; a third has a disturbed X-ray morphology. Using a radio luminosity limit equivalent to L_(1.4 Ghz) > 10^(23) W/Hz to calculate the radio-loud fraction, we find that this misses the majority of the radio detected galaxies in the sample. We determine integrated radio-to-X-ray flux ratios for the galaxies, GRx, which are shown to span a large range (factor of 100). We calculate the mass-weighted cooling times within 1 kpc, and find hints for an anticorrelation with the radio luminosity. We also calculate limits on k/f, where k is the ratio of the total particle energy to that of relativistic electrons radiating in the range 10 MHz-10 GHz and f is the volume filling factor of the plasma in the cavity. The k/f distribution is also broad, reflecting previous results for larger galaxy clusters. Lowering the X-ray flux limit, at the expense of less complete VLA and Chandra coverage, increases the size of our sample to 42 galaxies. Nuclear radio activity is detected in at least 34/42 of this extended sample.

Precession of the super-massive black hole in NGC 1275 (3C 84)?

The X-ray holes at the centre of the Perseus Cluster of galaxies are not all at the same position angle with respect to the centre of the cluster. This configuration would result if the jet inflating the bubbles is precessing, or moving around, and the bubbles detach at different times. The orientations which best fit the observed travel directions are an inclination of the precession axis to the line of sight of 120 degrees and an opening angle of 50 degrees. From the timescales for the bubbles seen in the cluster, the precession timescale, t_prec, is around 3.3x10^7 yrs. The bubbles rising up through different parts of the cluster may have interacted with the central cool gas, forming the whorl of cool gas observed in the temperature structure of the cluster. The dynamics of bubbles rising in fluids is discussed. The conditions present in the cluster are such that oscillatory motion, observed for bubbles rising in fluids on Earth, should take place. However the timescale for this motion is longer than that taken for the bubbles to evolve into spherical cap bubbles, which do not undergo a path instability, so such motion is not expected to occur.

Studying the X-ray hysteresis in GX 339−4: the disc and iron line over one decade

We report on a comprehensive and consistent investigation into the X-ray emission from GX 339−4. All public observations in the 11 year RXTE archive were analysed. Three different types of model ‐ single power law, broken power law and a disc + power law ‐ were fitted to investigate the evolution of the disc, along with a fixed Gaussian component at 6.4 keV to investigate any iron line in the spectrum. We show that the relative variation in flux and X-ray colour between the two best sampled outbursts are very similar. The decay of the disc temperature during the outburst is clearly seen in the soft state. The expected decay is SDisc ∝ T 4 ; we measure T 4.75±0.23 . This implies that the inner disc radius is approximately constant in the soft state. We also show a significant anticorrelation between the iron line equivalent width (EW) and the X-ray flux in the soft state while in the hard state the EW is independent of the flux. This results in hysteresis in the relation between X-ray flux and both line flux and EW. To compare the X-ray binary outburst to the behaviour seen in active galactic nuclei (AGN), we construct a disc fraction luminosity diagram for GX 339−4, the first for an X-ray binary. The shape qualitatively matches that produced for AGN. Linking this with the radio emission from GX 339−4 the change in radio spectrum between the disc and power-law-dominated states is clearly visible.

Using radio bubbles to constrain the matter content of AGN jets

We revisit a method to obtain upper limits on the jet matter content combining synchrotron self-absorption constraints and the large-scale bubble energy. We use both X-ray observations, which give limits on the jet power from the energies and time-scales of bubbles found in clusters of galaxies, and radio observations, which give limits on the magnetic field in the jets. Combining the two imposes constraints on the particle number density, and hence the jet content. Out of a sample of clusters which have clear radio bubbles, there are only two which have sufficient resolution in the radio images to give significant constraints, under the assumption that the jets are fairly steady. The results for M87 and Perseus indicate that the radio emitting region of the jet is electron-positron dominated, assuming that the minimum of the electron energy distribution, γ min ∼ 1.

A Global Study of X-ray Binaries

We present preliminary results on a global study of X-ray binaries using 14 Ms of data from the Rossi X-ray Timing Explorer satellite. Our initial study on GX 339-4 is recapped as an introduction to the methods used. We use a consistent analysis scheme for all objects, with three different spectral models to fit the powerlaw and disc components. We also take into account the possibility of a line being present in the data. The resulting almost 4000 observations allow the tracking of the spectral properties of the binaries as they evolve through an outburst. Our investigations concentrate on the disc and line properties of the binaries when in outburst. We also show the Disc-Fraction Luminosity diagram for the population of X-ray binaries studied which will enable us to further links with AGN.

Evolution of the disc radii during outburst of X-ray binaries as infered from thermal emission.

Compact object displays drastic spectral and timing changi ng from the beginning to the end of an outburst, showing the different efficiencies of accretio n processes. Black hole binaries hence exhibit schematically two different states in X-ray spectr a: he first dominated by a thermal component and the second by a hard powerlaw shape like. Whereas th hard component is often attributed to the emission of a radiatively inefficient coro na, the thermal component is interpreted as the emission of the optically thick accretion disc. The commonly accepted picture suggests that the observed tr ansition between hard and soft states is associated by a drop in the accretion efficiency of the ther mal component by a recession of the internal disc radius in hard states. However, recent studie s based on relativistically broadened iron line and the thermal component strength analysis would tend to show the presence of the disc in the vicinity of the horizon. By a reanalysis of archive spect ra where thermal emission is present, we tracked the values of the disc radii during outbursts amon g several sources. Indeed, whereas a constant inner radius would imply that the disc luminosity s hould monotonically depends on the temperature, we show that this relationship seems to deviat e at the lowest luminosities.