Black hole outflows King, A. R
Monthly Notices of the Royal Astronomical Society,
March 2010, Letnik:
402, Številka:
3
Journal Article
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I show that Eddington accretion episodes in active galactic nuclei (AGN) are likely to produce winds with velocities v∼ 0.1c and ionization parameters up to ξ∼ 104 (cgs), implying the presence of ...resonance lines of helium- and hydrogen-like iron. These properties are direct consequences of momentum and mass conservation, respectively, and agree with recent X-ray observations of fast outflows from AGN. Because the wind is significantly subluminal, it can persist long after the AGN is observed to have become sub-Eddington. The wind creates a strong cooling shock as it interacts with the interstellar medium of the host galaxy, and this cooling region may be observable in an inverse Compton continuum and lower excitation emission lines associated with lower velocities. The shell of matter swept up by the (‘momentum-driven’) shocked wind must propagate beyond the black hole's sphere of influence on a time-scale of ≲3 × 105 yr. Outside this radius, the shell stalls unless the black hole mass has reached the value Mσ implied by the M–σ relation. If the wind shock did not cool, as suggested here, the resulting (‘energy-driven’) outflow would imply a far smaller supermassive black hole mass than actually observed. In galaxies with large bulges the black hole may grow somewhat beyond this value, suggesting that the observed M–σ relation may curve upwards at large M. Minor accretion events with small gas fractions can produce galaxy-wide outflows with velocities significantly exceeding σ, including fossil outflows in galaxies where there is little current AGN activity. Some rare cases may reveal the energy-driven outflows which sweep gas out of the galaxy and establish the black hole–bulge mass relation. However, these require the quasar to be at the Eddington luminosity.
I suggest that the beaming factor in bright ultraluminous X-ray sources (ULXs) varies as , where is the Eddington ratio for accretion. This is required by the observed universal Lsoft∝T−4 relation ...between soft-excess luminosity and temperature, and is reasonable on general physical grounds. The beam scaling means that all observable properties of bright ULXs depend essentially only on the Eddington ratio , and that these systems vary mainly because the beaming is sensitive to the Eddington ratio. This suggests that bright ULXs are stellar-mass systems accreting at Eddington ratios of the order of 10–30, with beaming factors b≳ 0.1. Lower luminosity ULXs follow bolometric (not soft-excess) L∼T4 correlations and probably represent sub-Eddington accretion on to black holes with masses ∼10M⊙. High-mass X-ray binaries containing black holes or neutron stars and undergoing rapid thermal- or nuclear-time-scale mass transfer are excellent candidates for explaining both types. If the scaling for bright ULXs can be extrapolated to the Eddington ratios found in SS433, some objects currently identified as active galactic nuclei at modest redshifts might actually be ULXs (‘pseudo-blazars’). This may explain cases where the active source does not coincide with the centre of the host galaxy.
The Antarctic Ice Sheet (AIS) is out of equilibrium with the current anthropogenic‐enhanced climate forcing. Paleoenvironmental records and ice sheet models reveal that the AIS has been tightly ...coupled to the climate system during the past and indicate the potential for accelerated and sustained Antarctic ice mass loss into the future. Modern observations by contrast suggest that the AIS has only just started to respond to climate change in recent decades. The maximum projected sea level contribution from Antarctica to 2100 has increased significantly since the Intergovernmental Panel on Climate Change (IPCC) 5th Assessment Report, although estimates continue to evolve with new observational and theoretical advances. This review brings together recent literature highlighting the progress made on the known processes and feedbacks that influence the stability of the AIS. Reducing the uncertainty in the magnitude and timing of the future sea level response to AIS change requires a multidisciplinary approach that integrates knowledge of the interactions between the ice sheet, solid Earth, atmosphere, and ocean systems and across time scales of days to millennia. We start by reviewing the processes affecting AIS mass change, from atmospheric and oceanic processes acting on short time scales (days to decades), through to ice processes acting on intermediate time scales (decades to centuries) and the response to solid Earth interactions over longer time scales (decades to millennia). We then review the evidence of AIS changes from the Pliocene to the present and consider the projections of global sea level rise and their consequences. We highlight priority research areas required to improve our understanding of the processes and feedbacks governing AIS change.
Plain Language Summary
The Antarctic Ice Sheet (AIS) is an important component of the global climate system. Human activities have caused the atmosphere and especially the oceans to warm. However, the full effect of human caused climate change on the AIS has not currently been realized because the ice sheet responds on a range of time scales and to many different Earth processes. Modern observations show that West Antarctica has been melting at an accelerating rate since the 2000s, while the data for East Antarctica are less clear. Environmental records preserve the history of the climate and AIS, which extend beyond the instrumental record and reveal how the AIS responded to past climate warming. Estimates of how much the AIS will contribute to sea level rise by the Year 2100 have changed as a result of new information on how the AIS evolved in the past and research into the interactions between the ice sheet, solid Earth atmosphere, and ocean systems. This review brings together our knowledge of the major processes and feedbacks affecting the AIS and the evidence for how the ice sheet changed since the Pliocene. We consider the future estimates and consequences of global sea level rise from melting of the AIS and highlight priority research areas.
Key Points
The AIS is a highly dynamic component of the Earth system, evolving on a broad range of temporal and spatial scales
Paleoenvironmental evidence highlights the centennial to millennial response time scales of the AIS to atmospheric‐ocean forcing
Coupling feedbacks in Earth system components are required to reduce the uncertainty in AIS's contribution to past and future sea level rise
Africa, the ancestral home of all modern humans, is the most informative continent for understanding the human genome and its contribution to complex disease. To better understand the genetics of ...schizophrenia, we studied the illness in the Xhosa population of South Africa, recruiting 909 cases and 917 age-, gender-, and residence-matched controls. Individuals with schizophrenia were significantly more likely than controls to harbor private, severely damaging mutations in genes that are critical to synaptic function, including neural circuitry mediated by the neurotransmitters glutamine, γ-aminobutyric acid, and dopamine. Schizophrenia is genetically highly heterogeneous, involving severe ultrarare mutations in genes that are critical to synaptic plasticity. The depth of genetic variation in Africa revealed this relationship with a moderate sample size and informed our understanding of the genetics of schizophrenia worldwide.
Therapeutic plasma exchange (TPE) preconditioning with immunosuppressive therapy reduces ABO antibody titers, permitting engraftment of ABO‐incompatible (ABO‐I) kidney transplants. The posttransplant ...predictive role of ABO antibody titers for antibody‐mediated rejection (AMR) is unknown. This retrospective study evaluated 46 individuals who received TPE to permit ABO‐I kidney transplantation. ABO antibody titers were performed using donor‐type indicator red cells. Seven individuals (15.2%) experienced clinical or subclinical AMR. There was no significant difference between recipient blood group, number of pretransplant TPE and baseline titer between those with and without AMR. At 1–2 weeks posttransplant the median titer was 64 (range 4 – 512) among individuals with AMR and 16 (range 2 – 256) among individuals without AMR. Total agglutination reactivity score was significantly higher among individuals with AMR (p = 0.046). The risk of AMR was significantly higher among individuals with an elevated posttransplant titer of ≥64 (p = 0.006). The sensitivity of an elevated posttransplant titer was 57.1% with a specificity of 79.5%. The positive predictive value was 33.3% and the negative predictive value was 91.2%. Most individuals with AMR have an elevated titer, however, the positive predictive value of a high titer for AMR is poor.
In a retrospective study of 46 individuals who received therapeutic plasma exchange to permit ABO‐incompatible kidney transplantation, most individuals with antibody mediated rejection (AMR) have an elevated titer; however, the positive predictive value of a high titer for AMR is poor.
Rapid bifurcations in the plasma response to slowly varying n=2 magnetic fields are observed as the plasma transitions into and out of edge-localized mode (ELM) suppression. The rapid transition to ...ELM suppression is characterized by an increase in the toroidal rotation and a reduction in the electron pressure gradient at the top of the pedestal that reduces the perpendicular electron flow there to near zero. These events occur simultaneously with an increase in the inner-wall magnetic response. These observations are consistent with strong resonant field penetration of n=2 fields at the onset of ELM suppression, based on extended MHD simulations using measured plasma profiles. Spontaneous transitions into (and out of) ELM suppression with a static applied n=2 field indicate competing mechanisms of screening and penetration of resonant fields near threshold conditions. Magnetic measurements reveal evidence for the unlocking and rotation of tearinglike structures as the plasma transitions out of ELM suppression.
Abstract
We consider the Blandford–Znajek (BZ) mechanism for extracting black hole spin energy to drive astrophysical jets. Analyses of the BZ mechanism generally take no account of any electric ...charge on the black hole. But, as noted by Wald and others, if the medium surrounding the black hole is an ionized plasma with mobile charges, then a spinning hole quickly acquires an electric charge. The effect of this charge is to nullify the electric field structures which drive the BZ mechanism. Since jets are now observed in a wide variety of classes of accreting objects, most of which do not contain a central black hole, it seems likely that the jet-driving mechanism in all astrophysical objects uses energy directly from the accretion disk, rather than black hole spin.
Niche and neutral processes drive community assembly and metacommunity dynamics, but their relative importance might vary with the spatial scale. The contribution of niche processes is generally ...expected to increase with increasing spatial extent at a higher rate than that of neutral processes. However, the extent to what community composition is limited by dispersal (usually considered a neutral process) over increasing spatial scales might depend on the dispersal capacity of composing species. To investigate the mechanisms underlying the distribution and diversity of species known to have great powers of dispersal (hundreds of kilometres), we analysed the relative importance of niche processes and dispersal limitation in determining beta‐diversity patterns of aquatic plants and cladocerans over regional (up to 300 km) and continental (up to 3300 km) scales. Both taxonomic groups were surveyed in five different European regions and presented extremely high levels of beta‐diversity, both within and among regions. High beta‐diversity was primarily explained by species replacement (turnover) rather than differences in species richness (i.e. nestedness). Abiotic and biotic variables were the main drivers of community composition. Within some regions, small‐scale connectivity and the spatial configuration of sampled communities explained a significant, though smaller, fraction of compositional variation, particularly for aquatic plants. At continental scale (among regions), a significant fraction of compositional variation was explained by a combination of spatial effects (exclusive contribution of regions) and regionally‐structured environmental variables. Our results suggest that, although dispersal limitation might affect species composition in some regions, aquatic plant and cladoceran communities are not generally limited by dispersal at the regional scale (up to 300 km). Species sorting mediated by environmental variation might explain the high species turnover of aquatic plants and cladocerans at regional scale, while biogeographic processes enhanced by dispersal limitation among regions might determine the composition of regional biotas.
Intrinsic topological insulators are realized by alloying Bi2Te3 with Bi2Se3. Angle‐resolved photoemission and bulk transport measurements reveal that the Fermi level is readily tuned into the bulk ...bandgap. First‐principles calculations of the native defect landscape highlight the key role of anti‐site defects for achieving this, and predict optimal growth conditions to realize maximally resistive topological insulators.
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