83 research outputs found
Probing the central black hole in M87 with gamma-rays
Recent high-sensitivity observation of the nearby radio galaxy M87 have
provided important insights into the central engine that drives the large-scale
outflows seen in radio, optical and X-rays. This review summarizes the
observational status achieved in the high energy (HE;<100 GeV) and very high
energy (VHE; >100 GeV) gamma-ray domains, and discusses the theoretical
progress in understanding the physical origin of this emission and its relation
to the activity of the central black hole.Comment: Invited compact review to be published in Modern Physics Letters A;
19 pages, 4 figure
Linear Accelerating Superluminal Motion Model
Accelerating superluminal motions were detected recently by multi-epoch Very
Long Baseline Interferometry (VLBI) observations. Here, a Linear Accelerating
Superluminal Motion (LASM) model is proposed to interpret the observed
phenomena. The model provides a direct and accurate way to estimate the viewing
angle of a relativistic jet. It also predicts that both Doppler boosting and
deboosting effects may take place in an accelerating forward jet. The LASM
model is applied to the data of the quasar 3C 273, and the initial velocity,
acceleration and viewing angle of its three components are derived through
model fits. The variations of the viewing angle suggest that a supermassive
black hole binary system may exist in the center of 3C273. The gap between the
inner and outer jet in some radio loud AGNs my be explained in terms of Doppler
deboosting effects when the components accelerate to ultra-relativistic speed.Comment: 12 Pages, 3 Figues, 1 Table, Accepted for Publication by ApJ Lette
Accretion onto the Supermassive Black Hole in M87
Chandra X-ray observations of the giant elliptical galaxy M87 resolve the
thermal state of the hot interstellar medium into the accretion (Bondi) radius
of its central 3 10^9 Msun black hole. We measure the X-ray gas temperature and
density profiles and calculate the Bondi accretion rate, Mdot_Bondi \sim 0.1
Msun/yr. The X-ray luminosity of the active nucleus of M87 observed with
Chandra is L_{x, 0.5-7 \keV} \sim 7 \times 10^{40}erg/s. This value is much
less than the predicted nuclear luminosity, L_{Bondi} \sim 5 \times 10^{44}
erg/s, for accretion at the Bondi rate with a canonical accretion radiative
efficiency of 10%. If the black hole in M87 accretes at this rate it must do so
at a much lower radiative efficiency than the canonical value. The
multiwavelength spectrum of the nucleus is consistent with that predicted by an
advection-dominated flow. However, as is likely, the X-ray nucleus is dominated
by jet emission then the properties of flow must be modified, possibly by
outflows. We show that the overall energetics of the system are just consistent
with the predicted Bondi nuclear power. This suggests that either most of the
accretion energy is released in the relativistic jet or that the central engine
of M87 undergoes on-off activity cycles. We show that, at present, the energy
dumped into the ISM by the jet may reduce the accretion rate onto the black
hole by a factor \propto (v_j/c_s)^{-2}, where v_j is the jet velocity and c_s
the ISM sound speed, and that this is sufficient to account for the low nuclear
luminosity.Comment: emulateapj.sty, revised version, accepted by Ap
X-ray Spectral Survey of WGACAT Quasars, II: Optical and Radio Properties of Quasars with Low Energy X-ray Cut-offs
We have selected quasars with X-ray colors suggestive of a low energy
cut-off, from the ROSAT PSPC pointed archive. We examine the radio and optical
properties of these 13 quasars. Five out of the seven quasars with good optical
spectra show associated optical absorption lines, with two having high delta-v
candidate systems. Two other cut-off quasars show reddening associated with the
quasar. We conclude that absorption is highly likely to be the cause of the
X-ray cut-offs, and that the absorbing material associated with the quasars,
not intervening along the line-of-sight. The suggestion that Gigahertz Peaked
Sources are associated with X-ray cut-offs remains unclear with this expanded
sample.Comment: 17 pages, LaTeX, including 2 Tables and 1 figure. Ap.J. in pres
Caenorhabditis elegans Genomic Response to Soil Bacteria Predicts Environment-Specific Genetic Effects on Life History Traits
With the post-genomic era came a dramatic increase in high-throughput technologies, of which transcriptional profiling by microarrays was one of the most popular. One application of this technology is to identify genes that are differentially expressed in response to different environmental conditions. These experiments are constructed under the assumption that the differentially expressed genes are functionally important in the environment where they are induced. However, whether differential expression is predictive of functional importance has yet to be tested. Here we have addressed this expectation by employing Caenorhabditis elegans as a model for the interaction of native soil nematode taxa and soil bacteria. Using transcriptional profiling, we identified candidate genes regulated in response to different bacteria isolated in association with grassland nematodes or from grassland soils. Many of the regulated candidate genes are predicted to affect metabolism and innate immunity suggesting similar genes could influence nematode community dynamics in natural systems. Using mutations that inactivate 21 of the identified genes, we showed that most contribute to lifespan and/or fitness in a given bacterial environment. Although these bacteria may not be natural food sources for C. elegans, we show that changes in food source, as can occur in environmental disturbance, can have a large effect on gene expression, with important consequences for fitness. Moreover, we used regression analysis to demonstrate that for many genes the degree of differential gene expression between two bacterial environments predicted the magnitude of the effect of the loss of gene function on life history traits in those environments
Pentacycloundecane lactam vs lactone norstatine type protease HIV inhibitors: binding energy calculations and DFT study
The βLys66Tyr Variant of Human Hemoglobin as a Component of a Blood Substitute
It has been proposed that introducing tyrosine residues into human hemoglobin (e.g. βPhe41Tyr) may be able to reduce the toxicity of the ferryl heme species in extracellular hemoglobin-based oxygen carriers (HBOC) by facilitating long-range electron transfer from endogenous and exogenous antioxidants. Surface-exposed residues lying close to the solvent exposed heme edge may be good candidates for mutations. We therefore studied the properties of the βLys66Tyr mutation. Hydrogen peroxide (H2O2) was added to generate the ferryl protein. The ferryl state in βLys66Tyr was more rapidly reduced to ferric (met) by ascorbate than recombinant wild type (rwt) or βPhe41Tyr. However, βLys66Tyr suffered more heme and globin damage following H2O2 addition as measured by UV/visible spectroscopy and HPLC analysis. βLys66Tyr differed notably from therwt protein in other ways. In the ferrous state the βLys66Tyr forms oxy, CO, and NO bound heme complexes similar torwt. However, the kinetics of CO binding to the mutant was faster thanrwt, suggesting a more open heme crevice. In the ferric (met) form the typical met Hb acid-alkaline transition (H2O to-OH) appeared absent in the mutant protein. A biphasicity of cyanide binding was also evident. Expression in E. coli of the βLys66Tyr mutant was lower than therwt protein, and purification included significant protein heterogeneity. Whilst, βLys66Tyr and rwt autoxidised (oxy to met) at similar rates, the oxygen p50 for βLys66Tyr was very low. Therefore, despite the apparent introduction of a new electron transfer pathway in the βLys66Tyr mutant, the heterogeneity, and susceptibility to oxidative damage argue against this mutant as a suitable starting material for a HBOC
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