142 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
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
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
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
Nutrient limitations to bacterial and fungal growth during cellulose decomposition in tropical forest soils
Nutrients constrain the soil carbon cycle in tropical forests, but we lack knowledge on how these constraints vary within the soil microbial community. Here, we used in situ fertilization in a montane tropical forest and in two lowland tropical forests on contrasting soil types to test the principal hypothesis that there are different nutrient constraints to different groups of microorganisms during the decomposition of cellulose. We also tested the hypotheses that decomposers shift from nitrogen to phosphorus constraints from montane to lowland forests, respectively, and are further constrained by potassium and sodium deficiency in the western Amazon. Cellulose and nutrients (nitrogen, phosphorus, potassium, sodium, and combined) were added to soils in situ, and microbial growth on cellulose (phospholipid fatty acids and ergosterol) and respiration were measured. Microbial growth on cellulose after single nutrient additions was highest following nitrogen addition for fungi, suggesting nitrogen as the primary limiting nutrient for cellulose decomposition. This was observed at all sites, with no clear shift in nutrient constraints to decomposition between lowland and montane sites. We also observed positive respiration and fungal growth responses to sodium and potassium addition at one of the lowland sites. However, when phosphorus was added, and especially when added in combination with other nutrients, bacterial growth was highest, suggesting that bacteria out-compete fungi for nitrogen where phosphorus is abundant. In summary, nitrogen constrains fungal growth and cellulose decomposition in both lowland and montane tropical forest soils, but additional nutrients may also be of critical importance in determining the balance between fungal and bacterial decomposition of cellulose.This study is a product of the Andes Biodiversity and Ecosystem Research Group consortium (www.andesconservation.org) and was financed by the UK Natural Environment Research Council NE/G018278/1 to PM, a European
Union Marie-Curie Fellowship FP7-2012-329360 to ATN and ARC award DP170104091 to PM
Relativistic Outflow Drives Gamma-Ray Emission in 3C345
Aims: 3C345 was recently identified as a gamma-ray emitter, based on the
first 20 months of Fermi-LAT data and optical monitoring. In this paper, a
connection between the gamma-ray and optical variability of 3C345 and
properties of its parsec-scale radio emission is investigated. Methods: The
Fermi-LAT data of 3C345, covering an energy range of 0.1-300 GeV, were combined
with 32 Very Long Baseline Array observations of the object made at 43.2 GHz in
the period of January 2008 - March 2010. Results: The VLBA data reveal
morphology and kinematics of the flow on scales of up to ~5 milliarcseconds
(mas; deprojected linear distances of 380 parsecs). The brightness temperature,
T_b(r), measured along the jet first decreases with distance proportional to
r^-(0.95 +/-0.69) and later exhibits a break at ~0.3 mas, with T_b(r)
proportional to r^-(4.11 +/-0.85) at larger separations. Variations of the
gamma-ray, optical and parsec-scale radio emission show a similar long-term
trend persistent during the entire VLBA monitoring period. The gamma-ray and
optical variations on shorter time scales are related to structural changes in
the jet on scales of ~0.3 mas (~23 parsecs, deprojected), with the gamma-ray
and optical flares possibly related to the evolution of four distinct
superluminal components identified in the flow. Conclusions: The observations
indicate that both the quiescent and flaring components of the gamma-ray
emission are produced in a region of the jet of ~23 pc in extent. This region
may mark the Compton-loss dominated zone of the flow and its large extent may
favor the synchrotron self-Compton mechanism for gamma-ray production in the
relativistic jet of the quasar 3C345.Comment: 10 pages, 6 figures, 2 tables, accepted for publication in Astronomy
& Astrophysic
Enhancement of Late Successional Plants on Ex-Arable Land by Soil Inoculations
Restoration of species-rich grasslands on ex-arable land can help the conservation of biodiversity but faces three big challenges: absence of target plant propagules, high residual soil fertility and restoration of soil communities. Seed additions and top soil removal can solve some of these constraints, but restoring beneficial biotic soil conditions remains a challenge. Here we test the hypotheses that inoculation of soil from late secondary succession grasslands in arable receptor soil enhances performance of late successional plants, especially after top soil removal but pending on the added dose. To test this we grew mixtures of late successional plants in arable top (organic) soil or in underlying mineral soil mixed with donor soil in small or large proportions. Donor soils were collected from different grasslands that had been under restoration for 5 to 41 years, or from semi-natural grassland that has not been used intensively. Donor soil addition, especially when collected from older restoration sites, increased plant community biomass without altering its evenness. In contrast, addition of soil from semi-natural grassland promoted plant community evenness, and hence its diversity, but reduced community biomass. Effects of donor soil additions were stronger in mineral than in organic soil and larger with bigger proportions added. The variation in plant community composition was explained best by the abundances of nematodes, ergosterol concentration and soil pH. We show that in controlled conditions inoculation of soil from secondary succession grassland into ex-arable land can strongly promote target plant species, and that the role of soil biota in promoting target plant species is greatest when added after top soil removal. Together our results point out that transplantation of later secondary succession soil can promote grassland restoration on ex-arable land
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
SOS Response Induces Persistence to Fluoroquinolones in Escherichia coli
Bacteria can survive antibiotic treatment without acquiring heritable antibiotic resistance. We investigated persistence to the fluoroquinolone ciprofloxacin in Escherichia coli. Our data show that a majority of persisters to ciprofloxacin were formed upon exposure to the antibiotic, in a manner dependent on the SOS gene network. These findings reveal an active and inducible mechanism of persister formation mediated by the SOS response, challenging the prevailing view that persisters are pre-existing and formed purely by stochastic means. SOS-induced persistence is a novel mechanism by which cells can counteract DNA damage and promote survival to fluoroquinolones. This unique survival mechanism may be an important factor influencing the outcome of antibiotic therapy in vivo
Regional factors rather than forest type drive the community structure of soil living oribatid mites (Acari, Oribatida)
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