850 research outputs found
AGN have Underweight Black Holes and Reach Eddington
Eddington outflows probably regulate the growth of supermassive black holes
(SMBH) in AGN. I show that effect of the Rayleigh--Taylor instability on these
outflows means that SMBH masses are likely to be a factor of a few below the relation in AGN. This agrees with the suggestion by Batcheldor (2010)
that the relation defines an upper limit to the black hole mass. I
further argue that observed AGN black holes must spend much of their lives
accreting at the Eddington rate. This is already suggested by the low observed
AGN fraction amongst all galaxies despite the need to grow to the masses
required by the Soltan relation, and is reinforced by the suggested low SMBH
masses. Most importantly, this is the simplest explanation of the recent
discovery by Tombesi et al (2010a, b) of the widespread incidence of massive
ultrafast X--ray outflows in a large sample of AGN.Comment: MNRAS, in pres
An outburst scenario for the X-ray spectral variability in 3C 111
We present a combined Suzaku and Swift BAT broad-band E=0.6-200keV spectral
analysis of three 3C 111 observations obtained in 2010. The data are well
described with an absorbed power-law continuum and a weak (R~0.2) cold
reflection component from distant material. We constrain the continuum cutoff
at E_c~150-200keV, which is in accordance with X-ray Comptonization corona
models and supports claims that the jet emission is only dominant at much
higher energies. Fe XXVI Ly\alpha emission and absorption lines are also
present in the first and second observations, respectively. The modelling and
interpretation of the emission line is complex and we explore three
possibilities. If originating from ionized disc reflection, this should be
emitted at r_in> 50r_g or, in the lamp-post configuration, the illuminating
source should be at a height of h> 30r_g over the black hole. Alternatively,
the line could be modeled with a hot collisionally ionized plasma with
temperature kT = 22.0^{+6.1}_{-3.2} keV or a photo-ionized plasma with
log\xi=4.52^{+0.10}_{-0.16} erg s^{-1} cm and column density N_H > 3x10^23
cm^{-2}. However, the first and second scenarios are less favored on
statistical and physical grounds, respectively. The blue-shifted absorption
line in the second observation can be modelled as an ultra-fast outflow (UFO)
with ionization parameter log\xi=4.47^{+0.76}_{-0.04} erg s^{-1} cm, column
density N_H=(5.3^{+1.8}_{-1.3})x 10^{22} cm^{-2} and outflow velocity v_out =
0.104+/-0.006 c. Interestingly, the parameters of the photo-ionized emission
model remarkably match those of the absorbing UFO. We suggest an outburst
scenario in which an accretion disc wind, initially lying out of the line of
sight and observed in emission, then crosses our view to the source and it is
observed in absorption as a mildly-relativistic UFO.Comment: Accepted for publication in MNARS on July 1st 201
Quantum spin models with electrons in Penning traps
We propose a scheme to engineer an effective spin Hamiltonian starting from a
system of electrons confined in micro-Penning traps. By means of appropriate
sequences of electromagnetic pulses, alternated to periods of free evolution,
we control the shape and strength of the spin-spin interaction. Moreover, we
can modify the effective magnetic field experienced by the particle spin. This
procedure enables us to reproduce notable quantum spin systems, such as Ising
and XY models. Thanks to its scalability, our scheme can be applied to a fairly
large number of trapped particles within the reach of near future technology.Comment: 22 pages, 1 figure, added minor changes and typos, accepted for
publication in PR
Short term X-ray spectral variability of the quasar PDS 456 observed in a low flux state
We present an analysis of the 2013 Suzaku campaign on the nearby luminous
quasar PDS 456, covering a total duration of ~1 Ms and a net exposure of 455
ks. During these observations, the X-ray flux was suppressed by a factor of >10
in the soft X-ray band when compared to other epochs. We investigated the
broadband continuum by constructing a spectral energy distribution, making use
of the optical/UV photometry and hard X-ray spectra from the later
XMM-Newton/NuSTAR campaign in 2014. The high energy part of this low flux state
cannot be accounted for by self-consistent accretion disc and corona models
without attenuation by absorbing gas, which partially covers a substantial
fraction of the line of sight towards the X-ray source. Two absorption layers
are required, of column density and ,
with average covering factors of ~80% (with typical 5% variations) and 60%
(10-15%), respectively. In these observations PDS 456 displays significant
short term X-ray spectral variability, on timescales of ~100 ks, which can be
accounted for by variable covering of the absorbing gas. The partial covering
absorber prefers an outflow velocity of at
the >99.9% confidence level over the case where . This is
consistent with the velocity of the highly ionised outflow responsible for the
blueshifted iron K absorption profile. We therefore suggest that the partial
covering clouds could be the denser, or clumpy part of an inhomogeneous
accretion disc wind. Finally we estimate the size-scale of the X-ray source
from its variability. The radial extent of the X-ray emitter is found to be of
the order ~15-20 , although the hard X-ray (>2 keV) emission may
originate from a more compact or patchy corona of hot electrons, which is ~6-8
in size.Comment: 38 pages, 13 figures, accepted for publication in MNRA
Probing variability patterns of the Fe K line complex in bright nearby AGNs
The unprecedented sensitivity of current X-ray telescopes allows for the
first time to address the issue of the Fe K line complex variability patterns
in bright, nearby AGNs. We examine XMM-Newton observations of the brightest
sources of the FERO sample of radio-quiet type 1 AGNs with the aim of
characterizing the temporal behaviour of Fe K complex features. A systematic
mapping of residual flux above and below the continuum in the 4-9 keV range is
performed in the time vs energy domain, with the purpose of identifying
interesting spectral features in the three energy bands: 5.4-6.1 keV, 6.1-6.8
keV and 6.8-7.2 keV, corresponding respectively to the redshifted, rest frame
and blueshifted or highly ionized Fe Kalpha line bands. The variability
significance is assessed by extracting light curves and comparing them with
MonteCarlo simulations. The time-averaged profile of the Fe K complex revealed
spectral complexity in several observations. Red- and blue-shifted components
(either in emission or absorption) were observed in 30 out of 72 observations,
with an average ~90 eV for emission and ~ -30 eV for absorption
features. We detected significant line variability (with confidence levels
ranging between 90% and 99.7%) within at least one of the above energy bands in
26 out of 72 observations on time scales of ~6-30 ks. Reliability of these
features has been carefully calculated using this sample and has been assessed
at ~3sigma confidence level. This work increases the currently scanty number of
detections of variable, energy shifted, Fe lines and confirms the reliability
of the claimed detections. We found that the distribution of detected features
is peaked at high variability significances in the red- and blue-shifted energy
bands, suggesting an origin in a relativistically modified accretion flow.Comment: Accepted for publication in Astronomy & Astrophysic
Optomechanical Cooling of a Macroscopic Oscillator by Homodyne Feedback
We propose a simple optomechanical model in which a mechanical oscillator
quadrature could be "cooled" well below its equilibrium temperature by applying
a suitable feedback to drive the orthogonal quadrature by means of the homodyne
current of the radiation field used to probe its position.Comment: 9 pages, RevTeX, Figures available from authors, to appear in Phys.
Rev. Let
Tracing circumnuclear dense gas in H2O maser galaxies
A sample of 30 H2O extra-galactic maser galaxies with their published
HCN(J=1-0) and HCO+(J=1-0) observations has been compiled to investigate the
dense gas correlation with H2O maser emission. Our sample number exceeds the
size of the previous HCN samples studied so far by a factor of three, and it is
the first study on the possible relation with the dense gas tracer HCO+. We
find a strong correlation between normalized H2O maser emission luminosity
(LH2O\LCO) and normalized HCO+ line luminosity (LHCO+\LCO). Moreover, a weak
correlation has been found between LH2O\LCO and normalized HCN line luminosity
(LHCN\LCO). The sample is also studied after excluding Luminous and
Ultraluminous infrared galaxy (U)LIRG sources, and the mentioned correlations
are noticeably stronger. We show that 'Dense gas' fractions as obtained from
HCN and HCO+ molecules tightly correlate with maser emission, especially for
galaxies with normal IR luminosity(LIR< 10^11Lsun) and we show that HCO+ is a
better 'dense gas' tracer than HCN. Further systematic studies of these dense
gas tracers with higher transition level lines are vital to probe megamaser
physical conditions and to accurately determining how maser emission
interrelates with the dense gas
Targeting qubit states using open-loop control
We present an open-loop (bang-bang) scheme which drives an open two-level
quantum system to any target state, while maintaining quantum coherence
throughout the process. The control is illustrated by a realistic simulation
for both adiabatic and thermal decoherence. In the thermal decoherence regime,
the control achieved by the proposed scheme is qualitatively similar, at the
ensemble level, to the control realized by the quantum feedback scheme of Wang,
Wiseman, and Milburn [Phys. Rev. A 64, #063810 (2001)] for the spontaneous
emission of a two-level atom. The performance of the open-loop scheme compares
favorably against the quantum feedback scheme with respect to robustness,
target fidelity and transition times.Comment: 27 pages, 7 figure
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