1,055 research outputs found
Feedback-limited Accretion: Luminous Signatures from Growing Planets
Planets form in discs of gas and dust around stars, and keep growing by
accretion of disc material while available. Massive planets clear a gap in that
protoplanetary disc, but still accrete through spiral wakes. On its way to the
planet, the gas will settle on a \emph{circumplanetary} disc around the planet
and slowly accrete on to it. The energy of the accreted gas will be released,
heating the planet surroundings in a feedback process. For high enough
accretion rates the planet should be detectable at infrared wavelengths. We aim
to find whether detectable planet luminosities, , can occur when considering that the planet luminosity is
coupled to the accretion, and also to study which other effects has the
feedback on the dynamics of the circumplanetary and the gap regions. We model a
planet with mass ratio , orbiting at 10 AU from a solar mass star,
using a modified version of the 2D code FARGO-AD, which includes a prescription
for the accretion and feedback luminosity of the planet. We find that the
planetary feedback is able to partially deplete the circumplanetary disc, and
to reduce the accretion rate onto the planet. However, detectable luminosities
of are still produced. The
feedback also contributes to partially refilling the gap, to heat up the
coorbital region, and to perturb the orbital velocity of the gas.Comment: Submitted to MNRA
On the Nature of X-ray Surface Brightness Fluctuations in M87
X-ray images of galaxy clusters and gas-rich elliptical galaxies show a
wealth of small-scale features which reflect fluctuations in density and/or
temperature of the intra-cluster medium. In this paper we study these
fluctuations in M87/Virgo, to establish whether sound waves/shocks, bubbles or
uplifted cold gas dominate the structure. We exploit the strong dependence of
the emissivity on density and temperature in different energy bands to
distinguish between these processes. Using simulations we demonstrate that our
analysis recovers the leading type of fluctuation even in the presence of
projection effects and temperature gradients. We confirm the isobaric nature of
cool filaments of gas entrained by buoyantly rising bubbles, extending to 7' to
the east and south-west, and the adiabatic nature of the weak shocks at 40" and
3' from the center. For features of 5--10 kpc, we show that the central 4'x 4'
region is dominated by cool structures in pressure equilibrium with the ambient
hotter gas while up to 30 percent of the variance in this region can be
ascribed to adiabatic fluctuations. The remaining part of the central 14'x14'
region, excluding the arms and shocks described above, is dominated by
apparently isothermal fluctuations (bubbles) with a possible admixture (at the
level of about 30 percent) of adiabatic (sound waves) and by isobaric
structures. Larger features, of about 30 kpc, show a stronger contribution from
isobaric fluctuations. The results broadly agree with an AGN feedback model
mediated by bubbles of relativistic plasma.Comment: 16 pages, submitted to Ap
Discovery of multiple Lorentzian components in the X-ray timing properties of the Narrow Line Seyfert 1 Ark 564
We present a power spectral analysis of a 100 ksec XMM-Newton observation of
the narrow line Seyfert 1 galaxy Ark~564. When combined with earlier RXTE and
ASCA observations, these data produce a power spectrum covering seven decades
of frequency which is well described by a power law with two very clear breaks.
This shape is unlike the power spectra of almost all other AGN observed so far,
which have only one detected break, and resemble Galactic binary systems in a
soft state. The power spectrum can also be well described by the sum of two
Lorentzian-shaped components, the one at higher frequencies having a hard
spectrum, similar to those seen in Galactic binary systems. Previously we have
demonstrated that the lag of the hard band variations relative to the soft band
in Ark 564 is dependent on variability time-scale, as seen in Galactic binary
sources. Here we show that the time-scale dependence of the lags can be
described well using the same two-Lorentzian model which describes the power
spectrum, assuming that each Lorentzian component has a distinct time lag. Thus
all X-ray timing evidence points strongly to two discrete, localised, regions
as the origin of most of the variability. Similar behaviour is seen in Galactic
X-ray binary systems in most states other than the soft state, i.e. in the
low-hard and intermediate/very high states. Given the very high accretion rate
of Ark 564 the closest analogy is with the very high (intermediate) state
rather than the low-hard state. We therefore strengthen the comparison between
AGN and Galactic binary sources beyond previous studies by extending it to the
previously poorly studied very high accretion rate regime.Comment: 11 pages, 11 figures, accepted for publication in MNRA
Higher order glass-transition singularities in colloidal systems with attractive interactions
The transition from a liquid to a glass in colloidal suspensions of particles
interacting through a hard core plus an attractive square-well potential is
studied within the mode-coupling-theory framework. When the width of the
attractive potential is much shorter than the hard-core diameter, a reentrant
behavior of the liquid-glass line, and a glass-glass-transition line are found
in the temperature-density plane of the model. For small well-width values, the
glass-glass-transition line terminates in a third order bifurcation point, i.e.
in a A_3 (cusp) singularity. On increasing the square-well width, the
glass-glass line disappears, giving rise to a fourth order A_4 (swallow-tail)
singularity at a critical well width. Close to the A_3 and A_4 singularities
the decay of the density correlators shows stretching of huge dynamical
windows, in particular logarithmic time dependence.Comment: 19 pages, 12 figures, Phys. Rev. E, in prin
PG 1211+143: probing high frequency lags in a high mass AGN
We present the timing analysis of the four archived XMM-Newton observations
of PG 1211+143. The source is well-known for its spectral complexity,
comprising a strong soft-excess and different absorption systems. Soft energy
band (0.3-0.7 keV) lags are detected over all the four observations, in the
frequency range \nu \lsim 6 \times 10^{-4} Hz, where hard lags, similar to
those observed in black hole X-ray binaries, are usually detected in smaller
mass AGN. The lag magnitude is energy-dependent, showing two distinct trends
apparently connectable to the two flux levels at which the source is observed.
The results are discussed in the context of disk- and/or corona-reprocessing
scenarios, and of disk wind models. Similarities with the high-frequency
negative lag of 1H 0707-495 are highlighted, and, if confirmed, they would
support the hypothesis that the lag in PG 1211+143 represents the signature of
the same underlying mechanism, whose temporal characteristics scale with the
mass of the central object.Comment: 6 pages, 6 figures, accepted for publication in MNRAS Letter
Validity of Cognitive Assessment Tools for Older Adult Hispanics: A Systematic Review
A higher prevalence and incidence of dementia is found in Hispanic/Latino older adults. Therefore, valid instruments are necessary to assess cognitive functioning in this population group. Our aim was to review existing articles that have examined and reported on the validity of cognitive assessment tools in Hispanic/Latino population groups in the United States
Correlated X-ray/Ultraviolet/Optical variability in the very low mass AGN NGC 4395
We report the results of a one year Swift X-ray/UV/optical programme
monitoring the dwarf Seyfert nucleus in NGC 4395 in 2008-2009. The UV/optical
flux from the nucleus was found to vary dramatically over the monitoring
period, with a similar pattern of variation in each of the observed UV/optical
bands (spanning 1900 - 5500 {\AA}). In particular, the luminosity of NGC 4395
in the 1900 {\AA} band changed by more than a factor of eight over the
monitoring period. The fractional variability was smaller in the UV/optical
bands than that seen in the X-rays, with the X-ray/optical ratio increasing
with increasing flux. Pseudo-instantaneous flux measurements in the X-ray and
each UV/optical band were well correlated, with cross correlation coefficients
of >0.7, significant at 99.9 per cent confidence. Archival Swift observations
from 2006 sample the intra-day X-ray/optical variability on NGC 4395. These
archival data show a very strong correlation between the X-ray and b bands,
with a cross-correlation coefficient of 0.84 (significant at >99 per cent
confidence). The peak in the cross correlation function is marginally resolved
and asymmetric, suggesting that X-rays lead the b band, but by 1 hour. In
response to recent (August 2011) very high X-ray flux levels from NGC4395 we
triggered Swift ToO observations, which sample the intra-hour X-ray/UV
variability. These observations indicate, albeit with large uncertainties, a
lag of the 1900 {\AA} band behind the X-ray flux of ~400 s. The tight
correlation between the X-ray and UV/optical lightcurves, together with the
constraints we place on lag time-scale are consistent with the UV/optical
variability of NGC 4395 being primarily due to reprocessing of X-ray photons by
the accretion disc.Comment: 11 pages, 9 figures, 3 tables. Accepted for publication in MNRA
- …