551 research outputs found
Feedback Heating with Slow Jets in Cooling Flow Clusters
We propose a scenario in which a large fraction, or even most, of the gas
cooling to low temperatures of T<10^4 K in cooling flow clusters, directly
gains energy from the central black hole. Most of the cool gas is accelerated
to non-relativistic high velocities, v ~ 10^3-10^4 km/sec, after flowing
through, or close to, an accretion disk around the central black hole. A poorly
collimated wind (or double not-well collimated opposite jets) is formed.
According to the proposed scenario, this gas inflates some of the X-ray
deficient bubbles, such that the average gas temperature inside these bubbles
(cavities) in cooling flow clusters is kT_b ~< 100 keV. A large fraction of
these bubbles will be very faint, or not detectable, in the radio. The bright
rims of these weak smaller bubbles will appear as ripples. We suggest that the
X-ray ripples observed in the Perseus cluster, for example, are not sound
waves, but rather the rims of radio-faint weak bubbles which are only slightly
hotter than their environment. This scenario is incorporated into the moderate
cooling flow model; although not a necessary ingredient in that model, it
brings it to better agreement with observations. In the moderate cooling flow
model a cooling flow does exist, but the mass cooling rate is ~<10% of that in
old versions of cooling flow models.Comment: The Astrophysical Journal, in pres
Translocation of a polymer chain driven by a dichotomous noise
We consider the translocation of a one-dimensional polymer through a pore
channel helped by a motor driven by a dichotomous noise with time exponential
correlation. We are interested in the study of the translocation time, mean
velocity and stall force of the system as a function of the mean driving
frequency. We find a monotonous translocation time, in contrast with the mean
velocity which shows a pronounced maximum at a given frequency. Interestingly,
the stall force shows a nonmonotonic behavior with the presence of a minimum.
The influence of the spring elastic constant to the mean translocation times
and velocities is also presented.Comment: 11 pages, 7 figure
Study of contamination of liquid oxygen by gaseous nitrogen First quarterly report, 1 Jul. - 30 Sep. 1964
Analytical model development for contamination study of liquid oxygen by gaseous nitroge
SMAUG: a new technique for the deprojection of galaxy clusters
This paper presents a new technique for reconstructing the spatial
distributions of hydrogen, temperature and metal abundance of a galaxy cluster.
These quantities are worked out from the X-ray spectrum, modeled starting from
few analytical functions describing their spatial distributions. These
functions depend upon some parameters, determined by fitting the model to the
observed spectrum. We have implemented this technique as a new model in the
XSPEC software analysis package. We describe the details of the method, and
apply it to work out the structure of the cluster A1795. We combine the
observation of three satellites, exploiting the high spatial resolution of
Chandra for the cluster core, the wide collecting area of XMM-Newton for the
intermediate regions and the large field of view of Beppo-SAX for the outer
regions. We also test the validity and precision of our method by i) comparing
its results with those from a geometrical deprojection, ii) examining the
spectral residuals at different radii of the cluster and iii) reprojecting the
unfolded profiles and comparing them directly to the measured quantities. Our
analytical method yields the parameters defining the spatial functions directly
from the spectra. Their explicit knowledge allows a straightforward derivation
of other indirect physical quantities like the gravitating mass, as well as a
fast and easy estimate of the profiles uncertainties.Comment: 24 pages, 11 figures, 3 tables; emulateapj; accepted for publication
in the Astrophysical Journa
Using Magnetic Activity and Galactic Dynamics to Constrain the Ages of M Dwarfs
We present a study of the dynamics and magnetic activity of M dwarfs using
the largest spectroscopic sample of low-mass stars ever assembled. The age at
which strong surface magnetic activity (as traced by H-alpha) ceases in M
dwarfs has been inferred to have a strong dependence on mass (spectral type,
surface temperature) and explains previous results showing a large increase in
the fraction of active stars at later spectral types. Using spectral
observations of more than 40000 M dwarfs from the Sloan Digital Sky Survey, we
show that the fraction of active stars decreases as a function of vertical
distance from the Galactic plane (a statistical proxy for age), and that the
magnitude of this decrease changes significantly for different M spectral
types. Adopting a simple dynamical model for thin disk vertical heating, we
assign an age for the activity decline at each spectral type, and thus
determine the activity lifetimes for M dwarfs. In addition, we derive a
statistical age-activity relation for each spectral type using the dynamical
model, the vertical distance from the Plane and the H-alpha emission line
luminosity of each star (the latter of which also decreases with vertical
height above the Galactic plane).Comment: 8 pages, 5 figures, to appear in the proceedings of IAU 258: The Ages
of Star
Study of contamination of liquid oxygen by gaseous nitrogen third quarterly progress report, 1 jan. - 31 mar. 1965
Contamination of liquid oxygen by gaseous nitroge
The serendipituous discovery of a short-period eclipsing polar in 2XMMp
We report the serendipituous discovery of the new eclipsing polar 2XMMp
J131223.4+173659. Its striking X-ray light curve attracted immediate interest
when we were visually inspecting the source products of the 2XMMp catalogue.
This light curve revealed its likely nature as a magnetic cataclysmic variable
of AM Herculis (or polar) type with an orbital period of ~92 min, which was
confirmed by follow-up optical spectroscopy and photometry. 2XMMp
J131223.4+173659 probably has a one-pole accretion geometry. It joins the group
of now nine objects that show no evidence of a soft component in their X-ray
spectra despite being in a high accretion state, thus escaping ROSAT/EUVE
detection. We discuss the likely accretion scenario, the system parameters, and
the spectral energy distribution.Comment: Accepted for publication in A&
Radiative cooling, heating and thermal conduction in M87
The crisis of the standard cooling flow model brought about by Chandra and
XMM-Newton observations of galaxy clusters, has led to the development of
several models which explore different heating processes in order to assess if
they can quench the cooling flow. Among the most appealing mechanisms are
thermal conduction and heating through buoyant gas deposited in the ICM by
AGNs. We combine Virgo/M87 observations of three satellites (Chandra,
XMM-Newton and Beppo-SAX) to inspect the dynamics of the ICM in the center of
the cluster. Using the spectral deprojection technique, we derive the physical
quantities describing the ICM and determine the extra-heating needed to balance
the cooling flow assuming that thermal conduction operates at a fixed fraction
of the Spitzer value. We assume that the extra-heating is due to buoyant gas
and we fit the data using the model developed by Ruszkowski and Begelman
(2002). We derive a scale radius for the model of kpc, which is
comparable with the M87 AGN jet extension, and a required luminosity of the AGN
of a erg s, which is comparable to the observed AGN
luminosity. We discuss a scenario where the buoyant bubbles are filled of
relativistic particles and magnetic field responsible for the radio emission in
M87. The AGN is supposed to be intermittent and to inject populations of
buoyant bubbles through a succession of outbursts. We also study the X-ray cool
component detected in the radio lobes and suggest that it is structured in
blobs which are tied to the radio buoyant bubbles.Comment: 25 pages, 10 figures and 2 tables. Accepted for publication in Ap
Investigating the properties of AGN feedback in hot atmospheres triggered by cooling-induced gravitational collapse
Radiative cooling may plausibly cause hot gas in the centre of a massive
galaxy, or galaxy cluster, to become gravitationally unstable. The subsequent
collapse of this gas on a dynamical timescale can provide an abundant source of
fuel for AGN heating and star formation. Thus, this mechanism provides a way to
link the AGN accretion rate to the global properties of an ambient cooling
flow, but without the implicit assumption that the accreted material must have
flowed onto the black hole from 10s of kiloparsecs away. It is shown that a
fuelling mechanism of this sort naturally leads to a close balance between AGN
heating and the radiative cooling rate of the hot, X-ray emitting halo.
Furthermore, AGN powered by cooling-induced gravitational instability would
exhibit characteristic duty cycles (delta) which are redolent of recent
observational findings: delta is proportional to L_X/sigma_{*}^{3}, where L_X
is the X-ray luminosity of the hot atmosphere, and sigma_{*} is the central
stellar velocity dispersion of the host galaxy. Combining this result with
well-known scaling relations, we deduce a duty cycle for radio AGN in
elliptical galaxies that is approximately proportional to M_{BH}^{1.5}, where
M_{BH} is the central black hole mass. Outburst durations and Eddington ratios
are also given. Based on the results of this study, we conclude that
gravitational instability could provide an important mechanism for supplying
fuel to AGN in massive galaxies and clusters, and warrants further
investigation.Comment: Accepted for publication in MNRAS. 8 page
On the Nature of Feedback Heating in Cooling Flow Clusters
We study the feedback between heating and cooling of the intra-cluster medium
(ICM) in cooling flow (CF) galaxies and clusters. We adopt the popular view
that the heating is due to an active galactic nucleus (AGN), i.e. a central
black hole accreting mass and launching jets and/or winds. We propose that the
feedback occurs with the entire cool inner region (r <~ 5-30 kpc), where the
non-linear over-dense blobs of gas with a density contrast >~2 cool fast and
are removed from the ICM before experiencing the next major AGN heating event.
We term this scenario "cold-feedback". Some of these blobs cool and sink toward
the central black hole, while others might form stars and cold molecular
clouds. We derive the conditions under which the dense blobs formed by
perturbations might cool to low temperatures (T <~ 10^4 K), and feed the black
hole. The main conditions are found to be: (1) An over-dense blob must be
prevented from reaching an equilibrium position in the ICM: therefore it has to
cool fast, and the density profile of the ambient gas should be shallow; (2)
Non-linear perturbations are required: they might have chiefly formed by
previous AGN activity; (3) The cooling time of these non-linear perturbations
should be short relative to few times the typical interval between successive
AGN outbursts. (4) The blobs should be magnetically disconnected from their
surroundings, in order not to be evaporated by thermal conduction.Comment: Replaced wiht the version accepted by the Ap
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