1,268 research outputs found
Chandra Observation of the Interaction of the Radio Source and Cooling Core in Abell 2063
We present the results of a Chandra observation of the cooling core cluster
Abell 2063. Spectral analysis shows that there is cool gas (2 keV) associated
with the cluster core, which is more than a factor of 2 cooler than the outer
cluster gas (4.1 keV). There also is spectral evidence for a weak cooling flow,
Mdot ~ 20 Msun/yr. The cluster exhibits a complex structure in the center that
consists of several bright knots of emission, a depression in the emission to
the north of the center of the cluster, and a shell of emission surrounding it.
The depression in the X-ray emission is coincident with the position of the
north-eastern radio lobe of the radio source associated with the
cluster-central galaxy. The shell surrounding this region appears to be hotter,
which may be the result of a shock that has been driven into the gas by the
radio source. The power output of the radio source appears to be sufficient to
offset the cooling flow, and heating of the gas through shocks is a possible
explanation of how the energy transfer is established.Comment: Astrophysical Jounal, in press, 26 page with 9 figures, some in
color. Uses AASTEX late
New challenges for Adaptive Optics: Extremely Large Telescopes
The performance of an adaptive optics (AO) system on a 100m diameter ground
based telescope working in the visible range of the spectrum is computed using
an analytical approach. The target Strehl ratio of 60% is achieved at 0.5um
with a limiting magnitude of the AO guide source near R~10, at the cost of an
extremely low sky coverage. To alleviate this problem, the concept of
tomographic wavefront sensing in a wider field of view using either natural
guide stars (NGS) or laser guide stars (LGS) is investigated. These methods use
3 or 4 reference sources and up to 3 deformable mirrors, which increase up to
8-fold the corrected field size (up to 60\arcsec at 0.5 um). Operation with
multiple NGS is limited to the infrared (in the J band this approach yields a
sky coverage of 50% with a Strehl ratio of 0.2). The option of open-loop
wavefront correction in the visible using several bright NGS is discussed. The
LGS approach involves the use of a faint (R ~22) NGS for low-order correction,
which results in a sky coverage of 40% at the Galactic poles in the visible.Comment: 11 pages, 9 figures, 4 tables. Accepted for publication in MNRA
Untangling the X-ray Emission From the Sa Galaxy NGC1291 With Chandra
We present a Chandra ACIS-S observation of the nearby bulge-dominated Sa
galaxy NGC1291. The X-ray emission from the bulge resembles the X-ray emission
from a sub-class of elliptical and S0 galaxies with low L_X/L_B luminosity
ratios. The X-ray emission is composed of a central point-like nucleus, ~50
point sources that are most likely low mass X-ray binaries (LMXBs), and diffuse
gas detectable out to a radius of 120" (5.2 kpc). The diffuse gas has a global
temperature of 0.32^{+0.04}_{-0.03} keV and metallicity of 0.06 +/- 0.02 solar,
and both quantities marginally decrease with increasing radius. The hot gas
fills the hole in the HI distribution, and the softening of the spectrum of the
X-ray gas with radius might indicate a thermal coupling of the hot and cold
phases of the interstellar medium as previously suggested. The integrated X-ray
luminosity of the LMXBs, once normalized by the optical luminosity, is a factor
of 1.4 less than in the elliptical galaxy NGC4697 or S0 galaxy NGC1553. The
difference in L_{X,stellar}/L_B between the galaxies appears to be because of a
lack of very bright sources in NGC1291. No sources above 3 x 10^38 ergs/s were
found in NGC1291 when ~7 were expected from scaling from NGC4697 and NGC1553.
The cumulative L_{X,stellar}/L_B value including only sources below 1.0 x 10^38
ergs/s is remarkably similar between NGC1291 and NGC4697, if a recent surface
brightness fluctuation-determined distance is assumed for NGC4697. If this is a
common feature of the LMXB population in early-type systems, it might be used
as a distance indicator. Finally, a bright, variable (1.6-3.1 x 10^39 ergs/s)
source was detected at the optical center of the galaxy. Its spectrum shows
excess soft emission superimposed on a highly absorbed power law component,
similar to what has been found in several other low luminosity AGN (ABRIDGED).Comment: 13 pages in emulateapj5 style with 11 embedded Postscript figures;
minor revisions since last version; accepted by Ap
Global linear stability analysis of kinetic Trapped Ion Mode (TIM) turbulence in tokamak plasma using spectral method
Trapped ion modes (TIM) which belong to the family of ion temperature
gradient (ITG) modes, is one of the important ingredients in heat turbulent
transport at the ion scale in tokamak plasmas. It is essential to properly
estimate their linear growth rate to understand their influence on ion-scale
turbulent transport. A global linear analysis of a reduced gyro-bounce kinetic
model for trapped particle modes is performed, and a spectral method is
proposed to solve the dispersion relation. Importantly, the radial profile of
the particle drift velocity is taken into account in the linear analysis by
considering the exact magnetic flux {\psi} dependency of the equilibrium
Hamiltonian H_{eq}({\psi}) in the quasi-neutrality equation and equilibrium
gyro-bounce averaged distribution function F_{eq} . Using this spectral method,
linear growth-rates of TIM instability in presence of different temperature
profiles and precession frequencies of trapped ions, with an approximated
constant Hamiltonian and the exact {\psi} dependent equilibrium Hamiltonian,
are investigated. The growth-rate depends on the logarithmic gradient of
temperature \kappa_{T} , density \kappa_{n} and equilibrium Hamiltonian
\kappa_{\Lambda} . With the exact {\psi} dependent Hamiltonian, the growth
rates and potential profiles are modified significantly, compared to the cases
with approximated constant Hamiltonian. All the results from the global linear
analysis agree with a semi-Lagrangian based linear Vlasov solver with a good
accuracy. This spectral method is very fast and requires very less computation
resources compared to a linear version of Vlasov-solver based on a
semi-Lagrangian scheme
First Penning-trap mass measurement in the millisecond half-life range: the exotic halo nucleus 11Li
In this letter, we report a new mass for Li using the trapping
experiment TITAN at TRIUMF's ISAC facility. This is by far the shortest-lived
nuclide, , for which a mass measurement has ever been
performed with a Penning trap. Combined with our mass measurements of
Li we derive a new two-neutron separation energy of 369.15(65) keV: a
factor of seven more precise than the best previous value. This new value is a
critical ingredient for the determination of the halo charge radius from
isotope-shift measurements. We also report results from state-of-the-art
atomic-physics calculations using the new mass and extract a new charge radius
for Li. This result is a remarkable confluence of nuclear and atomic
physics.Comment: Formatted for submission to PR
Combined MASS-DIMM instrument for atmospheric turbulence studies
Several site-testing programs and observatories currently use combined
MASS-DIMM instruments for monitoring parameters of optical turbulence. The
instrument is described here. After a short recall of the measured quantities
and operational principles, the optics and electronics of MASS-DIMM,
interfacing to telescopes and detectors, and operation are covered in some
detail. Particular attention is given to the correct measurement and control of
instrumental parameters to ensure valid and well-calibrated data, to the data
quality and filtering. Examples of MASS-DIMM data are given, followed by the
list of present and future applications.Comment: Accepted by MNRAS, 11 pages, 8 figure
Deep 1.4 GHZ Follow Up of the Steep Spectrum Radio Halo in Abell 521
In a recent paper we reported on the discovery of a radio halo with very
steep spectrum in the merging galaxy cluster Abell 521 through observations
with the Giant Metrewave Radio Telescope (GMRT). We showed that the steep
spectrum of the halo is inconsistent with a secondary origin of the
relativistic electrons and supports a turbulent acceleration scenario. At that
time, due to the steep spectrum, the available observations at 1.4 GHz
(archival NRAO - Very Large Array - VLA CnB-configuration data) were not
adequate to accurately determine the flux density associated with the radio
halo. In this paper we report the detection at 1.4 GHz of the radio halo in
Abell 521 using deep VLA observations in the D-configuration. We use these new
data to confirm the steep-spectrum of the object. We consider Abell 521 the
prototype of a population of very-steep spectrum halos. This population is
predicted assuming that turbulence plays an important role in the acceleration
of relativistic particles in galaxy clusters, and we expect it will be unveiled
by future surveys at low frequencies with the LOFAR and LWA radio telescopes.Comment: 11 pages, 3 figures (figure 1 available in gif format only). Requires
aastex.cls - Accepted by Ap.
Resolving the Mystery of X-ray Faint Elliptical Galaxies: Chandra X-ray Observations of NGC 4697
Chandra observations of the X-ray faint elliptical galaxy NGC 4697 resolve
much of the X-ray emission (61% within one effective radius) into ~80 point
sources, of which most are low mass X-ray binaries (LMXBs). These LMXBs provide
the bulk of the hard emission and much of the soft emission as well. Of the
remaining unresolved emission, it is likely that about half is from fainter
LMXBs, while the other half (~23% of the total emission) is from interstellar
gas. Three of the resolved sources are supersoft sources. In the outer regions
of NGC 4697, eight of the LMXBs (about 25%) are coincident with candidate
globular clusters, which indicates that globulars have a high probability of
containing X-ray binaries compared to the normal stellar population. The X-ray
luminosities (0.3-10 keV) of the resolved LMXBs range from ~5e37 to ~2.5e39
ergs/s. The luminosity function of the LMXBs has a "knee" at 3.2e38 ergs/s,
which is roughly the Eddington luminosity of a 1.4 M_sun neutron star (NS);
this knee might be useful as a distance indicator. The highest luminosity
source has the Eddington luminosity of a ~20 M_sun black hole (BH). The
presence of this large population of NS and massive BH stellar remnants in this
elliptical galaxy shows that it (or its progenitors) once contained a large
population of massive main sequence stars.Comment: Astrophysical Journal Letters, in press. 4 pages in emulateapj5 style
with 4 embedded Postscript figures. Higher quality version of paper and
figures 1 and 2 are available at
http://www.astro.virginia.edu/~cls7i/papers/NGC4697-Chandra.ps.gz,
http://www.astro.virginia.edu/~cls7i/papers/NGC4697-Chandra_fig1.ps.gz, and
http://www.astro.virginia.edu/~cls7i/papers/NGC4697-Chandra_fig2.ps.g
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