831 research outputs found
Non-hydrostatic gas in the core of the relaxed galaxy cluster A1795
Chandra data on A1795 reveal a mild edge-shaped discontinuity in the gas
density and temperature in the southern sector of the cluster at r=60/h kpc.
The gas inside the edge is 1.3-1.5 times denser and cooler than outside, while
the pressure is continuous, indicating that this is a "cold front", the surface
of contact between two moving gases. The continuity of the pressure indicates
that the current relative velocity of the gases is near zero, making the edge
appear to be in hydrostatic equilibrium. However, a total mass profile derived
from the data in this sector under the equilibrium assumption, exhibits an
unphysical jump by a factor of 2, with the mass inside the edge being lower. We
propose that the cooler gas is "sloshing" in the cluster gravitational
potential well and is now near the point of maximum displacement, where it has
zero velocity but nonzero centripetal acceleration. The distribution of this
non-hydrostatic gas should reflect the reduced gravity force in the
accelerating reference frame, resulting in the apparent mass discontinuity.
Assuming that the gas outside the edge is hydrostatic, the acceleration of the
moving gas can be estimated from the mass jump, a ~ 800 h km/s/(10^8 yr). The
gravitational potential energy of this gas that is available for dissipation is
about half of its current thermal energy. The length of the cool filament
extending from the cD galaxy (Fabian et al.) may give the amplitude of the gas
sloshing, 30-40/h kpc. Such gas bulk motion might be caused by a disturbance of
the central gravitational potential by past subcluster infall.Comment: Minor text clarifications to correspond to published version. 5
pages, 1 figure in color, uses emulateapj.sty. ApJ Letters in pres
In-depth Chandra study of the AGN feedback in Virgo elliptical galaxy M84
Using deep Chandra observations of M84 we study the energetics of the
interaction between the black hole and the interstellar medium of this
early-type galaxy. We perform a detailed two dimensional reconstruction of the
properties of the X-ray emitting gas using a constrained Voronoi tessellation
method, identifying the mean trends and carrying out the fluctuation analysis
of the thermodynamical properties of the hot ISM. In addition to the PV work
associated with the bubble expansion, we identify and measure the wave energy
associated with the mildly supersonic bubble expansion. We show that, depending
on the age of the cavity and the associated wave, the waves can have a
substantial contribution to the total energy release from the AGN. The energy
dissipated in the waves tends to be concentrated near the center of M84 and in
the direction perpendicular to the bubble outflow, possibly due to the
interference of the waves generated by the expansion of northern and southern
bubbles. We also find direct evidence for the escape of radio plasma from the
ISM of the host galaxy into the intergalactic medium.Comment: 6 pages, ApJ in press, Nov. 1 200
Cold fronts in cool core clusters
Cold fronts have been detected both in merging and in cool core clusters,
where little or no sign of a merging event is present. A systematic search of
sharp surface brightness discontinuities performed on a sample of 62 galaxy
clusters observed with XMM-Newton shows that cold fronts are a common feature
in galaxy clusters. Indeed most (if not all) of the nearby clusters (z < 0.04)
host a cold front. Understanding the origin and the nature of a such frequent
phenomenon is clearly important. To gain insight on the nature of cold fronts
in cool core clusters we have undertaken a systematic study of all contact
discontinuities detected in our sample, measuring surface brightness,
temperature and when possible abundance profiles across the fronts. We measure
the Mach numbers for the cold fronts finding values which range from 0.2 to
0.9; we also detect a discontinuities in the metal profile of some clusters.Comment: 6 pages, 3 figures, for proceedings of "Heating vs. Cooling in
Galaxies and Clusters of Galaxies," eds H. Boehringer, P. Schuecker, G. W.
Pratt & A. Finoguenov, in Springer-Verlag series "ESO Astrophysics Symposia.
Resolving the Galactic X-ray background
We use Chandra deep observations of the Galactic Center (GC) region to
improve the constraints on the unresolved fraction of the Galactic X-ray
background (also known as the Galactic ridge X-ray emission). We emphasize the
importance of correcting the measured source counts at low fluxes for bias
associated with Poisson noise. We find that at distances of 2'-4' from Sgr A*
at least ~40% of the total X-ray emission in the energy band 4-8 keV originates
from point sources with luminosities L(2-10 keV)> 10^{31} erg/sec. From a
comparison of the source number-flux function in the GC region with the known
luminosity function of faint X-ray sources in the Solar vicinity, we infer that
Chandra has already resolved a large fraction of the cumulative contribution of
cataclysmic variables to the total X-ray flux from the GC region. This
comparison further indicates that most of the yet unresolved ~60% of the X-ray
flux from the GC region is likely produced by weak cataclysmic variables and
coronally active stars with L(2-10 keV)<10^{31} erg/sec. We conclude that the
bulk of the Galactic X-ray background is produced by discrete sources.Comment: Submitted to A&
Identification of black hole power spectral components across all canonical states
From a uniform analysis of a large (8.5 Ms) Rossi X-ray Timing Explorer data
set of Low Mass X-ray Binaries, we present a complete identification of all the
variability components in the power spectra of black holes in their canonical
states. It is based on gradual frequency shifts of the components observed
between states, and uses a previous identification in the black hole low hard
state as a starting point. It is supported by correlations between the
frequencies in agreement with those previously found to hold for black hole and
neutron stars. Similar variability components are observed in neutron stars and
black holes (only the component observed at the highest frequencies is
different) which therefore cannot depend on source-specific characteristics
such as the magnetic field or surface of the neutron star or spin of the black
hole. As the same variability components are also observed across the jet-line
the X-ray variability cannot originate from the outer-jet but is most likely
produced in either the disk or the corona. We use the identification to
directly compare the difference in strength of the black hole and neutron star
variability and find these can be attributed to differences in frequency and
strength of high frequency features, and do not require the absence of any
components. Black holes attain their highest frequencies (in the
hard-intermediate and very-high states) at a level a factor ~6 below the
highest frequencies attained by the corresponding neutron star components,
which can be related to the mass difference between the compact objects in
these systems.Comment: 17 pages, 16 figures, accepted for publication in Ap
ESO 3060170 -- a massive fossil galaxy group with a heated gas core?
We present a detailed study of the ESO 3060170 galaxy group combining
Chandra, XMM and optical observations. The system is found to be a fossil
galaxy group. The group X-ray emission is composed of a central dense cool core
(10 kpc in radius) and an isothermal medium beyond the central 10 kpc. The
region between 10 and 50 kpc (the cooling radius) has the same temperature as
the gas from 50 kpc to 400 kpc although the gas cooling time between 10 and 50
kpc (2 - 6 Gyr) is shorter than the Hubble time. Thus, the ESO 3060170 group
does not have a group-sized cooling core. We suggest that the group cooling
core may have been heated by a central AGN outburst in the past and the small
dense cool core is the truncated relic of a previous cooling core. The Chandra
observations also reveal a variety of X-ray features in the central region,
including a ``finger'', an edge-like feature and a small ``tail'', all aligned
along a north-south axis, as are the galaxy light and group galaxy
distribution. The proposed AGN outburst may cause gas ``sloshing'' around the
center and produce these asymmetric features. The observed flat temperature
profile to 1/3 R_vir is not consistent with the predicted temperature profile
in recent numerical simulations. We compare the entropy profile of the ESO
3060170 group with those of three other groups and find a flatter relation than
that predicted by simulations involving only shock heating, S r. This is direct evidence for the importance of non-gravitational
processes in group centers. We derive the mass profiles within 1/3 R_vir and
find the ESO 3060170 group is the most massive fossil group known (1 - 2 X
10 M). The M/L ratio of the system, ~ 150 at 0.3 R_vir, is
normal.Comment: 17 pages, 12 figures, to appear in ApJ. A high-resolution version can
be downloaded from http://cxc.harvard.edu/~msun/esoa.p
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