10,240 research outputs found
Adaptive binning of X-ray galaxy cluster images
We present a simple method for adaptively binning the pixels in an image. The
algorithm groups pixels into bins of size such that the fractional error on the
photon count in a bin is less than or equal to a threshold value, and the size
of the bin is as small as possible. The process is particularly useful for
generating surface brightness and colour maps, with clearly defined error maps,
from images with a large dynamic range of counts, for example X-ray images of
galaxy clusters. We demonstrate the method in application to data from Chandra
ACIS-S and ACIS-I observations of the Perseus cluster of galaxies. We use the
algorithm to create intensity maps, and colour images which show the relative
X-ray intensities in different bands. The colour maps can later be converted,
through spectral models, into maps of physical parameters, such as temperature,
column density, etc. The adaptive binning algorithm is applicable to a wide
range of data, from observations or numerical simulations, and is not limited
to two-dimensional data.Comment: 8 pages, 12 figures, accepted by MNRAS (includes changes suggested by
referee), high resolution version at
http://www-xray.ast.cam.ac.uk/~jss/adbin
Isothermal shocks in Abell 2199 and 2A 0335+096?
We report on a partially circular X-ray surface brightness discontinuity
found at about 55 kpc from the centre of Abell 2199 with Chandra X-ray
observatory observations. Unlike cold fronts found in other clusters, the
feature shows no significant temperature change across it but has an apparent
density jump. We therefore identify it as a weak isothermal shock associated
with the central AGN and the inflation of its radio bubbles, as found in the
Perseus cluster. We examine a similar feature at 40 kpc radius found by
Mazzotta et al in 2A 0335+096, and conclude that it too may be an isothermal
shock. The change in density if these are shocks implies a Mach number of ~1.5.
If the isothermal nature of these features is confirmed by deeper observations,
the implication is that such shocks are common in clusters of galaxies, and are
an important mechanism for the transport of energy from a central supermassive
black hole into the cluster core.Comment: 5 pages, accepted by MNRAS, includes minor changes suggested by
refere
A deeper X-ray study of the core of the Perseus galaxy cluster: the power of sound waves and the distribution of metals and cosmic rays
We make a further study of the very deep Chandra observation of the X-ray
brightest galaxy cluster, A426 in Perseus. We examine the radial distribution
of energy flux inferred by the quasi-concentric ripples in surface brightness,
assuming they are due to sound waves, and show that it is a significant
fraction of the energy lost by radiative cooling within the inner 75-100 kpc,
where the cooling time is 4-5 Gyr, respectively. The wave flux decreases
outward with radius, consistent with energy being dissipated. Some newly
discovered large ripples beyond 100 kpc, and a possible intact bubble at 170
kpc radius, may indicate a larger level of activity by the nucleus a few 100
Myr ago. The distribution of metals in the intracluster gas peaks at a radius
of about 40 kpc and is significantly clumpy on scales of 5 kpc. The temperature
distribution of the soft X-ray filaments and the hard X-ray emission component
found within the inner 50 kpc are analysed in detail. The pressure due to the
nonthermal electrons, responsible for a spectral component interpreted as
inverse Compton emission, is high within 40 kpc of the centre and boosts the
power in sound waves there; it drops steeply beyond 40 kpc. We find no thermal
emission from the radio bubbles; in order for any thermal gas to have a filling
factor within the bubbles exceeding 50 per cent, the temperature of that gas
has to exceed 50 keV.Comment: Accepted by MNRAS. Now includes evidence for further ancient bubble
at 170 kpc radius, and minor changes suggested by referee. A version with
good quality figures is available from
http://www-xray.ast.cam.ac.uk/papers/perdetail2.pd
ROSAT PSPC detection of soft X-ray absorption in GB 1428+4217: The most distant matter yet probed with X-ray spectroscopy
We report on a ROSAT PSPC observation of the highly-luminous z = 4.72
radio-loud quasar GB 1428+4217 obtained between 1998 December 11 and 17, the
final days of the ROSAT satellite. The low-energy sensitivity of the PSPC
detector was employed to constrain the intrinsic X-ray absorption of the
currently most distant X-ray detected object. Here we present the detection of
significant soft X-ray absorption towards GB 1428+4217, making the absorbing
material the most distant matter yet probed with X-ray spectroscopy. X-ray
variability by 25+-8 per cent is detected on a timescale of 6500 s in the rest
frame. The X-ray variation requires an unusually high radiative efficiency of
at least 4.2, further supporting the blazar nature of the source.Comment: 6 pages incl. 6 figures, accepted for publication in Monthly Notice
Limits from rapid TeV variability of Mrk 421
The extreme variability event in the TeV emission of Mrk 421, recently
reported by the Whipple team, imposes the tightest limits on the typical size
of the TeV emitting regions in Active Galactic Nuclei (AGN). We examine the
consequences that this imposes on the bulk Lorentz factor of the emitting
plasma and on the radiation fields present in the central region of this Active
Nucleus. No strong evidence is found for extreme Lorentz factors. However,
energetics arguments suggest that any accretion in Mrk 421 has to take place at
small rates, compatible with an advection-dominated regime.Comment: 5 pages (Latex MNRAS style), revised version, submitted to MNRA
Fuelling quasars with hot gas
We consider a model for quasar formation in which massive black holes are
formed and fuelled largely by the accretion of hot gas during the process of
galaxy formation. In standard hierarchical collapse models, objects about the
size of normal galaxies and larger form a dense hot atmosphere when they
collapse. We show that if such an atmosphere forms a nearly "maximal" cooling
flow, then a central black hole can accrete at close to its Eddington limit.
This leads to exponential growth of a seed black hole, resulting in a quasar in
some cases. In this model, the first quasars form soon after the first
collapses to produce hot gas. The hot gas is depleted as time progresses,
mostly by cooling, so that the accretion rate eventually falls below the
threshold for advection-dominated accretion, at which stage radiative
efficiency plummets and any quasar turns off. A simple implementation of this
model, incorporated into a semi-analytical model for galaxy formation,
over-produces quasars when compared with observed luminosity functions, but is
consistent with models of the X-ray Background which indicate that most
accretion is obscured. It produces few quasars at high redshift due to the lack
of time needed to grow massive black holes. Quasar fuelling by hot gas provides
a minimum level, sufficient to power most quasars at redshifts between one and
two, to which other sources of fuel can be added. The results are sensitive to
feedback effects, such as might be due to radio jets and other outflows.Comment: 12 pages, 6 figures, MN Latex style, accepted for publication in
MNRA
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