4,678 research outputs found
Low-luminosity X-ray sources and the Galactic ridge X-ray emission
Using the XMM-Newton Slew Survey, we construct a hard-band selected sample of
low-luminosity Galactic X-ray sources. Two source populations are represented,
namely coronally-active stars and binaries (ASBs) and cataclysmic variables
(CVs), with X-ray luminosities collectively spanning the range 10^(28-34) erg/s
(2-10 keV). We derive the 2-10 keV X-ray luminosity function (XLF) and volume
emissivity of each population. Scaled to the local stellar mass density, the
latter is found to be 1.08 +/- 0.16 x 10^28 erg/s/M and 2.5 +/- 0.6 x 10^27
erg/s/M, for the ASBs and CVs respectively, which in total is a factor 2 higher
than previous estimates. We employ the new XLFs to predict the X-ray source
counts on the Galactic plane at l = 28.5 deg and show that the result is
consistent with current observational constraints. The X-ray emission of faint,
unresolved ASBs and CVs can account for a substantial fraction of the Galactic
ridge X-ray emission (GRXE). We discuss a model in which roughly 80 per cent of
the 6-10 keV GRXE intensity is produced in this way, with the remainder
attributable to X-ray scattering in the interstellar medium and/or young
Galactic source populations. Much of the hard X-ray emission attributed to the
ASBs is likely to be produced during flaring episodes
XMM-Newton observations of the Galactic Centre Region - II: The soft thermal emission
We have extended our earlier study (Heard & Warwick 2013, Paper I) of the
X-ray emission emanating from the central 100 pc x 100 pc region of our Galaxy
to an investigation of several features prominent in the soft X-ray (2-4.5 keV)
band. We focus on three specific structures: a putative bipolar outflow from
the vicinity of Sgr A*; a high surface brightness region located roughly 12
arcmin to the north-east of Sgr A*; and a lower surface-brightness extended
loop feature seen to the south of Sgr A*. We show that all three structures are
thermal in nature and have similar temperatures (kT ~ 1 keV). The inferred
X-ray luminosities lie in the range (2 - 10) x 10^34 erg s^-1. In the case of
the bipolar feature we suggest that the hot plasma is produced by the
shock-heating of the winds from massive stars within the Central Cluster,
possibly collimated by the Circumnuclear Disc. Alternatively the outflow may be
driven by outbursts on Sgr A*, which follow tidal disruption events occurring
at a rate of roughly 1 every 4000 yr. The north-east enhancement is centred on
a candidate PWN which has a relatively hard non-thermal X-ray spectrum. We
suggest that the coincident soft-thermal emission traces the core of a new
thermal-composite supernova remnant, designated as SNR G0.13-0.12. There is no
clear evidence for an associated radio shell but such a feature may be masked
by the bright emission of the nearby Radio Arc and other filamentary
structures. SNR G0.13-0.12 is very likely interacting with the nearby molecular
cloud, G0.11-0.11, and linked to the Fermi source, 2FGL J1746.4-2851c. Finally
we explore a previous suggestion that the elliptically-shaped X-ray loop to the
south of Sgr A*, of maximum extent ~45 pc, represents the shell of a
superbubble located in the GC region. Although plausible, the interpretation of
this feature in terms a coherent physical structure awaits confirmation.Comment: 17 pages, 6 figures, accepted for publication in MNRA
An application of dynamic programming to pattern recognition
technique which has found only limited usage in the Industrial and Business Management fields which are home to many other of the OR techniques. Its main application has been in problems o
Recommended from our members
Rational expectations modelling in O.R
The conventional OR approach to managing a system is, in outline, firstly to create a model of the existing system, secondly, to investigate changes in the model which improve or control the behaviour of the model and thirdly, to implement these changes in the system. It is assumed that the model incorporating these changes will be a valid representation of the system after the changes, in as far as the original model was a valid representation of the original system, and can thus be used to assess the benefits and disbenefits arising from the changes
X-ray spectral complexity in narrow-line Seyfert 1 galaxies
We present a systematic analysis of the X-ray spectral properties of a sample
of 22 ``narrow-line'' Seyfert 1 galaxies for which data are available from the
ASCA public archive. Many of these sources, which were selected on the basis of
their relatively narrow H-beta line width (FWHM <= 2000 km/s), show significant
spectral complexity in the X-ray band. Their measured hard power-law continua
have photon indices spanning the range 1.6 - 2.5 with a mean of 2.1, which is
only slightly steeper than the norm for ``broad-line'' Seyfert 1s. All but four
of the sources exhibit a soft excess, which can be modelled as blackbody
emission (T_{bb} ~ 100 - 300 eV) superposed on the underlying power-law. This
soft component is often so strong that, even in the relatively hard bandpass of
ASCA, it contains a significant fraction, if not the bulk, of the X-ray
luminosity, apparently ruling out models in which the soft excess is produced
entirely through reprocessing of the hard continuum.
Most notably, 6 of the 22 objects show evidence for a broad absorption
feature centred in the energy range 1.1 - 1.4 keV, which could be the signature
of resonance absorption in highly ionized material. A further 3 sources exhibit
``warm absorption'' edges in the 0.7 - 0.9 keV bandpass. Remarkably, all 9
``absorbed'' sources have H-beta line widths below 1000 km/s, which is less
than the median value for the sample taken as a whole. This tendency for very
narrow line widths to correlate with the presence of ionized absorption
features in the soft X-ray spectra of NLS1s, if confirmed in larger samples,
may provide a further clue in the puzzle of active galactic nuclei.Comment: 13 pages, 5 figures, accepted for publication in MNRA
On characterising the variability properties of X-ray light curves from active galaxies
We review some practical aspects of measuring the amplitude of variability in
`red noise' light curves typical of those from Active Galactic Nuclei (AGN).
The quantities commonly used to estimate the variability amplitude in AGN light
curves, such as the fractional rms variability amplitude, F_var, and excess
variance, sigma_XS^2, are examined. Their statistical properties, relationship
to the power spectrum, and uses for investigating the nature of the variability
processes are discussed. We demonstrate that sigma_XS^2 (or similarly F_var)
shows large changes from one part of the light curve to the next, even when the
variability is produced by a stationary process. This limits the usefulness of
these estimators for quantifying differences in variability amplitude between
different sources or from epoch to epoch in one source. Some examples of the
expected scatter in the variance are tabulated for various typical power
spectral shapes, based on Monte Carlo simulations. The excess variance can be
useful for comparing the variability amplitudes of light curves in different
energy bands from the same observation. Monte Carlo simulations are used to
derive a description of the uncertainty in the amplitude expected between
different energy bands (due to measurement errors). Finally, these estimators
are used to demonstrate some variability properties of the bright Seyfert 1
galaxy Markarian 766. The source is found to show a strong, linear correlation
between rms amplitude and flux, and to show significant spectral variability.Comment: 14 pages. 12 figures. Accepted for publication in MNRA
The XMM-Newton slew survey in the 2-10 keV band
The XMM-Newton Slew Survey (XSS) covers a significant fraction of the sky in
a broad X-ray bandpass. Although shallow by contemporary standards, in the
`classical' 2-10 keV band of X-ray astronomy, the XSS provides significantly
better sensitivity than any currently available all-sky survey. We investigate
the source content of the XSS, focussing on detections in the 2-10 keV band
down to a very low threshold (> 4 counts net of background). At the faint end,
the survey reaches a flux sensitivity of roughly 3e-12 erg/cm2/s (2-10 keV).
Our starting point was a sample of 487 sources detected in the XMMSL1d2 XSS at
high galactic latitude in the hard band. Through cross-correlation with
published source catalogues from surveys spanning the electromagnetic spectrum
from radio to gamma-rays, we find that 45% of the sources have likely
identifications with normal/active galaxies, 18% are associated with other
classes of X-ray object (nearby coronally active stars, accreting binaries,
clusters of galaxies), leaving 37% of the XSS sources with no current
identification. We go on to define an XSS extragalactic hard band sample
comprised of 219 galaxies and active galaxies. We investigate the properties of
this extragalactic sample including its X-ray logN-logS distribution. We find
that in the low-count limit, the XSS is strongly affected by Eddington bias.
There is also a very strong bias in the XSS against the detection of extended
sources, most notably clusters of galaxies. A significant fraction of the
detections at and around the low-count limit may be spurious. Nevertheless, it
is possible to use the XSS to extract a reasonably robust sample of
extragalactic sources, excluding galaxy clusters. The differential logN-logS
relation of these extragalactic sources matches very well to the HEAO-1 A2
all-sky survey measurements at bright fluxes and to the 2XMM source counts at
the faint end.Comment: 16 pages, 13 figures, FITS table of XSS extragalactic sample
available from http://www.star.le.ac.uk/~amr30/Slew
Italy in the Australian news media, 2005-2012
Presents a systematic analysis of the coverage of Italian matters in the Australian news media in the period 2005-2012.
Executive summary
The study presents a systematic analysis of the coverage of Italian matters in the Australian news media in the period 2005-2012. As far as the authors are aware it is largest study of the representation of Italy in the news media of a foreign country ever undertaken.
The study was undertaken by the University of Canberra’s News and Media Research Centre (N&MRC) in association with the On. Marco Fedi, who commissioned research on coverage of Italian politics in selected Australian newspapers.
During the period covered by the study there was more than usual interest in Italian political developments in Australia, both because of their inherent newsworthiness and because of the newly-acquired right of direct participation of expatriate Italians in Australia in the 2006 and 2008 Italian general elections.
To obtain as wide a view of Italy in the Australian news as practicable both the press and television were included in the study.
Three well -known and respected newspapers, namely The Australian, the major national newspaper, The Sydney Morning Herald (SMH) published in Sydney and The Age published
in Melbourne were selected for the study. All editorial matter in all issues of these newspapers for the years 2005-2012 was searched using ‘Factiva’– a business information and research tool owned by Dow Jones & Company. The initial search for relevant news in the press generated a very large number of items (around 10,000) which were then assessed individually for relevance and eligibility for inclusion in the analysis. This process generated a data set of 5325 news items
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