205 research outputs found
The 4MOST facility simulator: instrument and science optimisation
This paper describes the design and implementation of a facility simulator
for the 4 metre Multi-Object Spectroscopic Telescope (4MOST) project, a new
survey instrument proposed for the ESO VISTA telescope. The 4MOST Facility
Simulator (4FS) has several roles, firstly to optimise the design of the
instrument, secondly to devise a survey strategy for the wide field design
reference surveys that are proposed for 4MOST, and thirdly to verify that
4MOST, as designed, can indeed achieve its primary science goals. We describe
the overall structure of the 4FS, together with details of some important 4FS
subsystems. We present the initial results from the 4FS which illustrate
clearly the value of having a functioning facility simulator very early in the
conceptual design phase of this large project.Comment: 7 pages, 7 figures, 1 tabl
The demographics and evolution of the absorbed AGN population.
It has become increasingly apparent that active galactic nuclei (AGN) have played a key role in the galaxy formation process, leading to the galaxy population we see to day. In order to understand better this inter-relationship, we must first measure the characteristics and evolution of the AGN population over cosmic timescales. Models of the AGN population which reproduce the spectrum and intensity of the extra-galactic X-ray background require a large population of faint AGN, the majority obscured by large column densities of cold material. In the local Universe, where we find mostly low luminosity objects, the obscured AGN make up 80% of the population. However, at higher redshifts, including the epoch when AGN and galaxies were forming most rapidly, the demographics of the obscured AGN population are still poorly understood. For this thesis, I have made a detailed examination of the AGN detected in several of the deepest XMM-Newton "blank-field" observations. I have carried out a detailed set of Monte-Carlo simulations in order to compare directly the X-ray properties of the observed AGN to the predictions of a number of AGN population models. For sources detected in the "13H" deep field, I find that the best fitting model contains AGN with a broad range of obscuration levels, but with significantly absorbed AGN making up at least 75% of the population. Furthermore, by examining the sources in XMM-Newton observations of the "CDFS" field, for which nearly complete redshift determinations are available, I find that the AGN absorption distribution exhibits little redshift or luminosity dependence. I confirm these findings by extending my study to a much larger AGN sample, and investigate field-to-field AGN source density variations
Long Term Wind-Driven X-Ray Spectral Variability of NGC 1365 with Swift
We present long-term (months-years) X-ray spectral variability of the Seyfert
1.8 galaxy NGC 1365 as observed by Swift, which provides well sampled
observations over a much longer timescale (6 years) and a much larger flux
range than is afforded by other observatories. At very low luminosities the
spectrum is very soft, becoming rapidly harder as the luminosity increases and
then, above a particular luminosity, softening again. At a given flux level,
the scatter in hardness ratio is not very large, meaning that the spectral
shape is largely determined by the luminosity. The spectra were therefore
summed in luminosity bins and fitted with a variety of models. The best fitting
model consists of two power laws, one unabsorbed and another, more luminous,
which is absorbed. In this model, we find a range of intrinsic 0.5-10.0 keV
luminosities of approximately 1.1-3.5 ergs/s, and a very large range of
absorbing columns, of approximately 10^22 - 10^24 cm^-2. Interestingly, we find
that the absorbing column decreases with increasing luminosity, but that this
result is not due to changes in ionisation. We suggest that these observations
might be interpreted in terms of a wind model in which the launch radius varies
as a function of ionising flux and disc temperature and therefore moves out
with increasing accretion rate, i.e. increasing X-ray luminosity. Thus,
depending on the inclination angle of the disc relative to the observer, the
absorbing column may decrease as the accretion rate goes up. The weaker,
unabsorbed, component may be a scattered component from the wind.Comment: 9 pages, 7 figures, accepted for publication in the Monthly Notices
of the Royal Astronomical Societ
Long-Term X-ray Spectral Variability of Seyfert Galaxies with Swift
We present analysis of the long-term X-ray spectral variability of Seyfert
galaxies as observed by Swift, which provides well-sampled observations over a
much larger flux range and a much longer timescale than any other X-ray
observatory. We examine long-term variability of three AGN: NGC 1365 (see
Connolly et al. 2014), Mkn 335 and NGC 5548. At high fluxes, the 0.5-10 keV
spectra soften with increasing flux, as seen previously within the 2-10 keV
band. However, at very low fluxes the sources also become very soft. We have
fitted a number of models to the data and find that both intrinsic luminosity
variability and variable absorption are required to explain the observations.
In some systems, e.g. NGC 1365, the best explanation is a two-component wind
model in which one component represents direct emission absorbed by a disc wind
wind, with the absorbing column inversely proportional to the intrinsic
luminosity, and the second component represents unabsorbed emission reflected
from the wind. In other AGN the situation is more complex.Comment: 6 pages, 5 figues, to appear in "Swift: 10 years of discovery",
Proceedings of Scienc
A deep Giant Metre-wave Radio Telescope 610-MHz survey of the 1^HXMM–Newton/Chandra survey field
We present the results of a deep 610-MHz survey of the 1^HXMM–Newton/Chandra survey area with the Giant Metre-wave Radio Telescope. The resulting maps have a resolution of ~7 arcsec and an rms noise limit of 60 μJy. To a 5σ detection limit of 300 μJy, we detect 223 sources within a survey area of 64 arcmin in diameter. We compute the 610-MHz source counts and compare them to those measured at other radio wavelengths. The well-known flattening of the Euclidean-normalized 1.4-GHz source counts below ~2 mJy, usually explained by a population of starburst galaxies undergoing luminosity evolution, is seen at 610 MHz. The 610-MHz source counts can be modelled by the same populations that explain the 1.4-GHz source counts, assuming a spectral index of −0.7 for the starburst galaxies and the steep spectrum active galactic nucleus (AGN) population. We find a similar dependence of luminosity evolution on redshift for the starburst galaxies at 610 MHz as is found at 1.4 GHz (i.e. 'Q'= 2.45^(+0.3)_(−0.4))
Finding counterparts for All-sky X-ray surveys with Nway: a Bayesian algorithm for cross-matching multiple catalogues
We release the AllWISE counterparts and Gaia matches to 106,573 and 17,665
X-ray sources detected in the ROSAT 2RXS and XMMSL2 surveys with |b|>15. These
are the brightest X-ray sources in the sky, but their position uncertainties
and the sparse multi-wavelength coverage until now rendered the identification
of their counterparts a demanding task with uncertain results. New all-sky
multi-wavelength surveys of sufficient depth, like AllWISE and Gaia, and a new
Bayesian statistics based algorithm, NWAY, allow us, for the first time, to
provide reliable counterpart associations. NWAY extends previous distance and
sky density based association methods and, using one or more priors (e.g.,
colors, magnitudes), weights the probability that sources from two or more
catalogues are simultaneously associated on the basis of their observable
characteristics. Here, counterparts have been determined using a WISE
color-magnitude prior. A reference sample of 4524 XMM/Chandra and Swift X-ray
sources demonstrates a reliability of ~ 94.7% (2RXS) and 97.4% (XMMSL2).
Combining our results with Chandra-COSMOS data, we propose a new separation
between stars and AGN in the X-ray/WISE flux-magnitude plane, valid over six
orders of magnitude. We also release the NWAY code and its user manual. NWAY
was extensively tested with XMM-COSMOS data. Using two different sets of
priors, we find an agreement of 96% and 99% with published Likelihood Ratio
methods. Our results were achieved faster and without any follow-up visual
inspection. With the advent of deep and wide area surveys in X-rays (e.g.
SRG/eROSITA, Athena/WFI) and radio (ASKAP/EMU, LOFAR, APERTIF, etc.) NWAY will
provide a powerful and reliable counterpart identification tool.Comment: MNRAS, Paper accepted for publication. Updated catalogs are available
at www.mpe.mpg.de/XraySurveys/2RXS_XMMSL2 . NWAY available at
https://github.com/JohannesBuchner/nwa
A tidal disruption flare in a massive galaxy? Implications for the fuelling mechanisms of nuclear black holes
We argue that the `changing look' AGN recently reported by LaMassa et al.
could be a luminous flare produced by the tidal disruption of a super-solar
mass star passing just a few gravitational radii outside the event horizon of a
nuclear black hole. This flare occurred in a massive,
star forming galaxy at redshift , robustly characterized thanks to
repeated late-time photometric and spectroscopic observations. By taking
difference-photometry of the well sampled multi-year SDSS Stripe-82
light-curve, we are able to probe the evolution of the nuclear spectrum over
the course of the outburst. The tidal disruption event (TDE) interpretation is
consistent with the very rapid rise and the decay time of the flare, which
displays an evolution consistent with the well-known behaviour (with
a clear superimposed re-brightening flare). Our analysis places constraints on
the physical properties of the TDE, such as the putative disrupted star's mass
and orbital parameters, as well as the size and temperature of the emitting
material. The properties of the broad and narrow emission lines observed in two
epochs of SDSS spectra provide further constraints on the circum-nuclear
structure, and could be indicative that the system hosted a moderate-luminosity
AGN as recently as a few years ago, and is likely undergoing residual
accretion as late as ten years after peak, as seen from the broad H
emission line. We discuss the complex interplay between tidal disruption events
and gas accretion episodes in galactic nuclei, highlighting the implications
for future TDE searches and for estimates of their intrinsic rates.Comment: 20 pages, 9 figures, 3 tables. Accepted for publication in MNRA
Herschel observations of gamma-ray burst host galaxies: implications for the topology of the dusty interstellar medium
Long-duration gamma-ray bursts (GRBs) are indisputably related to star
formation, and their vast luminosity in gamma rays pin-points regions of star
formation independent of galaxy mass. As such, GRBs provide a unique tool for
studying star forming galaxies out to high-z independent of luminosity. Most of
our understanding of the properties of GRB hosts (GRBHs) comes from optical and
near-infrared (NIR) follow-up observations, and we therefore have relatively
little knowledge of the fraction of dust-enshrouded star formation that resides
within GRBHs. Currently ~20% of GRBs show evidence of significant amounts of
dust along the line of sight to the afterglow through the host galaxy, and
these GRBs tend to reside within redder and more massive galaxies than GRBs
with optically bright afterglows. In this paper we present Herschel
observations of five GRBHs with evidence of being dust-rich, targeted to
understand the dust attenuation properties within GRBs better. Despite the
sensitivity of our Herschel observations, only one galaxy in our sample was
detected (GRBH 070306), for which we measure a total star formation rate (SFR)
of ~100Mstar/yr, and which had a relatively high stellar mass
(log[Mstar]=10.34+0.09/-0.04). Nevertheless, when considering a larger sample
of GRBHs observed with Herschel, it is clear that stellar mass is not the only
factor contributing to a Herschel detection, and significant dust extinction
along the GRB sightline (A_{V,GRB}>1.5~mag) appears to be a considerably better
tracer of GRBHs with high dust mass. This suggests that the extinguishing dust
along the GRB line of sight lies predominantly within the host galaxy ISM, and
thus those GRBs with A_{V,GRB}>1~mag but with no host galaxy Herschel
detections are likely to have been predominantly extinguished by dust within an
intervening dense cloud.Comment: 14 pages, 7 figures. Accepted for publication in A&
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