305 research outputs found
Shock break-out: how a GRB revealed the beginnings of a supernova
In February 2006, Swift caught a GRB in the act of turning into a supernova,
and made the first ever direct observations of the break-out and early
expansion of a supernova shock wave. GRB 060218 began with an exceptionally
long burst of non-thermal gamma-rays, lasting over 2000 s, as a jet erupted
through the surface of the star. While this was in progress, an optically-thick
thermal component from the shock wave of the supernova explosion grew to
prominence, and we were able to track the mildly relativistic expansion of this
shell as the blackbody peak moved from the X-rays into the UV and optical
bands. The initial radius of the shock implied that it was a blue supergiant
which had exploded, but the lack of Hydrogen emission lines in the supernova
spectrum indicated a more compact star. The most likely scenario is that the
shock ploughed into the massive stellar wind of a Wolf-Rayet progenitor, with
the shock breaking out and becoming visible to us once it reached the radius
where the wind became optically-thin. I present the Swift observations of this
landmark event, and discuss the new questions and answers it leaves us with.Comment: 6 pages, 4 figures, to appear in Phil. Trans. Roy. Soc. A,
Proceedings of the Royal Society Discussion meeting on Gamma-Ray Bursts,
September 18-20, 200
Cosmic Feedback from AGN
Accretion onto the massive black hole at the centre of a galaxy can feed
energy and momentum into its surroundings via radiation, winds and jets.
Feedback due to radiation pressure can lock the mass of the black hole onto the
M-sigma relation, and shape the final stellar bulge of the galaxy. Feedback due
to the kinetic power of jets can prevent massive galaxies greatly increasing
their stellar mass, by heating gas which would otherwise cool radiatively. The
mechanisms involved in cosmic feedback are discussed and illustrated with
observations.Comment: 9 pages, 13 figures, Proceedings IAU Symposium 267, Co-Evolution of
Central Black Holes and Galaxie
The h-index in Australian Astronomy
The Hirsch (2005) h-index is now widely used as a metric to compare
individual researchers. To evaluate it in the context of Australian Astronomy,
the h-index for every member of the Astronomical Society of Australia (ASA) is
found using NASA's Astrophysics Data System Bibliographic Services (ADS).
Percentiles of the h-index distribution are detailed for a variety of
categories of ASA members, including students. This enables a list of the top
ten Australian researchers by h-index to be produced. These top researchers
have h-index values in the range 53<h<77, which is less than that recently
reported for the American Astronomical Society Membership. We suggest that
membership of extremely large consortia such as SDSS may partially explain the
difference. We further suggest that many student ASA members with large h-index
values have probably already received their Ph.D.'s and need to upgrade their
ASA membership status. To attempt to specify the h-index distribution relative
to opportunity, we also detail the percentiles of its distribution by years
since Ph.D. award date. This shows a steady increase in h-index with seniority,
as can be expected.Comment: Accepted for publication in the Publications of the Astronomical
Society of Australi
The nature and origin of Seyfert warm absorbers
We collate the results of recent high resolution X-ray spectroscopic observations of 23 AGN, and use the resulting information to try to provide answers to some of the main open questions about warm absorbers: where do they originate, what effect do they have on their host galaxies, and what is their importance within the energetics and dynamics of the AGN system as a whole? We find that the warm absorbers of nearby Seyferts and certain QSOs are most likely to originate in outflows from the dusty torus, and that the kinetic luminosity of these outflows accounts for well under 1% of the bolometric luminosities of the AGN. Our analysis supports, however, the view that the relativistic outflows recently observed in two PG quasars have their origin in accretion disc winds, although the energetic importance of these outflows is similar to that of the Seyfert warm absorbers. We find that the observed soft X-ray absorbing ionisation phases fill less than 10% of the available volume. Finally, we show that the amount of matter processed through an AGN outflow system, over the lifetime of the AGN, is probably large enough to have a significant influence on the evolution of the host galaxy and of the AGN itself
Publication and citation statistics for UK astronomers
This article presents a survey of publication and citation statistics for 835
UK professional astronomers: the majority of academics and contract researchers
within the UK astronomical community. I provide histograms of these
bibliometrics for the whole sample as well as of the median values for the
individual departments. I discuss the distribution of top bibliometric
performers in the sample, and make some remarks on the usage of bibliometrics
in a real-world assessment exercise.Comment: 10 pages, 3 figures, added bibliograph
XMM-Newton observations of the Seyfert 1 AGN H0557-385
We present XMM-Newton observations of the Seyfert 1 AGN H0557-385. We have
conducted a study into the warm absorber present in this source, and using
high-resolution RGS data we find that the absorption can be characterised by
two phases: a phase with log ionisation parameter xi of 0.50 (where xi is in
units of ergs cm/s) and a column of 0.2e21 cm^-2, and a phase with log xi of
1.62 and a column of 1.3e22 cm^-2. An iron K alpha line is detected. Neutral
absorption is also present in the source, and we discuss possible origins for
this. On the assumption that the ionised absorbers originate as an outflow from
the inner edge of the torus, we use a new method for finding the volume filling
factor. Both phases of H0557-385 have small volume filling factors (< 1%). We
also derive the volume filling factors for a sample of 23 AGN using this
assumption and for the absorbers with log xi > 0.7 we find reasonable agreement
with the filling factors obtained through the alternative method of equating
the momentum flow of the absorbers to the momentum loss of the radiation field.
By comparing the filling factors obtained by the two methods, we infer that
some absorbers with log xi < 0.7 occur at significantly larger distances from
the nucleus than the inner edge of the torus.Comment: Accepted for publication in MNRA
Statistics of relativistically broadened Fe K-alpha lines in AGN
We present preliminary results on the properties of relativistically
broadened Fe K-alpha lines in a collection of more then 100 Active Galactic
Nuclei (AGN) observed by the XMM-Newton EPIC-pn camera. Our main conclusions
can be summarized as follows: a) we detect broad lines in about 25% of the
sample objects. This fraction increases to 42+/-13% if we consider only objects
with more than 10000 counts in the hard (2-10 keV) band, and to 50+/-32% for
the small sub-sample (6 objects) of type~1 Piccinotti AGN with optimal
XMM-Newton exposure (at least 200000 counts in the hard band); b) we find no
significant difference in the detection rate of broad lines between obscured
and unobscured AGN; c) the strongest relativistic profiles are measured in
low-luminosity (< 10^43 erg/s) AGN; d) Equivalent Widths (EWs) associated with
relativistic profiles in stacked spectra are ~150 eV for all luminosity
classes; e) models of relativistically broadened iron line profiles ("kyrline",
Dovciak et al. 2005), which include full relativistic treatment of the
accretion disk emission around a Kerr black hole in the strong gravity regime,
yield an average disk inclination angle ~30 degrees, and a radial dependence of
the disk emissivity profile ~-3. The distribution of EW is very broad, with
=2.4. We estimate that an investment of about 1 Ms of XMM-Newton time
would be required to put these results on a sound statistical basis.Comment: 4 pages, 9 figures, to appear in Astronomische Nachrichten,
Proceedings of the ESAC Workshop "Variable and broad lines around black
holes
The warm absorber and X-ray variability of the Seyfert 1 galaxy NGC 3516 as seen by the XMM-Newton RGS
We present a new analysis of the soft and medium energy X-ray spectrum of the
Seyfert 1 galaxy NGC 3516 taken with the Reflection Grating Spectrometer (RGS)
and European Photon Imaging Camera (EPIC) on board the XMM-Newton observatory.
We examine four observations made in October 2006. We investigate whether the
observed variability is due to absorption by the warm absorber and/or is
intrinsic to the source emission. We analyse in detail the EPIC-pn and RGS
spectra of each observation separately. The warm absorber in NGC 3516 is found
to consist of three phases of ionisation, two of which have outflow velocities
of more than 1000 km/s. The third phase (the least ionised one) is much slower
at 100 km/s. One of the high ionisation phases, with log xi of 2.4, is found to
have a partial covering fraction of about 60%. It has previously been suggested
that the passage of a cloud, part of a disk wind, in front of the source
(producing a change in the covering fraction) was the cause of a significant
dip in the lightcurve during one of the observations. From our modelling of the
EPIC-pn and RGS spectra, we find that variation in the covering fraction cannot
be solely responsible for this. We show that intrinsic change in the source
continuum plays a much more significant role in explaining the observed flux
and spectral variability than originally thought.Comment: 13 pages, 12 figures, accepted for publication in Astronomy and
Astrophysic
Absorption spectra of Fe L-lines in Seyfert 1 galaxies
Absorption L-lines of iron ions are observed, in absorption, in spectra of
Seyfert 1 galaxies by the new generation of X-ray satellites: Chandra (NASA)
and XMM-Newton (ESA). Lines associated to Fe23+ to Fe17+ are well resolved.
Whereas, those corresponding to Fe16+ to Fe6+ are unresolved. Forbidden
transitions of the Fe16+ to Fe6+ ions were previously observed, for the same
objects, in the visible and infra-red regions, showing that the plasma had a
low density. To interpret X-ray, visible and infra-red data, astrophysical
models assume an extended absorbing medium of very low density surrounding an
intense X-ray source. We have calculated atomic data (wavelengths, radiative
and autoionization rates) for n=2 to n'=3-4 transitions and used them to
construct refined synthetic spectra of the unresolved part of the L-line
spectra.Comment: 17 pages, 5 figures, Journal of Quantitative Spectroscopy and
Radiative Transfer, in pres
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