900 research outputs found
The evolutionary sequence of active galactic nuclei and galaxy formation revealed
Today, almost every galaxy spheroid contains a massive black hole: a remnant of, and testament to, a period in its evolution when it contained an active galactic nucleus (AGN). However, the sequence and timescales of the formation of the black hole and surrounding spheroid of stars are completely unknown, leaving a large gap in our knowledge of how the universe attained its present appearance. Here we present submillimeter observations of matched samples of X-ray absorbed and unabsorbed AGNs that have luminosities and redshifts characteristic of the sources responsible for most of the mass in present-day black holes. Strong submillimeter emission, an isotropic signature of copious star formation, is found only in the X-rayâabsorbed sample, ruling out orientation effects as the cause of the absorption. The space density and luminosity range of the X-rayâabsorbed AGNs indicate that they are undergoing the transition between a hidden growth phase and an unabsorbed AGN phase and imply that the X-rayâabsorbed period in the AGN's evolution coincides with the formation of the galaxy spheroid
Comparative Study of the Application of Chelating Resins for Rare Earth Recovery
The adsorption properties of chelating ion exchange resins containing mixed
sulfonic/phosphonic (SP), aminophosphonic (AP) or iminodiacetic (IDA) acid functional groups were investigated towards the rare earth elements (REE). The aim of this work was to determine the potential for such resins to assist in the isolation of a mixed rare earth product under conditions relevant to the hydrometallurgical processing of rare earth containing minerals. The selectivity of the resins towards La, Sm and Ho, versus the common impurity metals; Al, Fe and Th, was determined in sulfuric acid media. The chelating resins all displayed a similar selectivity with Fe and Th adsorbed in preference to the REE and Al (i.e.Thâ Fe>> REEâ Al). The IDA resin displayed a far superior performance compared to both phosphonic resins (SP and AP) as well as a strong acid cation exchange resin for the adsorption of REE in the presence of very high Na or Ca concentrations. Equilibrium and kinetic adsorption isotherms for La were measured and successfully modelled with all resins, and the elution characteristics of selected resins investigated in both batch and column operation. A number of areas were identified where these resins could be exploited to provide an advantage in the hydrometallurgical processing of RE
Freak observers and the measure of the multiverse
I suggest that the factor in the pocket-based measure of the
multiverse, , should be interpreted as accounting for equilibrium
de Sitter vacuum fluctuations, while the selection factor accounts for
the number of observers that were formed due to non-equilibrium processes
resulting from such fluctuations. I show that this formulation does not suffer
from the problem of freak observers (also known as Boltzmann brains).Comment: 6 pages, no figures; references adde
Catching NGC4051 in the low state with XMM-Newton
The Narrow Line Seyfert 1 galaxy NGC4051 shows unusual low flux states,
lasting several months, when the 2-10 keV X-ray spectrum becomes unusually hard
(photon index<1) while the spectrum at lower X-ray energies is dominated by a
large soft excess. A Chandra TOO of the low state has shown that the soft
excess and hard components are variable and well-correlated. The variability of
the hard component rules out an origin in a distant reflector. Here we present
results from a recent XMM-Newton TOO of NGC4051 in the low state, which allows
a much more detailed examination of the nature of the hard and soft spectral
components in the low state. We demonstrate that the spectral shape in the low
state is consistent with the extrapolation of the spectral pivoting observed at
higher fluxes. The XMM-Newton data also reveals the warm absorbing gas in
emission, as the drop in the primary continuum flux unmasks prominent emission
lines from a range of ion species.Comment: 4 pages, 4 figures. Proc. of the meeting: "The Restless High-Energy
Universe" (Amsterdam, The Netherlands), E.P.J. van den Heuvel, J.J.M. in 't
Zand, and R.A.M.J. Wijers Ed
Superstars and Giant Gravitons in M-theory
Following hep-th/0109127, we show that a certain class of BPS naked
singularities (superstars) found in compactifications of M-theory can be
interpreted as being composed of giant gravitons. More specifically, we study
superstars which are asymptotically AdS_7 x S^4 and AdS_4 x S^7 and show that
these field configurations can be interpreted as being sourced by continuous
distributions of spherical M2- and M5-branes, respectively, which carry
internal momenta and have expanded on the spherical component of the
space-time.Comment: 13 page
FIRI - a Far-Infrared Interferometer
Half of the energy ever emitted by stars and accreting objects comes to us in
the FIR waveband and has yet to be properly explored. We propose a powerful
Far-InfraRed Interferometer mission, FIRI, to carry out high-resolution imaging
spectroscopy in the FIR. This key observational capability is essential to
reveal how gas and dust evolve into stars and planets, how the first luminous
objects in the Universe ignited, how galaxies formed, and when super-massive
black holes grew. FIRI will disentangle the cosmic histories of star formation
and accretion onto black holes and will trace the assembly and evolution of
quiescent galaxies like our Milky Way. Perhaps most importantly, FIRI will
observe all stages of planetary system formation and recognise Earth-like
planets that may harbour life, via its ability to image the dust structures in
planetary systems. It will thus address directly questions fundamental to our
understanding of how the Universe has developed and evolved - the very
questions posed by ESA's Cosmic Vision.Comment: Proposal developed by a large team of astronomers from Europe, USA
and Canada and submitted to the European Space Agency as part of "Cosmic
Vision 2015-2025
Optical and X-ray Observations of M31N 2007-12b: An Extragalactic Recurrent Nova with a Detected
We report combined optical and X-ray observations of nova M31N 2007-12b.
Optical spectroscopy obtained 5 days after the 2007 December outburst shows
evidence of very high ejection velocities (FWHM H km
s). In addition, Swift X-ray data show that M31N 2007-12b is associated
with a Super-Soft Source (SSS) which appeared between 21 and 35 days
post-outburst and turned off between then and day 169. Our analysis implies
that M_{\rm WD} \ga 1.3 M in this system. The optical light curve,
spectrum and X-ray behaviour are consistent with those of a recurrent nova.
Hubble Space Telescope observations of the pre-outburst location of M31N
2007-12b reveal the presence of a coincident stellar source with magnitude and
color very similar to the Galactic recurrent nova RS Ophiuchi at quiescence,
where the red giant secondary dominates the emission. We believe that this is
the first occasion on which a nova progenitor system has been identified in
M31. However, the greatest similarities of outburst optical spectrum and SSS
behaviour are with the supposed Galactic recurrent nova V2491 Cygni. A
previously implied association of M31N 2007-12b with nova M31N 1969-08a is
shown to be erroneous and this has important lessons for future searches for
recurrent novae in extragalactic systems. Overall, we show that suitable
complementary X-ray and optical observations can be used not only to identify
recurrent nova candidates in M31, but also to determine subtypes and important
physical parameters of these systems. Prospects are therefore good for
extending studies of recurrent novae into the Local Group with the potential to
explore in more detail such important topics as their proposed link to Type Ia
Supernovae.Comment: 10 pages, 4 figures, accepted for publication in the Astrophysical
Journal. This paper - which replaces the original accepted paper - accounts
for the positional coincidence of M31N 2007-12b and M31N 1969-08a and uses
additional HST archival dat
The Seyfert-LINER Galaxy NGC 7213: An XMM-Newton Observation
We examine the XMM X-ray spectrum of the LINER-AGN NGC 7213, which is best
fit with a power law, K-alpha emission lines from Fe I, Fe XXV and Fe XXVI and
a soft X-ray collisionally ionised thermal plasma with kT=0.18 +0.03/-0.01 keV.
We find a luminosity of 7x10^(-4) L_Edd, and a lack of soft X-ray excess
emission, suggesting a truncated accretion disc. NGC 7213 has intermediate
X-ray spectral properties, between those of the weak AGN found in the LINER M81
and higher luminosity Seyfert galaxies. This supports the notion of a
continuous sequence of X-ray properties from the Galactic Centre through LINER
galaxies to Seyferts, likely determined by the amount of material available for
accretion in the central regions.Comment: 7 pages, 2 figures. To appear in From X-ray Binaries to Quasars:
Black Hole Accretion on All Mass Scales, ed. T. J. Maccarone, R. P. Fender,
and L. C. Ho (Dordrecht: Kluwer
Classical and Thermodynamic Stability of Black Branes
It is argued that many non-extremal black branes exhibit a classical
Gregory-Laflamme instability if, and only if, they are locally
thermodynamically unstable. For some black branes, the Gregory-Laflamme
instability must therefore disappear near extremality. For the black -branes
of the type II supergravity theories, the Gregory-Laflamme instability
disappears near extremality for but persists all the way down to
extremality for (the black D3-brane is not covered by the analysis of
this paper). This implies that the instability also vanishes for the
near-extremal black M2 and M5-brane solutions.Comment: 21 pages, LaTeX. v2: Various points clarified, typos corrected and
reference adde
Supersymmetry without Supersymmetry
We present four-dimensional M-theory vacua with N>0 supersymmetry which, from
the perspective of perturbative Type IIA string theory, have N=0. Such vacua
can appear when the compactifying 7-manifold is a U(1) fibration. The missing
superpartners are Dirichlet 0-branes. Someone unable to detect Ramond-Ramond
charge would thus conclude that these worlds have no unbroken supersymmetry. In
particular, the gravitinos (and also some of the gauge bosons) are 0-branes not
seen in perturbation theory but which curiously remain massless however weak
the string coupling.Comment: Latex, 16 pages. Minor revisions and reference adde
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