53,025 research outputs found
Observation of sub-Poisson photon statistics in the cavity-QED microlaser
We have measured the second-order correlation function of the cavity-QED
microlaser output and observed a transition from photon bunching to
antibunching with increasing average number of intracavity atoms. The observed
correlation times and the transition from super- to sub-Poisson photon
statistics can be well described by gain-loss feedback or enhanced/reduced
restoring action against fluctuations in photon number in the context of a
quantum microlaser theory and a photon rate equation picture. However, the
theory predicts a degree of antibunching several times larger than that
observed, which may indicate the inadequacy of its treatment of atomic velocity
distributions.Comment: 4 pages, 4 figure
Four hot DOGs eaten up with the EVN
Hot dust-obscured galaxies (hot DOGs) are a rare class of hyperluminous
infrared galaxies recently identified with the Wide-field Infrared Survey
Explorer (WISE) satellite. The majority of the ~1000-member all-sky population
should be at high redshifts (z~2-3), at the peak of star formation in the
history of the Universe. This class most likely represents a short phase during
galaxy merging and evolution, a transition from starburst- to AGN-dominated
phases. For the first time, we observed four hot DOGs with known mJy-level
radio emission using the European VLBI Network (EVN) at 1.7 GHz, in a hope to
find compact radio features characteristic to AGN activity. All four target
sources are detected at ~15-30 mas angular resolution, confirming the presence
of an active nucleus. The sources are spatially resolved, i.e. the flux density
of the VLBI-detected components is smaller than the total flux density,
suggesting that a fraction of the radio emission originates from larger-scale
(partly starburst-related) activity. Here we show the preliminary results of
our e-EVN observations made in 2014 February, and discuss WISE J1814+3412, an
object with kpc-scale symmetric radio structure, in more detail.Comment: 6 pages, 1 figure; appears in the proceedings of the 12th European
VLBI Network Symposium and Users Meeting (7-10 October 2014, Cagliari,
Italy), eds. A. Tarchi, M. Giroletti & L. Feretti. JREF Proceedings of
Science, PoS(EVN 2014)003,
http://pos.sissa.it/archive/conferences/230/003/EVN%202014_003.pd
Four hot DOGs in the microwave
Hot dust-obscured galaxies (hot DOGs) are a rare class of hyperluminous
infrared galaxies identified with the Wide-field Infrared Survey Explorer
(WISE) satellite. The majority of them is at high redshifts (z~2-3), at the
peak epoch of star formation in the Universe. Infrared, optical, radio, and
X-ray data suggest that hot DOGs contain heavily obscured, extremely luminous
active galactic nuclei (AGN). This class may represent a short phase in the
life of the galaxies, signifying the transition from starburst- to
AGN-dominated phases. Hot DOGs are typically radio-quiet, but some of them show
mJy-level emission in the radio (microwave) band. We observed four hot DOGs
using the technique of very long baseline interferometry (VLBI). The 1.7-GHz
observations with the European VLBI Network (EVN) revealed weak radio features
in all sources. The radio is free from dust obscuration and, at such high
redshifts, VLBI is sensitive only to compact structures that are characteristic
of AGN activity. In two cases (WISE J0757+5113, WISE J1603+2745), the flux
density of the VLBI-detected components is much smaller than the total flux
density, suggesting that ~70-90 per cent of the radio emission, while still
dominated by AGN, originates from angular scales larger than probed by the EVN.
The source WISE J1146+4129 appears a candidate compact symmetric object, and
WISE J1814+3412 shows a 5.1-kpc double structure, reminiscent of hot spots in a
medium-sized symmetric object. Our observations support that AGN residing in
hot DOGs may be genuine young radio sources where starburst and AGN activities
coexist.Comment: 8 pages, 4 tables, 1 figure; accepted for publication in the Monthly
Notices of the Royal Astronomical Societ
Two in one? A possible dual radio-emitting nucleus in the quasar SDSS J1425+3231
The radio-emitting quasar SDSS J1425+3231 (z=0.478) was recently found to
have double-peaked narrow [O III] optical emission lines. Based on the analysis
of the optical spectrum, Peng et al. (2011) suggested that this object harbours
a dual active galactic nucleus (AGN) system, with two supermassive black holes
(SMBHs) separated on the kpc scale. SMBH pairs should be ubiquitous according
to hierarchical galaxy formation scenarios in which the host galaxies and their
central black holes grow together via interactions and eventual mergers. Yet
the number of presently-confirmed dual SMBHs on kpc or smaller scales remains
small. A possible way to obtain direct observational evidence for duality is to
conduct high-resolution radio interferometric measurements, provided that both
AGN are in an evolutionary phase when some activity is going on in the radio.
We used the technique of Very Long Baseline Interferometry (VLBI) to image SDSS
J1425+3231. Observations made with the European VLBI Network (EVN) at 1.7 GHz
and 5 GHz frequencies in 2011 revealed compact radio emission at sub-mJy flux
density levels from two components with a projected linear separation of
\sim2.6 kpc. These two components support the possibility of a dual AGN system.
The weaker component remained undetected at 5 GHz, due to its steep radio
spectrum. Further study will be necessary to securely rule out a jet--shock
interpretation of the less dominant compact radio source. Assuming the dual AGN
interpretation, we discuss black hole masses, luminosities, and accretion rates
of the two components, using available X-ray, optical, and radio data. While
high-resolution radio interferometric imaging is not an efficient technique to
search blindly for dual AGN, it is an invaluable tool to confirm the existence
of selected candidates.Comment: 7 pages, 2 figures. Accepted for publication in Monthly Notices of
the Royal Astronomical Societ
Entanglement enhancement and postselection for two atoms interacting with thermal light
The evolution of entanglement for two identical two-level atoms coupled to a
resonant thermal field is studied for two different families of input states.
Entanglement enhancement is predicted for a well defined region of the
parameter space of one of these families. The most intriguing result is the
possibility of probabilistic production of maximally entangled atomic states
even if the input atomic state is factorized and the corresponding output state
is separable.Comment: accepted for publication in J. Phys.
Phase stability, ordering tendencies, and magnetism in single-phase fcc Au-Fe nanoalloys
Bulk Au-Fe alloys separate into Au-based fcc and Fe-based bcc phases, but
L1 and L1 orderings were reported in single-phase Au-Fe nanoparticles.
Motivated by these observations, we study the structural and ordering
energetics in this alloy by combining density functional theory (DFT)
calculations with effective Hamiltonian techniques: a cluster expansion with
structural filters, and the configuration-dependent lattice deformation model.
The phase separation tendency in Au-Fe persists even if the fcc-bcc
decomposition is suppressed. The relative stability of disordered bcc and fcc
phases observed in nanoparticles is reproduced, but the fully ordered L1
AuFe, L1 AuFe, and L1 AuFe structures are unstable in DFT.
However, a tendency to form concentration waves at the corresponding [001]
ordering vector is revealed in nearly-random alloys in a certain range of
concentrations. This incipient ordering requires enrichment by Fe relative to
the equiatomic composition, which may occur in the core of a nanoparticle due
to the segregation of Au to the surface. Effects of magnetism on the chemical
ordering are also discussed.Comment: 23 pages, 11 figure
Spectrum of low-lying configurations with negative parity
Spectrum of low-lying five-quark configurations with strangeness quantum
number and negative parity is studied in three kinds of constituent
quark models, namely the one gluon exchange, Goldstone Boson exchange, and
instanton-induced hyperfine interaction models, respectively. Our numerical
results show that the lowest energy states in all the three employed models are
lying at 1800 MeV, about 200 MeV lower than predictions of various
quenched three-quark models. In addition, it is very interesting that the state
with the lowest energy in one gluon exchange model is with spin 3/2, but 1/2 in
the other two models.Comment: Version published in Phys. Rev.
VLBI observation of the newly discovered z=5.18 quasar SDSS J0131-0321
Few high-redshift, radio-loud quasars are known to date. The extremely
luminous, radio-bright quasar, SDSS J013127.34-032100.1 was recently discovered
at a redshift of . We observed the source with high resolution very
long baseline interferometry (VLBI) at 1.7 GHz with the European VLBI Network
(EVN) and found a single compact radio component. We estimated a lower limit to
the brightness temperature of the detected radio component, T_B~10^{11} K.
Additionaly, when compared to archival radio data, the source showed
significant flux density variation. These two findings are indicative of the
blazar nature of the source.Comment: 5 pages, 2 figures. Accepted for publication in MNRAS Letter
Four dual AGN candidates observed with the VLBA
According to hierarchical structure formation models, merging galaxies are
expected to be seen in different stages of their coalescence. However,
currently there are no straightforward observational methods neither to select
nor to confirm a large number of dual active galactic nuclei (AGN) candidates.
Most attempts involve the better understanding of double-peaked narrow emission
line sources, to distinguish the objects where the emission lines originate
from narrow-line kinematics or jet-driven outflows from those which might
harbour dual AGN. We observed four such candidate sources with the Very Long
Baseline Array (VLBA) at 1.5 GHz with 10 milli-arcsecond angular
resolution where spectral profiles of AGN optical emission suggested the
existence of dual AGN. In SDSS J210449.13-000919.1 and SDSS J23044.82-093345.3,
the radio structures are aligned with the optical emission features, thus the
double-peaked emission lines might be the results of jet-driven outflows. In
the third detected source SDSS J115523.74+150756.9, the radio structure is less
extended and oriented nearly perpendicular to the position angle derived from
optical spectroscopy. The fourth source remained undetected with the VLBA but
it has been imaged with the Very Large Array at arcsec resolution a few months
before our observations, suggesting the existence of extended radio structure.
In none of the four sources did we detect two radio-emitting cores, a
convincing signature of duality.Comment: 35 pages, 3 figures, 2 tables, accepted for publication in Ap
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