87 research outputs found
Exponential asymptotics for line solitons in two-dimensional periodic potentials
As a first step toward a fully two-dimensional asymptotic theory for the
bifurcation of solitons from infinitesimal continuous waves, an analytical
theory is presented for line solitons, whose envelope varies only along one
direction, in general two-dimensional periodic potentials. For this
two-dimensional problem, it is no longer viable to rely on a certain recurrence
relation for going beyond all orders of the usual multi-scale perturbation
expansion, a key step of the exponential asymptotics procedure previously used
for solitons in one-dimensional problems. Instead, we propose a more direct
treatment which not only overcomes the recurrence-relation limitation, but also
simplifies the exponential asymptotics process. Using this modified technique,
we show that line solitons with any rational line slopes bifurcate out from
every Bloch-band edge; and for each rational slope, two line-soliton families
exist. Furthermore, line solitons can bifurcate from interior points of Bloch
bands as well, but such line solitons exist only for a couple of special line
angles due to resonance with the Bloch bands. In addition, we show that a
countable set of multi-line-soliton bound states can be constructed
analytically. The analytical predictions are compared with numerical results
for both symmetric and asymmetric potentials, and good agreement is obtained.Comment: 27 pages, 8 figures. To appear in Stud. Appl. Mat
From nonlocal gap solitary waves to bound states in periodic media
Solitary waves in one-dimensional periodic media are discussed employing the
nonlinear Schr\"odinger equation with a spatially periodic potential as a
model. This equation admits two families of gap solitons that bifurcate from
the edges of Bloch bands in the linear wave spectrum. These fundamental
solitons may be positioned only at specific locations relative to the
potential; otherwise, they become nonlocal owing to the presence of growing
tails of exponentially-small amplitude with respect to the wave peak amplitude.
Here, by matching the tails of such nonlocal solitary waves, higher-order
locally confined gap solitons, or bound states, are constructed. Details are
worked out for bound states comprising two nonlocal solitary waves in the
presence of a sinusoidal potential. A countable set of bound-state families,
characterized by the separation distance of the two solitary waves, is found,
and each family features three distinct solution branches that bifurcate near
Bloch-band edges at small, but finite, amplitude. Power curves associated with
these solution branches are computed asymptotically for large solitary-wave
separation, and the theoretical predictions are consistent with numerical
results.Comment: To appear in Proc. Roy. Soc. Lond.
Solitary waves and their linear stability in nonlinear lattices
Solitary waves in a general nonlinear lattice are discussed, employing as a
model the nonlinear Schr\"odinger equation with a spatially periodic nonlinear
coefficient. An asymptotic theory is developed for long solitary waves, that
span a large number of lattice periods. In this limit, the allowed positions of
solitary waves relative to the lattice, as well as their linear stability
properties, hinge upon a certain recurrence relation which contains information
beyond all orders of the usual two-scale perturbation expansion. It follows
that only two such positions are permissible, and of those two solitary waves,
one is linearly stable and the other unstable. For a cosine lattice, in
particular, the two possible solitary waves are centered at a maximum or
minimum of the lattice, with the former being stable, and the analytical
predictions for the associated linear stability eigenvalues are in excellent
agreement with numerical results. Furthermore, a countable set of
multi-solitary-wave bound states are constructed analytically. In spite of
rather different physical settings, the exponential asymptotics approach
followed here is strikingly similar to that taken in earlier studies of
solitary wavepackets involving a periodic carrier and a slowly-varying
envelope, which underscores the general value of this procedure for treating
multi-scale solitary-wave problems.Comment: To appear in Stud. Appl. Mat
Compton Thick AGN in the XMM-COSMOS survey
Heavily obscured, Compton Thick (CT, NH>10^24 cm^-2) AGN may represent an
important phase in AGN/galaxy co-evolution and are expected to provide a
significant contribution to the cosmic X-ray background (CXB). Through direct
X-ray spectra analysis, we selected 39 heavily obscured AGN (NH>3x10^23 cm^-2)
in the 2 deg^2 XMM-COSMOS survey. After selecting CT AGN based on the fit of a
simple absorbed two power law model to the XMM data, the presence of CT AGN was
confirmed in 80% of the sources using deeper Chandra data and more complex
models. The final sample of CT AGN comprises 10 sources spanning a large range
of redshift and luminosity. We collected the multi-wavelength information
available for all these sources, in order to study the distribution of SMBH and
host properties, such as BH mass (M_BH), Eddington ratio (\lambda_Edd), stellar
mass (M*), specific star formation rate (sSFR) in comparison with a sample of
unobscured AGN. We find that highly obscured sources tend to have significantly
smaller M_BH and higher \lambda_edd with respect to unobscured ones, while a
weaker evolution in M* is observed. The sSFR of highly obscured sources is
consistent with the one observed in the main sequence of star forming galaxies,
at all redshift. We also present optical spectra, spectral energy distribution
(SED) and morphology for the sample of 10 CT AGN: all the available optical
spectra are dominated by the stellar component of the host galaxy, and a highly
obscured torus component is needed in the SED of the CT sources. Exploiting the
high resolution Hubble-ACS images available, we conclude that these highly
obscured sources have a significantly larger merger fraction with respect to
other X-ray selected samples of AGN. Finally we discuss implications in the
context of AGN/galaxy co-evolutionary models, and compare our results with the
predictions of CXB synthesis models.Comment: Revised version after referee comments. Accepted for publication in
Astronomy & Astrophysics on 25 November 2014. 23 pages, 2 tables, 16 figure
Cold gas in massive early-type galaxies: The case of NGC 1167
We present a study of the morphology and kinematics of the neutral hydrogen
in the gas-rich (M_HI=1.5x10^{10}Msun), massive early-type galaxy NGC 1167,
which was observed with the Westerbork Synthesis Radio Telescope (WSRT). The HI
is located in a 160kpc disk (~3xD_25) and has low surface density (<2Msun
pc^{-2}). The disk shows regular rotation for r<65kpc but several signs of
recent and ongoing interaction and merging with fairly massive companions are
observed. No population of cold gas clouds is observed - in contrast to what is
found in some spiral galaxies. This suggests that currently the main mechanism
bringing in cold gas to the disk is the accretion of fairly massive satellite
galaxies, rather than the accretion of a large number of small gas clumps. NGC
1167 is located in a (gas-) rich environment: we detect eight companions with a
total HI mass of ~6x10^9Msun within a projected distance of 350kpc. Deep
optical images show a disrupted satellite at the northern edge of the HI disk.
The observed rotation curve shows a prominent bump of about 50km/s (in the
plane of the disk) at r=1.3xR_25. This feature in the rotation curve occurs at
the radius where the HI surface density drops significantly and may be due to
large-scale streaming motions in the disk. We suspect that both the streaming
motions and the HI density distribution are the result of the
interaction/accretion with the disrupted satellite. Like in other galaxies with
wiggles and bumps in the rotation curve, HI scaling describes the observed
rotation curve best. We suggest that interactions create streaming motions and
features in the HI density distribution and that this is the reason for the
success of HI scaling in fitting such rotation curves.Comment: 17 pages, 11 figures; A&A in pres
Hamiltonian form and solitary waves of the spatial Dysthe equations
The spatial Dysthe equations describe the envelope evolution of the
free-surface and potential of gravity waves in deep waters. Their Hamiltonian
structure and new invariants are unveiled by means of a gauge transformation to
a new canonical form of the evolution equations. An accurate Fourier-type
spectral scheme is used to solve for the wave dynamics and validate the new
conservation laws, which are satisfied up to machine precision. Traveling waves
are numerically constructed using the Petviashvili method. It is shown that
their collision appears inelastic, suggesting the non-integrability of the
Dysthe equations.Comment: 6 pages, 9 figures. Other author's papers can be downloaded at
http://www.lama.univ-savoie.fr/~dutykh
The XMM-Newton Serendipitous Survey. VI. The X-ray Luminosity Function
We present the X-ray luminosity function of AGN in three energy bands (Soft:
0.5-2 keV, Hard: 2-10 keV and Ultrahard: 4.5-7.5 keV). We have used the XMS
survey along with other highly complete flux-limited deeper and shallower
surveys for a total of 1009, 435 and 119 sources in the Soft, Hard and
Ultrahard bands, respectively. We have modeled the intrinsic absorption of the
Hard and Ultrahard sources (NH function) and computed the intrinsic X-ray
luminosity function in all bands using a Maximum Likelihood fit technique to an
analytical model. We find that the X-ray luminosity function (XLF) is best
described by a Luminosity-Dependent Density Evolution (LDDE) model. Our results
show a good overall agreement with previous results in the Hard band, although
with slightly weaker evolution. Our model in the Soft band present slight
discrepancies with other works in this band, the shape of our present day XLF
being significantly flatter. We find faster evolution in the AGN detected in
the Ultrahard band than those in the Hard band. The fraction of absorbed AGN in
the Hard and Ultrahard bands is dependent on the X-ray luminosity. We find
evidence of evolution of this fraction with redshift in the Hard band but not
in the Ultrahard band, possibly due to the low statistics. Our best-fit XLF
shows that the high-luminosity AGN are fully formed earlier than the less
luminous AGN. The latter sources account for the vast majority of the accretion
rate and mass density of the Universe, according to an anti-hierarchical black
hole growth scenario.Comment: 16 pages, 12 figures, accepted for publication in Astronomy and
Astrophysic
Recurrent radio emission and gas supply: the case of the radio galaxy B2 0258+35
Outlined is the discovery of a very faint, diffuse, low surface-brightness
(0.5 \mJybeam, 1.4 \mJyarcminsq on average) structure around the radio source
B2 0258+35 hosted by an HI-rich early-type galaxy (NGC 1167). Since B2 0258+35
is a young Compact Steep Spectrum (CSS) source, the newly discovered structure
could represent a remnant from an earlier stage of AGN activity.
We go on by explaining in detail all the possibilities for triggering the
radio activity in B2 0258+35 regarding gas accretion in a recurrent AGN
activity framework.
NGC 1167 hosts a very regular, extended and massive \HI\ disc that has been
studied in great detail. Previous studies of the \HI\ closer to the core seem
to go against the assumption of a circum-nuclear disc of \HI\ as the source of
the accreting gas.
We consider the cooling of gas from the hot, X-ray halo as a possible
alternative option for the fueling of the AGN, as suggested in the case of
other sources of similar radio power as B2 0258+35.
Estimates are given for the age of the faint diffuse emission as well as for
the current accretion rate, which are in good agreement with literature values.
If our assumptions about the accretion mechanism are correct, similar
large-scale, relic-like structures should be more commonly found around
early-type galaxies and this will be hopefully confirmed by the next generation
of sensitive, low-frequency radio surveys.Comment: 3 figures, accepted for publication in Astronomy and Astrophysics
(17.07.2012
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