5,801 research outputs found
The Nature of the H2-Emitting Gas in the Crab Nebula
Understanding how molecules and dust might have formed within a rapidly
expanding young supernova remnant is important because of the obvious
application to vigorous supernova activity at very high redshift. In previous
papers, we found that the H2 emission is often quite strong, correlates with
optical low-ionization emission lines, and has a surprisingly high excitation
temperature. Here we study Knot 51, a representative, bright example, for which
we have available long slit optical and NIR spectra covering emission lines
from ionized, neutral, and molecular gas, as well as HST visible and SOAR
Telescope NIR narrow-band images. We present a series of CLOUDY simulations to
probe the excitation mechanisms, formation processes and dust content in
environments that can produce the observed H2 emission. We do not try for an
exact match between model and observations given Knot 51's ambiguous geometry.
Rather, we aim to explain how the bright H2 emission lines can be formed from
within the volume of Knot 51 that also produces the observed optical emission
from ionized and neutral gas. Our models that are powered only by the Crab's
synchrotron radiation are ruled out because they cannot reproduce the strong,
thermal H2 emission. The simulations that come closest to fitting the
observations have the core of Knot 51 almost entirely atomic with the H2
emission coming from just a trace molecular component, and in which there is
extra heating. In this unusual environment, H2 forms primarily by associative
detachment rather than grain catalysis. In this picture, the 55 H2-emitting
cores that we have previously catalogued in the Crab have a total mass of about
0.1 M_sun, which is about 5% of the total mass of the system of filaments. We
also explore the effect of varying the dust abundance. We discuss possible
future observations that could further elucidate the nature of these H2 knots.Comment: 51 pages, 15 figures, accepted for publication in MNRAS, revised
Figure 12 results unchange
High-energy gamma-ray observations of the accreting black hole V404 Cygni during its June 2015 outburst
We report on Fermi/Large Area Telescope observations of the accreting black
hole low-mass X-ray binary V404 Cygni during its outburst in June-July 2015.
Detailed analyses reveal a possible excess of -ray emission on 26 June
2015, with a very soft spectrum above MeV, at a position consistent with
the direction of V404 Cyg (within the confidence region and a chance
probability of ). This emission cannot be associated with any
previously-known Fermi source. Its temporal coincidence with the brightest
radio and hard X-ray flare in the lightcurve of V404 Cyg, at the end of the
main active phase of its outburst, strengthens the association with V404 Cyg.
If the -ray emission is associated with V404 Cyg, the simultaneous
detection of keV annihilation emission by INTEGRAL requires that the
high-energy rays originate away from the corona, possibly in a
Blandford-Znajek jet. The data give support to models involving a
magnetically-arrested disk where a bright -ray jet can re-form after
the occurrence of a major transient ejection seen in the radio.Comment: 5 pages, 3 figures, accepted for publication in MNRA
Pairing Correlations in the Two-Dimensional Hubbard Model
We present the results of a quantum Monte Carlo study of the extended and
the pairing correlation functions for the two-dimensional Hubbard
model, computed with the constrained-path method. For small lattice sizes and
weak interactions, we find that the pairing correlations are
stronger than the extended pairing correlations and are positive when the
pair separation exceeds several lattice constants. As the system size or the
interaction strength increases, the magnitude of the long-range part of both
correlation functions vanishes.Comment: 4 pages, RevTex, 4 figures included; submitted to Phys. Rev. Let
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GENOME WIDE DNA METHYLATION PROFILING IS PREDICTIVE OF OUTCOME IN JUVENILE MYELOMONOCYTIC LEUKEMIA
Vlasov Description Of Dense Quark Matter
We discuss properties of quark matter at finite baryon densities and zero
temperature in a Vlasov approach. We use a screened interquark Richardson's
potential consistent with the indications of Lattice QCD calculations.
We analyze the choices of the quark masses and the parameters entering the
potential which reproduce the binding energy (B.E.) of infinite nuclear matter.
There is a transition from nuclear to quark matter at densities 5 times above
normal nuclear matter density. The transition could be revealed from the
determination of the position of the shifted meson masses in dense baryonic
matter. A scaling form of the meson masses in dense matter is given.Comment: 15 pages 4 figure
Detecting the rapidly expanding outer shell of the Crab Nebula: where to look
We present a range of steady-state photoionization simulations, corresponding
to different assumed shell geometries and compositions, of the unseen
postulated rapidly expanding outer shell to the Crab Nebula. The properties of
the shell are constrained by the mass that must lie within it, and by limits to
the intensities of hydrogen recombination lines. In all cases the
photoionization models predict very strong emission from high ionization lines
that will not be emitted by the Crab's filaments, alleviating problems with
detecting these lines in the presence of light scattered from brighter parts of
the Crab. The NIR [Ne VI] 7.652 m line is a particularly good
case; it should be dramatically brighter than the optical lines commonly used
in searches. The C IV doublet is predicted to be the strongest
absorption line from the shell, which is in agreement with HST observations. We
show that the cooling timescale for the outer shell is much longer than the age
of the Crab, due to the low density. This means that the temperature of the
shell will actually "remember" its initial conditions. However, the
recombination time is much shorter than the age of the Crab, so the predicted
level of ionization should approximate the real ionization. In any case, it is
clear that IR observations present the best opportunity to detect the outer
shell and so guide future models that will constrain early events in the
original explosion.Comment: Accepted by Ap
Measuring AGN Feedback with the Sunyaev-Zel'dovich Effect
One of the most important and poorly-understood issues in structure formation
is the role of outflows driven by active galactic nuclei (AGN). Using
large-scale cosmological simulations, we compute the impact of such outflows on
the small-scale distribution of the cosmic microwave background (CMB). Like
gravitationally-heated structures, AGN outflows induce CMB distortions both
through thermal motions and peculiar velocities, by processes known as the
thermal and kinetic Sunyaev-Zel'dovich (SZ) effects, respectively. For AGN
outflows the thermal SZ effect is dominant, doubling the angular power spectrum
on arcminute scales. But the most distinct imprint of AGN feedback is a
substantial increase in the thermal SZ distortions around elliptical galaxies,
post-starburst ellipticals, and quasars, which is linearly proportional to the
outflow energy. While point source subtraction is difficult for quasars, we
show that by appropriately stacking microwave measurements around early-type
galaxies, the new generation of small-scale microwave telescopes will be able
to directly measure AGN feedback at the level important for current theoretical
models.Comment: 12 pages, 12 figures, submitted to ApJ (comments welcome
Theory of monolayers with boundaries: Exact results and Perturbative analysis
Domains and bubbles in tilted phases of Langmuir monolayers contain a class
of textures knows as boojums. The boundaries of such domains and bubbles may
display either cusp-like features or indentations. We derive analytic
expressions for the textures within domains and surrounding bubbles, and for
the shapes of the boundaries of these regions. The derivation is perturbative
in the deviation of the bounding curve from a circle. This method is not
expected to be accurate when the boundary suffers large distortions, but it
does provide important clues with regard to the influence of various energetic
terms on the order-parameter texture and the shape of the domain or bubble
bounding curve. We also look into the effects of thermal fluctuations, which
include a sample-size-dependent effective line tension.Comment: replaced with published version, 21 pages, 16 figures include
The Nature of the H\u3csub\u3e2\u3c/sub\u3e-Emitting Gas in the Crab Nebula
Understanding how molecules and dust might have formed within a rapidly expanding young supernova remnant is important because of the obvious application to vigorous supernova activity at very high redshift. In previous papers, we have mapped the Crab nebula (the Crab) in a rotovibrational H2emission line, and then measured the molecular excitation temperature for a few of the brighter H2-emitting knots that we have found to be scattered throughout the Crab\u27s filaments. We found that H2 emission is often quite strong, correlates with optical low-ionization emission lines and has a surprisingly high excitation temperature. Here, we study Knot 51, a representative, bright example. It is a spatially isolated structure for which we have available long-slit optical and near-infrared (NIR) spectra covering emission lines from ionized, neutral and molecular gas, as well asHubble Space Telescope visible and Southern Astrophysical Research telescope NIR narrow-band images. We present a series of CLOUDYsimulations to probe the excitation mechanisms, formation processes and dust content in environments that can produce the observed H2 emission. There is still considerable ambiguity about the geometry of Knot 51, so we do not try for an exact match between model and observations. Rather, we aim to explain how the bright H2 emission lines can be formed from within a cloud of the size of Knot 51 that also produces the observed optical emission from ionized and neutral gas. Our models that are powered only by the Crab\u27s synchrotron radiation are ruled out because they are not able to reproduce the observed strong H2 emission coming from thermally populated levels. The simulations that come closest to fitting the observations (although they still have conspicuous discrepancies) have the core of Knot 51 almost entirely atomic with the H2 emission coming from just a trace molecular component, and in which there is extra heating. In this unusual environment, H2 forms primarily through H− by radiative detachment rather than by grain catalysis. In this picture, the 55 H2-emitting cores that we have previously catalogued in the Crab have a total mass of about 0.1 M⊙, which is about 5 per cent of the total mass of the system of filaments. We also explore the effect of varying the dust abundance. We discuss possible future observations that could further elucidate the nature of these H2 knots
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