7,353 research outputs found
CRASH: a Radiative Transfer Scheme
We present a largely improved version of CRASH, a 3-D radiative transfer code
that treats the effects of ionizing radiation propagating through a given
inhomogeneous H/He cosmological density field, on the physical conditions of
the gas. The code, based on a Monte Carlo technique, self-consistently
calculates the time evolution of gas temperature and ionization fractions due
to an arbitrary number of point/extended sources and/or diffuse background
radiation with given spectra. In addition, the effects of diffuse ionizing
radiation following recombinations of ionized atoms have been included. After a
complete description of the numerical scheme, to demonstrate the performances,
accuracy, convergency and robustness of the code we present four different test
cases designed to investigate specific aspects of radiative transfer: (i) pure
hydrogen isothermal Stromgren sphere; (ii) realistic Stromgren spheres; (iii)
multiple overlapping point sources, and (iv) shadowing of background radiation
by an intervening optically thick layer. When possible, detailed quantitative
comparison of the results against either analytical solutions or 1-D standard
photoionization codes has been made showing a good level of agreement. For more
complicated tests the code yields physically plausible results, which could be
eventually checked only by comparison with other similar codes. Finally, we
briefly discuss future possible developments and cosmological applications of
the code.Comment: 17 pages, 14 figures, accepted for pubblication in MNRAS, high res
figures available at
http://www.arcetri.astro.it/science/cosmology/IGM/radtrans.htm
CRASH3: cosmological radiative transfer through metals
Here we introduce CRASH3, the latest release of the 3D radiative transfer
code CRASH. In its current implementation CRASH3 integrates into the reference
algorithm the code Cloudy to evaluate the ionisation states of metals,
self-consistently with the radiative transfer through H and He. The feedback of
the heavy elements on the calculation of the gas temperature is also taken into
account, making of CRASH3 the first 3D code for cosmological applications which
treats self-consistently the radiative transfer through an inhomogeneous
distribution of metal enriched gas with an arbitrary number of point sources
and/or a background radiation. The code has been tested in idealized
configurations, as well as in a more realistic case of multiple sources
embedded in a polluted cosmic web. Through these validation tests the new
method has been proven to be numerically stable and convergent. We have studied
the dependence of the results on a number of physical quantities such as the
source characteristics (spectral range and shape, intensity), the metal
composition, the gas number density and metallicity.Comment: accepted for publication in MNRA
UV background fluctuations traced by metal ions at
Here we investigate how LyC-opaque systems present in the intergalactic
medium at can distort the spectral shape of a uniform UV background
(UVB) through radiative transfer (RT) effects. With this aim in mind, we
perform a multi-frequency RT simulation through a cosmic volume of
~cMpc scale polluted by metals, and self-consistently derive the ions
of all the species. The UVB spatial fluctuations are traced by the ratio of
He and H column density,
, and the ratio of C and Si optical depths, . We find that: (i)
spatially fluctuates through over-dense systems () with statistically
significant deviations \% in 18\% of the volume ; (ii) same
fluctuations in are also present in \% of the enriched domain (only
8\% of the total volume) and derive from a combination of RT induced effects
and in-homogeneous metal enrichment, both effective in systems with .Comment: Accepted for pub. in MNRAS after very minor re
Interpreting the Transmission Windows of Distant Quasars
We propose the Apparent Shrinking Criterion (ASC) to interpret the spatial
extent, R_w, of transmitted flux windows in the absorption spectra of high-z
quasars. The ASC can discriminate between the two regimes in which R_w
corresponds either to the physical size, R_HII, of the quasar HII region, or to
the distance, R^{max}_w, at which the transmitted flux drops to =0.1 and a
Gunn-Peterson (GP) trough appears. In the first case (HR regime), one can
determine the IGM mean HI fraction, x_HI; in the second (PR regime), the value
of R_w allows to measure the local photoionization rate and the local
enhancement of the photoionization rate, Gamma_G, due to nearby/intervening
galaxies. The ASC has been tested against radiative transfer+SPH numerical
simulations, and applied to 15 high-z (z>5.8) quasars sample from Fan et al.
(2006). All sample quasars are found to be in the PR regime; hence, their
observed spectral properties (inner flux profile, extent of transmission
window) cannot reliably constrain the value of x_HI. Four sample quasars show
evidence for a local enhancement (up to 50%) in the local photoionization rate
possibly produced by a galaxy overdensity. We discuss the possible
interpretations and uncertainties of this result.Comment: 10 pages, 2 figures, accepted for publication in MNRA
On the validity of the adiabatic approximation in compact binary inspirals
Using a semi-analytical approach recently developed to model the tidal
deformations of neutron stars in inspiralling compact binaries, we study the
dynamical evolution of the tidal tensor, which we explicitly derive at second
post-Newtonian order, and of the quadrupole tensor. Since we do not assume a
priori that the quadrupole tensor is proportional to the tidal tensor, i.e. the
so called "adiabatic approximation", our approach enables us to establish to
which extent such approximation is reliable. We find that the ratio between the
quadrupole and tidal tensors (i.e., the Love number) increases as the inspiral
progresses, but this phenomenon only marginally affects the emitted
gravitational waveform. We estimate the frequency range in which the tidal
component of the gravitational signal is well described using the stationary
phase approximation at next-to-leading post-Newtonian order, comparing
different contributions to the tidal phase. We also derive a semi-analytical
expression for the Love number, which reproduces within a few percentage points
the results obtained so far by numerical integrations of the relativistic
equations of stellar perturbations.Comment: 13 pages, 1 table, 2 figures. Minor changes to match the version
appearing on Phys. Rev.
Focusing on the extended X-ray emission in 3C 459 with a Chandra follow-up observation
6 pages, 4 figures. Reproduced with permission from Astronomy & Astrophysics. © 2019 ESO.Aims. We investigated the X-ray emission properties of the powerful radio galaxy 3C 459 revealed by a recent Chandra follow-up observation carried out in October 2014 with a 62 ks exposure. Methods. We performed an X-ray spectral analysis from a few selected regions on an image obtained from this observation and also compared the X-ray image with a 4.9 GHz VLA radio map available in the literature. Results. The dominant contribution comes from the radio core but significant X-ray emission is detected at larger angular separations from it, surrounding both radio jets and lobes. According to a scenario in which the extended X-ray emission is due to a plasma collisionally heated by jet-driven shocks and not magnetically dominated, we estimated its temperature to be ∼0.8 keV. This hot gas cocoon could be responsible for the radio depolarization observed in 3C 459, as recently proposed also for 3C 171 and 3C 305. On the other hand, our spectral analysis and the presence of an oxygen K edge, blueshifted at 1.23 keV, cannot exclude the possibility that the X-ray radiation originating from the inner regions of the radio galaxy could be intercepted by some outflow of absorbing material intervening along the line of sight, as already found in some BAL quasars.Peer reviewe
Can We Detect the Anisotropic Shapes of Quasar HII Regions During Reionization Through The Small-Scale Redshifted 21cm Power Spectrum?
Light travel time delays distort the apparent shapes of HII regions
surrounding bright quasars during early stages of cosmic reionization.
Individual HII regions may remain undetectable in forthcoming redshifted 21 cm
experiments. However, the systematic deformation along the line of sight may be
detectable statistically, either by stacking tomographic 21cm images of quasars
identified, for example, by JWST, or as small-scale anisotropy in the
three-dimensional 21cm power spectrum. Here we consider the detectability of
this effect. The anisotropy is largest when HII regions are large and expand
rapidly, and we find that if bright quasars contributed to the early stages of
reionization, then they can produce significant anisotropy, on scales
comparable to the typical sizes of HII regions of the bright quasars (approx.
30 Mpc and below). The effect therefore cannot be ignored when analyzing future
21cm power spectra on small scales. If 10 percent of the volume of the IGM at
redshift z=10 is ionized by quasars with typical ionizing luminosity of S= 5 x
10^{56} photons/second, the distortions can enhance by more than 10 percent the
21cm power spectrum in the radial (redshift) direction, relative to the
transverse directions. The level of this anisotropy exceeds that due to
redshift-space distortion, and has the opposite sign. We show that on-going
experiments such as MWA should be able to detect this effect. A detection would
reveal the presence of bright quasars, and shed light on the ionizing yield and
age of the ionizing sources, and the distribution and small-scale clumping of
neutral intergalactic gas in their vicinity.Comment: Version accepted by ApJ, with new fiducial model and improved
discussio
CRASH2: colored packets and other updates
In this paper we report on the improvements implemented in the cosmological
radiative transfer code CRASH. In particular we present a new multi-frequency
algorithm for spectra sampling which makes use of colored photon packets: we
discuss the need for the multi-frequency approach, describe its implementation
and present the improved CRASH performance in reproducing the effects of
ionizing radiation with an arbitrary spectrum. We further discuss minor changes
in the code implementation which allow for more efficient performance and an
increased precision.Comment: 9 pages, 6 figures, accepted for publication in MNRA
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