256 research outputs found
21cm Forest with the SKA
An alternative to both the tomography technique and the power spectrum
approach is to search for the 21cm forest, that is the 21cm absorption features
against high-z radio loud sources caused by the intervening cold neutral
intergalactic medium (IGM) and collapsed structures. Although the existence of
high-z radio loud sources has not been confirmed yet, SKA-low would be the
instrument of choice to find such sources as they are expected to have spectra
steeper than their lower-z counterparts. Since the strongest absorption
features arise from small scale structures (few tens of physical kpc, or even
lower), the 21cm forest can probe the HI density power spectrum on small scales
not amenable to measurements by any other means. Also, it can be a unique probe
of the heating process and the thermal history of the early universe, as the
signal is strongly dependent on the IGM temperature. Here we show what SKA1-low
could do in terms of detecting the 21cm forest in the redshift range z =
7.5-15.Comment: Accepted for publication in the SKA Science Book 'Advancing
Astrophysics with the Square Kilometre Array', to appear in 2015; 10 pages, 5
figures; the manuscript is based on Ciardi et al., 2013, MNRAS, 428, 175
Radio recombination lines from obscured quasars with the SKA
We explore the possibility of detecting hydrogen radio recombination lines
from 0 < z < 10 quasars. We compute the expected Hnalpha flux densities as a
function of absolute magnitude and redshift by considering (i) the range of
observed AGN spectral indices from UV to X-ray bands, (ii) secondary
ionizations from X-ray photons, and (iii) stimulated emission due to nonthermal
radiation. All these effects are important to determine the line fluxes. We
find that the combination of slopes: alpha_X,hard = -1.11, alpha_X,soft = -0.7,
alpha_EUV = -1.3, alpha_UV = -1.7, maximizes the expected flux, f_Hnalpha = 10
microJy for z = 7 quasars with M_AB = -27 in the n = 50 lines; allowed SED
variations produce variations by a factor of 3 around this value. Secondaries
boost the line intensity by a factor of 2 to 4, while stimulated emission in
high-z quasars with M_AB = -26 provides an extra boost to RRL flux observed at
nu = 1 GHz if recombinations arise in HII regions with T_e = 10^3-5 K, n_e =
10^3-5 cm^-3. We compute the sensitivity required for a 5sigma detection of
Hnalpha lines using the SKA, finding that the SKA-MID could detect sources with
M_AB < -27 (M_AB < -26) at z < 8 (z < 3) in less than 100 hrs of observing
time. These observations could open new paths to searches for obscured SMBH
progenitors, complementing X-ray, optical/IR and sub-mm surveys.Comment: 11 pages, 9 figures; to be published in Monthly Notices of the Royal
Astronomical Society Main Journa
Direction-Dependent Polarised Primary Beams in Wide-Field Synthesis Imaging
The process of wide-field synthesis imaging is explored, with the aim of
understanding the implications of variable, polarised primary beams for
forthcoming Epoch of Reionisation experiments. These experiments seek to detect
weak signatures from redshifted 21cm emission in deep residual datasets, after
suppression and subtraction of foreground emission. Many subtraction algorithms
benefit from low side-lobes and polarisation leakage at the outset, and both of
these are intimately linked to how the polarised primary beams are handled.
Building on previous contributions from a number of authors, in which
direction-dependent corrections are incorporated into visibility gridding
kernels, we consider the special characteristics of arrays of fixed dipole
antennas operating around 100-200 MHz, looking towards instruments such as the
Square Kilometre Array (SKA) and the Hydrogen Epoch of Reionization Arrays
(HERA). We show that integrating snapshots in the image domain can help to
produce compact gridding kernels, and also reduce the need to make complicated
polarised leakage corrections during gridding. We also investigate an
alternative form for the gridding kernel that can suppress variations in the
direction-dependent weighting of gridded visibilities by 10s of dB, while
maintaining compact support.Comment: 15 pages, 4 figures. Accepted for publication in JA
Multiple supermassive black hole systems: SKA's future leading role
Galaxies and supermassive black holes (SMBHs) are believed to evolve through
a process of hierarchical merging and accretion. Through this paradigm,
multiple SMBH systems are expected to be relatively common in the Universe.
However, to date there are poor observational constraints on multiple SMBHs
systems with separations comparable to a SMBH gravitational sphere of influence
(<< 1 kpc). In this chapter, we discuss how deep continuum observations with
the SKA will make leading contributions towards understanding how multiple
black hole systems impact galaxy evolution. In addition, these observations
will provide constraints on and an understanding of stochastic gravitational
wave background detections in the pulsar timing array sensitivity band (nanoHz
-microHz). We also discuss how targets for pointed gravitational wave
experiments (that cannot be resolved by VLBI) could potentially be found using
the large-scale radio-jet morphology, which can be modulated by the presence of
a close-pair binary SMBH system. The combination of direct imaging at high
angular resolution; low-surface brightness radio-jet tracers; and pulsar timing
arrays will allow the SKA to trace black hole binary evolution from separations
of a galaxy virial radius down to the sub-parsec level. This large dynamic
range in binary SMBH separation will ensure that the SKA plays a leading role
in this observational frontier.Comment: 11 pages, 4 figures. To be published in the proceedings of "Advancing
Astrophysics with the Square Kilometre Array", PoS(AASKA14)151, in pres
Cosmic Dawn and Epoch of Reionization Foreground Removal with the SKA
The exceptional sensitivity of the SKA will allow observations of the Cosmic
Dawn and Epoch of Reionization (CD/EoR) in unprecedented detail, both
spectrally and spatially. This wealth of information is buried under Galactic
and extragalactic foregrounds, which must be removed accurately and precisely
in order to reveal the cosmological signal. This problem has been addressed
already for the previous generation of radio telescopes, but the application to
SKA is different in many aspects.
In this chapter we summarise the contributions to the field of foreground
removal in the context of high redshift and high sensitivity 21-cm
measurements. We use a state-of-the-art simulation of the SKA Phase 1
observations complete with cosmological signal, foregrounds and
frequency-dependent instrumental effects to test both parametric and
non-parametric foreground removal methods. We compare the recovered
cosmological signal using several different statistics and explore one of the
most exciting possibilities with the SKA --- imaging of the ionized bubbles.
We find that with current methods it is possible to remove the foregrounds
with great accuracy and to get impressive power spectra and images of the
cosmological signal. The frequency-dependent PSF of the instrument complicates
this recovery, so we resort to splitting the observation bandwidth into smaller
segments, each of a common resolution.
If the foregrounds are allowed a random variation from the smooth power law
along the line of sight, methods exploiting the smoothness of foregrounds or a
parametrization of their behaviour are challenged much more than non-parametric
ones. However, we show that correction techniques can be implemented to restore
the performances of parametric approaches, as long as the first-order
approximation of a power law stands.Comment: Accepted for publication in the SKA Science Book 'Advancing
Astrophysics with the Square Kilometre Array', to appear in 201
The self-confinement of electrons and positrons from dark matter
Radiative emissions from electrons and positrons generated by dark matter
(DM) annihilation or decay are one of the most investigated signals in indirect
searches of WIMPs. Ideal targets must have large ratio of DM to baryonic
matter. However, such ``dark'' systems have a poorly known level of magnetic
turbulence, which determines the residence time of the electrons and positrons
and therefore also the strength of the expected signal. This typically leads to
significant uncertainties in the derived DM bounds. In a novel approach, we
compute the self-confinement of the DM-induced electrons and positrons. Indeed,
they themselves generate irregularities in the magnetic field, thus setting a
lower limit on the presence of the magnetic turbulence. We specifically apply
this approach to dwarf spheroidal galaxies. Finally, by comparing the expected
synchrotron emission with radio data from the direction of the Draco galaxy
collected at the Giant Metre Radio Telescope, we show that the proposed
approach can be used to set robust and competitive bounds on WIMP DM.Comment: 18 pages, 10 figures. v2: minor revision, matches published versio
Antenna beam characterization for the global 21-cm experiment LEDA and its impact on signal model parameter reconstruction
Cosmic dawn, the onset of star formation in the early universe, can in principle be studied via the 21-cm transition of neutral hydrogen, for which a sky-averaged absorption signal, redshifted to MHz frequencies, is predicted to be O(10-100) mK. Detection requires separation of the 21-cm signal from bright chromatic foreground emission due to Galactic structure, and the characterization of how it couples to instrumental response. In this work, we present characterization of antenna gain patterns for the Large-aperture Experiment to detect the Dark Ages (LEDA) via simulations, assessing the effects of the antenna ground-plane geometries used, and measured soil properties
MEQSILHOUETTE: a mm-VLBI observation and signal corruption simulator
The Event Horizon Telescope (EHT) aims to spatially resolve the silhouette (or shadow) of the supermassive black holes in the Galactic Centre (Sgr A⋆) and M87. The primary scientific objectives are to test general relativity in the strong-field regime and to probe accretion and jet-launch physics at event-horizon scales. This is made possible by the technique of very long baseline interferometry at (sub)millimetre wavelengths, which can achieve angular resolutions of order ∼ 10 μ-arcsec. However, this approach suffers from unique observational challenges, including scattering in the troposphere and interstellar medium; rapidly time-variable source structure in both polarized and total intensity; as well as non-negligible antenna pointing errors. In this, the first paper in a series, we present the MEQSILHOUETTE software package which is specifically designed to accurately simulate EHT observations. It includes realistic descriptions of a number of signal corruptions that can limit the ability to measure the key science parameters. This can be used to quantify calibration requirements, test parameter estimation and imaging strategies, and investigate systematic uncertainties that may be present. In doing so, a stronger link can be made between observational capabilities and theoretical predictions, with the goal of maximizing scientific output from the upcoming order of magnitude increase in EHT sensitivity
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