8 research outputs found
Hydrogen Epoch of Reionization Array (HERA)
The Hydrogen Epoch of Reionization Array (HERA) is a staged experiment to
measure 21 cm emission from the primordial intergalactic medium (IGM)
throughout cosmic reionization (), and to explore earlier epochs of our
Cosmic Dawn (). During these epochs, early stars and black holes
heated and ionized the IGM, introducing fluctuations in 21 cm emission. HERA is
designed to characterize the evolution of the 21 cm power spectrum to constrain
the timing and morphology of reionization, the properties of the first
galaxies, the evolution of large-scale structure, and the early sources of
heating. The full HERA instrument will be a 350-element interferometer in South
Africa consisting of 14-m parabolic dishes observing from 50 to 250 MHz.
Currently, 19 dishes have been deployed on site and the next 18 are under
construction. HERA has been designated as an SKA Precursor instrument.
In this paper, we summarize HERA's scientific context and provide forecasts
for its key science results. After reviewing the current state of the art in
foreground mitigation, we use the delay-spectrum technique to motivate
high-level performance requirements for the HERA instrument. Next, we present
the HERA instrument design, along with the subsystem specifications that ensure
that HERA meets its performance requirements. Finally, we summarize the
schedule and status of the project. We conclude by suggesting that, given the
realities of foreground contamination, current-generation 21 cm instruments are
approaching their sensitivity limits. HERA is designed to bring both the
sensitivity and the precision to deliver its primary science on the basis of
proven foreground filtering techniques, while developing new subtraction
techniques to unlock new capabilities. The result will be a major step toward
realizing the widely recognized scientific potential of 21 cm cosmology.Comment: 26 pages, 24 figures, 2 table
HI 21cm Cosmology and the Bi-spectrum: Closure Diagnostics in Massively Redundant Interferometric Arrays
New massively redundant low frequency arrays allow for a novel investigation
of closure relations in interferometry. We employ commissioning data from the
Hydrogen Epoch of Reionization Array to investigate closure quantities in this
densely packed grid array of 14m antennas operating at 100 MHz to 200 MHz. We
investigate techniques that utilize closure phase spectra for redundant triads
to estimate departures from redundancy for redundant baseline visibilities. We
find a median absolute deviation from redundancy in closure phase across the
observed frequency range of about 4.5deg. This value translates into a
non-redundancy per visibility phase of about 2.6deg, using prototype
electronics. The median absolute deviations from redundancy decrease with
longer baselines. We show that closure phase spectra can be used to identify
ill-behaved antennas in the array, independent of calibration. We investigate
the temporal behavior of closure spectra. The Allan variance increases after a
one minute stride time, due to passage of the sky through the primary beam of
the transit telescope. However, the closure spectra repeat to well within the
noise per measurement at corresponding local sidereal times (LST) from day to
day. In future papers in this series we will develop the technique of using
closure phase spectra in the search for the HI 21cm signal from cosmic
reionization.Comment: 32 pages. 11 figures. Accepted to Radio Scienc
Hydrogen Epoch of Reionization Array (HERA)
Photo shows a view of Moki Canyon trail down sand dunes near Red Cove Spring. Geology: Wingate, Todilto, and Navajo sandstone, San Juan County, Utah, 1927. Photograph from Herbert E. Gregory Book 9: Utah, California, 1924-1928
The hydrogen epoch of reionization array dish III: measuring chromaticity of prototype element with reflectometry
Spectral structures due to the instrument response is the current limiting factor for the experiments attempting to detect the redshifted 21 cm signal from the Epoch of Reionization (EoR). Recent advances in the delay spectrum methodology for measuring the redshifted 21 cm EoR power spectrum brought new attention to the impact of an antenna’s frequency response on the viability of making this challenging measurement. The delay spectrum methodology provides a somewhat straightforward relationship between the time-domain response of an instrument that can be directly measured and the power spectrum modes accessible to a 21 cm EoR experiment. In this paper, we derive the explicit relationship between antenna reflection coefficient (S 11 ) measurements made by a Vector Network Analyzer (VNA) and the extent of additional foreground contaminations in delay space. In the light of this mathematical framework, we examine the chromaticity of a prototype antenna element that will constitute the Hydrogen Epoch of Reionization Array (HERA) between 100 and 200 MHz. These reflectometry measurements exhibit additional structures relative to electromagnetic simulations, but we find that even without any further design improvement, such an antenna element will support measuring spatial k modes with line-of-sight components of k ∥ > 0.2h Mpc − 1 . We also find that when combined with the powerful inverse covariance weighting method used in optimal quadratic estimation of redshifted 21 cm power spectra the HERA prototype elements can successfully measure the power spectrum at spatial modes as low as k ∥ > 0.1h Mpc − 1 . This work represents a major step toward understanding the HERA antenna element and highlights a straightforward method for characterizing instrument response for future experiments designed to detect the 21 cm EoR power spectrum
H i 21 cm Cosmology and the Bi-spectrum: Closure Diagnostics in Massively Redundant Interferometric Arrays
New massively redundant low frequency arrays allow for a novel investigation
of closure relations in interferometry. We employ commissioning data from the
Hydrogen Epoch of Reionization Array to investigate closure quantities in this
densely packed grid array of 14m antennas operating at 100 MHz to 200 MHz. We
investigate techniques that utilize closure phase spectra for redundant triads
to estimate departures from redundancy for redundant baseline visibilities. We
find a median absolute deviation from redundancy in closure phase across the
observed frequency range of about 4.5deg. This value translates into a
non-redundancy per visibility phase of about 2.6deg, using prototype
electronics. The median absolute deviations from redundancy decrease with
longer baselines. We show that closure phase spectra can be used to identify
ill-behaved antennas in the array, independent of calibration. We investigate
the temporal behavior of closure spectra. The Allan variance increases after a
one minute stride time, due to passage of the sky through the primary beam of
the transit telescope. However, the closure spectra repeat to well within the
noise per measurement at corresponding local sidereal times (LST) from day to
day. In future papers in this series we will develop the technique of using
closure phase spectra in the search for the HI 21cm signal from cosmic
reionization