SARAO News

PAPER – the Precision Array for Probing the Epoch of Reionisation project

The primary goal of PAPER was to detect emission from the neutral gas that pervaded the Universe before the first galaxies and black holes were formed. This ‘epoch of reionization’, as it is called, is the last frontier in observational cosmology.

Observations with PAPER were used to set the first physically interesting limits on emission from the neutral hydrogen during this key epoch in cosmic structure formation. Besides providing important constraints on the very early evolution of the Universe, the PAPER observations were helping to define the techniques and instrumentation that will translate into the design of a next generation ‘Hydrogen Epoch of Reionization Array’ (HERA), and eventually, the low-frequency SKA.

Beyond the cosmological studies, PAPER has produced unprecedented images of the southern radio sky at low frequency. PAPER comprises a unique combination of very wide field (‘full-sky imaging’), reasonable spatial resolution, and wide frequency range. Spectacular images of the closest powerful radio galaxy, Centaurus A, have been made with PAPER, revealing very large structure in this giant radio galaxy, some 200 kpc in extent. This is ten times the size of the entire Milky Way. The PAPER data provide interesting insight into the interaction between these giant radio ‘jets’ emanating from the massive black hole at the centre of the Centaurus A galaxy, with the large scale intergalactic medium surrounding the galaxy.

(kpc = kiloparsec, a unit that astronomers use to measure distances between parts of a galaxy, or within groups of galaxies; 1 kpc = 3 000 light years)

PAPER involved a close collaboration with South African astronomers and engineers on all aspects of the project, from design and construction, to scientific discovery.

About PAPER

PAPER operated on the Karoo site in South Africa for over four years since 2009. The Karoo provides a unique environment on planet Earth, with remarkably low interference from man-made radio transmissions, thereby enabling sensitive observations at low radio frequencies.

The array doubled in size each year, so that by the time of decommissioning in March 2015 PAPER consisted of 128 dipole antennas arranged in a grid formation over a 300 metre clearing. The correlator, a custom built array of supercomputers responsible for processing data received from the antennas, was among the world’s largest and most powerful correlators used for radio astronomy. The effective annual doubling in collecting area dramatically increased the combined sensitivity of the instrument and thereby improved the probability of making a detection of the faint Epoch of Reionization emissions – a major scientific breakthrough.

PAPER operated two arrays, the primary science instrument in the Karoo and a secondary, smaller, sister array in Green Bank, West Virginia in the USA. While the northern hemisphere installation of 32 antennas was primarily intended as a development test bed, it also allowed data from the full sky (northern and southern hemispheres) to be gathered.

Scientists from the SKA Project office working on PAPER collaborated with partners at American research institutions, such as the National Radio Astronomy Observatory, the University of California, Berkeley and the University of Pennsylvania.

PAPER website: http://eor.berkeley.edu

The former PAPER antennas on the Losberg site have been repurposed in two ways since the start of the HERA build:

• The dipoles from the PAPER antennas are being modified and installed above HERA dishes, and the exact same signal path is being employed with the higher collecting area HERA reflectors. This allows data to be taken with the exact same signal chain and back end as was used in the PAPER experiment. This phased approach allows us to understand the new dishes with the well-understood PAPER system as well as to allow time until a new feed and architecture could be deployed. The changeover to the new architecture is scheduled to be in place for the start of the 2018/19 observing season.
• The second use for the old PAPER antennas was to do comparative studies between HERA-19 (the first HERA build phase) and a similar “PAPER-19” configuration placed just next to it. By gathering exactly the same data, in the same configuration, but with two different instruments, researchers can better characterise the behavioural and sensitivity differences between them.

The technique used to analyse the data from the PAPER experiment showed promising results, with the only major limitation being sensitivity. The 350 element HERA array was designed to address this; to get more collecting area on the sky. Estimates are that an array the size of HERA-19 is about as sensitive as PAPER-128. This added sensitivity massively improves the chances of making a detection of the Epoch of Reionization.

The HERA collaboration is made up of a number of different partner institutions, many of which are US based and include the researchers who worked on PAPER, as well as the US contingent of researchers who worked on MWA. US partners include University of California at Berkeley (PI institution), MIT, University of Washington, Arizona State University, Brown University, UC Los Angeles, NRAO, and the University of Pennsylvania. Alongside South Africa, other non-US partners include Scuola Nationale Superiore (Italy) and the University of Cambridge, UK. A list of participants and collaborators can be found on the HERA website at http://reionization.org/team/

More on the technical design: http://reionization.org/science/technical-design/

HERA only observes for a cumulative quarter of the year. There are two reasons for this:

1. During the winter, the galactic plane is directly overhead in the South African sky. The galaxy is an extreme source of “foreground interference,” and essentially drowns out the sought after EoR signal. Observing is therefore only done in the 6 month period of the year where the galactic plane is lowest in the sky.
2. During the day, the sun charges Earth’s ionosphere, which blocks any extra-terrestrial low frequency signals from entering the atmosphere, and also retains terrestrial signals. Therefore, HERA only observes at night, when the ionospheric charge is not as significant.

PAPER

1. PAPER-64 Constraints On Reionization II: The Temperature Of The z=8.4 Intergalactic Medium. Jonathan C. Pober, Zaki S. Ali, Aaron R. Parsons, Matthew McQuinn, James E. Aguirre, Gianni Bernardi, Richard F. Bradley, Chris L. Carilli, Carina Cheng, David R. DeBoer, Matthew R. Dexter, Steven R. Furlanetto, Jasper Grobbelaar, Jasper Horrell, Daniel C. Jacobs, Patricia J. Klima, Saul A. Kohn, Adrian Liu, David H. E. MacMahon, Matthys Maree, Andrei Mesinger, David F. Moore, Nima Razavi-Ghods, Irina I. Stefan, William P. Walbrugh, Andre Walker, Haoxuan Zheng. The Astrophysical Journal, Volume 809, Issue 1, article id. 62, 11 pp. (2015). DOI:10.1088/0004-637X/809/1/62.

We present constraints on both the kinetic temperature of the intergalactic medium (IGM) at z = 8.4, and on models for heating the IGM at high-redshift with X-ray emission from the first collapsed objects. These constraints are derived using a semi-analytic method to explore the new measurements of the 21 cm power spectrum from the Donald C. Backer Precision Array for Probing the Epoch of Reionization (PAPER), which were presented in a companion paper, Ali et al. Twenty-one cm power spectra with amplitudes of hundreds of mK2 can be generically produced if the kinetic temperature of the IGM is significantly below the temperature of the cosmic microwave background (CMB); as such, the new results from PAPER place lower limits on the IGM temperature at z = 8.4. Allowing for the unknown ionization state of the IGM, our measurements find the IGM temperature to be above ≈5 K for neutral fractions between 10% and 85%, above ≈7 K for neutral fractions between 15% and 80%, or above ≈10 K for neutral fractions between 30% and 70%. We also calculate the heating of the IGM that would be provided by the observed high redshift galaxy population, and find that for most models, these galaxies are sufficient to bring the IGM temperature above our lower limits. However, there are significant ranges of parameter space that could produce a signal ruled out by the PAPER measurements; models with a steep drop-off in the star formation rate density at high redshifts or with relatively low values for the X-ray to star formation rate efficiency of high redshift galaxies are generally disfavored. The PAPER measurements are consistent with (but do not constrain) a hydrogen spin temperature above the CMB temperature, a situation which we find to be generally predicted if galaxies fainter than the current detection limits of optical/NIR surveys are included in calculations of X-ray heating.

1. The precision array for probing the epoch of re-ionisation: eight station results. Aaron R. Parsons, Donald C. Backer, Griffin S. Foster, Melvyn C. H. Wright, Richard F. Bradley, Nicole E. Gugliucci, Chaitali R. Parashare, Erin E. Benoit, James E. Aguirre, Daniel C. Jacobs, Chris L. Carilli, David Herne, Mervyn J. Lynch, Jason R. Manley, and Daniel J. Werthimer. The Astronomical Journal, Volume 139, Number 4. http://dx.doi.org/10.1088/0004-6256/139/4/1468

We are developing the Precision Array for Probing the Epoch of Re-ionization (PAPER) to detect 21 cm emission from the early universe, when the first stars and galaxies were forming. We describe the overall experiment strategy and architecture and summarize two PAPER deployments: a four-antenna array in the low radio frequency interference (RFI) environment of Western Australia and an eight-antenna array at a prototyping site at the NRAO facilities near Green Bank, WV. From these activities we report on system performance, including primary beam model verification, dependence of system gain on ambient temperature, measurements of receiver and overall system temperatures, and characterization of the RFI environment at each deployment site. We present an all-sky map synthesized between 139 MHz and 174 MHz using data from both arrays that reaches down to 80 mJy (4.9 K, for a beam size of 2.15e–5 sr at 156 MHz), with a 10 mJy (620 mK) thermal noise level that indicates what would be achievable with better foreground subtraction. We calculate angular power spectra (Cℓ) in a cold patch and determine them to be dominated by point sources, but with contributions from galactic synchrotron emission at lower radio frequencies and angular wavemodes. Although the sample variance of foregrounds dominates errors in these power spectra, we measure a thermal noise level of 310 mK at ℓ = 100 for a 1.46 MHz band centered at 164.5 MHz. This sensitivity level is approximately 3 orders of magnitude in temperature above the level of the fluctuations in 21 cm emission associated with re-ionization.

1. Constraining Polarized Foregrounds for EoR Experiments I: 2D Power Spectra from the PAPER-32 Imaging Array. Kohn, S. A.; Aguirre, J. E.; Nunhokee, C. D.; Bernardi, G.; Pober, J. C.; Ali, Z. S.; Bradley, R. F.; Carilli, C. L.; DeBoer, D. R.; Gugliucci, N. E.; Jacobs, D. C.; Klima, P.; MacMahon, D. H. E.; Manley, J. R.; Moore, D. F.; Parsons, A. R.; Stefan, I. I.; Walbrugh, W. P. The Astrophysical Journal, Volume 823, Issue 2, article id. 88, 10 pp. (2016). DOI:10.3847/0004-637X/823/2/88.

Current generation low-frequency interferometers constructed with the objective of detecting the high-redshift 21 cm background aim to generate power spectra of the brightness temperature contrast of neutral hydrogen in primordial intergalactic medium. Two-dimensional (2D) power spectra (power in Fourier modes parallel and perpendicular to the line of sight) that formed from interferometric visibilities have been shown to delineate a boundary between spectrally smooth foregrounds (known as the wedge) and spectrally structured 21 cm background emission (the EoR window). However, polarized foregrounds are known to possess spectral structure due to Faraday rotation, which can leak into the EoR window. In this work we create and analyze 2D power spectra from the PAPER-32 imaging array in Stokes I, Q, U, and V. These allow us to observe and diagnose systematic effects in our calibration at high signal-to-noise within the Fourier space most relevant to EoR experiments. We observe well-defined windows in the Stokes visibilities, with Stokes Q, U, and V power spectra sharing a similar wedge shape to that seen in Stokes I. With modest polarization calibration, we see no evidence that polarization calibration errors move power outside the wedge in any Stokes visibility to the noise levels attained. Deeper integrations will be required to confirm that this behavior persists to the depth required for EoR detection.

1. Multiredshift Limits on the 21 cm Power Spectrum from PAPER. Jacobs, Daniel C.; Pober, Jonathan C.; Parsons, Aaron R.; Aguirre, James E.; Ali, Zaki S.; Bowman, Judd; Bradley, Richard F.; Carilli, Chris L.; DeBoer, David R.; Dexter, Matthew R.; Gugliucci, Nicole E.; Klima, Pat; Liu, Adrian; MacMahon, David H. E.; Manley, Jason R.; Moore, David F.; Stefan, Irina I.; Walbrugh, William P. The Astrophysical Journal, Volume 801, Issue 1, article id. 51, 10 pp. (2015). (ApJ Homepage) DOI: 10.1088/0004-637X/801/1/51

The epoch of the reionization (EoR) power spectrum is expected to evolve strongly with redshift, and it is this variation with cosmic history that will allow us to begin to place constraints on the physics of reionization. The primary obstacle to the measurement of the EoR power spectrum is bright foreground emission. We present an analysis of observations from the Donald C. Backer Precision Array for Probing the Epoch of Reionization (PAPER) telescope, which place new limits on the H i power spectrum over the redshift range of 7.5\lt z\lt 10.5, extending previously published single-redshift results to cover the full range accessible to the instrument. To suppress foregrounds, we use filtering techniques that take advantage of the large instrumental bandwidth to isolate and suppress foreground leakage into the interesting regions of k-space. Our 500 hr integration is the longest such yet recorded and demonstrates this method to a dynamic range of 104. Power spectra at different points across the redshift range reveal the variable efficacy of the foreground isolation. Noise-limited measurements of Δ2 at k = 0.2 hr Mpc-1 and z = 7.55 reach as low as (48 mK)2 (1σ). We demonstrate that the size of the error bars in our power spectrum measurement as generated by a bootstrap method is consistent with the fluctuations due to thermal noise. Relative to this thermal noise, most spectra exhibit an excess of power at a few sigma. The likely sources of this excess include residual foreground leakage, particularly at the highest redshift, unflagged radio frequency interference, and calibration errors. We conclude by discussing data reduction improvements that promise to remove much of this excess.

1. New Limits on 21 cm Epoch of Reionization from PAPER-32 Consistent with an X-Ray Heated Intergalactic Medium at z = 7.7. Parsons, Aaron R.; Liu, Adrian; Aguirre, James E.; Ali, Zaki S.; Bradley, Richard F.; Carilli, Chris L.; DeBoer, David R.; Dexter, Matthew R.; Gugliucci, Nicole E.; Jacobs, Daniel C.; Klima, Pat; MacMahon, David H. E.; Manley, Jason R.; Moore, David F.; Pober, Jonathan C.; Stefan, Irina I.; Walbrugh, William P. The Astrophysical Journal, Volume 788, Issue 2, article id. 106, 21 pp. (2014). DOI: 10.1088/0004-637X/788/2/106

We present new constraints on the 21 cm Epoch of Reionization (EoR) power spectrum derived from three months of observing with a 32 antenna, dual-polarization deployment of the Donald C. Backer Precision Array for Probing the Epoch of Reionization in South Africa. In this paper, we demonstrate the efficacy of the delay-spectrum approach to avoiding foregrounds, achieving over eight orders of magnitude of foreground suppression (in mK2). Combining this approach with a procedure for removing off-diagonal covariances arising from instrumental systematics, we achieve a best 2σ upper limit of (41 mK)2 for k = 0.27 h Mpc-1 at z = 7.7. This limit falls within an order of magnitude of the brighter predictions of the expected 21 cm EoR signal level. Using the upper limits set by these measurements, we generate new constraints on the brightness temperature of 21 cm emission in neutral regions for various reionization models. We show that for several ionization scenarios, our measurements are inconsistent with cold reionization. That is, heating of the neutral intergalactic medium (IGM) is necessary to remain consistent with the constraints we report. Hence, we have suggestive evidence that by z = 7.7, the H I has been warmed from its cold primordial state, probably by X-rays from high-mass X-ray binaries or miniquasars. The strength of this evidence depends on the ionization state of the IGM, which we are not yet able to constrain. This result is consistent with standard predictions for how reionization might have proceeded.

• A Flux Scale for Southern Hemisphere 21 cm Epoch of Reionization Experiments. Jacobs, Daniel C.; Parsons, Aaron R.; Aguirre, James E.; Ali, Zaki; Bowman, Judd; Bradley, Richard F.; Carilli, Chris L.; DeBoer, David R.; Dexter, Matthew R.; Gugliucci, Nicole E.; Klima, Pat; MacMahon, Dave H. E.; Manley, Jason R.; Moore, David F.; Pober, Jonathan C.; Stefan, Irina I.; Walbrugh, William P. The Astrophysical Journal, Volume 776, Issue 2, article id. 108, 14 pp. (2013). DOI: 10.1088/0004-637X/776/2/108.

We present a catalog of spectral measurements covering a 100-200 MHz band for 32 sources, derived from observations with a 64 antenna deployment of the Donald C. Backer Precision Array for Probing the Epoch of Reionization (PAPER) in South Africa. For transit telescopes such as PAPER, calibration of the primary beam is a difficult endeavor and errors in this calibration are a major source of error in the determination of source spectra. In order to decrease our reliance on an accurate beam calibration, we focus on calibrating sources in a narrow declination range from -46° to -40°. Since sources at similar declinations follow nearly identical paths through the primary beam, this restriction greatly reduces errors associated with beam calibration, yielding a dramatic improvement in the accuracy of derived source spectra. Extrapolating from higher frequency catalogs, we derive the flux scale using a Monte Carlo fit across multiple sources that includes uncertainty from both catalog and measurement errors. Fitting spectral models to catalog data and these new PAPER measurements, we derive new flux models for Pictor A and 31 other sources at nearby declinations; 90% are found to confirm and refine a power-law model for flux density. Of particular importance is the new Pictor A flux model, which is accurate to 1.4% and shows that between 100 MHz and 2 GHz, in contrast with previous models, the spectrum of Pictor A is consistent with a single power law given by a flux at 150 MHz of 382 ± 5.4 Jy and a spectral index of -0.76 ± 0.01. This accuracy represents an order of magnitude improvement over previous measurements in this band and is limited by the uncertainty in the catalog measurements used to estimate the absolute flux scale. The simplicity and improved accuracy of Pictor A’s spectrum make it an excellent calibrator in a band important for experiments seeking to measure 21 cm emission from the epoch of reionization.

• Imaging on PAPER: Centaurus A at 148 MHz. Stefan, Irina I.; Carilli, Chris L.; Green, David A.; Ali, Zaki; Aguirre, James E.; Bradley, Richard F.; DeBoer, David; Dexter, Matthew; Gugliucci, Nicole E.; Harris, D. E.; Jacobs, Daniel C.; Klima, Pat; MacMahon, David; Manley, Jason; Moore, David F.; Parsons, Aaron R.; Pober, Jonathan C.; Walbrugh, William P. Monthly Notices of the Royal Astronomical Society, Volume 432, Issue 2, p.1285-1293 DOI: 10.1093/mnras/stt548

We present observations taken with the Precision Array for Probing the Epoch of Reionization (PAPER) of the Centaurus A field in the frequency range 114-188 MHz. The resulting image has a 25 arcmin resolution, a dynamic range of 3500 and an rms of 0.5 Jy beam-1 (for a beam size of 25 arcmin × 23 arcmin). A spectral index map of Cen A is produced across the full band. The spectral index distribution is qualitatively consistent with electron reacceleration in regions of excess turbulence in the radio lobes, as previously identified morphologically. Hence, there appears to be an association of `severe weather’ in radio lobes with energy input into the relativistic electron population. We compare the PAPER large-scale radio image with the X-ray image from the ROSAT All Sky Survey. There is a tentative correlation between radio and X-ray features at the end of the southern lobe, some 200 kpc from the nucleus, as might be expected from inverse Compton (IC) scattering of the cosmic microwave background by the relativistic electrons also responsible for the radio synchrotron emission. The magnetic fields derived from the (possible) IC and radio emission are of similar magnitude to fields derived under the minimum pressure assumptions, ˜ 1 μG. However, the X-ray field is complex, with large-scale gradients and features possibly unrelated to Cen A. If these X-ray features are unrelated to Cen A, then these fields are lower limits.

• Opening the 21 cm Epoch of Reionization Window: Measurements of Foreground Isolation with PAPER. Pober, Jonathan C.; Parsons, Aaron R.; Aguirre, James E.; Ali, Zaki; Bradley, Richard F.; Carilli, Chris L.; DeBoer, Dave; Dexter, Matthew; Gugliucci, Nicole E.; Jacobs, Daniel C.; Klima, Patricia J.; MacMahon, Dave; Manley, Jason; Moore, David F.; Stefan, Irina I.; Walbrugh, William P. The Astrophysical Journal Letters, Volume 768, Issue 2, article id. L36, 6 pp. (2013). DOI: 10.1088/2041-8205/768/2/L36.

We present new observations with the Precision Array for Probing the Epoch of Reionization with the aim of measuring the properties of foreground emission for 21 cm epoch of reionization (EoR) experiments at 150 MHz. We focus on the footprint of the foregrounds in cosmological Fourier space to understand which modes of the 21 cm power spectrum will most likely be compromised by foreground emission. These observations confirm predictions that foregrounds can be isolated to a “wedge”-like region of two-dimensional (k , k ∥)-space, creating a window for cosmological studies at higher k ∥ values. We also find that the emission extends past the nominal edge of this wedge due to spectral structure in the foregrounds, with this feature most prominent on the shortest baselines. Finally, we filter the data to retain only this “unsmooth” emission and image its specific k ∥ modes. The resultant images show an excess of power at the lowest modes, but no emission can be clearly localized to any one region of the sky. This image is highly suggestive that the most problematic foregrounds for 21 cm EoR studies will not be easily identifiable bright sources, but rather an aggregate of fainter emission.

• New 145 MHz Source Measurements by PAPER in the Southern Sky. Jacobs, Daniel C.; Aguirre, James E.; Parsons, Aaron R.; Pober, Jonathan C.; Bradley, Richard F.; Carilli, Chris L.; Gugliucci, Nicole E.; Manley, Jason R.; van der Merwe, Carel; Moore, David F.; Parashare, Chaitali R. The Astrophysical Journal Letters, Volume 734, Issue 2, article id. L34, 6 pp. (2011). DOI:10.1088/2041-8205/734/2/L34

We present observations from the Precision Array for Probing the Epoch of Reionization (PAPER) in South Africa, observed in 2010 May and September. Using two nights of drift scanning with PAPER’s 60° FWHM beam we have made a map covering the entire sky below +10° declination with an effective center frequency of 145 MHz, a 70 MHz bandwidth, and a resolution of 26′. A 4800 deg2 region of this large map with the lowest Galactic emission reaches an rms of 0.7 Jy. We establish an absolute flux scale using sources from the 160 MHz Culgoora catalog. Using the 408 MHz Molonglo Reference Catalog (MRC) as a finding survey, we identify counterparts to 480 sources in our maps and compare our fluxes to the MRC and to 332 sources in the Culgoora catalog. For both catalogs, the ratio of PAPER to catalog flux averages to 1, with a standard deviation of 50%. This measured variation is consistent with comparisons between independent catalogs observed at different bands. The PAPER data represent new 145 MHz flux measurements for a large number of sources in the band expected to encompass cosmic reionization and represents a significant step toward establishing a model for removing foregrounds to the reionization signal.