Raman Research Institute, Bangalore, India

Ravi Subrahmanyan (RRI, Bangalore)

Ron Ekers & Aaron Chippendale (CAS)A Raghunathan & Nipanjana Patra (RRI, Bangalore)

‘All-sky’ EoR spectrumor

‘Global’ EoR spectrumor

EoR ‘monopole’ spectrum

While we have MWA, PAPER, LOFAR…. attempting to detect the structures in the neutral fraction at different cosmic times during reionization

There are some of us CORE, EDGES….attempting to detect spectral features in the background light arising from cosmological evolution of the neutral gas – the EoR monopole spectrum.

Obs. Freq 350 MHz130 MHz

These are attempting to detect the power spectrum of spatial distribution in the neutral hydrogen.

Historically the first suggestion for experimental Historically the first suggestion for experimental detection of the EoR spectrum was in detection of the EoR spectrum was in

Shaver et al. 1999Shaver et al. 1999

Redshift

Re-ionization

Cosmic Time

30 mK

z = 8.5

is the expected size of the ‘step’ in the radio background assuming ionization of preheated neutral hydrogen.

Loeb & Zaldarriaga 2004

the gas is in absorption that peaks at z ~ 50 and is effectively transparent at z < 20

50 MHz – 10 MHz

Without any heating from the first bound objects:

At lower redshifts z < 25:

• While TK < TCMB : Ly- from the first bound objects may drive Ts -> TK

• Radiative and shock heating of the neutral gas from the first light and structure formation drive TK > TCMB

• Reionization reduces the neutral fraction of the gas

Gnedin & Shaver 2004

Furlanetto 2006

Pritchard & Loeb 2008

The form of the EoR component of the radio background spectrum depends on the evolution in the sources of reionization and the reionization process itself.

Detection of the EoR spectrum involves

• A measurement of the mean sky spectrum in the frequency range 45-200 MHz (z = 6-30)

• And search for the presence of predicted spectral features in the observations

Galactic component ≈ 650 K

Extragalactic component ≈ 200 K

at 100 MHz

The EoR spectrum is deeply buried in foregrounds

It is close to impossible to accurately model and subtract the foreground to isolate the EoR spectrum

So why do we even try to detect the EoR component of So why do we even try to detect the EoR component of the radio background?the radio background?

The EoR spectrum may have features – in a restricted class The EoR spectrum may have features – in a restricted class of models – that are relatively sharp in frequency space of models – that are relatively sharp in frequency space compared to the foregrounds.compared to the foregrounds.

Such signatures may be detectable in sensitive Such signatures may be detectable in sensitive measurements of the radio background spectrum in the measurements of the radio background spectrum in the presence of the orders-of-magnitude larger additive presence of the orders-of-magnitude larger additive foreground.foreground.

For Tsys = 500 K, BW = 1 MHz, rms sensitivity of 10 mK is For Tsys = 500 K, BW = 1 MHz, rms sensitivity of 10 mK is potentially possible in an integration time of 1 hr and using a potentially possible in an integration time of 1 hr and using a single antenna element. The EoR sky-average spectrum single antenna element. The EoR sky-average spectrum measurement requires a high-quality system and calibration measurement requires a high-quality system and calibration strategy, not a large collecting area telescope.strategy, not a large collecting area telescope.

Developing such a precision elements for this difficult Developing such a precision elements for this difficult background spectrum detection is developing good antenna background spectrum detection is developing good antenna elements for interferometer detections of EoR power spectra.elements for interferometer detections of EoR power spectra.

EDGES = Experiment to detect the global EoR signature (Bowman, Rogers, Hewitt 2008)

Suh et al 2004

A four-point antenna

A single linear polarization was observed100-200 MHz band (z = 6 to 13)

Switch between antenna and calibrated noise source

Systematics: • linear polarization of sky• multi-path propagation from sky to antenna• ground reflections of receiver noise• reflections in signal path within the electronics cables and modules

may contribute to ripples in the band

Observations at Murchison Shire, WA

with 7th order polynomial subtracted

75 mK rms

Sensitivity to EoR signatures is limited by systematics

For instantaneous reionization: EoR step is not greater than about 450 mK

COsmological Reionization Experiment - Mk 1

Aaron Chippendale2009 PhD Sydney Uni

Frequency independent antenna

To avoid spectral features arising from frequency dependent response to brightness temperature variations on the sky.

114 to 228 MHz z = 5.2 to 11.5

• Square pyramidal 2-arm log-spiral

• Structural bandwidth of 20:1

• Structure built from styrofoam + glue + paint

• 0.52 mm diameter copper wire

• Wooden base

Correlation receiver

To reject additive receiver noise contribution in the recorded sky spectrum

Recorded raw spectra are differences between sky and noise source

Noise source is switched ON (high power) and OFF (ambient temperature) every second.

Calibrated sky spectra = OFF/(ON-OFF)

System and calibration stability is extremely goodSystematics at 1% level are precisely repeatable after 24 hrs LST

Calibrated sky spectra OFF/(ON-OFF)

Calibrated spectra show a periodic ripple phase-locked to the rate at which the wires wind around the antenna!

Requires a rotating platform for the antenna!

COsmological Reionization Experiment - Mk 2

Reject components in the measured spectrum that arise from antenna emission and losses in transmission line from antenna

This requires that the power splitter be moved up towards the source and possibly outside the antenna

=> Space beam splitters

Make an interferometer measurement of the EoR spectral signatures.

87.5 to 175 MHz z = 7.1 to 15.2

X

Semi-transparent resistive screen

Reflection / Transmission properties of a resistive wire mesh for normal incidence 0.1 mm carbon wire mesh of 5 cm grid

30 degrees incidence

60 degrees incidence

Expected response to a 100 K sky

System: 2-m baseline interferometerWith a 5m x 10m carbon wire gridGrid size = 5 cmWire dia 0.1 mm

Expected response to a sky model

System: 2-m baseline interferometerWith a 5m x 10m carbon wire gridGrid size = 5 cmWire dia 0.1 mm

Current limits are at 0.5 K levelCurrent limits are at 0.5 K levelCurrent sensitivity is a factor 1000 below the Current sensitivity is a factor 1000 below the foregrounds. foregrounds. Need to go to at least factor of 10 lower in sensitivity to Need to go to at least factor of 10 lower in sensitivity to make useful measurements.make useful measurements.

The difficulty in going to lower sensitivity is The difficulty in going to lower sensitivity is systematics.systematics.

Current attempt is to use short spacing Current attempt is to use short spacing interferometers, space beam splitters, frequency interferometers, space beam splitters, frequency independent antennas.independent antennas.

toto

Reject frequency dependent contributions from sky Reject frequency dependent contributions from sky sources, antenna structural losses, transmission losses, sources, antenna structural losses, transmission losses, receiver noise, together with their multi-path receiver noise, together with their multi-path propagation to the spectrometer.propagation to the spectrometer.

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