Raman Research Institute, Bangalore, India

1. ‘All-sky’ EoR spectrum or ‘Global’ EoR spectrum or EoR ‘monopole’ spectrum Ravi Subrahmanyan (RRI, Bangalore) Ron Ekers & Aaron Chippendale (CAS) A Raghunathan & Nipanjana Patra (RRI, Bangalore) Raman Research Institute, Bangalore, India

2. 350 MHz 130 MHz Obs. Freq These are attempting to detect the power spectrum of spatial distribution in the neutral hydrogen. 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.

3. Redshift Cosmic Time Re-ionization 30 mK z = 8.5 • Historically the first suggestion for experimental detection of the EoR spectrum was in Shaver et al. 1999 is the expected size of the ‘step’ in the radio background assuming ionization of preheated neutral hydrogen.

4. Without any heating from the first bound objects: Loeb & Zaldarriaga 2004 50 MHz – 10 MHz the gas is in absorption that peaks at z ~ 50 and is effectively transparent at z < 20

5. Gnedin & Shaver 2004 • At lower redshifts z < 25: • While TK < TCMB : • Ly-a 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 Furlanetto 2006

6. 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

7. 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

8. So why do we even try to detect the EoR component of the radio background? • The EoR spectrum may have features – in a restricted class of models – that are relatively sharp in frequency space compared to the foregrounds. • Such signatures may be detectable in sensitive measurements of the radio background spectrum in the presence of the orders-of-magnitude larger additive foreground. • 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 single antenna element. The EoR sky-average spectrum measurement requires a high-quality system and calibration strategy, not a large collecting area telescope. • Developing such a precision elements for this difficult background spectrum detection is developing good antenna elements for interferometer detections of EoR power spectra.

9. EDGES = Experiment to detect the global EoR signature (Bowman, Rogers, Hewitt 2008) A four-point antenna • A single linear polarization was observed • 100-200 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 Suh et al 2004

10. 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

11. COsmological Reionization Experiment - Mk 1 114 to 228 MHz z = 5.2 to 11.5 • Frequency independent antenna • To avoid spectral features arising from frequency dependent response to brightness temperature variations on the sky. Aaron Chippendale 2009 PhD Sydney Uni • 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

12. 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)

15. System and calibration stability is extremely good Systematics at 1% level are precisely repeatable after 24 hrs LST Calibrated sky spectra OFF/(ON-OFF)

16. 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!

17. COsmological Reionization Experiment - Mk 2 87.5 to 175 MHz z = 7.1 to 15.2 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.

18. X Semi-transparent resistive screen

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

20. 30 degrees incidence 60 degrees incidence

21. Expected response to a 100 K sky System: 2-m baseline interferometer With a 5m x 10m carbon wire grid Grid size = 5 cm Wire dia 0.1 mm

22. Expected response to a sky model System: 2-m baseline interferometer With a 5m x 10m carbon wire grid Grid size = 5 cm Wire dia 0.1 mm

23. Status: • Current limits are at 0.5 K level Current sensitivity is a factor 1000 below the foregrounds. Need to go to at least factor of 10 lower in sensitivity to make useful measurements. • The difficulty in going to lower sensitivity is systematics. • Current attempt is to use short spacing interferometers, space beam splitters, frequency independent antennas. to • Reject frequency dependent contributions from sky sources, antenna structural losses, transmission losses, receiver noise, together with their multi-path propagation to the spectrometer.