Outline of my Talk

1. Development of a Zero-Spacing Interferometric technique for detecting the EoR signal 11-12-2013A. RaghunathanCollaborators: NipanjanaPatra, N. Udaya Shankar, Ravi Subrahmanyan, R.D. EkersTHE METRE WAVELENGTH SKY CONFERENCE

2. Outline of my Talk Introduction to EoR Challenges in the measurement Use of interferometer for the detection of EoR signal Concept of Zero-spacing interferometer Brief description of the receiver system built based on the zero-spacing interferometric technique Demonstration of the technique developed

3. Introduction : Epoch of reionization (EoR) is used to describe the period in the thermal history of universe during which the universe underwent a phase change from neutral to ionized state as a consequence of the formation of the first stars and galaxies. This epoch began with the generation of first generation of stars and galaxies. Hence studying the EoR has far reaching implications on understanding the structure formation of universe. Epoch of Reionization EoR Schematic outline of cosmic history courtesy : S.G.Djorgovski et.al

4. Gnedin & Shaver 2004 Nature of EoR signature • The first generation stars had great impact on the neutral hydrogen of IGM through their electromagnetic radiation • They ionized and altered their spin temperature • Thus 21cm line of neutral hydrogen is seen today either in emission or absorption against cosmic microwave background depending upon its spin temperature • These features manifest as a sharp step of 20 – 30 mK in the 3K CMB spectrum as shown in the Fig. • Due to the expansion of the Universe, 21cm line is red shifted by factor of 8 - 15 to octave band 87.5 to 175 MHz

5. Challenges in the measurement of EoR signal Measurement of the EoR signal requires • The sky to have uniform brightness distribution and • The instrument to have frequency independent spectral characteristics Unfortunately sky exhibits spatial variation in its brightness due to the distribution of sourcesand the measuring instrument inherently possesses frequency dependent response due to impedance mismatch between the low noise amplifier and the antenna and its frequency dependent noise characteristics In addition, sky is several orders of magnitude brighter than the EoR signal,. Isolating the latter from the total noise measured is difficult

6. Use of Interferometer for the measurement of EoR signal • Interferometer can be a strong candidate for measuring the EoR signal since its output response a) is free, in principle, from all the frequency dependent instrumental effects b) to the model sky, when averaged over the entire LST range of 0 to 24 hrs, is identical to the interferometer response to a constant brightness sky. thus satisfying both the requirements of the EoR detection However, it has inherently poor sensitivity for Uniform Sky

7. Response of a two element interferometer to uniform sky The interferometer response to a uniform sky is function of baseline length. For long baselines the response for uniform sky tends towards zero. To get max. response to the uniform sky component, we need to have a very short baseline -- tending towards zero

8. Semi-transparent screen Reflected ray Transmitted ray Direct ray • θf D D Antenna –A2 Antenna -A1 Zero spacing Interferometer - concept Image of A2 In this concept as proposed by R.D. Ekers and Ravi Subrahmanyan, a semitransparent screen is placed at the geometric centre of the two elements of the interferometer. Under this condition, we observe that the image of Ant-2 coincides with Ant-1 as shown and vice versa If the transmitted and reflected rays of the incident sky signal on the screen reach the antennas with identical phase relationship, then this configuration simulates the performance of a Zero-baseline interferometer

9. V1 V2 Incident signal θ -θ Zero spacing Interferometer concept….contd. Reflected signal Transmitted signal For isotropic sky radiation of equal magnitude on either side of the screen (±θ), the interferometer produces two complex products Product 1 proportional to Cos (ΔΦ) , Sin (ΔΦ) Product 2 proportional to Cos (ΔΦ) , Sin (-ΔΦ) where ΔΦ is the phase angle difference between the reflected and transmitted signals.

10. Zero spacing Interferometer concept….contd. When the antenna outputs are added together the real components add up in phase and imaginary components cancel each other due to the symmetry of sky on either side of screen For a perfectly conducting screen, ΔΦ = 90 deg. Due to this, the interferometer with a perfectly conducting screen does not produce any correlated output for the zero base-line component of the sky brightness. To overcome this limitation, screen is made RESISTIVE so that ΔΦ takes up a value close to 0 / 180 deg. so that screen and two element interferometer together respond to uniform isotropic radiation of the sky.

11. Screen design Investigation was carried out (Astrakhan, 1968) in order to understand the transmission and reflection properties of the planar wire grids as a func. of i) radiation parameters like frequency, polarization, angle of incidence and ii) grid parameters like diameter of the wire, grid spacing and conductivity of the wire 2. It was found that for obtaining maximum correlator output, half the incident power should be absorbed in the screen and the remaining half should be equally divided between the reflected and transmitted signals 3. The optimal grid parameters which result in this behaviour of the screen over the band 87.5 – 175 MHz are Conductivity of wire σ = 60570 S/m ; grid spacing=0.2m wire radius = 0.1 mm

12. Fabrication of the screen • A wooden framework with nails is prepared to draw and stretch wires in both X and Y directions. • After stretching, wires are glued to flex base material at every cross-over and finally nails are removed. • Then in each segment of wire, a resistor is introduced to meet the required conductivity of the wire

13. Screen characteristics : Simulation and measurement Results Transmission coefficient as a function of frequency a) Simulation b) Measurement (a) (b)

14. Screen characterisics : Simulation and measurement Results contd. Phase of the transmission coefficient as a function of frequency a) Simulation b) Measurement ( E-Polarization)

15. Semi-transparent Screen Broad band Dipole antenna Receiver system of Zero-Base line Interferometer Phase switching Mechanism Calibration Signal High Dynamic range and high resolution Receiver System Data Acquisition System Ethernet link Computer

16. Frequency Independent octave bandwidth Dipole antenna Frequency range : 87.5 –175 MHz Return Loss : < -15 dB Dispersion in the 3 dB beamwidth over the band : < 2.5 %

17. Digital spectro-correlator A 6 layer high frequency (250 MHz ) 12 bit analog to digital converter card 1024 channel Virtex-5 FPGA based In-House developed correlator which produces both auto and cross power spectra of both the antennas

18. Steps taken to minimize the pass-band ripple in the output spectrum • In the FE design  Spurious levels were maintained @ < 50 dB • Effect of sampler noise is minimized • Out of band aliasing components like RFI is suppressed by 50 dB • Long term and short term stabilities are maintained for LO with the help of i) Good phase noise signal generator ii) GPS signal 5. System modules were closely spaced to minimize the spectral features arising out of reflections due to impedance mismatch

19. Field setup Deployment of semi-transparent resistive screen in the field in between two fat dipole antennas to form a zero baseline interferometer.

20. Sky observation with Zero-spacing interferometer • Sky data collected is cleaned for RFI using median filtering, Hampel filtering and polynomial fitting to produce a spectrum as shown in the figure

21. Demonstration of the screen effect • Sky data is collected with and without the screen during the transit of galactic centre and one of the spectral channels of the output spectrum is plotted as a function of time • A positive bias in the response when the screen is introduced, is an effect of the screen on the interferometer response that arises from the formation of a zero spacing • This demonstrates the enhancement of sensitivity of the interferometer for uniform sky

22. Conclusion • We have developed a zero-spacing interferometer technique to enhance the sensitivity of a two element interferometer and successfully demonstrated it by building a receiver system and making sky observation in the frequency range 87.5 – 175 MHz • The absence of receiver noise, antenna noise and fringes in the averaged response spectrum makes this the preferred configuration for the detection of spectral signatures in the sky background

23. Acknowledgement I thank everyone in the Radio astronomy lab, workshop ( main and basement) of RRI and Gauribidanur for extending their invaluable help while building the receiver and conducting the experiment in the field.