PAPER: THE PRECISION ARRAY TO PROBE THE EPOCH OF REIONIZATION

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1 PAPER: THE PRECISION ARRAY TO PROBE THE EPOCH OF REIONIZATION Nicole Gugliucci 1 Advisor: Rich Bradley 2 1 Department of Astronomy, University of Virginia, Charlottesville, VA neg9j@virginia.edu 2 National Radio Astronomy Observatory Technology Center, Charlottesville, VA rbradley@nrao.edu ABSTRACT The Precision Array to Probe the Epoch of Reionization is a collaborative effort between the University of Virginia, the National Radio Astronomy Observatory, the University of California at Berkeley, the University of Pennsylvania, and Curtin University in Western Australia. This array of low frequency radio antennas will be used to detect neutral hydrogen in the universe from a time before it was ionized by the first stars and galaxies. We have been using a prototype array in Green Bank, West Virginia, which has expanded from eight to sixteen antennas this year. This larger array presents new calibration challenges and opportunities, including a look at the impact of the ionosphere on our astronomical observations. I plan to expand the Green Bank array to longer baselines in the near future to further this exploration and aide in making a catalog of the sky at our telescope s receiving frequencies around 150 MHz. Introduction Science Goal In the past decade, much has been learned about the state of the universe just 300,000 years after the Big Bang, during the period of recombination of elementary particles to make hydrogen. Sensitive probes of the cosmic microwave background (CMB) by space-based telescopes such as WMAP (Wilkinson Microwave Anisotropy Probe) 1 and COBE (Cosmic Microwave Background Explorer) 2, and a host of ground-based experiments have ushered in the age of precision cosmology, where fundamental properties of the universe and its origins can be probed in an experimental way. Astronomers are now turning their sights to the next important period in the history of the universe, the epoch of reionization (EoR). This is the period when the neutral hydrogen that pervaded all of space after recombination was ionized, or broken down into its constituent protons and electrons, by the ultraviolet light from the first stars in the first galaxies (Loeb 2006). Today the intergalactic medium is completely ionized. The very first galaxies are so faint to us today that JWST (James Webb Space Telescope) 3 will only begin to probe this era of galaxy formation. However, much effort has been put into simulating possible formation scenarios, and their effects on the reionization process (e.g. Barkana & Loeb (2001); Iliev et al. (2007)). Different parameters of the early universe can be changed, such as star formation rates, gas clumping factors, and x-ray luminosity, which affect the time and duration of the reionization process. A measurement of the neutral hydrogen as it is ionized will better constrain this period of evolution (Furlanetto, Oh, & Briggs 2006). Hydrogen has a spectral transition at 21 cm in the rest frame of the gas, which in the radio band of the electromagnetic spectrum. We can map this widespread neutral hydrogen on the sky in two dimensions. However, by mapping hydrogen at different redshifts, we also get a picture of the universe at different times during this epoch 3 Gugliucci 1

2 of reionization. This provides a three-dimensional map of the universe during the era of galaxy building. Thus, the evolution of the ionization process can be understood. (Due to the finite speed of light, the light that we see from a distant object takes a finite amount of time to reach us. Thus we see that object as it was some time ago. Due to the fact that we live in an expanding universe, it is as if objects are all moving away from each other. We measure a Doppler shift, such that the frequency of light that we receive is at a lower frequency than it was emitted, or we say that it is redshifted. Thus, measuring the spectral line of hydrogen at different frequencies corresponds to different distances and time periods.) Constraints have already been set for the time frame for reionization by observations of distant quasars and of the CMB. By measuring the absorption of quasar light by neutral hydrogen along the line of sight, astronomers have determined that reionization was complete by a redshift of approximately 6 (Becker et al. 2001; White et al. 2003; Fan et al. 2006), or one billion years after the Big Bang. The polarization of the cosmic microwave background signal, as measured by WMAP, indicates that the midpoint of reionization occurred around a redshift of 10 (Spergel et al. 2007). These and other lines of evidence suggest that reionization is a complicated process that may have spanned redshifts from 6 to 14 (or 12.7 to 13.3 billion years ago). These redshifts mean that the transition for the neutral hydrogen spectral line falls within the frequency range MHz. Although the very first radio astronomers used such low frequencies in their early experiments, this range has been largely ignored for the past few decades while the centimeter bands have been explored due to the early study of local neutral hydrogen and the desire for better spatial resolution. In the frequency ranges for this experiments, challenges arise from excessive man-made interference (RFI) and a refractive and unstable ionosphere, which begins to come into play below 1 GHz. The ionosphere is the layer of the earth s atmosphere that has been ionized by solar radiation. Densities are generally higher during the day than at night and during solar maximum. The global distribution of electrons around the ionosphere changes daily, such that the area of highest ionization is at the subsolar point. Thus, the density is also seasonal for a particular location. Note that solar maximum is scheduled to occur in 2012, or around the time that many low frequency arrays will be gearing up for full operations. In addition to the heightened electron density, sudden ionospheric disturbances can be triggered by solar flares, in which the ionization increases dramatically over the course of minutes, then slowly recovers to normal densities over a few hours (Kunitsyn & Tereshchenko 2003). Ionosondes have traditionally been used to probe the height and structure of the different ionospheric layers. A radio pulse is sent at different frequencies which probes different heights by reflecting back the waves. These challenges will have to be overcome by the next generation of radio telescopes that are slated to explore this region of the electromagnetic spectrum, such as the Murchison Widefield Array (MWA) 4, Long Wavelength Array (LWA) 5, and Low Frequency Array (LOFAR) 6. Early experiments will be designed to measure the power spectrum of the fluctuations of the neutral hydrogen signal. Project goals PAPER (Precision Array to Probe the Epoch of Reionization) is a collaboration among scientists, engineers, and students at the National Radio Astronomy Observatory (NRAO), the University of Virginia (UVa), the University of California at Berkeley (UCB), University of Pennsylvania, and Curtin University in Western Australia. The project philosophy is that of a targeted experiment towards the detection of the power spectrum of the neutral hydrogen signal, and a staged approach to deployment and detection. PAPER is a radio interferometer, or a system of radio telescopes electronically linked together to make one larger telescope. An interferometer is sensitive to the spatial frequency of the distribution of flux on the sky, and measurements are made of the amplitude and phase at each spatial frequency that is sampled by the antenna spacings, or baselines. These data, known as visibilities, are related to the sky brightness by means of a Fourier transform. The observed visibilities, after careful calibration, can be deconvolved to produce an image of the sky (Taylor, Carilli, & Perley 1999). Each individual element of PAPER is a sleeved dipole upon a ground screen in the shape of a trough reflector. It is designed to have a clean reception pattern on the sky over the frequency range necessary for this experiment (see Fig. 1) Gugliucci 2

3 An antenna of this design can receive signals from all over the sky simultaneously, thus giving the array the ability to make full-sky maps in a short amount of time. In order to reach the sensitivity for the EoR signal from the redshifted neutral hydrogen, we will deploy at least 128 antennas in the radio-quiet zone of Western Australia. The array makes use of a correlator, which brings the signals from each antenna together, and it is being designed and built at UCB in collaboration with CASPER (Center for Astronomy Signal Processing and Electronics Research 7 ). Fig. 1. Photograph of the current antenna design used in Green Bank, WV. The sleeved dipole is made of copper tubes and aluminum plates, with foam separators. The groundscreen is made of steel and wire mesh. The signal of the EoR will be on the level of 10 mk, or approximately 10 6 times fainter than foreground sources such as the synchrotron emission from the Milky Way Galaxy and bright extragalactic sources. 8 This calls for careful calibration of the instrument, as well as precise foreground removal. To achieve these ends, we have built an engineering array located within the National Radio Quiet Zone 9 at the NRAO site in Green Bank, West Virginia. A small array allows one to evaluate instrument performance and develop effective imaging algorithms, and this leads to changes in design for the next iteration of construction. The current array features sixteen antennas. At present, each antenna is fitted with Fluxes in radio astronomy may be quoted in Jankys, or W m 2 Hz 1. However, this is can be interchangeable with brightness temperature, measured in Kelvin, which is the temperature that the emitting object would have if the emission was thermal in nature. 9 a dual-polarization amplifier, but we only record one polarization at a time. The correlator has similarly been built from a small system to its current, 32-input configuration. The deployment in Western Australia will also occur in stages, with 32 antennas arriving this fall, a 64-antenna array soon to follow, and 128 antennas in Early work in the low frequency regime was more likely to be limited by sensitivity and source confusion rather than by the ionosphere (Kassim et al. 2007, and references therein). New telescopes, such as PAPER, will have better resolution and sensitivity than previous experiments, so the ionosphere does become an important factor. The ionosphere can affect the measurements of source positions, the achievable resolution of the image, or the ability to make an image at all in serious cases of scintillation. The scale of these effects is dependent on array size and geometry. For example, ionospheric disturbances are shown to be worse along longer baselines (Kassim et al. 2007). However, long baselines are crucial to improving resolution and reducing imaging confusion. We need to test the limits of our array such that we can use the longest baselines available to us to reduce source confusion, but not be restricted by ionospheric disturbances that would further reduce image quality and source detection. We also want to determine what ionospheric disturbances our instrument is sensitive to, in terms of the size and time of variability, and correct for those in later stages. Considering a simple model of a plane-parallel ionosphere, the excess path length through a homogeneous slab is L = L νhz 2 n e, (1) where L is the size of the slab (Rao 1999). Typical path lengths range from 350 to 3500 m and there is an important ν 2 dependence. In the plane-parallel case, rays coming from the same source will have the same differential path length added on, such that there is no change in the phase difference between two antennas from the case where the plane-parallel ionospheric slab was not present at all. In a classic demonstration of Snell s law, the source position merely appears to have shifted. This becomes more complicated in the case of a curved earth and curved ionosphere, which is obviously closer to reality. In this case, there is a path length difference, so there is also a phase Gugliucci 3

4 difference between the originally parallel rays. So, you have refractions that are different depending on the sky position. The zenith angle of the source is changed by the amount z = 2 sin z 3r 0 ( νp ) ( h ) ( i cos 2 z + 2h ) 3/2 i, ν r 0 r 0 (2) where the ionosphere is modeled as a layer of thickness h with a parabolic distribution of electron density with maximum height h i, ν p is the plasma frequency, ν is the observing frequency, z is the zenith angle of the source at the top of the atmosphere, and r 0 is the radius of the Earth (Thompson, Moran, & Swenson 2001). This is a good starting point for a model ionosphere. To probe further into the problems that the ionosphere can cause, we can look at previous low frequency surveys. The more recent work done with the 74 MHz system on the Very Large Array (VLA) has uncovered many of the ionospheric problems with which future arrays will have to cope (Kassim et al. 2007). Optical astronomers have long been familiar with the idea of an isoplanatic patch, or the area of sky over which wavefront errors are loosely correlated. Kassim et al. (2007) defines it more specifically as the region of sky that is defined by a phase difference less than one radian at a particular observing frequency. For the VLSS (VLA Low-Frequency Sky Survey) 10, there were multiple isoplanatic patches within the primary beam of An isoplanatic patch is typically a few degrees in size, or 10s km wide at a height of 400 km. PAPER has a primary beam of 120 so we will see many isoplanatic patches within each image. The ionosphere is home to irregularities at different size scales. The wedge effect is the product of irregularities on the 1000 km scale and causes an overall astrometric shift in the image. The wedge is most prominent in the morning, since the ionosphere is reacting to the sun, increasing the ionized content. This is time variable on the scale of minutes, so short-timescale self-calibration is needed to alleviate that effect. TIDs (Traveling Ionospheric Disturbances) are structures on size scales of 100s km that move through a given patch of sky on a time-scale of 90 minutes. This has been shown to cause linear phase gradients down the arms of the VLA. Phase distortion of up to 600 degrees were observed on 10 the bright extragalactic radio source Virgo A on timescales of 10s of minutes. Self-calibration on a bright source will remove this effect near the bright source, but the sources in the rest of the field of view may not be detected due to the refractive effects. Smaller scale irregularities could cause complete loss of data, such as during scintillation events, or other errors that have not yet been probed by current arrays. I have proposed to explore the effects of the ionosphere on PAPER imaging with metrics such as source position and image quality. This is a two-pronged approach, where both simulations and data analysis will be useful. The ionosphere can be modeled by its effects on the phases of the incoming signals, using what is already known about its behavior and introducing irregularities on different size scales. The results of this can then be compared to images made with our Green Bank array, giving us a better handle on how to remove such aritifacts from the interferometric data. Observations and Data Analysis We worked with an engineering array of eight antennas through most of 2008 in Green Bank, WV, with a prototype correlator built at Berkeley. This was expanded to sixteen antennas in October. I have been involved in many aspects of this deployment which involves choosing and surveying antenna positions, building antennas and amplifiers, cutting and measuring cable, operating the correlator, and analyzing the data. This new array included a check with surveying equipment on all of the field positions and orientations of the antennas. This requires new position calibration for all of the antennas using a new imaging toolset, AIPY (Astronomical Imaging in Python). This is being developed by UCB graduate student Aaron Parsons, and has functionality for doing basic calibration, measuring antenna positions, modeling the beam pattern, and creating snapshot or full-sky images. The positions are found by phase calibration and fitting the data to a model sky. To illustrate this concept, a two-antenna interferometer (see Figure 2) has a response to a point source as V λ = I s (ŝ) exp ( 2πi b ŝ/λ)dω. (3) The visibility is dependent on the source position (ŝ), the baseline vector between two antennas ( b), the source flux (I s ), and the wavelength of the radiation (λ). The phase is the term in the expo- Gugliucci 4

5 Fig. 2. A two-antenna interferometer with baseline vector b and source position ŝ, which go into the calculation of the phase of the visibilities. By Condon & Ransom from Essential Radio Astronomy ( shared under Creative Commons License. nential, thus is dependent on the sky and array geometry. To find the antenna positions from the data, each baseline in the array (120 for a sixteen antenna array) is fit with a model of the brightest sources in the sky, and all baselines need to give a good fit for the positions to be accurately derived. This process requires a good starting point from surveying techniques and careful monitoring of the fitting process. Preliminary results show difference between surveyed and fit positions from a few to 10s of feet. This may indicate a problem with the reference frame used for surveying or some unknown effects on the fitting process that are currently under investigation. Early results from sixteen antenna data before repositioning showed the first effects of the ionosphere after initial calibration. Figure 3a shows the difference in the position of Cassiopeia A, a supernova remnant that is always above the horizon from Green Bank, from where it should be in the sky. This can be compared to Figure 3b, which is a model of the changing position of Cas A due to refraction in a spherical ionosphere using Equation 2 with parameters from Thompson, Moran, & Swenson (2001). When comparing the model to the data, we see the same shape and scale of refraction affecting the position of Cas A. We can improve this model with satellite data, which uses two frequencies to measure the TEC, or total electron content, along lines of sight. One such system, called DORIS 11, which is a system 11 Doppler Orbitography by Radiopositioning Integrated on Fig. 3. (a) Data on Cas A from PAPER-16GB, where the two lines are the shifts in right ascension and declination from the actual position due to the ionosperic refraction. Plot by A. Parsons. (b) Model from Eqn. 2 with ν p = 12 MHz, h i = 350 km, and h = 225 km. of dual-frequency receivers on board various scientific satellites and 51 ground transmitters evenly placed throughout the globe. These data, including ionosphere corrections, are freely available on the web via the International DORIS Service. From these, I can derive the total electron content over almost any part of the Earth and determine large scale gradients and patterns. An example of this data, corrected to show the TEC directly overhead a portion of the globe, shows a clear gradient in the ionization content in Figure 4. We can apply the ionospheric monitoring data to our models and compare against PAPER data, such as that from Figure 3b. The modeling will prepare for data analysis, where I will try and match up image artifacts and position shifts with ionospheric irregularities on all scales. As shown above, the 16-antenna array is a good testbed for this. Many of these ionospheric effects are exaggerated as baselines get longer (Kassim et al. 2007). At the moment, the longest baselines for PAPER are approximately 270 meters. I have marked out positions for outriggers extending beyond the original circle (see Figure 5) which will improve the resolution of PA- PER and further probe ionospheric errors. The longest baseline possible with this configuration is 460 m, compared to 277 m with the current configuration. I will start with the outriggers close to the array and move them further out systematically. Satellite, Gugliucci 5

6 Fig. 4. Total electron content values (in units of TECU, or electrons/m 2 ) over a large swath of the continental US at one point in time. Position offsets on the order 10s of arcminutes are already detectable and will be refined with these longer baselines. Fig. 5. Positions of 16-antenna array plus tracks for three outriggers to move out to longer distances. AutoCad drawing by P. Klima. With enough sensitivity, we hope to create a catalog of the entire sky as seen from Green Bank, complete with positions, fluxes, and spectral indices of the brightest radio sources, for future use as flux calibrators. In addition to being probes of the ionosphere, these foreground point sources need to be removed from our data before we can detect the EoR. This will also create a catalog of sources with accurate positions, fluxes, and spectral indices over the bandwidth of our instrument. Conclusions and Future Work Our initial experiments with PAPER have helped to bring us closer to our goal of a precision instrument for detecting the EoR and imaging with a new type of interferometer. We will continue to develop calibration strategies and imaging techniques for our unique problem. The development of the AIPY package provides a unique testbed for new methods and ideas. The two-pronged approach of ionospheric modeling and data analysis will allow me to develop a better understanding of the ionosphere and its effects on imaging arrays and the EoR experiments to be carried out in the next few years. This will, along the way, develop a catalog of point sources in both the Northern and Southern skies. The brings the potential of exploring extragalactic radio source populations over a wide bandwidth and searches for transient sources. We will continue to use the Green Bank array as a test bed for new technologies and a place to carry out experiments. I will continue learning and developing data analysis and imaging techniques for all-sky maps and snapshot imaging. We will soon tackle the problem of polarimetry with a dipole array, and explore a largely unknown polarization signal from the Galaxy in the foreground. Eventually, we will turn to modeling and subtracting out a brightness distribution of the sky, in order to detect the power spectrum of the EoR from within the noise. The deployments to Western Australia will follow the staged approach, with 32 antennas arriving this fall. This will allow us greater sensitivity for imaging and more antenna pairs, or baselines, for use in calibration and data modeling, in addition to a quieter radio environment. This will lead us towards snapshot imaging, removal of foregrounds, and a catalog of sources in the southern sky. This will be followed by a 64-antenna array and then a 128-antenna array in The latter will bring us to the required sensitivity for the EoR experiment. If funding allows, we will expand it to an even larger array to gain even more sensitivity. Observations below 30 MHz become difficult because of refraction, scintillation, and increasing opacity of the Earth s ionosphere. However, high angular resolution observations of neutral hydrogen at redshifts up to 100 (corresponding to 10 MHz observing frequency) would probe the universe billion years ago, just 17 million years after the Big Bang. At this time, the neutral hydrogen signal would be seen in absorption against the CMB and would trace the very first large-scale, dark-matter dominated structures in Gugliucci 6

7 the universe. In order to escape the effects of the Earth atmosphere, as well as RFI, designs are being drafted for the first radio observatory on the Moon. Members of the PAPER team are collaborating with MIT on such a concept called the Lunar Array for Radio Cosmology (LARC) along with scientists from Harvard, UCB, University of Washington and NASA s Jet Propulsion Laboratory. LARC is also collaborating with a similar project, DALI (The Dark Ages Lunar Interferometer) which includes members from the Naval Research Lab, University of Colorado, NASA s Goddard Spaceflight Center, Harvard, and others. The development of PAPER will help open the path for the next few decades of research into the formative years of the universe. Thompson, A. Richard, Moran, James M., & Swenson, George W., Jr. 2001, Interferometry and Synthesis in Radio Astronomy White, Richard L., Becker, Robert H., Fan, Xiaohui, & Strauss, Michael A. 2003, AJ, 126, 1 REFERENCES Barkana R. & Loeb, A. 2001, PhR, 349, 125 Becker, R. H., Fan, X., White, R. L. & 27 coauthors 2001, AJ, 122, 2850 Fan, X., Strauss, M. A., Becker, R. H., White, R. L., Gunn, J. E., Knapp, G. R., Richards, G. T., Schneider, D. P., Brinkmann, J., & Fukugita, M. 2006, AJ, 132, 117 Furlanetto, S. R., Oh, S. P., & Briggs, F. H. Iliev, I. T., Mellema, G., Shapiro, P. R., McDonald, P., & Pen, U.-L. 2007, ASPC, 380, 3 Kassim, N. E., Lazio, T. Joseph W., Erickson, W. C., Perley, R. A., Cotton, W. D., Greisen, E. W., Cohen, A. S., Hicks, B., Schmitt, H. R., & Katz, D. 2007, ApJS, 172, 686 Kunitsyn, V.E. & Tereshchenko, E.D. 2003, Springer-Verlag, Ionospheric Tomography Loeb, A. 2006, SciAm, 295, 46 Rao, A. P. 2001, Ionospheric effects in Radio Astronomy from Low Frequency Summer School at GMRT in 1999 Spergel, D. N., Bean, R., Dor, O., Nolta, M. R., Bennett, C. L., Dunkley, J., Hinshaw, G., Jarosik, N., Komatsu, E., Page, L., Peiris, H. V., Verde, L., Halpern, M., Hill, R. S., Kogut, A., Limon, M., Meyer, S. S., Odegard, N., Tucker, G. S., Weiland, J. L., Wollack, E., & Wright, E. L. 2007, ApJS, 170, 377 Taylor, G. B., Carilli, C. L., & Perley, R. A. 1999, ASPC, 180 This 2-column preprint was prepared with the AAS LATEX macros v5.2. Gugliucci 7