------------------------------------------------------------------------ CenA_ATCA_V6.tex Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-MailScanner-Information: Please contact the postmaster@aoc.nrao.edu for more information X-MailScanner: Found to be clean X-MailScanner-SpamCheck: not spam, SpamAssassin (not cached, score=-4, required 5, autolearn=disabled, RCVD_IN_DNSWL_MED -4.00) X-MailScanner-From: rprother@physics.adelaide.edu.au X-Spam-Status: No % arXiv:0906.3766v1 % % ****** Start of file apssamp.tex ****** % % This file is part of the APS files in the REVTeX 4 distribution. % Version 4.0 of REVTeX, August 2001 % % Copyright (c) 2001 The American Physical Society. % % See the REVTeX 4 README file for restrictions and more information. % % TeX'ing this file requires that you have AMS-LaTeX 2.0 installed %\documentclass[onecolumn,prd,letterpaper,unsortedaddress,superscriptaddress,epsfig]{revtex4} \documentclass[twocolumn,prd,,showpacs,preprintnumbers,amsmath,amssymb,unsortedaddress,epsfig,floatfix]{revtex4} %\usepackage{epsfig} \setlength{\topmargin}{-0.5in} \setlength{\oddsidemargin}{0in} \setlength{\textwidth}{6.5in} \setlength{\textheight}{9.in} %\voffset=0.5in % added to solve format problem of unknown origin \usepackage{graphicx} \usepackage{dcolumn} \usepackage{amsmath} \usepackage{amssymb} \usepackage{pslatex} %\usepackage{epsfig} \usepackage[dvips,hypertex,bookmarks]{hyperref} %\usepackage{html} %\usepackage{url} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% % Hyphenation \hyphenation{brems-strahl-ung author another created financial paper re-commend-ed Post-Script} \def\deg{$^\circ$~} % Custom symbols \def\abs#1{\left| #1 \right|} \def\pderiv#1#2{{\partial #1 \over \partial #2}} \newcommand{\sci}[2]{#1\times 10^{#2}} \newcommand{\degree}{^\circ} \newcommand{\sign}{\textrm{sign}} \renewcommand{\Re}{\textrm{Re}\,} \renewcommand{\Im}{\textrm{Im}\,} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% % Variables \newcommand{\w}{\omega} \newcommand{\incsig}{x_\textrm{inc}} \newcommand{\dW}{\partial W} \newcommand{\dOn}{\partial\Omega\,\partial\nu} \newcommand{\DEC}{\eta_\omega} \newcommand{\power}[1]{\left(#1^2\right)_\omega} \newcommand{\vdp}{\varepsilon_0} \newcommand{\vvec}{\mathbf{v}} \newcommand{\infint}{\int_{-\infty}^{+\infty}} \newcommand{\beq}{\begin{equation}} \newcommand{\eeq}{\end{equation}} % Comment out a block of lines between \comment{ and } \newcommand{\comment}[1]{} \begin{document} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %\comment{ %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \title{LUNASKA Experiment Observational Limits on UHE Neutrinos from Centaurus A and the Galactic Center} \author{ C.~W.~James$^{1,*}$, R.~J.~Protheroe$^{1}$, R.~D.~Ekers$^{2}$, J.~Alvarez-Mu\~niz,$^{3}$ R.~A.~McFadden$^{4,2}$, C.~J.~Phillips$^{2}$, P.~Roberts$^{2}$, J.~D.~Bray$^{1,2}$ } \vspace{2mm} \noindent \affiliation{ $^{1}$School of Chemistry \& Physics, Univ.\ of Adelaide, Australia.\\ $^2$Australia Telescope National Facility, Epping, Australia.\\ $^3$Dept.\ Fisica de Particulas \& IGFAE, Univ. Santiago de Compostela, Spain. \\ $^4$School of Physics, Univ.\ of Melbourne, Australia.\\ $^*$Present address: Radboud Universiteit Nijmegen, The Netherlands. } \begin{abstract} We present the first observational limits to the ultra-high energy (UHE) neutrino flux from the Galactic Center, and from Centaurus A which is the nearest active galactic nucleus. These results are based on our ``Lunar UHE Neutrino Astrophysics using the Square Kilometer Array'' (LUNASKA) project experiments at the Australia Telescope Compact Array (ATCA). We also derive limits for the previous experiments and compare these limits with expectations for acceleration and super-heavy dark matter models of the origin of UHE cosmic rays. \end{abstract} \pacs{98.70.Sa, 95.55.Vj, 98.54.Cm} % PACS, the Physics and Astronomy Classification Scheme. % Neutrinos in astronomical observations 95.85.Ry, % Cosmic rays 96.50.S-, % galactic and extragalactic 98.70.Sa, % radiowaves, 41.20.Jb, 84.40.-x, % Cosmology 98.80.-k, % Dark matter 95.35.+d % ANITA paper PACS numbers: 98.70.Sa, 95.55.Vj % 98.70.Sa Cosmic rays (including sources, origin, acceleration, and interactions) (see also 26.40.+r Cosmic ray nucleosynthesis-in Nuclear astrophysics) % 95.55.Vj Neutrino, muon, pion, and other elementary particle detectors; cosmic ray detectors (see also 29.40.-n Radiation detectors-in Nuclear physics) % 95.85.Ry Neutrino, muon, pion, and other elementary particles; cosmic rays % 96.50.S- Cosmic rays (see also 94.20.wq Solar radiation and cosmic ray effects) % 98.70.Sa Cosmic rays (including sources, origin, acceleration, and interactions) % 41.20.Jb Electromagnetic wave propagation; radiowave propagation % 84.40.-x Radiowave and microwave (including millimeter wave) technology % 95.35.+d Dark matter (stellar, interstellar, galactic, and cosmological) % 98.54.Cm Active and peculiar galaxies and related systems (including BL Lacertae objects, blazars, Seyfert galaxies, Markarian galaxies, and active galactic nuclei) \maketitle %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %\section{Introduction} Arrival directions of the UHE cosmic rays (CR) detected by the Pierre Auger experiment above $5.6\times 10^{19}$~eV have been found to be statistically correlated with positions of nearby active galactic nuclei (AGN) \cite{AugerScience07}, and a few of the arrival directions appear to be clustered around Centaurus A, our nearest active galactic nucleus (AGN) at a distance of $\sim$3.7~Mpc. This has led to speculation that Centaurus A may be responsible for some of the UHE CR. However, the flux is extremely low, and so the nature of the sources of UHE CR remains at present unknown. As well as observing UHE CR directly, an alternative means of exploring the origin of UHE CR is to search for UHE neutrinos. Cosmic rays of sufficient energy will interact (e.g.\ via pion photoproduction) with photons of the 2.725~K cosmic microwave background (CMB) radiation, with the resulting energy-loss producing a cut-off in the spectrum at around $\sim10^{20}$~eV from a distant source \cite{Greisen_Zatsepin_Kuzmin}. These same interactions produce ``cosmogenic'' neutrinos from the decay of unstable secondaries \cite{BerezinskyGZK_ProtheroeJohnson96_Engel01}. As well as these cosmogenic neutrinos, UHE neutrinos are also expected to be produced by acceleration sources of UHE CR, and some information on the CR spectrum at the sources is imprinted on the spectrum of cosmogenic neutrinos \cite{Protheroe04}. Of course, neutrinos are not deflected by magnetic fields, and so should point back to where they were produced. See refs.~\cite{ProtheroeClay2004_Falcke2004_SKAscienceCase} for recent reviews of UHE CR production scenarios and radio techniques for high-energy cosmic ray and neutrino astrophysics. %\subsection{The Lunar Cherenkov Technique} In our present work we use the lunar Cherenkov technique \cite{Dagkesamanskii}, in which the Moon is used as a UHE neutrino target and Earth-based radio telescopes are used to detect coherent radio Cherenkov emission produced by neutrino-induced cascades in the lunar regolith. A high-energy particle interacting in a dense medium will produce a cascade of secondary particles which develops an excess negative charge by entrainment of electrons from the surrounding material and positron annihilation in flight. The charge excess is proportional to the number of particles in electromagnetic cascades, which in turn is proportional to the energy of the primary particle. Askaryan~\cite{Askaryan} first noted this effect and predicted the Cherenkov emission process in dense dielectric media to be coherent at radio frequencies where the wavelength is comparable to the dimensions of the shower, and this effect has been confirmed experimentally \cite{Saltzberg_GorhamSAND01}. At wavelengths comparable to the width of the shower, the coherent emission is in a narrow cone about the Cherenkov angle, while for wavelengths comparable to the shower length the coherent emission is nearly isotropic \cite{Alvarez-Muniz06}. The lunar Cherenkov technique aims to utilize the outer layers of the Moon, nominally the regolith which is a sandy layer of ejecta covering the Moon to a depth of $\sim$10~m, as a suitable medium to observe the Askaryan effect. Since the regolith is comparatively transparent at radio frequencies, coherent Cherenkov emission from cascades due to sufficiently high-energy neutrino interactions in the regolith should be detectable as nanosecond-scale pulses by Earth-based radio-telescopes. \begin{figure} \begin{center} \includegraphics[width=0.49\textwidth]{ATCA_MoonAperture.eps} %\includegraphics[width=0.49\textwidth, clip=true]{ATCA_Feb8_mean_22_bw_small.eps} %\includegraphics[width=0.49\textwidth, clip=true]{ATCA_May8_mean_23_bw_small.eps} \caption{Contours of effective area (km$^2$) as a function of UHE neutrino arrival direction for $10^{23}$~eV neutrinos for limb-pointing configuration (May 2008). The `+' marks the position of peak effective area; the Moon is at the center $(\eta,\xi)$=(0,0); telescope pointing direction $(\eta,\xi)$=(0.183$^\circ$,0.183$^\circ$).} \label{ATCA_instant} \end{center} \end{figure} %\section{Description of the Experiment} The aim of the LUNASKA project is to develop further the lunar Cherenkov technique for UHE neutrino astronomy, and to influence the design of the Square Kilometer Array \cite{SKA} so that UHE neutrino observations may be possible. Our experiment was carried out at the ATCA \cite{ATCA} which is an aperture synthesis telescope located at latitude $-$30$^\circ$ near Narrabri, Australia. It consists of six identical $22$~m dishes of which we have used three with baselines ranging from $\sim$100~m to $\sim$750~m. The ATCA has a half-power beam that matches the lunar disk at 1.4~GHz and it provided us with $600$~MHz (1.2--1.8 GHz) of bandwidth. Being an array the ATCA also provided both strong timing discrimination against terrestrial radio-frequency interference (RFI), and gave a large effective area and high sensitivity while seeing the entire moon \cite{Ekers08}. In order to perform a search for nanosecond-duration lunar Cherenkov pulses, we had to build specialized hardware, including dedispersion filters as such pulses suffer dispersion in the Earth's ionosphere -- our experiment is the first to correct for this in real-time. To detect and store candidate events in real time we used field-programmable gate array based analog-to-digital converters developed by the Australia Telescope National Facility each of which could digitize and perform simple logic on two data streams at a rate of $2.048$~GHz. The signal was passed to both a running buffer of length 256 samples and a real-time trigger algorithm. We triggered independently, with a maximum rate of 1040 Hz, at each antenna. On fulfilling the trigger conditions, the buffer was returned to the control room and recorded. We calibrated the system sensitivity using the thermal emission from the lunar disk, and the system clocks using correlated emission from the quasar 3C273. Full details of the experiment are given in refs.\ \cite{James_PhD2009_James_PRD2009}. The observations described here cover two observing periods, February and May 2008. The February 2008 observations were tailored to target a broad ($\gtrsim 20^{\circ}$) region of the sky near the Galactic Center, both a potential accelerator of UHE CR and also a potential source of UHE CR, gamma-rays and neutrinos through its dark matter halo. Based on simulation results \cite{JamesProtheroe09}, we pointed the antennas at the lunar center in February to achieve the greatest total effective aperture and sensitivity to an isotropic or very broadly-distributed flux. Our May 2008 observing period targeted Centaurus A only, and by pointing the telescopes at the portion of the lunar limb closest to Centaurus A we achieved maximum sensitivity for this source. We recorded a total of 98307 3-fold coincidences within 4 microsecond windows, and using our 0.5 nanosecond timing capability we are able to exclude all but 60 events as coming from the Moon. These remaining events occurred during short intervals of high RFI and were excluded after a check on wavefront curvature and visual inspection of the pulses. We calculated the effective areas at $10^{21}$, $10^{22}$, and $10^{23}$~eV as in ref.\ \cite{JamesProtheroe09}. This is shown in Fig.\ \ref{ATCA_instant} for $10^{23}$~eV neutrinos and the limb-pointing configuration, and we see that the sensitivity has a characteristic kidney shape peaking at $\sim$$15^\circ$ away from the Moon along the line extending from the Moon's center to the telescope pointing position on the lunar limb. For the center-pointing configuration (not shown), the sensitivity pattern forms an annulus, with peak exposure around $15^{\circ}$--$20^{\circ}$ degrees from the Moon. For both configurations, the sensitivity pattern broadens with increasing primary particle energy as the increased strength of the pulses produced allows the telescopes to be sensitive to a wider range of interaction geometries. Combining the instantaneous aperture, e.g.\ as shown in Fig.\ \ref{ATCA_instant}, with the known telescope-pointing positions on the Moon and the Moon's position itself at the time of observing, allows us to a calculate the exposure $[At](E_\nu;\alpha,\delta)$ (area-time product) at $10^{21}$, $10^{22}$, and $10^{23}$~eV as a function of celestial coordinates $(\alpha,\delta)$. \begin{figure*} \begin{center} %\includegraphics[width=0.7\textwidth]{ATCA_combined_23_paper_small.eps} \includegraphics[width=0.7\textwidth]{ATCA_DirectionalExposure23.eps} \caption{(color online) The exposure of the 2008 LUNASKA UHE $\nu$ detection experiment using the ATCA to $10^{23}$~eV neutrinos. The small circles show the directions of UHE CR events above $5.6\times 10^{19}$~eV detected by Auger \cite{AugerScience07}} \label{ATCA_combined_exposure} \end{center} \end{figure*} %\section{Results} The experiment with the greatest exposure to UHE neutrinos in the $10^{21}$ to $10^{23}$~eV range is the Antarctic Impulsive Transient Antenna Experiment (ANITA) \cite{ANITA} but this experiment is only sensitive to the declination range $-$10$^{\circ} < \delta < $+15$^{\circ}$. Other experiments for which we can obtain a neutrino flux limit from Centaurus A are the pioneering experiment at Parkes \cite{Parkes}, the Radio Ice Cherenkov Experiment (RICE) \cite{RICE} and the Goldstone Lunar UHE Neutrino Experiment (GLUE) \cite{GLUE}. The directional dependence of the exposures of the original Parkes experiment and GLUE were calculated previously in ref.\ \cite{JamesProtheroe09}. RICE was a Cherenkov radio experiment embedded in Antarctic ice at the South Pole and had a much lower neutrino energy threshold than the three lunar Cherenkov experiments, albeit with only a slowly-increasing exposure with neutrino energy. However, it had a very long observation time of several years as compared to several days for the lunar Cherenkov experiments, which makes up for the lower instantaneous effective aperture. RICE was mostly sensitive to down-going neutrino events, and so to declinations $\delta$$<$0$^\circ$. Taking the instantaneous effective volume as a function of neutrino arrival direction at $10^{22}$~eV \cite{Besson08_private} as indicative of the relative sensitivity at all energies, the exposure in celestial coordinates can be calculated for the known observation time of 7.4$\times$$ 10^7$~s, using the UHE neutrino-nucleon interaction cross-section \cite{gandhixsections} to convert between effective volume and effective area. %The combined exposure from Parkes, GLUE, RICE, and our LUNASKA ATCA observations to UHE The exposure from our LUNASKA ATCA observations to UHE neutrinos was calculated at all three energies, and is shown in Fig.\ \ref{ATCA_combined_exposure} for $10^{23}$~eV. In this figure the nominal declination range of the ANITA observations is also given, and in this range the exposure of ANITA (not shown) dominates. The concentration of exposure about Centaurus A, and the broad Galactic Centre region (nominally Sagittarius A), both of which are outside ANITA's sensitive declination range, is due to the targeting of these regions in our experiment. The individual and total exposures of these experiments to UHE $\nu$ from Centaurus A and Sgr A are given in Table \ref{ATCA_exposures_table} \begin{table} \caption{Experimental exposures (km$^2$-days) of GLUE, RICE and the LUNASKA ATCA observations to UHE neutrinos at discrete energies from the Galactic Center and Centaurus A .} \begin{center} \begin{tabular}{l c c c c c c c } %\begin{tabular}{l l l l l l l l l l} \hline \hline $E_{\nu}$~~~~~~ & {G.C.~~} & & & ~~ & {Cen A} & & \\ (eV) & GLUE & RICE & ATCA & ~~ & GLUE & RICE & ATCA \\ \hline $10^{21}$ & 0.5 & 195 & 2.9 & ~~ & 0.015 & 242 & 6.9 \\ $10^{22}$ & 14 & 333 & 54 & ~~ & 2.1 & 242 & 111 \\ $10^{23}$ & 175 & 710 & 409 & ~~ & 43 & 512 & 745 \\ \hline \hline \end{tabular} \end{center} \label{ATCA_exposures_table} \end{table} The model-independent 90\% confidence limit to the neutrino flux at energy $E_\nu$ from a putative point source at position $(\alpha,\delta)$ can be obtained from the exposure using $E_\nu F_\nu(E_\nu) \le 2.3/[At](E_\nu;\alpha,\delta)$. \begin{figure} \begin{center} \includegraphics[width=0.5\textwidth]{ATCA_CenA_LimitAltPlot.eps} \caption{(color online) Neutrino flux limits for Centaurus A from the present work: 2008 LUNASKA experiments, and based on GLUE and RICE experiments. Neutrino flux predictions of two AGN models for UHE CR production as labeled: KOT08 \cite{Kachelriess0904.0590}; CH08 \cite{CuocoHannestad023007}.} \label{FluxLimitsCenA} \end{center} \end{figure} %\section{Summary and Conclusion} In Fig.~\ref{FluxLimitsCenA} we show the all-flavor neutrino flux limits for Centaurus A. With Centaurus A only 3.7~Mpc away, and with the pion photoproduction energy-loss distance on the CMB minimizing at $\sim$12~Mpc above $10^{11}$~GeV (e.g.\ ref.~\cite{Protheroe04}) for rectilinear propagation one would observe UHE CR almost unattenuated by pion photoproduction interactions on the CMB. Cuoco \& Hannestad \cite{CuocoHannestad023007} have predicted the flux of UHE neutrinos from the Centaurus A jets ('CH08' in Fig.~\ref{FluxLimitsCenA}) using a model of an optically thick pion photoproduction source described by Mannheim {\it et al.}\ \cite{MannheimProtheroeRachen01} in which protons are injected with an $E^{-1.7}$ spectrum and produce an $E^{-2.7}$ flux of escaping neutrons which is normalized to the CR flux estimated to be due to Centaurus~A by assuming 2 of the Auger events above $6\times 10^{19}$ eV are due to Centaurus~A. The $\nu/n$ normalization depends on the poorly-known gamma-ray opacity. Kachelriess et al.\ \cite{Kachelriess0904.0590} consider several scenarios giving rise to various power-law spectra of protons as a result of pion photoproduction and cascading in accretion disk radiation: electromagnetic acceleration associated with kG scale magnetic fields near the core of Centaurus A; shock acceleration in the jet; and shock acceleration in the hot spots. Some of the scenarios are excluded as a result of over-producing TeV gamma-rays, and we plot ('KOT09') the result for acceleration near the core with an $E^{-2}$ proton spectrum. \begin{figure} \begin{center} \includegraphics[width=0.5\textwidth]{ATCA_GC_LimitAltPlot.eps} \caption{(color online) Neutrino flux limits for the Galactic Centre from the present work: 2008 LUNASKA experiments, and based on GLUE and RICE experiments. SHDM models (see text) with $M_X=10^{14}$ GeV and $M_X=10^{16}$ GeV (as labeled). Limit at $10^9$~GeV: Auger limit on the {\em cosmic ray} flux from a point source at the Galactic Centre \cite{Auger0607382}.} \label{FluxLimitsGC} \end{center} \end{figure} In Fig.~\ref{FluxLimitsGC} we show all-flavor neutrino flux limits for the Galactic Center. We consider the possibility that the Galaxy's dark matter halo is composed partly of SHDM, which decays or annihilates into particles which cascade into neutrinos, photons, and nucleons. We use fragmentation functions \cite{AloisioBerezinskyKachelriess094023} for $M_X=10^{16}$ GeV and normalize the sum of the gamma-ray plus nucleon components to the Auger limit on the {\em cosmic ray} flux from a point source at the Galactic Centre \cite{Auger0607382}. We plot the corresponding neutrino flux shown as the curve labeled ``$10^{16}$ GeV''. We also show the expected neutrino flux assuming $M_X=10^{14}$ GeV. Of course whether or not the neutrino (and cosmic ray) flux from SHDM annihilation appears point-like will depend on the radial distribution of the dark matter halo \cite{EvansFarrarSarkar0103085}. If this distribution is cusped the angular distribution will be narrow, approximating to a point source, but in the case of other models the angular distribution can be broadened to up to $\sim$$60^\circ$ half width. In comparison with experiments such as ANITA, our methods to improve sensitivity to certain specifically targeted regions were successful. We have reported the first neutrino flux limits above $10^{21}$ eV from Centaurus~A and the Galactic Center. For Centaurus A, the limit from our 2008 LUNASKA experiments using the ATCA is currently the strongest at primary neutrino energies of $10^{22}$--$10^{23}$~eV and above. While not ruling out any of the current theories of UHE CR production we have proved the viability of the lunar Cherenkov technique for targeted observation of specific UHE neutrino source candidates. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %\begin{acknowledgments} The Australia Telescope Compact Array is part of the Australia Telescope which is funded by the Commonwealth of Australia for operation as a National Facility managed by CSIRO. This research was supported by the Australian Research Council's Discovery Project funding scheme (project numbers DP0559991 and DP0881006). 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