Detection of Cosmic Rays at Ultra-High Energies with Phase I of the Square Kilometre Array (SKADS) Olaf Scholten and Heino Falcke April 13, 2007
1 abstract Currently under development by an international consortium, the Square Kilometre Array (SKA) is a centimeter- and meter-wavelength telescope envisioned as being one of a suite of new, large telescopes for the 21 st century. Construction of the SKA will likely proceed in phases, with the first phase, Phase I, having a sensitivity of 10% of the final SKA. This article summarizes the science related to the detection of Ultra-High Energy (UHE) cosmic rays that will be obtained with the Phase I SKA. 2 Scientific case There exist two rather different mechanisms for radio emission from showers triggered by UHE cosmic rays or neutrinos. The geosynchrotron radiation mechanism has recently been confirmed with new digital radio techniques (1; 2; 3; 4; 5). Here we will focus on the second mechanism which applies to showers in dense media, such as ice, salt, and lunar regolith, since it is most promising for detecting cosmic rays of the highest energies. It is the direction where a real break-through could be made already with SKA-I. When Ultra High Energy (UHE) particles impinge on a dense dielectric medium, the Askaryan effect (6), coherent Čerenkov radiation, produces an electro-magnetic pulse (EMP). Our interest is in UHE particles hitting the moon were the duration of the EMP is nano-seconds. At a wavelength of the size of the charge distribution, the radiation reaches full coherence, which, for the case of the moon is in the range of a few GHz. At much lower frequencies where the wavelength is of comparable size as the length of the shower, the angular spread reaches its maximum and the detection efficiency reaches an optimum (7). In fact, the sensitivities reached are so high (partly due to the enormous size of the moon and partly due to the sensitive radio wave detection with SKA-I) that one expects to measure copious amounts of UHE cosmic rays & neutrinos impinging on the moon. The SKA-I measurements can be the first to unequivocally falsify or prove the existence of the Greisen-Zatsepin-Kuzmin (GZK)-limit (8) (at an energy of about 6 10 19 ev). It is predicted that at energies above the GZK limit, protons (cosmic rays) will produce pions when interacting with the 2.7 K microwave background. The universe thus becomes opaque for cosmic rays at energies exceeding this limit which reflects in a sharply reduced flux of cosmic rays and, at the same time, a strongly increased flux of neutrinos resulting from the decay of the pions created off the CMB. This predicted 1
GZK-neutrino flux can be measured with good statistics with SKA-I. Also cosmic rays above the GZK-cut off can be measure with a sensitivity several orders of magnitude better than the extrapolated flux from lower energies. Seeing a small number of cosmic ray events with the accompanying neutrino flux will, for the first time, affirm the GZK mechanism. If these UHE particles originate from a cataclysmic event on astronomical scale, of which one expects only very few within 50Mpc, the direction of the cosmic rays will unambiguously point towards it. At these energies the bending of the trajectories of charged particles due to the interstellar magnetic fields can be ignored. If, on the other hand, these UHE particles rather originate from omni-present super-heavy particles, maybe a remnant of the big bang, the direction should be random. Either of these will be an exciting new discovery. Not seeing any evidence for the GZK limit would be equally exciting. This will indicate physics beyond the standard model such as Lorentz invariance braking at the highest energies! 3 Proposed Observations The optimal window for observing radio signals from UHE cosmic ray or neutrino impacts on the moon lies roughly between 80 and 500 MHz (7). The lower limit is determined by the strong increase of the Galactic background noise at lower frequencies while the, not so sharply defined, upper limit is determined by the fact that the detection efficiency is proportional to the inverse-third power of the frequency. In Fig. 1 the predicted sensitivities are given for observations in the low, 100-300 MHZ, (LFB) and the medium, 300-500 MHz (MFB) frequency bands. The sensitivity is defined as the minimum flux of particles in an energy bin of size E = E which will induce a single detected event. We have assumed here a 100% moon coverage and a detection threshold of 6 σ where σ is the amplitude of the noise per band and 1 year observing time with the moon in field of view. The detection efficiency can be improved by dividing the full frequency band in smaller bands and requiring that the EMP comes from a single place on the moon and at the same time for all bands. Observations should be performed over a rather large bandwidth, from the lowest frequencies up to 2 GHz. The pulse shape -or equivalently the frequency response- will be necessary to be able to distinguish neutrino and cosmic ray induced pulses. The pulse seen in the low-frequency band will serve as a trigger to look for the corresponding signal in the higher frequency bands where it is expected to be less pronounced as compared to the system 2
noise. 4 Anticipated Results E 2 dn/de [GeV/cm 2 /sr/s] 10-8 CR 10-9 10-10 10-11 10-12 SKA-I MFB LOFAR E 2 dn/de [GeV/cm 2 /sr/s] RICE 10-5 ANITA 10-6 10-7 10-8 10-9 WB SKA-I MFB FORTE LOFAR 10-13 SKA-I LFB 10-10 SKA-I LFB 10-1 1 10 10 2 10 3 10 4 E [10 20 ev] 10-1 1 10 10 2 10 3 10 4 10 5 E [10 20 ev] Figure 1: Comparing the detection limits of different systems, LOFAR, and SKA-I, for UHE cosmic rays (left) and neutrinos (right). For the SKA measurements two frequency bands are considered, see text. The detection limits for cosmic rays are compared to present data (10), where the grey band is an extrapolation. For neutrinos (right) the limits are compared with various models, in particular, WB (12) (vertical bars), GZK (13) (dotted thin line), and TD (14) (solid thin line). Limits from the RICE (15), ANITA (16), and FORTE (17) experiments are also shown. From Fig. 1 it is clear the the sensitivity is such that we can measure the rather well established flux of cosmic rays at an energy below the GZK limit. This will serve as a calibration of our flux measurement. Above the GZK limit we are sensitive to a flux of four orders of magnitude lower than a simple extrapolation of the flux at smaller energies. For the predicted flux of GZK neutrinos we should observe several hundreds of events. From the amplitude, the frequency spectrum, and the polarization direction of the EMP, the energy and angle of the shower can be reconstructed and will distinguish between neutrino and cosmic ray induced showers. It is therefore important to have the observations over a wide frequency domain. 3
5 Competition The Lunar Orbiting Radio Detector (LORD) project is a collaboration between Russia (LPI, Lavochkin Association, MSU, JINR) and Sweden (ISP) (18) which plans to launch a satellite in an orbit around the Moon. The launch is foreseen for 2009. The objective of the LORD mission is multi-fold. Apart from measuring the electromagnetic pulses from cosmic rays impinging on the Moon, also the lunar electro magnetic environment, the lunar seismicity, and the space plasma will be investigated. For the detection of the radio pulse from cosmic rays impinging on the moon the satellite is scheduled to carry an antenna system which is sensitive to the frequency range of 100 to 1000 MHz. Because of its relative proximity to the moon (distance less than 1000 km) it is very sensitive to radio pulses from the moon. The large bandwidth contributes to the sensitivity of the probe. However, more importantly, it offers the possibility to investigate the frequency dependence of the signal (or, equivalently, the time-dependence of the pulse). The expected detection sensitivity of LORD for UHE cosmic particles is comparable to that of the SKA. References [1] H. Falcke and P. Gorham, Astropart. Phys. 19, 477 (2003). [2] D.A. Suprun, P.W. Gorham, J.L. Rosner, Astropart. Phys. 20, 157 (2003). [3] T. Huege and H. Falcke, Astropart. Phys. 24, 116 (2005). [4] H. Falcke et al., Nature 435, 313 (2005). [5] D. Ardouin et al., Proceedings of the 29 th Int. Cosmic ray Conf., Pune, India (2005), astro-ph/0510170. [6] G. A. Askaryan, Sov. Phys. JETP 14, 441 (1962); 21, 658 (1965). [7] O. Scholten et al., (astro-ph/0508580), Astropart. Phys. 26, 219 (2006). [8] K. Greisen, Phys. Rev. Lett. 16, 748 (1966); G.T. Zatsepin, V.A. Kuzmin, Pis ma Zh. Eksp. Teor. Fiz. 4, 114 (1966), [JETP. Lett. 4, 78 (1966)]. 4
[9] H. Falcke, P. Gorham, R.J. Protheroe, Prospects for radio detection of ultra-high energy cosmic rays and neutrinos, SKA Science case. [10] M. Takeda et al., Astropart. Phys. 19, 447 (2003); http://wwwakeno.icrr.u-tokyo.ac.jp/agasa/ [11] R.U. Abbasi et al., Phys. Rev. Lett. 92, 151101 (2004). [12] J. Bahcall and E. Waxman, Phys. Rev. D 64, 64 (2001). [13] R. Engel, D. Seckel, T. Stanev, Phys. Rev. D 64, 93010 (2001). [14] R.J. Protheroe, T. Stanev, Phys. Rev. Lett. 77, 3708 (1996). [15] RICE Collaboration, I. Kravchenko et al., Astropart. Phys. 20, 195 (2003). [16] S.W. Barwick et al., Phys. Rev. Lett. 96, 171101 (2006). [17] N.G. Lehtinen, P.W. Gorham, A.R. Jacobson and R.A. Roussel-Dupre, Phys. Rev. D 69, 013008 (2004). [18] LORD Satellite mission, see web-page http://www.physics.irfu.se/newpages/satellites/ 5