Observation of Ultrahigh-Energy Cosmic Rays with the ANITA Balloon-Borne Radio Interferometer

Abstract

We report the observation of sixteen cosmic ray events of mean energy of 1.5 × 10 19 eV, via radio pulses originating from the interaction of the cosmic ray air shower with the Antarctic geomagnetic field, a process known as geosynchrotron emission. We present the first ultra-wideband, far-field measurements of the radio spectral density of geosynchrotron emission in the range from 300-1000 MHz. The emission is 100% linearly polarized in the plane perpendicular to the projected geomagnetic field. Fourteen of our observed events are seen to have a phase-inversion due to reflection of the radio beam off the ice surface, and two additional events are seen directly from above the horizon.The origin of ultra-high energy cosmic rays (UHECR) remains a mystery decades after their discovery [1,2]. Key to the solution will be increased statistics on events of high enough energy (≥ 3 × 10 19 eV) to elucidate the endpoint of the UHECR energy spectrum as seen at Earth. The primary difficulty is the extreme rarity of events at these energies. Despite steady progress with experiments such as the Pierre Auger Observatory, there remains room for new methodologies. Cosmic rays have been detected for decades via impulsive radio geosynchrotron emission [3,[5][6][7][8][9][10][11][12][13][14][15][16]] but until now not in this crucial energy range, which offers the possibility of pointing the UHECRs back to their sources. We present data from the Antarctic Impulsive Transient Antenna (ANITA) [21] which represents the first entry of radio techniques into this energy range. We find 16 UHECR events, at least 40% of which are above 10 19 eV, and we show compelling evidence of their origin as geosynchrotron emission from cosmic-ray showers. Our results indicate degree-scale precision for reconstruction of the UHECR arrival direction, lending strong credence to efforts to develop radio geosynchrotron detection as a competitive method of UHECR particle astronomy.Geosynchrotron emission arises when the electron-positron particle cascade initiated by a primary cosmic ray encounters the Lorentz force in the geomagnetic field. The resulting acceleration deflects the electrons and positrons and they begin to spiral in opposite directions around the field lines [17,18]. In air, the particles' radiation length is of order 40 g cm −2 , a kilometer or less at the altitudes of air shower maximum development. Particle trajectories form partial arcs around the field lines before they lose enough energy to drop out of the shower. The meter-scale longitudinal thickness of the shower particle 'pancake' is comparable to radio wavelengths below several hundred MHz; thus the ensemble behavior of all of the cascade particles yields forward-beamed synchrotron emission which is partially or fully coherent in the radio regime. Therefore, the resulting radio impulse power grows quadratically with primary particle energy, and at the highest energies, yields radio pulses that are detectable at large distances. Current systems under development for detection of thes...

Method for high precision reconstruction of air showerXmaxusing two-dimensional radio intensity profiles

“…CROME [33] and ANITA [34] detected GHz emission at the Cherenkov angle, which can be interpreted as relativistic compression of the airshower radio emission. Full Cherenkov rings were found by LOFAR in the 110-190 MHz range [21].…”

Measuring a Cherenkov ring in the radio emission from air showers at 110–190MHz with LOFAR

“…Hereby emission is amplified for a specific angle with respect to the shower axis [9,10]. Time compression of the pulse also allows shower emission to be detectable at GHz frequencies where coherence would otherwise be lost [11,12]. The inset shows the construction of a high-band element in which the bow-tie shaped antennas are mounted before they are packed in weather-proof foil.…”

Characteristics of Four Upward-Pointing Cosmic-Ray-like Events Observed with ANITA

“…We have previously reported the observation of ultrahigh-energy (UHE) cosmic-ray (CR) air showers detected from suborbital altitudes with the Antarctic Impulsive Transient Antenna (ANITA) balloon payload [1] during our first flight in 2007 [2]. The initial blind neutrino-search analysis that led to their identification in the data found 16 events in a signal box with an expected background of 1.6 events.…”