- Email: [email protected]
ANITA and the Highest Energy Cosmic Rays
Available online at www.sciencedirect.com
Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 553 www.elsevier.com/locate/npbps
ANITA and the Highest Energy Cosmic Rays E.W. Grashorn for the ANITA Collaboration Department of Physics, Ohio State University, 191 W. Woodruff Ave. Columbus OH 43210
Abstract ANITA 1 observed 16 cosmic ray triggered radio impulses during 35 days aloft in its 2007-2008 flight. Keywords: Ultra-high Energy, Cosmic Ray, Radio The origin of ultra-high energy cosmic rays (UHECR) remains a mystery, which could be solved with increased statistics on the highest energy events. The primary difficulty is the extreme rarity of events at these energies (≥ 3 × 1019 eV, unaffected by galactic and extragalactic magnetic fields). Cosmic rays have been detected for decades via impulsive radio geosynchrotron emission [1], which arises when the electronpositron particle cascade initiated by a primary cosmic ray encounters the Lorentz force in the geomagnetic field. Previous observations were not in this crucial energy range, UHECRs could point to their sources. The Antarctic Impulsive Transient Antenna (ANITA [1]) long-duration balloon payload circles the Antarctic continent at altitudes of 35-37 km, observing 1.5M km2 of ice. During flight, ANITA records all nanosecondduration radio impulses over a 200-1200 MHz band. The direction of detected signals, determined by pulsephase interferometric mapping, is localized to an angular ellipse of 0.3◦ × 0.8◦ (elevation × azimuth) [1] which is projected back onto the continent to determine the origin of the pulse. ANITA’s mission is the detection of ultra-high energy neutrinos via linearly-polarized coherent radio Cherenkov pulses. Most impulsive signals are of known anthropogenic origin and rejected. For its first flight ANITA’s trigger system was sensitive to both vertically and horizontally polarized (Vpol and Hpol) signals. Radio pulses of neutrino origin favor Vpol due to the geometry of the radio Cherenkov cone; the Hpol information was used as a sideband test
for our blind neutrino analysis. Our results were surprising: while the neutrino analysis (Vpol) gave a null result, 16 Hpol events were observed [1]. These events are randomly distributed around ANITA’s flight path uncorrelated in location to human activity or to each other, but with similar radio pulse profiles and frequency spectra (Fig. 1). Their measured planes of polarization are
Figure 1: The time domain impulses (top), and the averaged frequency spectra (bottom) for the cosmic ray events observed by ANITA.
perpendicular to the local geomagnetic field, indicating UHECR origin. With two exceptions, the events reconstruct to locations on the surface of the ice (the exceptions originate above the horizon, but below the horizontal [1]). This is the first radio-triggered detection of UHECRs and the first radio detection of UHECRs above 1019 eV, which suggests this method as a competitive method of UHECR particle astronomy. After optimization for UHECR observation, we estimate a new 30 day flight of ANITA could detect hundreds of geosynchrotron events, with 60-80 above 1019 eV, and ∼ 10 above the nominal GZK cutoff energy [1]. [1] S. Hoover, et al., Phys. Rev. Lett. 105 (2010) 151101.
0920-5632/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2012.09.190
Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 553 www.elsevier.com/locate/npbps
ANITA and the Highest Energy Cosmic Rays E.W. Grashorn for the ANITA Collaboration Department of Physics, Ohio State University, 191 W. Woodruff Ave. Columbus OH 43210
Abstract ANITA 1 observed 16 cosmic ray triggered radio impulses during 35 days aloft in its 2007-2008 flight. Keywords: Ultra-high Energy, Cosmic Ray, Radio The origin of ultra-high energy cosmic rays (UHECR) remains a mystery, which could be solved with increased statistics on the highest energy events. The primary difficulty is the extreme rarity of events at these energies (≥ 3 × 1019 eV, unaffected by galactic and extragalactic magnetic fields). Cosmic rays have been detected for decades via impulsive radio geosynchrotron emission [1], which arises when the electronpositron particle cascade initiated by a primary cosmic ray encounters the Lorentz force in the geomagnetic field. Previous observations were not in this crucial energy range, UHECRs could point to their sources. The Antarctic Impulsive Transient Antenna (ANITA [1]) long-duration balloon payload circles the Antarctic continent at altitudes of 35-37 km, observing 1.5M km2 of ice. During flight, ANITA records all nanosecondduration radio impulses over a 200-1200 MHz band. The direction of detected signals, determined by pulsephase interferometric mapping, is localized to an angular ellipse of 0.3◦ × 0.8◦ (elevation × azimuth) [1] which is projected back onto the continent to determine the origin of the pulse. ANITA’s mission is the detection of ultra-high energy neutrinos via linearly-polarized coherent radio Cherenkov pulses. Most impulsive signals are of known anthropogenic origin and rejected. For its first flight ANITA’s trigger system was sensitive to both vertically and horizontally polarized (Vpol and Hpol) signals. Radio pulses of neutrino origin favor Vpol due to the geometry of the radio Cherenkov cone; the Hpol information was used as a sideband test
for our blind neutrino analysis. Our results were surprising: while the neutrino analysis (Vpol) gave a null result, 16 Hpol events were observed [1]. These events are randomly distributed around ANITA’s flight path uncorrelated in location to human activity or to each other, but with similar radio pulse profiles and frequency spectra (Fig. 1). Their measured planes of polarization are
Figure 1: The time domain impulses (top), and the averaged frequency spectra (bottom) for the cosmic ray events observed by ANITA.
perpendicular to the local geomagnetic field, indicating UHECR origin. With two exceptions, the events reconstruct to locations on the surface of the ice (the exceptions originate above the horizon, but below the horizontal [1]). This is the first radio-triggered detection of UHECRs and the first radio detection of UHECRs above 1019 eV, which suggests this method as a competitive method of UHECR particle astronomy. After optimization for UHECR observation, we estimate a new 30 day flight of ANITA could detect hundreds of geosynchrotron events, with 60-80 above 1019 eV, and ∼ 10 above the nominal GZK cutoff energy [1]. [1] S. Hoover, et al., Phys. Rev. Lett. 105 (2010) 151101.
0920-5632/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2012.09.190