Radio Detection of Cosmic Rays

Introduction

Short nanoseconds radio frequency (RF) pulses are emitted from cosmic ray (CR) induced air showers which can be recorded with an array of antennas to determine properties of the primary cosmic rays, referred as the "Radio-Detection (RD)" of cosmic rays. The early experiments such as LOPES, CODALEMA, Tunka-Rex, LOFAR and AERA have already demonstrated the RD of cosmic rays around and above 100 PeV. However, below 100 PeV, strength of the induced signal starts falling beneath the overwhelming background noise, hence the detection becomes gradually more challenging. We aim to address these technical challenges in the RD-technique with sophisticated beamforming (BF) approach to lower the detection threshold to as low as possible. We have developed tools in the NuRadioReco framework to execute near-field beamforming with the CR-induced RF-signals and designing an optimized array-layout with these tools for the radio upgradation of the GRAPES-3 experiment, in India. The Square Kilometre Array (SKA-Low) array commissioned as an exceptionally dense array with thousands of SKALA-antennas, has commendable potential to suppress the RD threshold down to as low as 1 PeV using BF-technique. This may also facilitate detection of gamma rays at PeV energies or the PeVatrons which may serve as a distinctive science case for SKA-Low array. We are working on to extend the BF-approach and generalize the tools being initially developed for the radio-extension of the GRAPES-3 experiment to the cosmic ray science-case of the SKA-Low array.

FIG. 1. Tentative array layout of the proposed phased array system. (To be further extended)

FIG. 2. Radio footprint of a vertical 1016 eV Iron shower in 30-80 MHz band (array in FIG.1 is also portrayed in the footprints)

FIG. 3. Radio footprint of the same shower in 300-500 MHz band, apparition of Cherenkov ring is evidential. (array in FIG.1 is also portrayed in the footprints

The sources of radio impulses from CR air-showers are relatively extended, relativistically moving and generally resides in the radiative near field region (Fresnel region) of the detection site. In comparison to the applications in radio astronomy, the point source and plane wave approximations do not hold anymore and the signal strength drops off rapidly away from the core location, shown in figure below. The nature of these weak CR-induced signals differs substantially from faint astronomical signals, so detection of the former is comparatively more challenging. Due to these technical complicacies, highly sensitive radio telescopes or interferometers such as the LOFAR, LOFAR Super Station (LSS), MWA have not applied the BF-technique in the RD of cosmic rays.

Most of these instruments serve primarily for radio astronomical purposes and cosmic ray detection as secondary objective, that is also at a higher detection threshold.

FIG. A typical CoREAS simulation for a 10 PeV Proton shower with core at (0, 0). Left Profile depicts left-quadrant of the simulated radio footprint. Right Profile portrays the simulated signal pulses at different antenna-locations after processed through SKALA-detector response in NuRadioReco, 100-200 MHz band. No noise has been injected. Note that, the strength of the signal falls of rapidly away from the core (0, 0).

About Me

Subhadip Saha

Prime Minister's Research Fellow (Cycle 8, December, 2021)

Department of Physics, Indian Institute of Technology, Kanpur.

Supervisor

Prof. Pankal Jain

Head, Department of Space Science & Astronomy

Indian Institute of Technology, Kanpur.

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