Cosmic rays are rays of particles and radiation emitted from various astrophysical environments, for instance shocks, Active Galactic Nuclei (AGN), Puslar Wind Nebulae and Supernova Remnants. We can observe these cosmic rays from the Earth, and their spectrum takes on a distinctive power-law shape with a peak at a few GeV (billions of electron volts, eV), and ‘knee’ features around 4 and 400 PeV (1 PeV = 1015 eV = 1 000 000 000 000 000 eV), with an ‘ankle’ at 1 EeV (1 EeV = 1018 eV). The lower energy Cosmic Rays are thought to originate within our galaxy, while higher energy ones come from further afield, providing astronomers with a different way to probe the cosmos, without using conventional observations of electromagnetic radiation.
These cosmic rays are a useful tool for astronomers as they present the Universe in a different way to conventional electromagnetic observations. The features and underlying physics highlighted by conventional photons compared to cosmic rays tend to be different. For example, photons are excellent to observe thermalized regions and accretion disks (e.g. in X-rays), or regions of star formation (optical and UV radiation), while cosmic rays tend to trace different underlying physics – notably astrophysical systems where non-thermal processes dominate, particularly regions of particle acceleration. Astronomers can therefore use cosmic rays alongside conventional methods to get a better understanding of a range of physical processes in various environments (see also multi-messenger astronomy).
Cosmic rays are composed of gamma-rays (high-energy photons), nuclei (of elements up to Iron), electrons, positrons and protons. The cosmic rays impact on the Earth’s atmosphere and collide with atmospheric nuclei, creating huge showers of secondary charged ‘daughter’ particles. These secondary particles travel faster than the speed of light in the atmosphere, and so create a faint glow of Cherenkov radiation which can be detected by ground-based telescope arrays (e.g. HESS, Magic) in a distinctive ring-pattern. Measuring the properties of this ring can tell us about the direction from which the original cosmic ray came, and its energy. Similar techniques can be used by detecting the Cherenkov light produced as the secondary particles travel through water (e.g. HAWC). Direct detection of the incident cosmic ray gamma-rays is also possible using orbital observatories, such as the Fermi satellite.
While these techniques have provided astronomers with important results and observations over the last few years, their ability to probe the very highest of energies is limited. Future detectors, such as the Cherenkov Telescope Array (CTA) will go some way to probe these higher energies, but there will always be the limitation of very few cosmic ray events to detect above hundred of TeV, causing random (poisson) fluctuations to dominate the observations and severely limit the science that can be done.
To overcome this problem, astronomers need to either make observations for a very long period of time, or use very large areas of telescopes to detect the very few cosmic ray events at high energies incident on the Earth per square kilometer per second. A novel idea proposed in a recent paper is to use smartphones (observing ultra-high energy cosmic rays with smartphones, Whiteson et al. 2015). This work shows that the device in your pocket can actually work quite effectively as a cosmic ray detector and, if less than 1% of smartphones collected cosmic ray data for only a few hours each day, their combined effort would have significantly better observational power than today’s state-of-the-art detectors (for example the Pierre Auger Observatory).
The idea is to use the camera of a typical smartphone. The authors look particularly at the iPhone 6 and the Samsung Galaxy S III, and show that the cameras in these devices are sensitive to some of the weakly charged particles produced in the cosmic ray showers (muons). The figure below shows how the phone camera responds to a beam of these muons passing through it, which can clearly be seen (top to bottom).
The group plan to provide an app which people can install on to their smartphone. When inactive, the phone will monitor the camera and analyze any images where a number of pixels are activated above some threshold. The data from these activated pixels only is then saved along with the time and geographical location and altitude of the phone, and uploaded to a remote central computer when a WiFi network is in range.
If enough phones in an area are involved in the scheme, data can be correlated to reproduce the ring pattern of muons resulting from an air shower, allowing the energy and source direction of the incident cosmic ray to be calculated. As there are so many phones spread across large areas of the Earth, their combined effort in a scheme like this could lead to a very large effective telescope area, able to detect many more cosmic ray events than is possible with atmospheric Cherenkov telescope arrays, providing useful sensitivities to higher energies.
Of course, there would be many technical details to be refined for such a scheme to work (such as making sure the timestamps in many different phones can be coordinated, and that geographical and altitude information is good enough), though tests have indicated that the systems available on most modern smartphones could be sufficient. Privacy concerns may also be an issue – while only activated pixels are saved during a cosmic ray shower detection, the idea that a phone’s exact location at a certain time is transmitted to some remote central computer may be uncomfortable for many users.
If these hurdles can be overcome, however, this could present a very exciting idea for the future of gamma-ray astronomy and citizen science!