This award funds the theoretical research activities of Professor Glennys Farrar at New York University.
Ultra-high-energy cosmic rays (UHECR's) are cosmic rays (i.e., subatomic particles) with extreme kinetic energies that not only vastly exceed their rest masses, but also exceed the energies typical of other cosmic rays. These cosmic rays are significant for a number of reasons. First, the origin of their huge energies remains a mystery which touches on numerous aspects of astrophysics. Second, these huge energies vastly exceed those accessible in traditional accelerator experiments. Finally, the energies associated with such UHECR's often violate the so-called "GZK bound" which provides a theoretical upper limit on the energies that cosmic rays may have. Understanding how such UHECR's manage to violate the GZK bound is an important area of research. In her work, Professor Farrar aims to develop combinations of astrophysics analysis methods in order to constrain the sources and possible composition of UHECR's, using recent data from the AUGER experiment. She also wishes to use these techniques in order to map out the galactic magnetic field by exploiting massive new data sets of rotation measures of extragalactic sources in combination with synchrotron emission data from the WMAP experiment.
This work is also envisioned to have significant broader impacts. First, it will lead to the development of computational infrastructure (including the development of state-of-the-art techniques for optimizing fits involving many parameters and hundreds of millions of measured data points, new techniques for rapid simulation of particle cascades, and new probabilistic methods of event reconstruction that have implications for pattern recognition problems more broadly). Second, the PI will continue her work with the New York Schools Cosmic Particle Telescope (NYSCPT) Project (a partnership with local New York City schools): this includes further development of the next phase of NYSCPT as well as delivering public lectures on NYSCPT and mentoring NYSCPT participants.
The unifying theme of most of the research supported under this grant has been to discover the origin and nature of Ultrahigh Energy Cosmic Rays (UHECRs), and to exploit UHECRs to study particle physics at energies inaccessible to accelerator experiments. The Pierre Auger Observatory has made measurements of an unprecedented quality, of the particle showers produced when UHECRs collide in the upper atmosphere. It has become increasingly clear that the size of the signal at the ground is much larger than predicted by conventional particle physics, however it was not previously known whether this discrepancy might be curable by assuming the UHECRs are heavy nuclei or making some clever combination of minor "tweaks" in the particle physics modeling. Farrar and her postdoc Jeff Allen (supported by other funding) have now shown that no combination of composition and conventional physics can explain the totality of the observations – both the longitudinal development in the atmosphere and the strength of the ground signal (Fig. 1). They have identified what phenomenon is required to explain the data (a suppression of the production or decay of π0 mesons) and have given examples of possible new physical mechanisms that can explain the data. They have also identified ways that future observations can discriminate between different mechanisms. How and where UHECRs are produced is one of the major questions in Astrophysics. In order to determine a UHECR’s souce, its arrival direction must be corrected for deflection in the Milky Way’s magnetic field. The Galactic magnetic field (GMF) was not previously well-enough known for this to be reliably done, and a major achievement under this grant has been to determine the GMF. Normally, a magnetic field is measured in situ but of course it is impossible to carry a magnetometer into the Galaxy to measure its field. So, indirect methods must be used. The PI and postdoc Ronnie Jansson pioneered a powerful new approach which takes advantage of the existence of two different types of observables: one sensitive to the line-of-sight component and the other to the transverse component of the magnetic field. Based on theoretical considerations, we formulated a general description with a total of 35 parameters to characterize the large-scale coherent field, small scale random field, and a new component we called a striated field. The coherent field has disk and halo components along the lines considered before, as well as – for the first time – allowing for the possibility of a vertical component extending to large distances above the disk. We allowed for a striated field, because that can arise when a random field is stretched or compressed preferentially in one direction, and constrained the 35 parameters of the model using over 10,000 observables whose variance, which is primarily astrophysical, we inferred from the data (Figs 2-4). The new GMF model fits data dramatically better than any previous model, so finally it is possible to predict the arrival directions of UHECRs from interesting source candidates (Fig. 5). Prior to this project, Farrar and A. Gruzinov showed that conventional "favorite" candidates for accelerating UHECRs – Active Galactic Nuclei (AGNs) and Gamma Ray Bursts – have difficuly explaining the total flux of UHECRs and the observed density of sources if UHECRs are protons and they argued for the existence of a previously undetected class of flares, as intense as the most powerful AGNs but with lifetimes of a few months or years. Farrar and graduate student Sjoert vanVelzen searched archival SDSS data and found two clear-cut examples of such flares, most likely caused by the tidal disruption of a star by the supermassive black hole at the center of a galaxy. VanVelzen and Farrar made the first solid observational determination of the rate of these flares. Two other examples of probable Tidal Disruption Flares have been discovered since then with the Swift-BAT satellite and Pan-Starrs; in the former case the jet is apparently beaming directly toward us, giving direct evidence of the existence of a jet potentially capable of accelerating UHECRs. Farrar has modeled the spectral energy distributions of the SDSS flares and obtained an excellent fit; the results indicate the flares readily surpass the minimum power requirement for accelerating UHECRs (Fig. 6). Other projects supported in part by this grant have been using the Chandra X-ray satellite to investigate possible UHECR sources, simulating the merger of the Bullet cluster in order to constrain possible long-range modifications of Gravity or exotic interactions of Dark Matter, improving the limit on the mass of a possible long-lived or stable gluino (a new particle predicted by supersymmetry) and work by postdoc Jon Roberts to explain the PAMELA positron excess by focussing in the Solar System magnetic field; he predicts that due to the changing solar cycle, AMS should see a decreased positron flux compared to PAMELA.
