Black holes (BHs) and ultra-high energy cosmic rays (UHECRs) are two extremes of the universe that link particle physics and astrophysics. BHs are the most efficient power generators in the universe while UHECRs are the most energetic particles ever detected. As we showed previously, a major fraction of the power of BHs is channeled into radio-emitting plasma jets, which are also efficient particle accelerators. Are BHs also responsible for UHECRs? This long-standing question could be answered soon, through the dawn of cosmic ray astronomy. The giant Auger observatory has now shown for the first time that the arrival directions of UHECRs are non-isotropic, potentially pointing back to their sources of origin. BHs turned out to be major suspects, but other sources could still also be responsible. To address this conclusively and to establish cosmic ray astronomy as a productive new field in the coming years, we need to increase statistics, expand current observatories, and have complementary all-sky radio surveys available to identify sources, since radio emission traces particle acceleration sites. Here, techniques pioneered by the Low-Frequency Array (LOFAR) promise major advances. First of all, working on LOFAR we uncovered a new technique to detect UHECRs with radio antennas and verified it experimentally. The technique promises to increase the number of high-quality events by almost an order of magnitude and provides much improved energy and direction resolution. We now want to implement this technique in Auger, combining LOFAR and AUGER know-how. Secondly, LOFAR and soon other SKA pathfinders will significantly improve all-sky radio surveys with high sensitivity, resolution, and image quality. Hence, we will use LOFAR to understand the astrophysics of UHECR source candidates and compile a radio-based catalog thereof. We start with jets from BHs and move later to other sources. Together this will allow us to identify UHECR sources and study them in detail.
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