ALERT -- The Apertif-LOFAR Exploration of the Radio Transient Sky

© JvL

In ERC Consolidator project ALERT, we will survey the radio sky to catch extreme astrophysical explosions. Up to recently our view of that radio sky was largely static, based on a Universe that is unvarying on million-year timescales. But if you take build a high speed radio camera with frames every millisecond, as illustrated in the figure, then you suddenly find flashes of bright, brief radio emission.

These snapshots are monochromatic but if one takes that bright pixel and looks at its behaviour of color versus time, an important effect shows up: dispersion. At lower observed frequencies, the radio waves traverse the medium more slowly, and arrive later. This slope immediately indicates how many electrons the burst encountered.

Some bursts travel through less than the maximum electron depth expected in our Galaxy, and are thus produced a few kpc away. These bright bursts are produced by intermittent pulsars, sometimes just one pulse every hour.

But some other bursts encountered 30 times more electrons than expected from our Milky Way. That suggests a distance of over 2 Gpc. Several of these extragalactic, or "Lorimer" bursts are now known. They must be extremely luminous. Their nature is a complete mystery.

By finding both kinds of fast radio transient sources, using Apertif and LOFAR, ALERT aims to determine what powers these populations of extragalactic bursts and intermittent pulsars.