ALERT, the Apertif Lofar Exploration of the Radio Transient Sky, will investigate the Northern Sky with unprecedented speed and precision, to determine the nature of some of the most powerful flashes and explosions in the Universe.

FRB 121102

We observed the repeater FRB 121102 (Spitler et al. 2016) while commissioning the Apertif Radio Transient System (ARTS; van Leeuwen 2014) on Westerbork. Starting at UTC 2017-08-31 06:23, we recorded 300 MHz of bandwidth from the central set of dipoles in the Apertif phased array feeds, from a single dish. The observation was the first with ARTS toward this source and lasted for 2 hours.

Data were coherently dedispersed at the known DM of 557 pc/cc, and channelised. These filterbank data were next searched offline for radio bursts, both in time and over a limited dispersion-measure range. At barycentric MJD 57996.2656372 ARTS detected a bright FRB, its first, of fluence 35 +- 10 Jy ms, peak flux 24 +- 7 Jy, and a FWHM of 1.3 +- 0.2 ms, at optimized DM of 555 pc/cc. No further bursts were discovered in the observing session.

Following the detection of multiple bright pulses from FRB 121102 at higher frequencies with the Green Bank Telescope (ATel #10675) on 26 August, this detection indicates the FRB source may be in a phase of outburst.

To start estimation of the false-positive rate, we pointed with the same setup off-source for 2 hours. Our pipeline reported ~25 candidates at the same S/N ratio as the aforementioned burst, or higher. Visual inspection showed all of these to be either RFI, or not persistent after varying the time and/or frequency sampling, in constrast to the burst above. In 10 further hours of on-source follow-up, the same approach also did not detect any convincing events.

This project received funding from the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013) / ERC Grant Agreement n. 617199, from the Netherlands Organisation for Scientific Research (NWO-M 614.061.613), from the Netherlands Institute for Radio Astronomy ASTRON, from the Netherlands Research School for Astronomy NOVA (ARTS), and from the Netherlands eScience Center (ASDI 027.015.G09).