XV Latin American Regional IAU Meeting (Cartagena de Indias, Colombia, 3-7 October 2016) Editors: Mario Armando Higuera Garzón & Santiago Vargas Domínguez RevMexAA (Serie de Conferencias), 49, 7–10 (2017) AN OVERVIEW OF FAST RADIO BURSTS Gustavo E. Romero 1,2 RESUMEN Presento una breve descripci´ on del estado actual del conocimiento de las fulguraciones r´apidas de or´ ıgen c´ osmico detectadas en radio frecuencias en a˜ nos recientes. Se trata de eventos epis´ odicos de producci´on de radio ondas que duran alrededor de 1 ms. Su origen parece ser extragal´ actico. En este art´ ıculo resumo la evidencia que apoya esta afirmaci´ on y discuto algunos modelos propuestos a la fecha para explicar la naturaleza de estos acontecimientos. ABSTRACT I offer a short review of the current understanding of fast radio bursts: episodic flares of radio waves detected at low frequencies with durations of about 1 ms. These events seem to have an extragalactic origin. I summarize the evidence supporting this statement and I discuss some of the theoretical models proposed so far to explain their nature. Key Words: acceleration of particles — radiation mechanisms: non-thermal — relativistic processes — turbulence 1. INTRODUCTION Fast radio bursts (FRBs) are transient flashes of cosmic radio waves with durations of a few ms that are detected at cm-wavelengths. The first event was discovered by Lorimer et al. (2007) using archival data from a pulsar survey at 1.4 GHz from 2001. The identification was possible because of the large dispersion measure (DM) of the event, which showed a quadratic frequency dependent delay, suggesting dispersion of the signal through a cold plasma. The event occurred in the direction of the Magellanic clouds. The Galactic DM from this line of sight is estimated to be of 25 cm -3 pc, whereas the FRB presented DM= 375 cm -3 pc. The largest DM of a pulsar in the Small Magellanic Cloud is of 205 cm -3 pc. Hence, whatever caused the FRB is thought to be behind this galaxy, with un upper limit of z ∼ 0.3. The duration of the event was 5 ms, with a flux den- sity of 30 ± 10 Jy. It was a single spike, with no rep- etition. Subsequent monitoring of the region showed no further events in the DM range of 0 to 500 cm -3 pc. Since Lorimer et al.’s discovery, about 20 FRBs have been detected, most of them by Parkes radio telescope (see the list in the FRBCAT: Petroff et al. 2016 3 ). A repeating FRB has been also found with 1 Instituto Argentino de Radioastronom´ ıa (IAR), CONICET-CIC, CC 5, 1894 Villa Elisa, Provincia de Buenos Aires, Argentina (gustavo.esteban.romero@gmail.com). 2 FCAyG, Universidad Nacional de La Plata, Paseo del Bosque s/n, 1900 La Plata, Buenos Aires, Argentina (romero@fcaglp.unlp.edu.ar). 3 The catalogue is available on-line at Arecibo 305-m telescope in Puerto Rico (Spitler et al. 2016) and a counterpart for this source was re- cently detected (Chatterjee et al. 2017). If these bursts occur, as everything indicates, at cosmologi- cal distances, then they are the most luminous radio events in the known universe. Dedicated instruments are under construction with the aim of increasing the number of detections. In the meanwhile, the ulti- mate nature of these events remains an utter mys- tery. In what follows I will review the main facts and the current best ideas about FRBs and their origin. 2. FACTS The DM is defined as the integrated column den- sity of free electrons along the light of sight to a source: DM =  d 0 n e dl. (1) The radiation of an extragalactic source pass through the ionized gas of the interstellar (ISM) and the intergalactic medium (IGM), where the free elec- trons make the group velocity of the signal frequency dependent. Higher radio frequency radiation arrives, then, before the radiation emitted at the same time but lower frequencies from the source. All observed FRBs show a dispersion delay δt ∝ DM ν -2 . This is in agreement with the effects of a cold plasma, sug- gesting an effect of the IGM. The DM obtained for FRBs are in the range 375-1600 cm -3 pc which cor- respond to distances of 0.5-4 Gpc in a FLRW metric, http://www.astronomy.swin.edu.au/pulsar/frbcat/. 7