Sometimes, being an astrophysicist is an exercise in international detective work. Piecing together the evidence is complicated, because observations are often made after a key event, the experiments are not generally repeatable and, when it comes to telescopes, it is all about location, location, location. Three papers1–3 in this issue of Nature report exactly this situation in the detection of a phenomenon called a fast radio burst (FRB) coming from a source in our Galaxy. Intriguingly, the FRB was accompanied by a burst of X-rays4–6. The discovery was made and understood by piecing together observations from multiple space- and ground-based telescopes, and should help us to work out the mechanisms that drive these spectacular events.
The name ‘fast radio bursts’ is a good description of what they are: bright bursts of radio waves with durations roughly at the millisecond scale. First discovered7 in 2007, their short-lived nature makes it particularly challenging to detect them and to determine their position on the sky. The smorgasbord of theories8 that has been proposed to explain FRBs has, until recently, outpaced our discovery of actual FRB events. The majority of these theories invoke some kinds of stellar remnant as FRB sources. In particular, highly magnetized young neutron stars known as magnetars have emerged as leading candidates, because their strong magnetic fields could act as ‘engines’ that drive FRBs.A key approach to testing these progenitor theories is to associate FRBs with other astronomical phenomena. It is therefore crucial to narrow down the potential positions of FRBs to small regions of the sky, so that the associations are unambiguous. Until now, however, there has been no observational evidence directly linking FRBs with magnetars. The detection reported in the three new papers offers the first such evidence, thereby providing vital clues that will help us understand the origins of at least some FRBs.
The timeline for the observations of these results is as follows. On 27 April 2020, two space observatories — the Neil Gehrels Swift Observatory and the Fermi Gamma-ray Space Telescope — detected multiple bursts of X-ray/γ-ray emissions coming from the Galactic magnetar SGR 1935+2154. On the following day, the same region of the sky was in view of ground-based telescopes in the Western Hemisphere. Two radio telescopes — the Canadian Hydrogen Intensity Mapping Experiment (CHIME) and the Survey for Transient Astronomical Radio Emission 2 (STARE2), in the United States — detected an FRB from that sky region. The FRB has since been named FRB 200428.The CHIME team was the first to announce the detection, and it loosely localized the event to a region that contains SGR 1935+2154 — thereby hinting at the exciting first association of an FRB with a known Galactic source. These findings are reported1in Nature by the CHIME/FRB Collaboration. The announcement prompted the STARE2 scientists to check their own data, and to confirm the discovery of FRB 200428; these findings are described in Nature by Bochenek et al.2. However, Bochenek and colleagues found the FRB to be about 1,000 times brighter than had been announced by the CHIME/FRB Collaboration. This discrepancy was resolved after the CHIME/FRB Collaboration recalibrated its data, whereupon it found the brightness to be the same as that determined by Bochenek and co-workers1,2.In addition, several space telescopes and detectors reported an X-ray burst coming from SGR 1935+2154 at the same time as FRB 200428. These included the European Space Agency’s INTEGRAL space telescope4, Russia’s Konus detector aboard NASA’s Wind spacecraft5, and the Chinese Insight space observatory6.
Later that day, the sky region of interest came into view of the extremely sensitive Five-hundred-meter Aperture Spherical Radio Telescope (FAST) located in China, which had been observing SGR 1935+2154 in the previous weeks. As reported by Lin et al.3, FAST did not detect any FRB activity coming from SGR 1935+2154, even though the Fermi Gamma-ray Space Telescope detected multiple X-ray bursts during that time. However, two days later, FAST detected an FRB at the same location as FRB 200428, and localized the event to a small region around SGR 1935+2154. Each of the experiments described above thus played a part in the detection of FRB 200428, the measurement of its brightness, and the association of the FRB with SGR 1935+2154.FRB 200428 is the first FRB for which emissions other than radio waves have been detected, the first to be found in the Milky Way, and the first to be associated with a magnetar. It is also the brightest radio burst from a Galactic magnetar that has been measured so far — which potentially solves a key puzzle in this field. Before the discovery of FRB 200428, the absence of X-ray and γ-ray bursts from repeating FRBs lent weight to certain magnetar theories of the origins of FRBs. But because no bright radio bursts had been observed coming from Galactic magnetars, it seemed unlikely that magnetars could be FRB sources at all. The discovery of FRB 200428 proves that magnetars can indeed drive FRBs. Moreover, FRB 200428 is the first Galactic radio burst that is as bright as the FRBs observed in other, nearby galaxies, which also provides much-needed evidence that magnetars could be the sources of extragalactic FRBs.Intriguingly, there are several mechanisms by which magnetars can drive FRBs, each of which has a distinct observational signature. The new results thus open up a host of exciting problems to explore. For example, what theoretical mechanism could give rise to such bright, yet rare, radio bursts with X-ray counterparts? One promising possibility is that a flare from a magnetar collides with the surrounding medium and thereby generates a shock wave9,10 (Fig. 1). Observations of nearby rapidly star-forming galaxies will be crucial for finding events similar to FRB 200428, to help pin down the actual mechanism.
Figure 1 | A potential mechanism for the formation of fast radio bursts. A bright, millisecond-long burst of radio waves, known as a fast radio burst (FRB), has been detected1–3 coming from a highly magnetized stellar remnant (a magnetar) in our Galaxy. The radio waves were accompanied by X-ray emissions4–6. One possible mechanism9,10 for the formation of such an FRB is that the magnetar produces a submillisecond-long flare of electrons and other charged particles, which collides with particles that had been emitted from previous flares (note that the collision occurs a great distance away from the magnetar; this distance is not shown to scale). The collision generates an outward-moving shock front, which in turn produces huge magnetic fields. Electrons gyrate around the magnetic field lines, and thereby emit a burst of radio waves. The shock wave also heats the electrons, which causes them to emit X-rays.
Other magnetar-driven FRB mechanisms would produce accompanying neutrino bursts11. There is therefore scope for truly multi-messenger astronomy — the coordinated use of fundamentally different signal types, such as electromagnetic radiation and neutrinos — to provide another key clue to this cosmic mystery. Moreover, the discovery highlights the need for international scientific cooperation in astronomy, and for sky coverage from multiple locations.
Source: http://feeds.nature.com/~r/nature/rss/current/~3/l78qH5-yRqE/d41586-020-03018-5
