The origin of the ultra-short but extremely high-energy cosmic radio flashes is one of the great mysteries of astronomy. But now it could be resolved. For the first time astronomers detected such a “fast radio burst” not in another galaxy but in our Milky Way – and this enabled them to narrow down its source more precisely. Accordingly, this short radio pulse emanated from a magnetar about 30,000 light years away. It is a neutron star that rotates quickly and has extremely strong magnetic fields. These celestial objects have long been suspected as possible sources of the radio flashes, now this seems to be confirmed.
Fast radio bursts (FRB) only last a few milliseconds, but during this time they release as much energy as our sun does in a whole day. It wasn’t until 2007 that the Parkes Radio Telescope in Australia happened to catch some of these ultra-short cosmic radio pulses. However, because no other radio telescope initially detected such flashes and there was no astronomical explanation for the phenomenon, it remained unclear whether these flashes were actually of cosmic origin – it was possible that it was just atmospheric disturbances or signals from Earth. In 2014, however, the Arecibo radio telescope in Puerto Rico also picked up such a signal. Since then, astronomers have detected dozens of fast radio bursts with a wide variety of radio telescopes. But because all of them came from other galaxies and thus great distances, their source remained unclear. However, the polarization characteristics of the radio signals indicated that they came from a strongly magnetized environment.
Magnetar in the Milky Way Center as the originator
Now, for the first time, astronomers have succeeded in detecting a fast radio burst from our own galaxy. This signal was captured by the Canadian Hydrogen Intensity Mapping Experiment (CHIME) in British Columbia. This system consists of four semicircular arched reflector ensembles the size of a football field, which continuously search the sky in the frequency range from 400 to 800 megahertz. On April 28, 2020, the CHIME telescope registered a short but strong radio pulse with two clearly defined peaks only milliseconds apart. Closer analysis of the signal showed that this radio pulse had all the classic characteristics of an FRB. However, in contrast to the previous radio pulses of this type, this one came from an area only around 30,000 light-years away near the center of the Milky Way. This is the first fast radio burst from our own galaxy, as the researchers from the CHIME collaboration explain.
More importantly, astronomers were able to trace this radio signal back to an astronomical source for the first time. Because the day before the ultra-short radio pulse, a magnetar had awakened to new activity at its place of origin. The neutron star, christened SGR 1935 + 2154, repeatedly emitted strong pulses of X-rays that were detected by several space telescopes. “There was some excitement in the astronomical community over this magnetar that had suddenly become so active in the X-ray field, and our collaboration was asked to open their eyes,” said Kiyoshi Masui of the Massachusetts Institute of Technology (MIT) and a member of the CHIME collaboration . When the CHIME telescope actually detected a radio burst from this magnetar the next day, the sensation was perfect. This is the first time that a fast radio burst has been associated with bursts of radiation in other wavebands, and the first time that its source can be identified. “If this radio pulse were to come from any other object in the vicinity of the magnetar, it would be a pretty big coincidence,” says Masui.
Mechanism still unclear
Similar to the fast radio bursts from extragalactic sources, the radio pulse, baptized FRB 200428, also released large amounts of energy in a very short time. “Only the measurement of its brightness confirmed that this was not a normal radio pulse,” explains Masui. According to the calculations of the CHIME team, the short signal from the center of the Milky Way released the energy of 10 to the 34th power in the radio wave range – that is 3000 times more than any other radio pulse previously registered by a magnetar in our galaxy. The enormous intensity of this radio burst was also confirmed by measurements from another radio telescope, the Survey for Transient Astronomical Radio Emission 2 (STARE2) in the USA. The STARE2 team headed by Christopher Bochenek from the California Institute of Technology in Pasadena also detected FRB 200428 and determined that it was several thousand times more energetic than any radio pulse from the cancer pulsar – the strongest known radio source in the Milky Way to date. “The event on April 28, 2020 at Magnetar SGR 1935 + 2154 clearly demonstrates that magnetars can produce far stronger radio pulses than have been observed in our galaxy up to now,” write the researchers of the CHIME collaboration.
According to astronomers, this could confirm that at least some fast radio bursts are due to magnetars. FRB 200428 is still one to two orders of magnitude weaker than the extragalactic radio flashes captured so far. Nevertheless, the current observation, coupled with the FRB-typical features of this radio flash, suggest that magnetars can trigger such fast radio bursts, according to the researchers. The mechanism by which such a neutron star produces these radio pulses is, however, unclear: “The models for magnetars as sources for fast radio bursts form two main classes,” explains the CHIME team. On the one hand, the intense radio bursts could arise within the magnetosphere of the magnetar, and the ultra-short duration and the sharp separation of the two peaks of FRB 200428 would suit this. According to the other class of models, the fast radio burst only arises in the vicinity of the magnetar, as electrons and other particles ejected during radiation bursts collide with remnants of earlier bursts at a distance from the magnetar. This creates a strongly magnetized shock front from which the radio waves then emanate.
Astronomers do not yet know which mechanism actually takes place on magnetars and whether this really leads to the observed fast radio bursts. They also suspect that the strongest radio flashes detected so far may be due to another source. “We keep our eyes peeled for more active magnetars, but our main focus now is on exploring this one source more closely and finding out what it can tell us about the origin of the fast radio bursts,” says Masui.
Source: The CHIME / FRB Collaboration, Nature, doi: 10.1038 / s41586-020-2863-y; Christopher Bochenek (California Institute of Technology, Pasadena) et al., Nature, doi: 10.1038 / s41586-020-2872-x