Artist's conception of the recently discovered magnetar in the constellation Vulpecula. Green lines are magnetic field structures. Polar light rays denote radio, gamma ray and x-ray emissions. These arise after the crust-cracking star quakes. (From Phys. Today, January, 2021, p. 15)
Magnetars are highly magnetized young neutron stars each left over from the explosion of a star on the order of ten times the solar mass. They possess magnetic fields on the order of 100 million times stronger than the Earth's. The strain induced by the magnetic field increases until its abruptly relieved in as "starquake". This event then gives rise to the characteristic bursts of X-rays and γ-rays that signal the magnetar.
Of the approximately thirty magnetars currently known in our Galaxy and the Magellanic Clouds, five have exhibited transient radio pulsations. A subset of these are known as FRBs or fast radio bursts, which are millisecond-duration bursts of radio waves arriving from cosmological distances - some of which have been seen to repeat. A leading model for repeating FRBs is that they are extragalactic magnetars, powered by their intense magnetic fields.
In the latest issue of Physics Today the finding of a galactic FRB and its connection to a magnetar in our own galaxy has now been reported. (January, p. 13). This followed an international effort which determined that the signal received coincided with x-ray and gamma-ray emissions from the same location. Specifically, the site corresponds to a magnetar in the constellation Vulpecula. The findings provide new observational constraints on FRB origin theories, as well as a direction for future research.
On April 27, 2020, the Burst Alert Telescope aboard NASA's Neil Gehrels Swift observatory detected multiple bursts of x-rays and gamma rays from the magnetar SGR 1935+2154. This was in the context of the Canadian Hydrogen Intensity Mapping Experiment (CHIME) FRB project. The combination of this two-component bright radio burst and the estimated distance to SGR 1935+2154 together imply a burst energy at 400 to 800 megahertz of approximately 3 × 1034 erg, which is three orders of magnitude higher than the burst energy of any radio-emitting magnetar detected thus far. Such a burst coming from a nearby galaxy - at a distance of less than approximately 12 megaparsecs- would ordinarily be indistinguishable from a typical FRB.
However, the specific signatures- in terms of variation in brightness and frequency (see below):
Disclosed these signals were exceptional and not an ordinary FBR. In particular, the energy signal from SGR 1935+2154 indicated common features with giant radio pulses (GRPs) emitted by pulsars. A key to the team's finding was confirmation by the CHIME radio telescope in British COlumbia which detected the FRB coming from the same direction as the signal. (The signal, though well outside the radio telescope's bandwidth field, was only detected on account of its extreme brightness.) Further confirmation then followed thanks to the alert monitoring of data by a Caltech grad student, Christopher Bochenek. This was in the course of his daily inspection of data collected by radio telescopes and making up the university's Survey for Transient Radio Emission 2 catalog.
Despite the latest finding the physics of the connection between magnetars and FRBs remains sketchy. Thus, how exactly a magnetar can produce FRBs is still being debated. One proposed hypothesis is that a star quake (see top graphic) causes a magnetar to generate short-lived flares of electrons and other charged particles that collide with those emitted form previous flares - thus creating a shock front and enormous magnetic fields. In this dynamic, electrons swirling around the magnetic field lines emit bursts of radio waves while the heated electrons emit x-rays.
Another hypothesis is that the starquake triggers disturbances in the magnetic field lines near the magnetar's surface. These disturbances then induce relativistic particles to stream from the magnetosphere and then generate radio emissions.
According to Caltech grad student Bochenek (ibid.): "We knew the source would need to be a compact object and have strong magnetic fields and that magnetars and neutron stars create coherent radio emissions. But it's not like anyone expected magnetars to make such bright radio emission, before the discovery of FRBs."
All of which points the need for further research, specifically focused on the specific mechanisms and circumstances that drive magnetars to develop FRBs, and more generally determine how coherent radio emissions can arise under extreme conditions. For example, with respect to the original hypothesis of particle collisions noted above, is it possible for shocks to arise via a variation in the physical conditions (plasma) such that the following form for the Vlasov -Boltzmann equation applies:
St (<f>) = - e/m <{d E + v/c x d B} ¶ d f / ¶ v >
For which turbulent shocks can be engendered. A powerful shock front would be the necessary result and attendant on it intense magnetic fields with the potential to excite the FRBs. Any resolution of the mechanisms will certainly require many more observations - to understand the physics behind FRBs. One notable motivation for future investigations would be a search for non-radio counterparts, say in nearby galaxies for similar events.
In this case, one could conjecture that a coincident neutrino burst detected from the same source as an FRB might provide evidence of a different magnetar-driven FRB mechanism. In this case perhaps one involving an ultra-relativistic shock wave interacting with thermal synchrotron photons.
Meanwhile, the FRB source found in Vulpecula could answer other open-ended questions, such as: How frequently do the signals repeat over years to decades? And: How distant and energetic are the bursts? It's an exciting new arena for astrophysics and one which portends new insights into the workings of our universe.
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