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26th of May 2016
The ATCA finds that an Interstellar Manatee is a Magnetised Zombie
by Jamie Farnes (Radboud University Nijmegen)
The large radio nebulae W50 has previously been compared to an interstellar manatee. Astronomers are uncertain about the origins of this weird manatee, but magnetic fields may have played a role. Astronomers have now used the Australia Telescope Compact Array (ATCA) to study the formation of this object, which covers 2 x 1 degrees on the sky (by contrast the Moon is 0.5 degrees in diameter), by creating a large mosaic using 198 separate telescope pointings. In combination with images of the ionised hydrogen gas (which emits the Hα spectral line) from the IPHAS survey, the study found an inverted relationship between the magnetic fields and ionised hydrogen. This revealed gaps in the ionised gas that covers the object. By peeking through these gaps, it was possible to measure the magnetic fields all the way towards this object. W50 is indeed magnetised, with a loop magnetic field surrounding the nebula, which together with the ionised hydrogen features tells us that W50 is most likely an old supernova remnant - the corpse of a dead star that underwent a cataclysmic explosion ~20,000 years ago - that has been reanimated by jets from a microquasar at its centre. As W50 has come back to life, this suggests there may be other Zombie supernova remnants out there waiting to be discovered. The study has been submitted to the Monthly Notices of the Royal Astronomical Society.

Figure caption: A multiwavelength view of W50 (the Manatee nebulae) with microquasar SS433 at the centre. The magnetic fields were observed using the ATCA. Left panel: This image highlights the size of the full radio nebulae. The plot shows ionised hydrogen gas (red), radio intensity (green), and the linear polarisation (faint, in blue), which lights up the magnetic fields seen with the ATCA. Right panel: Similar to left panel, but this time highlighting the magnetic fields. The plot also shows ionised hydrogen gas (red), but with the other colours inverted, giving radio intensity (blue), and the linear polarisation/magnetic fields (now in green). There is an inverted relationship between red and green, showing that the ionised gas is nearer to us than the supernova remnant, and acts as a "Faraday screen". This "depolarises" the observed magnetic fields and makes them even harder to detect. The emission we do see is due to holes in these Faraday curtains.

Reference: Farnes, J., Gaensler, B., Purcell, C., Sun, X., Haverkorn, M., Lenc, E., O'Sullivan, S., Akahori, T. 2016, MNRAS, submitted

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