Ashley Chrimes

Introduction: Deciphering the origin of fast radio bursts (FRBs) has been a key aim in transient and radio astronomy, ever since their discovery. The detection of FRB-like bursts from Galactic magnetar SGR1935 has recently furthered interest in magnetar-based FRB progenitor models. If we assume that SGR1935 is typical of all FRB sources, and that all magnetars are capable of producing FRBs, we conclude that the distribution of magnetars on their hosts should be similar to the distribution of magnetars on the Milky Way! However, many FRB models invoke neutron star systems, but not magnetars specifically. We can more generally ask, how does the distribution of neutron stars on the Milky Way compare to extragalactic transients, in particular FRBs?

Aims: 1. Create a synthetic image of the Milky Way, as if viewed from an extragalactic, face-on vantage point. 2. Place Galactic neutron star systems on the map, using best estimates of their heliocentric distance and direction on the sky. 3. Measure the Galactic neutron stars as if they were producing transients – make offset, enclosed flux and Flight distributions. 4. Compare Galactic neutron stars on the face-on disc of the Milky Way to extragalactic transients, including FRBs.

Image construction: We split the Galaxy into three components: the spiral arms, the disc and the bulge/bar, and restrict all analyses to ‘our half’ of the Milky Way. This reduces distance uncertainties and incompleteness, and should not bias the results as the Galaxy is expected to be approximately symmetric on a global scale. Arms: H-2 region masers are assigned a position using the BESSEL distance calculator. We assume that the maser bolometric luminosity is proportional to the optical arm luminosity. Disc: An exponential disc profile. Bar/bulge: A Sersic profile convolved with a bar profile mapped using Gaia. The components are weighted using colour estimates for the Milky Way to produced B and I-band images.

Galactic populations: We now place Galactic neutron star systems on the Milky Way image. Magnetars are drawn from the McGill catalogue, pulsars are from the ANTF database (restricted to high luminosities for uniformity across the map), and XRBs are from INTEGRAL surveys, which have high completeness out to the maximum heliocentric distances being considered here. The system locations on the map are 2D projections of their 3D location. The distance estimates are from parallaxes, dispersion measures, HI column densities and other techniques, depending on the population in question.

Results: We now measure the neutron stars as if they were transients on other galaxies. Physical offset, half-light radius normalised offset, and enclosed fluxes for magnetars versus extragalactic transients and other Galactic systems are shown, and a maximum galactocentric offset of 12kpc is assumed. The fraction of light is another useful tool. To generate an F-light value for each pixel, we decide which pixels will be ‘associated’ with the Galaxy (< 12kpc in this case). Associated pixels are ranked by value in a cumulative sum, and normalised by the total cumulative flux such that the brightest pixel takes the value 1. Unassociated pixels are assigned 0. A cumulative distribution of F-light for pixels underlying transients which are uniformly sampled from the light is a one-to-one line. We perform Andersen-Darling tests between each Galactic sample and the extragalactic transients. We find that – of all the extragalactic transients – Galactic neutron stars are the best match for their distribution on their host galaxy, which 75 per cent of the tests made yielding a 2-sigma level consistency. The next best matching transients are type Ia supernovae, at around 33 per cent. This is further evidence, from an independent method, that FRBs originate from neutron star systems.