Structures in the rotation measure sky

Abstract

Coherent structures in the distribution of the Faraday rotation measure (RM) of extragalactic radio sources are isolated using wavelet transformation techniques. A new algorithm of wavelet analysis for data points non‐uniformly distributed on a sphere is developed and implemented. Signatures of the magnetic fields in the local (Orion) arm, the Sagittarius arm (and its extension, the Carina arm), the synchrotron Loop I and, possibly, the Perseus arm have been revealed using the RM catalogues of Simard‐Normandin et al. (551 sources) and Broten et al. (663 sources). Unlike earlier analyses of the RM sky, our approach has allowed us to assess the stability of the results with respect to modifications of the data sample. Only the aforementioned features remain stable under mild sample modifications. We consider separately low‐latitude sources at |b|<10° and, using the model of electron density distribution of Cordes et al., we estimate magnetic field strength by comparing the model wavelet transform with that of the real data. Independent estimates of the mean magnetic field strength in the Orion arm using low‐ and high‐latitude sources converge to 1.4±0.3 μG. Rotation measures of low‐latitude sources provide a clear indication of a magnetic field reversal at a distance 0.6–1 kpc towards the Galactic Centre. Our analysis has revealed for the first time the extension of the reversal in the Carina arm. Low‐latitude sources from the catalogue of Broten et al. indicate a magneto‐ionic structure in the direction of the Perseus arm with the magnetic field direction reversed with respect to that in the Orion arm. The extent of the region with reversed magnetic field near the Sun is 3 kpc or more in the azimuthal direction. The average pitch angle of magnetic field in the nearby spiral arms is 15°, and the mean field strength in the Sagittarius–Carina and Perseus arms is 1.7±0.3 μG and 1.4±1.2 μG, respectively. The line‐of‐sight magnetic field in Loop I is estimated as 0.9±0.3 μG. We find firm evidence of a dominant even symmetry of the local mean magnetic field with respect to the Galactic equator. Our results are compatible with a moderate large‐scale north–south asymmetry, with the magnetic field in the southern hemisphere being stronger in a region of at least 3 kpc in size. It cannot be excluded, however, that the asymmetry is local and results from vertical bending of magnetic lines in a region of about 400 pc in size, with the Sun being located close to the top of a magnetic loop, the magnetic field of which is 0.5 μG stronger than the average field.

Can the magnetic field in the Orion arm inhibit the growth of instabilities in the bow shock of Betelgeuse?

“…It also conforms to typical field strengths found in the spiral arms of other galaxies (Fletcher et al 2011;Vallée 2011). We repeated the simulation for magnetic field strengths of 1.4 µG (conform to Frick et al 2001) and 5.0 µG (conform to Opher et al 2009) to investigate the quantitative effect of the field strength on the result.…”

“…The motion of the star through the ISM wass included by letting the ISM stream pass the star with a velocity v = 28.3 km s −1 . We assumed the ISM to be cold, so the We run the simulation with magnetic field strengths of 0, 1.4, 3.0, and 5.0 µG, covering the parameter space of estimated magnetic field strengths in the Orion arm of the Milky Way (Frick et al 2001;Opher et al 2009;Heerikhuisen & Pogorelov 2011). The magnetic field was set parallel to the direction of motion and fixed at the upper z-boundary.…”

The large-scale magnetic field in the fourth Galactic quadrant

“…Somewhat surprisingly, these two effects have never been fully taken into account in discussions of the large-scale component of the Galactic magnetic field based on RM estimates of PSR and/or EGS (see e.g., Rand & Kulkarni 1989;Rand & Lyne 1994;Indrani & Deshpande 1999;Han et al 1999;Frick et al 2001;Han et al 2006;Brown et al 2007;Men et al 2008;Vallée 2008a). From time to time, one finds references to unexpected deviations from smoothness in the RM-distributions, as e.g., in the so-called "Carina-anomaly" mentioned by Han et al (2006).…”

Rotation Measures of Extragalactic Sources behind the Southern Galactic Plane: New Insights into the Large‐Scale Magnetic Field of the Inner Milky Way

“…1) of the Sagittarius-Carina spiral arm is reversed relative to the local arm (Thomson & Nelson 1980;Simard-Normandin & Kronberg 1980;Lyne & Smith 1989), implying a large-scale field reversal between the local arm and the Q1 component of the Sagittarius-Carina arm. Some evidence suggests that this reversed field extends into the fourth quadrant (Q4, 270 l 360 ) component of the SagittariusCarina arm (e.g., Rand & Lyne 1994;Han et al 1999;Frick et al 2001).…”