Eyes on the Polarized Sky, Feet on the Ground Jeroen Stil, University of Calgary On Behalf of the SWG Cosmic Magnetism NSF

How do we turn our dreams into reality? Far from the Galactic plane: } Galactic Faraday rotation Extragalactic Faraday rotation RM errors (present data) of order 10 rad m-2 with some exceptions Several extragalactic applications require error on RRM as small as 1 rad m-2 Outskirts of clusters (1+z)-2 dependence for internal Faraday rotation Significant progress to be made by Improving sampling by > 2 orders of magnitude across the sky Reducing RM errors by 1 order of magnitude (all-sky)

Where Is the Plasma In which Faraday Rotation Occurs? rad m-2 |f| 300 rad m-2 225 rad m-2 150 rad m-2 75 rad m-2 0 rad m-2 Harvey-Smith et al. (2011), 1 RM per square degree Association of Faraday rotation with visible sources depends critically on density of the RM grid and RM errors Much easier if a single “screen” is responsible for most of the Faraday rotation Or avoid these objects, e.g. X-ray clusters (presentation by T. Akahori this meeting) Interesting physics in sources that mix synchrotron emission and Faraday rotation See also Vacca et al. (2014) PoS(AASKA14)114

Extragalactic examples of internal Faraday effect from mixed relativistic/thermal plasma Polarization angle l2 (m2) l2 (m2) Polarized emission in MACS J0717.5+3745 Examples of poor 2 fits, indicating that a simple Faraday screen model is not applicable (Bonafede et al. 2009) Thermal plasma in the radio lobes of Cen A (O’Sullivan et al. 2013) Note importance of broad, continuous l2 coverage

Resolution in Faraday Depth Curves mark resolution and sensitivity to extended structure in Faraday depth for surveys with bandwidth indicated. Markers indicate lowest observed frequency 100 MHz, 600 MHz, 1 GHz and 2 GHz. Depolarization resolved unresolved BW = 1540 MHz BW = 770 MHz BW = 50 MHz Resolved, extended Faraday Depth structure only if source falls under a curve and above the black line. BW = 100 MHz resolved unresolved RM error is 1 rad m-2 at 10s detection threshold in p Is 1 GHz bandwidth sufficient? No: Farnsworth et al. (2011) Compare O’Sullivan et al. (2012), Anderson et al. (2016), Kim et al. (2016) See discussion in Haverkorn et al. (2014)

Galactic Foreground Estimates in SKA Surveys Extragalactic RM dispersion (Schnitzeler 2010) 1/sq degr. 25/sq degr. 1000/sq degr. 300/sq degr. 3000/sq degr. -500 rad m-2 +500 rad m-2 Error in RRM due to statistical error in galactic foreground, averaging sources within annulus with inner radius 10” and outer radius q. RM structure function with slope 0.7 on scales < 1o. Curves start at minimum of 5 sources. Must choose inner radius such as not to include extragalactic RM! 20 40 60 rad m-2 Oppermann et al. (2012) Galactic Faraday rotation and error map

Galactic Foreground estimates in SKA Surveys High-Galactic latitude Faraday rotation of diffuse Galactic emission from GALFACTS. 30 RMs per square degree, errors of individual RMs 1 – 5 rad m-2 Topology of the foreground matters, and isotropic weighting of the data may underestimate the foreground in case of filaments and edges. Also: structure and variability of the ionosphere for SKA low (Loi et al. 2015)

Extended Background Sources Differential Faraday rotation over angular size of background radio galaxy for galaxies more distant than ~ 5 Mpc. Targets of opportunity from chance alignments. Observe higher frequency to avoid complete depolarization. Complex Faraday rotation is the norm. Confusion about line of sight if foreground galaxy is not observed directly. Double-lobed radio source behind UGC 10288 Irwin et al. (2013) NGC 1310 in front of Fornax A (Fomalont et al. 1989) Radio haloes/relics and radio lobes as extended polarized screens in galaxy clusters

What do we learn if we don’t have a model for ne? The one application with a well established electron density model based on pulsar dispersion measures is the large-scale magnetic field of the Milky Way. FRBs can provide large scale dispersion measures possibly rotation measures. Position accuracy, redshift and Faraday rotation by the host provide challenges. Applications that do not require an electron density model or dispersion measure: Average direction of magnetic field along line of sight, reversals, geometry Structure functions (Simonetti et al. 1984) Physical models and |RM|/sRM, coherence scale (Murgia et al. 2004) Polarization gradients (Gaensler et al. 2011) Limit to Alfven speed (Stil & Hryhoriw 2016) Most likely, we have not exhausted the possibilities.

Calibration Statistical analysis depends on uniform data quality and small, well-understood (systematic) errors. Polarization purity across the field of view is crucial because residual instrumental polarization appears as artificial peaks with small |RM|, needed for a lot of extragalactic science. Need a pipeline that does off-axis polarization calibration and dedicated commissioning work (holography).

Conclusions A dense RM grid connects all aspects of cosmic magnetism science Wide-band surveys offer more information, not just better information Can do science without ne model Uniformity of RM grid and RM variability science limited by quality of wide-field polarization calibration, time-variable ionosphere