the ionosphere, and LOFAR Charlotte Sobey csobey@mpifr-bonn.mpg.de Max-Planck-Institut f¨ ur Radioastronomie 5th Dec 2012 Charlotte Sobey. MPIfR. TKP meeting Amsterdam. 05/12/12
Calibration of High-Precision Faraday Rotation Measurements from LOFAR (A&A sub.) Motivation: why pulsar polarimetry and RMs? ionFR: introduction to model for ionospheric RM Comparisons: LOFAR observations vs. model 2 Pulse modulation of PSR B0823+26 Serendipity! surprising observation of pulsar switching ‘quiet’ to ‘bright’ mode... emission mechanism Charlotte Sobey. MPIfR. TKP meeting Amsterdam. 05/12/12
rotation observed in the ISM from pulsars/EG sources ∆PA = RM λ2, where PA = 1 2 tan−1 U Q RM = 0.81 observer source ne B · dl rad m−2 Yields information about the magnetic field of our galaxy Interesting for many reasons, including HECR propagation Figure: L: Faraday rotation (Wikipedia), R: schematic Galactic magnetic field magnitude & direction (van Eck et al. 2011) Charlotte Sobey. MPIfR. TKP meeting Amsterdam. 05/12/12
code to model ionospheric Faraday rotation Ionosphere also magnetised plasma: ∆PA = (RMion + RMISM + RMIGM + RMint )λ2 ionFR written by Carlos Sotomayor in python available on github! Inputs: RA & DEC of source, coordinates of telescope, obs date Calculate and output: ionospheric RM at IPP using TEC maps & IGRF11 0.2 0.4 0.6 0.8 Latitude [◦] 70◦S 50◦S 30◦S 10◦S 10◦N 30◦N 50◦N 70◦N 70◦S 50◦S 30◦S 10◦S 10◦N 30◦N 50◦N 70◦N 08 UT [hr] 10 UT [hr] 12 UT [hr] 14 UT [hr] 70◦S 50◦S 30◦S 10◦S 10◦N 30◦N 50◦N 70◦N 16 UT [hr] 18 UT [hr] 20 UT [hr] 22 UT [hr] A IPP ζ ’ O h R thin shell Ionosphere Earth Radio Source ζ Figure: L: Center for Orbit Determination in Europe (CODE), M: ionospheric piercing point (C. Sotomayor), R: Carlos Sotomayor (courtesy Aris N.) Charlotte Sobey. MPIfR. TKP meeting Amsterdam. 05/12/12
variation: hourly Ionospheric (TEC) dynamic on minutes – years: time dependent calibration required Figure: Daily RM variation as viewed from LOFAR Superterp toward Cas A. L: 11th April 2009 (closer to solar min), R: 11th April 2011 (closer to solar max). (C. Sotomayor) Charlotte Sobey. MPIfR. TKP meeting Amsterdam. 05/12/12
variation: yearly Ionospheric (TEC) dynamic on minutes – years: time dependent calibration required Essential for monitoring RMs in the ISM over several epochs Apr 98 Mar 00 Feb 02 Jan 04 Dec 05 Nov 07 Oct 09 Sep 11 0 2 4 6 8 10 |φion | [rad m−2] Apr 98 Mar 00 Feb 02 Jan 04 Dec 05 Nov 07 Oct 09 Sep 11 0 5 10 15 20 25 |φion | [rad m−2] Figure: Weekly averages of the maximum and minimum (blue and red lines, respectively) absolute ionospheric Faraday depth. L: towards CasA viewed from LOFAR, R: towards Eta Carinae viewed from average of SKA core sites in Western Australia and South Africa. (C. Sotomayor) Charlotte Sobey. MPIfR. TKP meeting Amsterdam. 05/12/12
obtained using RM-synthesis, excellent for use with LOFAR observations −100 −50 0 50 100 φ [rad m−2] 0.0 0.2 0.4 0.6 0.8 1.0 F(φ) −20 −10 0 10 20 φ [rad m−2] 0.0 0.2 0.4 0.6 0.8 1.0 F(φ) Figure: Examples of Faraday spectra from two LOFAR observations. L: 3-minute Superterp HBA observation (119–125 MHz) of PSR B2217+47 with FWHM 6.6 rad m−2. R: 3-minute Superterp LBA observation (58–64 MHz) of PSR B1919+21 with FWHM of 0.84 rad m−2. (C.Sobey, J. Hessels, PWG) Charlotte Sobey. MPIfR. TKP meeting Amsterdam. 05/12/12
at sunrise and sunset with Superterp Observations of PSR B0834+06 over several hours with Superterp HBAs in tied-array mode to measure RM as function of time Comparison: LOFAR observations vs. ionFR model 16 17 18 19 20 21 22 23 UT Time [Hours] 25.5 26.0 26.5 27.0 27.5 28.0 28.5 φobserved [rad m−2] 11/04/2011 : CODE PSR B0834 + 06 TEC & IGRF11 4 5 6 7 8 UT Time [Hours] 20/10/2011 : CODE 0.5 1.0 1.5 2.0 2.5 3.0 φion [rad m−2] PSR B0834 + 06 TEC & IGRF11 Figure: L: B0834+06 observed 7×10 mins every hour on 11th April 2011 over sunset at 120–126 MHz, R: B0834+06 observed 20×3 mins every 10 mins on 20th October 2011 over sunrise at 129–140 MHz. (C.Sobey, J. Hessels, C. Sotomayor, LOFAR PWG) Charlotte Sobey. MPIfR. TKP meeting Amsterdam. 05/12/12
B0834+06 Corrected for ionospheric RM (minimum χ2 offset between model and obs) Trend of RM with latitude as measured from Superterp, FR606 & SE607 suggests 3D model required; airmass comparison 48 50 52 54 56 Latitude [◦N] 25.0 25.1 25.2 25.3 25.4 25.5 25.6 φISM [rad m−2] 1.3 1.4 1.5 1.6 1.7 1.8 1.9 Airmass Superterp FR606 SE607 PSR B0834 + 06 (CODE) PSR B0834 + 06 (ROB) Airmass Figure: RM of the ISM towards PSR B0834+06, as determined from HBA observations using the LOFAR FR606, Superterp and SE607 stations. (C. Sobey, J. Hessels, C. Sotomayor, LOFAR PWG) Charlotte Sobey. MPIfR. TKP meeting Amsterdam. 05/12/12
Observing pulsars in polarisation interesting, in particular to derive magnetic field of Milky Way Need to correct for ionospheric contribution, especially if monitoring RMs over long timescales (years) ionFR can predict ionospheric Faraday rotation Comparisons between the model and observations seem very good, higher time resolution and finer gridding for TEC maps better LOFAR can measure high-precision RMs, calibrate for ionosphere using ionFR – in future 3D modelling Bring on cycle 0 observations! 2/3 observations of pulsars with known and unknown RMs Charlotte Sobey. MPIfR. TKP meeting Amsterdam. 05/12/12
of B0823+26 I LOFAR observation similar to those before (IQUV), BUT, intermittent pulsar Serendipity: in one observation (3 min) observed to switch between quiet to bright Figure: L: I(time,phase), M: Single pulse profiles, R: Average bright pulse profile (C. Sobey, J. Hessels, PWG) Charlotte Sobey. MPIfR. TKP meeting Amsterdam. 05/12/12
of B0823+26 II LOFAR observation in total intensity (3 hr, HBA, 48 MHz, I) Again, in one observation, switch between quiet to bright mode Interesting: previously thought ‘off’ mode actually just ‘quiet’ (emission still present) Figure: L: I(time,phase) quiet mode, M: I(time,phase) bright mode, R: Single pulse profiles (C. Sobey, J. Hessels, LOFAR PWG) Charlotte Sobey. MPIfR. TKP meeting Amsterdam. 05/12/12
Two rare observations during mode change of PSR B0823+26 (esp. with polarisation) High sensitivity of LOFAR allowed detection of pulse in previously thought ‘off’ mode Provides information about magnetosphere emission – how to produce mode change within a single pulse? Rapid change in magnetic field and/or electron density? Thank you for listening! Charlotte Sobey. MPIfR. TKP meeting Amsterdam. 05/12/12
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