1. Aristeidis Noutsos University of Manchester

2. The LOFAR Ionosphere See Ger’s talk, in Hamburg last year. Variations of ~3 rad m –2 were observed in the Stokes images of PSR J0218+4232, during the 1989-92 solar maximum. The ionospheric RM contribution can be as much as ~ 5 rad m –2. Ionospheric TEC fluctuations can hamper accurate RM determination: ‣ relative TEC varies in short time scales ‣ absolute TEC varies more slowly (responsible for RM variations) Strong, highly polarised pulsars can be used to calibrate LOFAR: ‣ we need to average a number of pulses, depending on pulsar strength.

3. Observation proposal We have put together a WSRT proposal to observe 13 highly polarised, northern pulsars in the LFFE band (115-180 MHz). Requested time: 48 h. (Noutsos, Stappers, de Bruyn, Haverkorn) We would like to record simultaneously Full-Stokes filterbank data with PuMa II (Δf ~ 20 MHz) this will allow us to perform high-time-resolution phase-resolved analysis ( δ t < 50 μ s) Aperture synthesis data this will allow us to perform “RM synthesis” on pulsars with no detectable radio pulsation e.g. the ms-binary PSR J0218+4232 and the scattered PSR B1937+21

4. Karastergiou (2009): ‣ Scattering simulations on polarization profiles. ‣ Scattering affects pulsar PA and RM profiles. ‣ RM variations are largest near steep gradients of PA profile. ‣ Since τ scat ~ λ 4, this effect will be strong in the LFFE. RM PA I,L,V Establish a number of physical properties that remain largely unexplored at low frequencies: total flux degree of polarization phase-resolved RM etc. Vela Kennet & Melrose (2009): ‣ Generalised Faraday Rotation generates V in PSR magnetospheres ‣ GFR scales as λ 3. It should be evident in the LFFE, if present. Goals (I) Gould & Lyne (1998): ‣ Multi-wavelength measurements for 280 pulsars. ‣ Interesting changes in linear and circular. ‣ Lowest observing frequency = 230 MHz. ‣ WSRT can show us what happens below that. f (MHz) L % PSR B0950+08

5. Goals (II) Investigate their suitability as LOFAR polarization calibrators: we would like to use them regularly for polarization calibration What makes a good pulsar calibrator? Only pulsars with Dec>0º were considered. LOFAR sensitivity diminishes at large ζº DM had to be small (<100 pc cm –3 ) At low frequencies scattering smears the pulse profiles Linear flux > 100 mJy A flat PA profile is desirable steep PAs / OPM may cause RM variation Increasing L towards low frequencies Candidate PSRs have to produce a high-S/N integrated profile in ~ 1 min (or even faster!) ζ°

6. LOFAR sensitivity The integration time needed for good polarization s/n depends on S. For LOFAR 18+18, 60 σ / L chan is actually easy to achieve for strong, highly polarised pulsars: e.g. B1929+10 produces it in 3 sec! That s/n gives σ PA = 0.9º. We need such low errors for phase-resolved RM variations and GFR tests. Assumptions: LOFAR 18+18 sensitivity (Nijboer & Pandey-Pommier 2009) 128 channels across 25 MHz 60σ/Lchannel Pulse width and L/I hold at ~100 MHz

7. S 100 10 4 mJy (L/I) 600 0.3 α? DM 71 pc cm – 3 Δt 60 σ 0.3 s PSR B0531+21 Notes: S 100 from Malofeev et al. (2000) L/I from Gould & Lyne (1998) α from Lorimer et al. (1995) profiles from EPN database

8. S 100 1080 mJy (L/I) 230 0.11 α–1.7 DM6 pc cm –3 Δt 60 σ 52 s f (MHz) L % PSR B0809+74

9. S 100 620 mJy (L/I) 400 0.29 α–1.6 DM 20 pc cm – 3 Δt 60 σ 7 s f (MHz) L % PSR B0823+26 B e w a r e ! I t s w i t c h e s o f f s o m e t i m e s.

10. S 100 1040 mJy (L/I) 400 0.03 α–2.7 DM 13 pc cm – 3 Δt 60 σ 501 s f (MHz) L % PSR J0837+0610

11. S 100 2030 mJy (L/I) 230 0.31 α–1.7 DM3 pc cm –3 Δt 60 σ 3 s f (MHz) L % PSR B0950+08

12. S 100 1280 mJy (L/I) 230 0.25 α–1.5 DM 5 pc cm – 3 Δt 60 σ 7 s f (MHz) L % PSR B1133+16

13. S 100 260 mJy (L/I) 230 0.44 α–1.8 DM9 pc cm –3 Δt 60 σ 76 s f (MHz) L % PSR B1237+25

14. S 100 1280 mJy (L/I) 400 0.15 α–1.5 DM 20 pc cm – 3 Δt 60 σ 10 s f (MHz) L % PSR B1508+55

15. S 100 400 mJy (L/I) 230 0.57 α–2.1 DM 35 pc cm – 3 Δt 60 σ 29 s f (MHz) L % PSR B1541+09

16. S 100 1900 mJy (L/I) 400 0.29 α–1.9 DM 12 pc cm – 3 Δt 60 σ 2 s f (MHz) L % PSR B1919+21

17. S 100 660 mJy (L/I) 400 0.79 α–1.7 DM 3 pc cm – 3 Δt 60 σ 3 s f (MHz) L % PSR B1929+10

18. S 100 60 mJy (L/I) 230 0.49 α–0.7 DM 23 pc cm – 3 Δt 60 σ 421 s f (MHz) L % PSR B2021+51

19. S 100 1200 mJy (L/I) 400 0.05 α–2.8 DM 44 pc cm – 3 Δt 60 σ 67 s f (MHz) L % PSR B2217+47

20. psrΔt int (sec)p L (=L/I)S 100 (mJy)δ (=W/P) 0531+210.30.30100000.09 0809+74520.1110800.03 0823+2670.296200.01 0837+06105010.0310400.02 0950+0830.3120300.04 1133+1670.2512800.03 1237+25760.442600.04 1508+55100.1512800.015 1541+09290.574000.06 1919+2120.2919000.02 1929+1030.796600.03 2021+514210.49600.015 2217+47670.0512000.01

21. σ RM RM LOFAR 18+18 RM-sensitivity Simulation RM fit = 5 rad m –2

22. LOFAR 18+18 RM-sensitivity Simulation f = 150 MHz bw = 25 MHz (continuous) Δ f = 128 channels Δ(RM) = 0.001 rad m-2 Actual function? Stepping: resolution artefact?

23. RM determination issues In the LOFAR observing band, scattering will play an important role (see my science talk?) Scattering screen PA profile pulsarobserver

24. RM determination issues Noutsos et al. (2009; accepted) Δ RM p–p ≈ 25 rad m –2 A number of high-DM pulsars show large RM variation across the pulse Scattering But, we still see significant variation in a few lower-DM pulsars: PSR J2048–1616 DM = 12 pc cm –3 Δ RM p–p ≈ 17 rad m –2 DM = 478 pc cm –3 PSR J1644 – 4559 1.4 GHz

25. Caveat! If ΔRM is large, the PA may wrap across the profile. RM determination issues We need to address this issue: I. In the pulsar polarization calibration procedure II. In the calculation of RMs from pulsars found with LOFAR Possible solution: We model the scattering tails and de-convolve the Stokes profiles calculate RM from pulse-averaged Stokes