Aristeidis NoutsosUniversity of Manchester

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.

Observation proposalWe 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 (δt < 50 μs) • Aperture synthesis datathis 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

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 τscat ~ λ4, 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 λ3. 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

Goals (II) Investigate their suitability as LOFAR polarization calibrators:• we would like to use them regularly for polarization calibrationWhat 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 desirablesteep 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°

LOFAR sensitivity

The integration time needed for good polarization s/n depends on S. For LOFAR 18+18,

60σ / Lchan is actually easy to achieve for strong, highly polarised pulsars: e.g. B1929+10 produces it in 3 sec!That s/n gives σPA = 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

S100 620 mJy(L/I)400 0.29

α –1.6

DM20 pc cm–3

Δt60σ 7 s

f (MHz)

L %

PSR B0823+26

Beware! It sw

itches o

ff some tim

es.

S1002030 mJy

(L/I)230 0.31α –1.7

DM3 pc cm–3

Δt60σ 3 s

f (MHz)

L %

PSR B0950+08

S1001280 mJy

(L/I)230 0.25α –1.5

DM5 pc cm–3

Δt60σ 7 s

f (MHz)

L %

PSR B1133+16

S100 400 mJy(L/I)230 0.57

α –2.1

DM35 pc cm–3

Δt60σ 29 s

f (MHz)

L %

PSR B1541+09

S100 660 mJy(L/I)400 0.79

α –1.7

DM3 pc cm–3

Δt60σ 3 s

f (MHz)

L %

PSR B1929+10

psr Δtint (sec)

pL (=L/I) S100 (mJy) δ (=W/P)

0531+21 0.3 0.30 10000 0.090809+74 52 0.11 1080 0.030823+26 7 0.29 620 0.010837+0610

501 0.03 1040 0.02

0950+08 3 0.31 2030 0.041133+16 7 0.25 1280 0.031237+25 76 0.44 260 0.041508+55 10 0.15 1280 0.0151541+09 29 0.57 400 0.061919+21 2 0.29 1900 0.021929+10 3 0.79 660 0.032021+51 421 0.49 60 0.0152217+47 67 0.05 1200 0.01

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?

RM determination issues

Noutsos et al. (2009; accepted)

ΔRMp–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 J2048–1616DM = 12 pc cm–3 ΔRMp–p ≈ 17 rad m–2

DM = 478 pc cm–3

PSR J1644 – 4559 1.4 GHz

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 procedureII. 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