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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
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