Aristeidis Noutsos University of Manchester. The LOFAR Ionosphere See Ger’s talk, in Hamburg last...
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Transcript of Aristeidis Noutsos University of Manchester. The LOFAR Ionosphere See Ger’s talk, in Hamburg last...
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