Pulsar broadening measurements at low frequencies with LOFAR

Kimon ZagkourisUniversity of Oxford

In collaboration with the LOFAR Pulsar Working Group

Ierapetra June 2014

Image Credit: David A. Aguilar (CfA) / NASA / ESA

Pulsar Scattering• Interstellar medium (ISM) is not

uniform or isotropic• The ISM causes radio waves to:

• Disperse• Scatter

• Scattering → “exponential like” tail.• Thin/thick screen or a uniformly distribution medium.• τd να

B1831-03 Löhmer et al. (2004)

Lorimer and Kramer (2005)

Pulsar Scattering• Observed profile → I ⨂ ISMs ⨂ DMs ⨂ RsTraditional Measuring τd:

• Higher frequency profile → no scattering.• Convolve it with the ISM and instrument functions.• Fit on the observed profile.• Repeat for a range of τd. Best fit → τd .

Drawbacks:• Requires high frequency not scattered profile.• Profile evolution → Wrong τd.

Intrinsic profile

Observed profile

ISM ⨂ DMs ⨂ Rs

CLEAN based method

Developed by Bhat et al (2003)..

1. Find maximum of the profile.2. Multiply maximum with a gain factor (e.g. 5%).3. Convolve this with ISMs ⨂ DMs ⨂ Rs.4. subtract it from the profile.5. Repeat until the residual profile is noise like

• Repeated for a range of τd values. • Best τd → best noise like residual.

B2111+46 @ 122 MHz (LOFAR HBA)

Scattered “Cleaned”

Best value for τd:• Γ → skewness of the residual.• Fr → positivity of the residual.• Pvalue → Kolmogorov-Smirnov test value.• Nf → total number of iterations.

Minimising Γ + Fr → best τd value.

CLEAN method:• Finds the value of τd for a given ISM screen

model.• Finds the best screen model.• No high frequency profile needed.• Can return the intrinsic profile.

CLEAN based method

LOFAR Observations• Bhat et al. (2004) measurements below DM=100 are

from Δνd. 2π τd Δνd ≈ 1.

• LOFAR:• LBA 30 – 80 MHz• HBA 120 – 240 MHz• 80 MHz bandwidth.• Continuous band coverage.

• If τd ≈ P0 → pulsar might not be detected.

• LOFAR ideal to explore 10 – 200 DM region with direct measurements of τd.

• Measure α within LOFAR band.• Check for deviations in low frequency regime.

• LOFAR observed ~100 pulsars.

• 30 – 40 candidates for scattering measurement.

• Only 22 had scattering tails and enough SNR.

• 4 – 16 independent measurements within the band.

• 3 pulsars had their τd measured for the first time at these

frequencies.

• All 22 sources had only sporadically measurements at low

frequencies.

• Measured α within a continuous frequency band for the

first time!

This work

LOFAR Observations - Results

τ d

Frequency dependency of τdTheory suggests:• Kolmogorov α ≈ -4.4• Gaussian α ≈ -4

Measurements:• Bhat (2004) α ≈ -3.86 ± 0.16• This work α ≈ -2.64 ± 1.28

Difference could be because:• We used a thin screen model in all cases.• Different scattering procedure at low frequencies.• Screen truncation Cordes and Lazio (2001) →

“flatter” spectrum.• Strong profile evolution.• Multiple screens.• Low SNR -> bigger error for τd and α. Direct measurements of τd (Lewandowski

2013). Purple filled circles are points of this work.

Τ d (m

sec)

Frequency (MHz)

B2217+47

Τ d (m

sec)

Frequency (MHz)

This work

B0611+22

Possible break!

Some pulsars (e.g. B0611+22) indicate possible break in the powerlaw. Mid-high frequency observations needed.

Three pulsar showed a steeper spectrum than expected α ≈ -5.7. Possibly a thin screen model is not the best choice for these cases.

Intriguing results

Conclusions and Future work

The story so far:• LOFAR is great to study scattering!• τd and α for 22 pulsars at low

frequencies.• Not all pulsars can be used for

scattering measurements.• Indications for a different scattering

behavior at low frequencies.• CLEAN based method → deconvolved

profile → useful for pulsars used in timing experiments.

The road ahead:• LOFAR Cycle 1 and 2 observations -> more

scattering measurements.• Time variability of scattering.• Telescopes such as (GBT, Arecibo, GMRT, LWA,

MWA) can fill in the frequency gaps to probe for breaks in the power law.

• Southern looking telescopes will help increase coverage and the analysis’ statistics.

• Cyclic spectroscopy (Demorest 2011) can measure the scattering timescales much more accurate and is the next thing to try!

Thank you!