Pulsar broadening measurements at low frequencies with LOFAR Kimon Zagkouris University 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. τ d  ν α B Löhmer et al. (2004) Lorimer and Kramer (2005)

Pulsar Scattering Observed profile → I ⨂ ISMs ⨂ DMs ⨂ Rs Traditional Measuring τ d : Higher frequency profile → no scattering. Convolve it with the ISM and instrument functions. Fit on the observed profile. Repeat for a range of τ d. Best fit → τ d. Drawbacks: Requires high frequency not scattered profile. Profile evolution → Wrong τ d. 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 τ d values. Best τ d → best noise like residual.

122 MHz (LOFAR HBA) Scattered“Cleaned” Best value for τ d : Γ → skewness of the residual. F r → positivity of the residual. P value → Kolmogorov-Smirnov test value. N f → total number of iterations. Minimising Γ + F r → best τ d value. CLEAN method: Finds the value of τ d 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 Δν d. 2π τ d Δν d ≈ 1. LOFAR: LBA 30 – 80 MHz HBA 120 – 240 MHz 80 MHz bandwidth. Continuous band coverage. If τ d ≈ P 0 → pulsar might not be detected. LOFAR ideal to explore 10 – 200 DM region with direct measurements of τ d. 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 τ d 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 τdτd

Frequency dependency of τ d Theory suggests: Kolmogorov α ≈ -4.4 Gaussian α ≈ -4 Measurements: Bhat (2004) α ≈ ± 0.16 This work α ≈ ± 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 τ d and α. Direct measurements of τ d (Lewandowski 2013). Purple filled circles are points of this work.

Τ d (msec) Frequency (MHz) B Τ d (msec) Frequency (MHz) This work B Possible break! Some pulsars (e.g. B ) indicate possible break in the powerlaw. Mid- high frequency observations needed. Three pulsar showed a steeper spectrum than expected α ≈ 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! τ d 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!