Pulsar broadening measurements at low frequencies with LOFAR

1. 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

2. Pulsar Scattering • Interstellar medium (ISM) is not uniform or isotropic • The ISM causes radio waves to: • Disperse • Scatter Lorimer and Kramer (2005) • Scattering → “exponential like” tail. • Thin/thick screen or a uniformly distribution medium. • τdνα B1831-03 Löhmer et al. (2004)

3. Pulsar Scattering • Observed profile → I ⨂ ISMs ⨂ DMs ⨂ Rs Intrinsic profile • Traditional 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. ISM ⨂ DMs ⨂ Rs Observed profile

4. CLEAN based method • Developed by Bhat et al (2003).. • Find maximum of the profile. • Multiply maximum with a gain factor (e.g. 5%). • Convolve this with ISMs ⨂ DMs ⨂ Rs. • subtract it from the profile. • Repeat until the residual profile is noise like • Repeated for a range of τd values. • Best τd → best noise like residual.

5. CLEAN based method B2111+46 @ 122 MHz (LOFAR HBA) • 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. Scattered “Cleaned”

6. LOFAR Observations • Bhat et al. (2004) measurements below DM=100 are from Δνd. 2πτd 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 αwithin LOFAR band. • Check for deviations in low frequency regime.

7. LOFAR Observations - Results • 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 τdmeasured for the first time at these frequencies. • All 22 sources had only sporadically measurements at low frequencies. • Measured αwithin a continuous frequency band for the first time! This work τd

9. Frequency dependencyof τd • Theory 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 ofτd(Lewandowski 2013). Purple filled circles are points of this work.

10. Intriguing results B0611+22 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. This work Τd (msec) B2217+47 Possible break! Frequency (MHz) Τd (msec) Frequency (MHz)

11. Conclusions and Future work • 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! • 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. Thank you!