Optimizing pulsar timing arrays

Optimizing pulsar timing arrays

Cees Bassa1

Ben Stappers1, Gemma Janssen1, Michael Kramer2,Ramesh Karuppusamy2, KJ Lee2, Roy Smits3,

Kuo Liu4, Delphine Perrodin5, Mark Purver1

1Jodrell Bank, Manchester, UK, 2MPIfR, Bonn, Germany,3ASTRON, The Netherlands, 4CNRS, Orléans, France, 5INAF, Cagliari, Italy

January 20th, 2013

http://www.epta.eu.org http://www.leap.eu.org

Cees Bassa (JBCA) Optimizing pulsar timing arrays January 20th, 2013 1 / 12

Part I: Optimal observation scheduling

Based on Lee et al. (2012, MNRAS 423, 2642)

Problem:Given a set of millisecond pulsars and a set telescopes, each having a fixedamount of observing time, how should the observing time be split among thetelescopes and the pulsars to maximize the detection of gravitational waves?

Approach:Need to characterize the noise and its dependence on pulsar, telescope, andschedule parameters.


Stochastic GW background detection significance using Hellings & Downsformalism (after Jenet et al. 2005, correlations between timing residuals frompairs of pulsars)

Timing residuals contains both GW signal as well as sources of noise:

• Radiometer noise (white) σ0

• Pulse jitter noise (white) σJ

• Unmodelled timing noise (red) σr with spectral index β

Scaling with telescope and schedule:

• White noise components: σ2w =

σ20

G2τ2 +σ2

Jτ2

(telescope gain G, observing time τ)

• No dependence on scheduling and telescope for red noise


Case A: 1,2σ0 = 10−7, 1,2σJ = 0, τ1,2 = 1, G1,2 = 1Case C: τ1 = 1.5, τ2 = 0.5, 2τ1 = 0

Case B: G1 = 2Case D: G1 = 2, 1σJ = 10−7


Solid: PPTA, dashed: EPTA, dot-dashed: NANOGravThin: unoptimized, thick: optimizedUsing Hobbs et al. 2010 uncertainties as σ0, bi-weekly sessions of 20 h each


Main conclusions:

• Optimized schedule able to detect 2× to 3× weaker GW background

• Should spend more time on weaker pulsars

• More observing time needed to beat down jitter noise

• Big telescope should observe weak pulsars, small telescopes brighter ones(High gain improves radiometer noise, low gain to integrate down jitter)

• Information about jitter and red noise is needed for more pulsars


Part 2: Large European Array for Pulsars (LEAP)

LEAP Project:Coherently add pulsar observations from thefive 100 m class European telescopes toincrease aperture and TOA accuracy.

Combine telescopes to form a tied array andcreate a single telescope with the equivalentsize of a 200 m dish.

Comparable in aperture to the illuminatedArecibo dish, but able to cover −30◦ < δ < 90◦.


In a tied array telescope signals from different telescopes are corrected fordifferences in time delay, then added in phase.

Time delays due to

• differences in geometry

• observatory clocks

• instruments (cable lengths)

• atmospheric conditions

Total time delay needs to be known to afraction of a wavelength (21 cm = 0.7 ns)→ observe phase calibrators

Advantages:

• Larger aperture will improve TOA accuracy

• Ability to time weaker pulsars

• Calibration of instrumental delays between telescopes


Observations:

• monthly 24 h sessions

• 20-30 millisecond pulsars

• simultaneously with all 4 (soon 5) EPTA telescopes(new/upgraded instruments at all observatories)

• observe pulsars and phase calibrators

• 128 MHz of bandwidth @ 1400 MHz, 8 bit sampling

• baseband data (raw voltages) recorded on disk

Processing:

• data transferred to Jodrell Bank

• correlate calibrator (pulsar) data to obtain time/phase offsets(dedicated storage/processing clusters in place)

• apply time/phase offsets to pulsar data and add coherently

• generate TOAs using standard software (dspsr, psrchive)


SUM/EB/JB/WB


Summary

LEAP project is:

• increasing TOA accuracy

• providing high signal-to-noise data(studies of noise characteristics)

• helping to understand timing systematics between telescopes

• calibrating instrumental delays between EPTA telescopes

Thank you!


Optimizing pulsar timing arrays

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