The Parkes Pulsar Timing Array Project

1
The Parkes Pulsar Timing Array Project
R. N. Manchester
Australia Telescope National Facility, CSIRO
Sydney Australia
Summary

• Project outline
• Recent instrumental developments
• Current results
• Future prospects

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The Parkes Pulsar Timing Array Project
Collaborators

• Australia Telescope National Facility, CSIRO,
  Sydney
• Dick Manchester, George Hobbs, David Champion,
  John Sarkissian, John Reynolds, Mike Kesteven,
  Warwick Wilson, Grant Hampson, Andrew Brown,
  Jonathan Khoo, (Russell Edwards, David Smith)
• Swinburne University of Technology, Melbourne
• Matthew Bailes, Willem van Straten, Joris
  Verbiest, Ramesh Bhat, Sarah Burke, Andrew
  Jameson
• University of Texas, Brownsville
• Rick Jenet
• University of California, San Diego
• Bill Coles
• Franklin Marshall College, Lancaster PA
• Andrea Lommen
• University of Sydney, Sydney
• Daniel Yardley, Sam To
• National Observatories of China, Beijing
• Johnny Wen
• Peking University, Beijing
• Kejia Lee
• Southwest University, Chongqing
• Xiaopeng You
• Curtin University, Perth

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The PPTA Project Goals

• To detect gravitational waves of astrophysical
  origin
• To establish a pulsar-based timescale and to
  investigate irregularities in terrestrial
  timescales
• To improve on the Solar System ephemeris used
  for barycentric correction

To achieve these goals we need weekly
observations of 20 MSPs over five - ten years
with TOA precisions of 100 ns for 10 pulsars
and lt 1 ?s for rest

• Modelling and detection algorithms for GW
  signals
• Measurement and correction for interstellar and
  Solar System propagation effects
• Implementation of radio-frequency interference
  mitigation techniques

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The PPTA Project

• Using the Parkes 64-m radio telescope at three
  frequencies, 685 MHz, 1400 MHz and 3100 MHz, to
  observe 20 MSPs
• Observations at 2 - 3 week intervals using
  10/50cm and 20cm receivers
• Using polyphase digitial filterbanks and
  baseband recording systems
• Regular observations commenced in mid-2004 using
  WBC, PDFB1 and CPSR2
• New digital filterbank systems with higher time
  and frequency resolution PDFB2 commissioned in
  March 2007, PDFB3 in February 2008
• New baseband recorder system (APSR) currently
  being commissioned
• All data stored in PSRFITS format
• mySQL database and processing pipeline -
  PSRCHIVE programs
• Timing analysis - TEMPO2
• GW simulations, detection algorithms and
  implications, galaxy evolution studies

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Sky Distribution of Millisecond Pulsars
P lt 20 ms and not in globular clusters
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New Observing Systems
CABB Processor Board

• PDFB2, PDFB3 Digital Polyphase Filterbank
  systems
• APSR Baseband recording system

• Bandwidths 64, 128, 256, 512 or 1024 MHz
• 8-bit digitisation of 2 or 4 inputs
• On-line folding at pulsar period
• 2048 channels x 2048 phase bins for 4-ms pulsar,
  more for longer periods
• Up to 8192 channels or bins 4M max product
• Real-time RFI rejection - adaptive FIR filters
• Search mode - streamed data
• Front-end for APSR system, 16 baseband chan.

PDFB3/APSR Block Diagram
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PDFB3 - Initial Results
PSR B1641-45 (0.455 sec) 2048 bins x 2048
channels at 20cm 256 MHz bandwidth Raw spectrum
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PDFB3 PSR J0437-4715 at 10cm
Pulse period 5.75 ms 1GHz bandwidth 2048 bins x
2048 channels
Raw Spectrum
Calibrated Spectrum
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PDFB3 Search mode

• 1-, 2-, 4- or 8-bit digitisation
• 1, 2 or 4 polarisation products
• 128 - 2048 channels
• Sampling interval 32 ?s - 10 ms (Maximum rate
  dept on configuration)
• Data files in PSRFITS format

Not yet fully commissioned
Vela at 1.4 GHz, 256 MHz bandwidth, 4-bit data
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APSR Next Generation Baseband System

• Swinburne/ATNF collaboration. Uses PDFB3 as a
  frontend.
• 16 baseband channels over 64 - 1024 MHz
• 8-bit digitisation for 64 - 256 MHz, 4-bit for
  512 MHz, 2-bit for 1024 MHz
• 4 x 10 Gb ethernet to 16 dual-quad-core Dell
  processors
• Quasi-real-time coherent dedispersion and
  folding
• Web-based interface.

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PDFB3 Real-time RFI Mitigation
System devised by Mike Kesteven, implemented by
Andrew Brown and Grant Hampson

• Reference antenna pointed at RFI source
• 2048 channels across band
• Signalreference cross-correlation subtracted
  from data
• Filter updates at 1ms intervals to follow
  tropospheric multi-path propagation
• Single reference can be applied to both
  polarisations of telescope signal
• Main application (so far) Digital TV signals in
  50cm band
• 4-m reference antenna pointed toward Mt Ulandra,
  200 km south of Parkes

64 MHz at 50cm (653-617 MHz)
RFI Mitigation Off
Mt Ulandra TV signal
50cm Receiver
Cross-Correlation
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PSR J0437-4715 at 50cm
RFI Mitigation On
Stokes I
S/N 1710!
RFI Mitigation Off
S/N 1040 in 4 min

• Pulsar signal under RFI is recovered with no
  (evident) perturbation
• Improvement in S/N will be greater with weaker
  pulsars
• Expect to apply to regular 50cm observations
  with APSR soon

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Pipeline Processing of PPTA data

• Data transferred from Parkes to Epping over 1Gb
  ethernet, usually within a day or two of
  observations, stored on RAIDs
• mySQL database used to keep track of data and
  processing status
• PSRCHIVE programs used for data processing
• PPTA1 script Automatic zapping of narrow-band
  and impulsive RFI
• PPTA2 script Polarisation and bandpass
  calibration, scrunching in frequency time and
  polarisation, computation of TOAs using analytic
  profile templates, TEMPO2 residual plots.
• Manual data checking and correction, flagging of
  unrecoverable data
• PPTA3 script Computation and application of DM
  corrections to TOAs, recomputation of TEMPO2
  residual plots

System developed by George Hobbs, David Champion
and Xiaopeng You
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Analytic Profile Templates (all 20cm)
PSR J0437-4715
PSR J17130747
PSR J1744-1134
PSR J18570943
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Timing Residuals for PSR J0437-4715

• PDFB2 at 10cm
• 1 GHz bandwidth
• 1.2 years data span
• 64 min obs time
• 211 TOAs
• Weighted fit for 9 parameters ?, ?, F0, F1,
  Kepler binary parameters
• No DM correction
• Reduced ?2 2.87

Rms timing residual 56 ns!!
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PPTA Pulsars One year of PDFB2 data

• Timing data at 2 -3 week intervals at 10cm or
  20cm
• TOAs from 64-min observations (except
  J18570943, J19392134, J2124-3358, each 32 min)
• Uncorrected for DM variations
• Solve for position, F0, F1, Kepler parameters if
  binary
• Four pulsars with rms timing residuals lt 200 ns,
  eleven lt 1 ?s
• Best results on J0437-4715 (56ns), J1909-3744
  (130 ns), J19392134 (150ns)

Best timing results yet!
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DM Corrections

• 10cm/20cm and 50cm CPSR2 data
• About 4 yr data span
• Routinely applied to all 20 pulsars

?DM of 10-3 corresponds to ?t 2.1 ?s at 20cm
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Timing Residuals

• PDFB1/PDFB2 data
• 3-year data span
• DM-corrected

10cm/20cm, reduced ?2 5.67
20cm, reduced ?2 1.40
Rms residuals roughly double 1-year PDFB2 results.
Still some systematic errors.
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What next?

• Investigate reasons for outliers and correct if
  possible
• Correct for feed cross-coupling where necessary
  (MB data)
• Develop and use frequency-dependant analytic
  templates
• Systematic frequency dependence of TOAs across
  band commonly observed - can be as large as 2 ?s
• Probably intrinsic profile variation, but may
  have instrumental component
• Calibrate and remove instrumental delays
• Pulse-modulated calibration signal - 1 ms
  periodicity
• Analyse using standard TOA methods - can also
  check algorithms
• Measurement precision a few nanoseconds
• Avoid need for jumps between instruments
• More observing!

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Future Prospects

• We are now approaching the level of TOA
  precision that is required to achieve the main
  goals of PTA projects
• Good chance that detection of nanoHertz GW will
  be achieved with a further 5 - 10 years of data
  if current predictions are realistic
• Major task is to eliminate all sources of
  systematic error - good progress, but not there
  yet
• So far, intrinsic pulsar period irregularities
  are not a limiting factor
• Progress toward all goals will be enhanced by
  international collaboration - more (precise) TOAs
  and more pulsars are better!
• Current efforts will form the basis for detailed
  study of GW and GW sources by future instruments
  with higher sensitivity