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P. Demorest Amaldi 8, June 2009 1

Key Challenges and Advances for Pulsar

Timing

Paul Demorest, NRAO

June 25, 2009

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Pulsar Timing Challenges

P. Demorest Amaldi 8, June 2009 2

Context: Rough GW detection requirement is 2040 MSPs timingat 100 ns precision. Current PTA projects have 3 pulsars at100 ns, 1020 at 1 s. What challenges do we face in achieving

this order of magnitude improvement in timing results?

Talk outline:

Pulsar Timing Overview

Timing Signal-to-Noise Ratio

Systematic Effects

Future Prospects

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Pulsars

P. Demorest Amaldi 8, June 2009 3

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Radio Telescopes

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Observations

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Basic observing process:

q Incoming radio wave is amplified then digitized

q . . . split into many frequency channels

q . . . coherently dispersion-correctedq . . . folded modulo pulsar spin period

Resulting in pulse profiles/lightcurves (flux vs spin phase):

-0.05

0

0.05

0.1

0.15

0.2

0.25

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

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Raw Timing Data

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Each observation is a cube containing radio flux as a fn of time(min), radio freq (MHz), and spin phase (103 turns).

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Measuring Times of Arrival

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Combine data profiles:

-0.05

0

0.05

0.1

0.15

0.2

0.25

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

with high-SNR templates:

-5

0

5

10

15

20

25

30

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

to get the shift between the two, or time of arrival (TOA).

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Timing Model Fits

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TOAs are then fit to a timing model including spin, astrometric,binary, etc parameters:

-100

-50

0

50

100

150

200

2004.5 2005 2005.5 2006 2006.5 2007 2007.5 2008 2008.5

Residual(us)

Year

-150

-100

-50

0

50

100

150

200

2004.5 2005 2005.5 2006 2006.5 2007 2007.5 2008 2008.5

Residual(us)

Year

-10-8

-6

-4

-2

0

2

4

6

2004.5 2005 2005.5 2006 2006.5 2007 2007.5 2008 2008.5

Residual(us)

Year

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TOA uncertainty

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Basic time-of-arrival uncertainty calculation depends on pulseprofile SNR:

t

W

SNR=WSpsr

1B

Aeff

kTsys

q Intrinsic to each pulsar:

3 W Pulse sharpness (

width)

3 Spsr Pulsar radio flux

q Instrumental parameters:

3 B Radio bandwidth

3 Total observation time

3 Aeff Effective telescope area

3 Tsys Receiver system temperature

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Improving timing uncertainties

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Timing of most MSPs is SNR-limited. How can we improve things?

q Intrinsic pulsar factors Find more/better pulsars! One of the

current best timers (J19093744) was only discovered in 2003.

q Instrumental improvements

3 GW timing programs currently occupy 5% of largeradio telescopes time.

3 B Current-gen backends 50100 MHz, next-gen1 GHz, next-next-gen 1-3 GHz.

3Aeff Build new/larger telescopes

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Systematic Timing Effects

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Any process that shifts or distorts the pulse shape:

q Local

3 Polarimetry and calibration

3 Manmade radio-frequency interference3 Instrumental artifacts (quantization, etc)

q Interstellar medium

3 Dispersion measure variation with time

3 Multipath propagation effects (scattering/scintillation)

q Intrinsic to pulsar

3 Intrinsic pulse shape variations

3 Single-pulse jitter

3 Timing noise

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Interstellar medium

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Ionized interstellar gas affects radio wave via plasma dispersion

relation.

These effects are strong functions of RF, whereas GW effect is

achromatic.

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Dispersion variation

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Total electron column density varies due to motions of Earth, PSR,and ISM:

(PSR B1937+21; Ramachandran et al. 2006)

Can be measured/removed with timing measurements at widely

separated RFs.

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Scattering/Scintillation

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Electron density variation transverse to the line-of-sight causesconstructive/destructive interference:

(Walker et al. 2008)

Typical scales kHzMHz,T sechours. Determinedby LOS/ISM velocity and ISM

length scales.

Effectively a (time-varying) filter on the signal. This affects profile

shapes!

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ISM corrections

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How to deal with this?

q ISM effects decrease at higher RF:

3 dispersion2

3 scattering 43 BUT Spsr 1.8

q

Current standard approach: optimize set of observing freqs. tobalance these effects.

q More advanced approaches currently under development will

measure scintillation patterns and correct TOAs.3 New wideband backends will help this effort.

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Historical improvement

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Pulsar timing Moores Law:

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Current and Future Work

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q Ongoing pulsar searches at Parkes, Arecibo (PALFA), GBT, . . .

q Backend instrumentation: Next-gen backends currently being

tested include GUPPI (NRAO), DFB3 (ATNF), APSR

(Swinburne). These are approaching receiver-limited BW.

q Current data analysis efforts focus on polarization/calibration

and ISM issues.

q New telescopes: EVLA, ATA, FAST, MeerKAT, ASKAP. . . (and

eventually SKA)