Pulsar Astronomy and Astrophysics Frontiers

R. N. ManchesterCSIRO Astronomy and Space Science

Australia Telescope National Facility, Sydney

Summary• Recent results from pulsar searches

• Pulsar timing – glitches and period fluctuations

• The Parkes Pulsar Timing Array (PPTA) project

Spin-Powered Pulsars: A Census

Data from ATNF Pulsar Catalogue, V1.41 (www.atnf.csiro.au/research/pulsar/psrcat)

(Manchester et al. 2005)

• Currently 1973 known (published) pulsars

• 1788 rotation-powered disk pulsars

• 167 in binary systems

• 236 millisecond pulsars

• 141 in globular clusters

• 8 X-ray isolated neutron stars

• 15 AXP/SGR

• 20 extra-galactic pulsars

• For most pulsars P ~ 10-15

• MSPs have P smaller by about 5 orders of magnitude

• Most MSPs are binary, but few normal pulsars are

• c = P/(2P) is an indicator of pulsar age

• Surface dipole magnetic field ~ (PP)1/2

The P – P Diagram.

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P = Pulsar period

P = dP/dt = slow-down rate.

Galactic Disk pulsars

Great diversity in the pulsar population!

Recent Pulsar Searches

• HTRU Parkes 20cm multibeam search

- Mid-latitude survey

- RRATs

• More RRATs from the Parkes Multibeam Survey

• Radio detections of Fermi sources

• Fermi blind search

HTRU Parkes multibeam search• New digital backend system for the 13-beam 20cm Parkes system

• 1024 channels and 64 s sampling (cf., PMPS 96 channels, 250 s)

Survey in three parts:

- High-latitude survey:

Dec < +10o, 270s/pointing

- Mid-latitude survey:

-120o < l < +30o, |b| < 15o, 540s

- Low-latitude survey:

-80o < l +30o, |b| < 3.5o, 4300s

(Keith et al. 2011)Mid-latitude survey ~30% complete27 pulsars detected so far, including 5 MSPs

PSR J1622-4950:a radio-loud magnetar

• Discovered in Parkes HTRU survey

• P = 4.3 s, P = 1.7 x 10-11

• Bs = 2.8 x 1014 G

• c = 4 kyr

• Spin-down lum, E ~ 8.5 x 1033 erg s-1

(Levin et al. 2010)

Radio (1.4 GHz) variability

Chandra X-ray• Radio emission flat spectrum, highly variable both in flux density and pulse shape

• X-ray source detected by Chandra, luminosity ~ 2.5 x 1033 erg s-1

• Possible SNR association

ATCA 5.5 GHz

A magnetar in X-ray quiescence detected through its radio pulsations

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HTRU RRATs Search

(Burke-Spolaor et al. 2011)11 new RRATs discovered!

• HTRU survey data searched for isolated dispersed pulses• Identified as Rotating Radio Transients (RRATs)

1451 sources!

~100 pulsars!!

Fermi Gamma-ray Pulsars

• 98 pulsars now have detectable -ray emission

- 7 detected by EGRET prior to Fermi launch in June 2008

• 30 are known young radio pulsars, e.g. Vela pulsar

• 13 are known radio millisecond pulsars (MSPs)

• 25 (young) pulsars discovered in blind -ray searches

- 3 of these detected in deep radio searches

• 30 MSPs detected in radio searches of -ray sources!!

The Vela Pulsar

(Abdo et al. 2009)

• Strong radio pulsar associated with Vela SNR

• P = 89.3 ms, c = 11.3 kyr

• E = 6.9x1036 erg/s

• Brightest -ray source

• -ray pulses detected by SAS-2 (1975), COS-B (1988), EGRET (1994), Fermi (2009)

• Double -ray profile

• P1 lags radio by 0.14 periods

• UV double pulse between -ray main peaks

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Now 30 previously known young radio pulsars have -ray pulse detections

Fermi Detections of Known MSPs

(Abdo et al. 2009)

• Many MSPs have relatively high values of E/d2

• Searches at positions of known MSPs using radio timing ephemeris

• 13 MSPs detected!

• Generally -ray pulse morphology and relationship to radio profiles similar to young pulsars

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Blind Searches for Pulsars in Fermi Data• Many unidentified Fermi sources that have -ray properties consistent with those of known pulsars

• Some have associations with SNR, X-ray point sources, etc., but no known pulsar

• Computationally impossible to search directly for periodicities – long data spans and not many photons

• Time differences between photons up to a few weeks apart searched for periodicities

• Once pulsations are detected, can do a timing analysis and get accurate period, period derivative and position

25 pulsars detected!

Fermi – CTA1 Pulsar

(Abdo et al. 2008)

First gamma-ray pulsar found in a blind search!

PSR J0007+7303

Fermi Blind-search Pulsars

(Abdo et al. 2009, Saz Parkinson et al. 2010 )

• 25 mostly young, high-E pulsars

• Have pulse profiles very similar to radio-selected sample

• Three have been detected as faint radio pulsars

• PSR J1907+0602 detected at Arecibo, only 3 Jy!

• Most have low upper limits on S1400

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17(2010)

GBT Survey for pulsars associated with Fermi gamma-ray sources

(Hessels et al. 2011)

• GBT 100m telescope at 350 MHz, 100 MHz bw, 4096 chan., 81.92 s samp. int.• 50 Fermi sources observed, observation time/pointing 32 min

10 MSPs discovered, P range: 1.6 ms – 7.6 ms

Now 30 MSPs detected from radio searches of -ray sources!

E/d2 – Period Dependence

(Abdo et al., 2009)• -ray pulses detected: red dot• -ray point source: green triangle

• Radio-selected sample

• Most high E/d2 pulsars have detected -ray pulsed emission, for both young pulsars and MSPs

• But some are not detected

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(Abdo et al. 2010)

• Two thirds of -ray pulsars are also detected at radio wavelengths

• All pulsars with E > 1037 erg s-1 are detected in both bands

• Many have similar radio and -ray pulse profiles

• Some high-E/d2 radio pulsars are not (yet) detected by Fermi

Radio – Beaming

• Radio beams for high-E pulsars are wide!

• For high E pulsars, both radio and -ray emission regions are in the outer magnetosphere, sometimes but not always co-located

J0034-0534

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(Ravi et al. 2010)

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Pulsar Glitches

• Sudden increase in spin rate of neutron star (); typically ~ 1 - 5000 x 10-9

• Usually accompanied by increase in slow-down rate (|)• Increase in | often decays more-or-less exponentially with timescale in range 1 – 500 days

(Espinoza et al. 2011)

• Probably due to sudden transfer of angular momentum to NS crust from faster rotating interior superfluid

..

Two Giant GlitchesPSR B2334+61:• Timed at Xinjiang Astronomical Observatory

• P ~ 0.495 s, c ~ 41 kyr

• Glitch in 2005, ~ 20.5 x 10-6

• Two exp. decays observed, d ~ 20 d, d ~ 150 d

• Permanent increase in slow-down ~ 1.1%

• Also increase in by factor of four

• Possible ~350-day oscillation in after glitch

PSR J1718-3718:(Yuan et al. 2010)

• Timed at Parkes, at 1.4 and 3 GHz

• P ~ 3.8 s, c ~ 34 kyr, Bs ~ 7 x 1013 G

• Glitch in 2007, ~ 33.2 x 10-6

• Little change in at glitch

• Significant decrease in at glitch

- very unusal and not easily explained

. ...

...

(Manchester & Hobbs 2011)

J1846-0258 in SNR Kes 75

• Youngest known pulsar – c ~ 800 yr

• Discovered at X-rays, no radio detection

• P ~ 326 ms, centred in SNR Kes 75

• Large glitch ~ 4 x 10-6 in 2006

• Burst in X-rays at same time

• Large increase in slow-down rate after glitch

• Over-decay so that less than pre-glitch extrapolation

• Change in braking index: n(pre) = 2.65 +/- 0.01, n(post) = 2.16 +/- 0.13

Change in magnetic structure and particle outflow at time of glitch

(Livingstone et al. 2010,2011)

Pulsar Timing Arrays • A Pulsar Timing Array (PTA) is an array of pulsars widely distributed on the sky that are being timed with high precision with frequent observations over a long data span

• PTA observations have the potential to detect a stochastic gravitational wave background from binary SMBHs in the cores of distant galaxies

• Requires observations of ~20 MSPs over 5 – 10 years; could give the first direct detection of gravitational waves!

• PTA observations can improve our knowledge of Solar system properties, e.g. masses and orbits of outer planets and asteroids

• PTA observations can detect instabilities in terrestrial time standards and establish an ensemble pulsar timescale (EPT)Idea first discussed by Hellings & Downs (1983),

Romani (1989) and Foster & Backer (1990)

Clock errors

All pulsars have the same TOA variations: monopole signature

Solar-System ephemeris errors

Dipole signature

Gravitational waves

Quadrupole signature

Can separate these effects provided the PTA contains a sufficient number of widely distributed pulsars

Global Effects in a PTAThe three main global timing effects that can be observed with a PTA have different spatial signatures on the sky

Detecting a Stochastic GW Background

TEMPO2 simulation of timing-residual correlations due to a GW background for the PPTA pulsars

Hellings & Downs correlation function

(Hobbs et al. 2009)

• A stochastic background of GWs in the Galaxy independently modulates both the pulse period emitted from a pulsar and the period observed at Earth

• In a PTA, the modulations from GWs passing over the pulsars are uncorrelated

• GWs passing over the Earth produce a correlated modulation of the signal from the different pulsars – it is this correlation that enables us to detect GWs!

• The quadrupolar nature of GWs results in a characteristic correlation signature in the timing residuals from pulsar pairs which, for an isotropic stochastic background, is dependent only on the angle between the pulsars

• The uncorrelated GWs passing over the pulsars reduces the maximum correlation to 0.5• It also introduces a “self-noise” in the correlations which is independent of ToA precision

Major Pulsar Timing Array Projects European Pulsar Timing Array (EPTA)

• Radio telescopes at Westerbork, Effelsberg, Nancay, Jodrell Bank, (Cagliari)

• Currently used separately, but plan to combine for more sensitivity

• High-quality data (rms residual < 2.5 s) for 9 millisecond pulsars

North American pulsar timing array (NANOGrav)

• Data from Arecibo and Green Bank Telescope

• High-quality data for 17 millisecond pulsars

Parkes Pulsar Timing Array (PPTA)

• Data from Parkes 64m radio telescope in Australia

• High-quality data for 20 millisecond pulsars

Observations at two or three frequencies required to remove the effects of interstellar dispersion

The Parkes Pulsar Timing Array Project• Using the Parkes 64-m radio telescope to observe 20 MSPs

• ~25 team members – principal groups: Swinburne University (Melbourne; Matthew Bailes), University of Texas (Brownsville; Rick Jenet), University of California (San Diego; Bill Coles), CASS, ATNF (Sydney; RNM, GH)

• Observations at 2 – 3 week intervals at three frequencies: 732 MHz, 1400 MHz and 3100 MHz

• New digital filterbank systems and baseband recorder system

• Regular observations commenced in mid-2004

• Timing analysis – PSRCHIVE and TEMPO2

• GW simulations, detection algorithms and implications, galaxy evolution studies

The PPTA Pulsars

Best result so far – PSR J0437-4715 at 10cm

• Observations of PSR J0437-4715 at 3100 MHz

• 1 GHz bandwidth with digital filterbank systems (PDFB1, 2 and 4)

• 3.1 years data span

• 374 ToAs, each 64 min observation time

• Weighted fit for 12 parameters using TEMPO2

• No dispersion correction

• Reduced 2 = 2.46

Rms timing residual 55 ns!

14 Years of Timing PSR J0437-4715• Data from FPTM, CPSR1, CPSR2, WBC, PDFB1,2,4 (Verbiest et al. 2008 + PPTA)

• Offsets between instruments determined from overlapping/adjacent data and then held fixed

• Fit for position, pm, F0, F1, binary parameters

• Clear evidence for long-term (“red”) period variations – origin?

Current status: • Timing data at 2 -3 week intervals at 10cm or 20cm

• PDFB2, 4 (1), spans 2.3 – 4.0 years

• TOAs from 64-min observations (mostly; some 32 min)

• Uncorrected for DM variations

• Solve for position, F0, F1, Kepler parameters if binary

• Four pulsars with rms timing residuals < 200 ns, 13 with < 1 s

• Best results on J0437-4715 (55 ns), and J1909-3744 (95 ns)Getting better, but more

work to be done!

* Needs DM corrections # PCM calibration

Effect of Dispersion Measure Variations• PSR J1045-4509

• Six years of timing at 20cm (1.4 GHz) and 50cm (700 MHz)

• Correlated residual variations with -2 dependence – due to variations in interstellar dispersion

• Must be removed for PTA applications

• PSR J1045-4509: DM correction reduces post-fit residuals by ~50%

• Observed DM variations interesting for ISM studies

20cm

50cm

Before DM Correction

After DM Correction

20cm post-fit

Polarisation Calibration• 20cm feed has significant cross-polar coupling (~ –10db)

• Results in parallactic-angle dependence of pulse profile

• Cross-coupling can be measured and profiles corrected using PSRCHIVE routines (PCM and PAC)

• Results in large improvement for highly polarised pulsars, e.g. PSR J1744-1134

• 3 years of PDFB2/4 data at 20cm

• Before PCM correction: Rms residual = 487 ns Reduced 2 = 19.0

• After PCM correction: Rms residual = 195 ns Reduced 2 = 3.1

Measuring Planet Masses with Pulsar Timing• Timing analysis uses Solar-System ephemeris (from JPL)• Error in planet mass leads to sinusoidal term in timing residuals• Obs of four pulsars, data from Parkes (CPSR2), Arecibo, Effelsberg:

J0437-4715 – (P) 13.5 yr J1744-1134 – (P) 14.7 yr J1857+0943 – (P,A,E) 23.8 yr J1909-3744 – (P) 6.8 yr

• Tempo2 modified to solve for planet mass using all four data sets simultaneously• Jupiter is best candidate:

(Champion et al., 2010)Best published value: (9.547919 ± 8) × 10-4 Msun

Pulsar timing result: (9.547922 ± 2) × 10-4 Msun

Unpub. Galileo result: (9.54791915 ± 11) × 10-4 Msun

More pulsars, more data span, should give best available value!

MJupiter = 5 x 10-10 MSun

(Wen et al. 2010)

Stochastic GWB Detection with PTAs

• SMBH binary merger rate in galaxies is constrained by PTA observations

• Model predictions for GW by Jaffe & Backer (JB03) and Sesana et al. (S0809)

• Two cases: equal 109 M binary, equal 1010 M binary

Δ Obs. limit by Jenet et al. (J06) × 20 psrs, 100 ns, 5 years ☐ 20 psrs, 500 ns, 10 years O 20 psrs, 100 ns, 10 years 100 psrs, 100 ns, 10 years

100 psrs, 10 ns, 10 years

JB03S0809

J06

J06

SKA will detect GWs!

The Gravitational Wave Spectrum

An Ensemble Pulsar Timescale (EPT)

• Terrestrial time defined by a weighted average of cesium clocks at time centres around the world

• TAI is (nearly) real-time atomic timescale

• Revised by reweighting to give BIPMxxxx

• Current best pulsars give a 10-year stability (z) comparable to TT(NIST) – TT(PTB) – two of the best atomic timescales

• Pulsar timescale is not absolute, but can reveal irregularities in TAI and other terrestrial timescales

• Analysis of “corrected” Verbiest et al. data sets for 18 MSPs using TEMPO2 and Cholesky method (Coles et al. 2010) to optimally deal with red timing noise

TAI – BIPM2010

EPT(PPTA2010) – Relative to TAI

EPT BIPM2010

First realisation of a pulsar timescale with accuracy comparable to atomic timescales!

(Hobbs et al. 2010)

Summary Several on-going pulsar searches are gradually increasing the number of known pulsars, especially millisecond pulsars

The Fermi Gamma-ray Observatory has increased the number of known -ray-emitting pulsars by an order of magnitude

Radio and -ray emission regions for high-E pulsars and MSPs are both high in the pulsar magnetosphere – sometimes co-located

Pulsar Timing Arrays have the potential to detect nHz gravitational waves and to establish the most precise long-term standard of time

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, e.g. SKA

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GW from Formation of Primordial Black-holes• Black holes of low to intermediate mass can be formed at end of the inflation era from collapse of primordial density fluctuations

• Intermediate-mass BHs (IMBH) proposed as origin of ultra-luminous X-ray sources; lower mass BHs may be “dark matter”

• Collapse to BH generates a spectrum of gravitational waves depending on mass

(Saito & Yokoyama 2009)

Pulsar timing can already rule out

formation of black holes in mass range

102 – 104 M!

Radio and -ray Beaming• Approximate sky coverage by “top-hat” fan beams (integral over of two-dimensional beam pattern)

• r and g are equivalent widths of radio and -ray beams respectively

• c is the angular width of the overlap region

• For a random orientation of rotation axes:

the relative number of pulsars detectable in band i is proportional to i

the relative number of pulsars detectable in both bands is proportional to c

In all cases r >= c

(Ravi, Manchester & Hobbs 2010)

Radio – -ray Beaming• For the highest Edot pulsars, r >~ g

• This implies that the radio beaming fraction fr is comparable to or greater than the -ray beaming fraction fg

• For OG and TPC models, fg ~ 1.0

• For lower Edot Sample G pulsars, fr >~ 0.57 – includes several MSPs

• Even high-altitude radio polar-cap models (e.g., Kastergiou & Johnston

2007) are unlikely to give fr >~ fg ~ 1

• Therefore …

(Manchester 2005, Ravi et al. 2010)

For high Edot pulsars, it is probable that the radio emission region is located in the outer magnetosphere Radio pulse profiles are formed in a similar way to -ray profiles with caustic effects important

(Ravi, Manchester & Hobbs 2010)

Radio – -ray Beaming• Two samples:

G: All pulsars found (or that could be found) in the Fermi 6-month blind search (Abdo et al. 2010)

R: High Edot radio pulsars searched by LAT for -ray emission (Abdo et al. 2010)

• Fraction of G and R samples with Edot > given value observed at both bands plotted as function of Edot

• 20/35 Sample G pulsars detected in radio band

• 17/201 Sample R pulsars detected in -ray band

For both samples, the

highest E pulsars a

re

detected in both bands.

Vela Pulsar Gamma-Ray Spectrum• Integrated spectrum from Fermi LAT

• Power-law with exponential cutoff

• Power-law index = 1.38 ± 0.08

• Exp. cutoff freq. Ec ~ 1.4 Gev

• Super-exponential cutoff excluded

• Implies that emission from high altitude in pulsar magnetosphere

PSR B0833-45

Modelling of -ray pulse profiles• Two main models:

Outer-Gap modelSlot-Gap or Two-Pole Caustic model

(Watters et al. 2009)

• OG model in red• TPC model in green• 500 km altitude PC emission (radio) in aqua

Blind Detection of PSR J1022-5746

• Most energetic blind Tc 4.6 kyr• HESS association - PWN

(Abdo et al. 2009)

PTA Pulsars: Timing Residuals

• 30 MSPs being timed in PTA projects world-wide• Circle size ~ (rms residual)-1

• 12 MSPs being timed at more than one observatory

Sky positions of all known MSPs suitable for PTA studies

• In the Galactic disk (i.e. not in globular clusters) • Short period and relatively strong – circle radius ~ S1400/P • ~60 MSPs meet criteria, but only ~30 “good” candidates• Current searches finding some potentially good PTA pulsars

Fermi Observations of Known Pulsars • In pre-Fermi era, seven pulsars known to emit -ray pulses

• Fermi scans whole sky every 3 hours – detected photons tagged with time, position and energy

• Timing consortium using radio telescopes at Parkes, Green Bank, Arecibo, Nancay and Nanshan – timing solutions for 300+ pulsars with high E/d2 (E = 42IP/P3)

• Photons with directions within PSF of known radio pulsar selected

• Total data span usually many months, few x 1000 photons

• Folded at known pulsar period and tested for periodicity

• For detected sources, can form mean pulse profile in different energy bands and (for stronger sources) spectra for different time bins across pulse profile

.. .

PSR J1048-5832

• P = 123.7 ms

• c = 20.3 kyr

• E = 2x1036 erg/s

• Marginal EGRET detection

PSR J2229+6114

• P = 51.6 ms

•c = 10.5 kyr

• E = 2x1037 erg/s

• X-ray profile double but single at -ray

Fermi Detections of Young Radio Pulsars

(Abdo et al. 2009)Now 30 previously known young radio

pulsars have -ray pulse detections

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Gravitational Waves• Prediction of general relativity and other theories of gravity

• Generated by acceleration of massive objects

• Propagate at the speed of light

(K. Thorne, T. Carnahan, LISA Gallery)

• Astrophysical sources:

Inflation era fluctuations

Cosmic strings

BH formation in early Universe

Binary black holes in galaxies

Black-hole coalescence and infall

Coalescing double-neutron-star binaries

Compact X-ray binaries

These sources create a stochastic GW background in the Galaxy

Detection of Gravitational Waves• Generated by acceleration of massive objects in Universe, e.g. binary black holes• Huge efforts over more than four decades to detect gravitational waves• Initial efforts used bar detectors pioneered by Weber• More recent efforts use laser interferometer systems, e.g., LIGO, VIRGO, LISA

• Two sites in USA• Perpendicular 4-km arms• Spectral range 10 – 500 Hz• Initial phase now operating• Advanced LIGO ~ 2014

LISALIGO• Orbits Sun, 20o behind the Earth• Three spacecraft in triangle• Arm length 5 million km• Spectral range 10-4 – 10-1 Hz• Planned launch ~2020

Timing Stability of MSPs

• 10-year data span for 20 PPTA MSPs

• Includes 1-bit f/b, Caltech FPTM and CPSR2 data

• z: frequency stability at different timescales

• For “white” timing residuals, expect z ~ -3/2

• Most pulsars roughly consistent with this out to 10 years

• Good news for PTA projects!

(Verbiest et al. 2009)

100 ns

10 s

Single-source Detection

Need better sky distribution of pulsars - international PTA collaborations are

important!

Localisation with PPTASensitivity

(Anholm et al. 2008)

• First realistic sensitivity curve for a PTA system!

• Computed GW strains for SMBH binary systems in Virgo cluster

• PPTA can’t expect to detect individual systems - but SKA will!

(Yardley et al. 2010)

Studies of MSP and binary parameters and evolution Pulsar astrometry Pulsar polarisation and emission mechanisms Interstellar medium – ne fluctuations and magnetic fields Tests of gravitational theories Galaxy and SMBH evolution and mergers Instrumental and software development

- Low-noise broad-band receivers- Ultra-fast signal processing systems- Timing analysis systems and simulations- RFI mitigation

PTA Spin-offs

(Yan et al. 2010)

PTA projects have many secondary objectives: