Pulsar Astronomy

1
Pulsar Astronomy and Astrophysics Frontiers
R. N. Manchester
CSIRO 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

2
Spin-Powered Pulsars A Census

• 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

Data from ATNF Pulsar Catalogue, V1.41
(www.atnf.csiro.au/research/pulsar/psrcat)
(Manchester et al. 2005)
3
.
The P P Diagram
Galactic Disk pulsars
P Pulsar period P dP/dt slow-down rate
.
.

• 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
• tc P/(2P) is an indicator of pulsar age
• Surface dipole magnetic field (PP)1/2

.
.
.
Great diversity in the pulsar population!
4
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

5
HTRU Parkes multibeam search

• New digital backend system for the 13-beam 20cm
  Parkes system
• 1024 channels and 64 ms sampling (cf., PMPS 96
  channels, 250 ms)

• Survey in three parts
• High-latitude survey
• Dec lt 10o, 270s/pointing
• Mid-latitude survey
• -120o lt l lt 30o, b lt 15o, 540s
• Low-latitude survey
• -80o lt l 30o, b lt 3.5o, 4300s

Mid-latitude survey 30 complete 27 pulsars
detected so far, including 5 MSPs
(Keith et al. 2011)
6
PSR J1622-4950a radio-loud magnetar
Radio (1.4 GHz) variability

• Discovered in Parkes HTRU survey
• P 4.3 s, P 1.7 x 10-11
• Bs 2.8 x 1014 G
• tc 4 kyr
• Spin-down lum, E 8.5 x 1033 erg s-1

.
.

• 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

Chandra X-ray
ATCA 5.5 GHz
A magnetar in X-ray quiescence detected through
its radio pulsations
(Levin et al. 2010)
7
HTRU RRATs Search

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

(Burke-Spolaor et al. 2011)
8
1451 sources!
100 pulsars!!
9
Fermi Gamma-ray Pulsars

• 98 pulsars now have detectable g-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 g-ray
  searches
• - 3 of these detected in deep radio searches
• 30 MSPs detected in radio searches of g-ray
  sources!!

10
The Vela Pulsar

• Strong radio pulsar associated with Vela SNR
• P 89.3 ms, tc 11.3 kyr
• E 6.9x1036 erg/s
• Brightest g-ray source
• g-ray pulses detected by SAS-2 (1975), COS-B
  (1988), EGRET (1994), Fermi (2009)
• Double g-ray profile
• P1 lags radio by 0.14 periods
• UV double pulse between g-ray main peaks

.
Now 30 previously known young radio pulsars have
g-ray pulse detections
(Abdo et al. 2009)
11
Fermi Detections of Known MSPs

• Many MSPs have relatively high values of E/d2
• Searches at positions of known MSPs using radio
  timing ephemeris
• 13 MSPs detected!
• Generally g-ray pulse morphology and
  relationship to radio profiles similar to young
  pulsars

.
(Abdo et al. 2009)
12
Blind Searches for Pulsars in Fermi Data

• Many unidentified Fermi sources that have g-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!
13
Fermi CTA1 Pulsar
First gamma-ray pulsar found in a blind search!
PSR J00077303
(Abdo et al. 2008)
14
Fermi Blind-search Pulsars
.

• 25 mostly young, high-E pulsars
• Have pulse profiles very similar to
  radio-selected sample
• Three have been detected as faint radio pulsars
• PSR J19070602 detected at Arecibo, only 3 mJy!
• Most have low upper limits on S1400

(Abdo et al. 2009, Saz Parkinson et al. 2010 )
15
17
(2010)
16
GBT Survey for pulsars associated with Fermi
gamma-ray sources

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

10 MSPs discovered, P range 1.6 ms 7.6 ms
(Hessels et al. 2011)
Now 30 MSPs detected from radio searches of g-ray
sources!
17
.
E/d2 Period Dependence

• Radio-selected sample
• Most high E/d2 pulsars have detected g-ray
  pulsed emission, for both young pulsars and MSPs
• But some are not detected

.
(Abdo et al., 2009)

• g-ray pulses detected red dot
• g-ray point source green triangle

18
Radio g Beaming
J0034-0534

• Two thirds of g-ray pulsars are also detected at
  radio wavelengths
• All pulsars with E gt 1037 erg s-1 are detected
  in both bands
• Many have similar radio and g-ray pulse profiles
• Some high-E/d2 radio pulsars are not (yet)
  detected by Fermi

.
.
.
(Abdo et al. 2010)

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

.
(Ravi et al. 2010)
19
Pulsar Glitches

• Sudden increase in spin rate of neutron star
  (n) typically Dn/n 1 - 5000 x 10-9
• Usually accompanied by increase in slow-down
  rate (n)
• Increase in n often decays more-or-less
  exponentially with timescale in range 1 500 days

.
.

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

(Espinoza et al. 2011)
20
Two Giant Glitches
PSR B233461

• Timed at Xinjiang Astronomical Observatory
• P 0.495 s, tc 41 kyr
• Glitch in 2005, Dn/n 20.5 x 10-6
• Two exp. decays observed, td 20 d, td 150 d
• Permanent increase in slow-down Dn/n 1.1
• Also increase in n by factor of four
• Possible 350-day oscillation in n after glitch

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

• Timed at Parkes, at 1.4 and 3 GHz
• P 3.8 s, tc 34 kyr, Bs 7 x 1013 G
• Glitch in 2007, Dn/n 33.2 x 10-6
• Little change in n at glitch
• Significant decrease in n at glitch
• - very unusal and not easily explained

.
..
(Manchester Hobbs 2011)
21
J1846-0258 in SNR Kes 75

• Youngest known pulsar tc 800 yr
• Discovered at X-rays, no radio detection
• P 326 ms, centred in SNR Kes 75
• Large glitch Dn/n 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 n less than pre-glitch
  extrapolation
• Change in braking index n(pre) 2.65 /- 0.01,
  n(post) 2.16 /- 0.13

(Livingstone et al. 2010,2011)
Change in magnetic structure and particle outflow
at time of glitch
22
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)
23
Global Effects in a PTA
The three main global timing effects that can be
observed with a PTA have different spatial
signatures on the sky

• 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
24
Detecting a Stochastic GW Background

• 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

Hellings Downs correlation function
TEMPO2 simulation of timing-residual correlations
due to a GW background for the PPTA pulsars
(Hobbs et al. 2009)
25
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 lt 2.5 ms) 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
26
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

27
The PPTA Pulsars
28
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!
29
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?

30
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 lt 200 ns,
  13 with lt 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
31
Effect of Dispersion Measure Variations
Before DM Correction

• PSR J1045-4509
• Six years of timing at 20cm (1.4 GHz) and 50cm
  (700 MHz)
• Correlated residual variations with n-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 post-fit
20cm
50cm
After DM Correction
32
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 c2 19.0
• After PCM correction
• Rms residual 195 ns
• Reduced c2 3.1

33
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
• J18570943 (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

DMJupiter 5 x 10-10 MSun
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
(Champion et al., 2010)
More pulsars, more data span, should give best
available value!
34
Stochastic GWB Detection with PTAs
J06
J06

• 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
• ? 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

JB03
S0809
SKA will detect GWs!
(Wen et al. 2010)
35
The Gravitational Wave Spectrum
36
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
37
EPT(PPTA2010) Relative to TAI
EPT
BIPM2010
First realisation of a pulsar timescale with
accuracy comparable to atomic timescales!
(Hobbs et al. 2010)
38
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 g-ray-emitting pulsars by an
  order of magnitude
• Radio and g-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

.
39
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

Pulsar timing can already rule out formation of
black holes in mass range 102 104 M?!
(Saito Yokoyama 2009)
40
Radio and g-ray Beaming

• Approximate sky coverage by top-hat fan beams
  (integral over f of two-dimensional beam pattern)
• Qr and Qg are equivalent widths of radio and
  g-ray beams respectively
• Qc 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 Qi
• the relative number of pulsars detectable in
  both bands is proportional to Qc
• In all cases Qr gt Qc

(Ravi, Manchester Hobbs 2010)
41
Radio g-ray Beaming

• For the highest Edot pulsars, Qr gt Qg
• This implies that the radio beaming fraction fr
  is comparable to or greater than the g-ray
  beaming fraction fg
• For OG and TPC models, fg 1.0
• For lower Edot Sample G pulsars, fr gt 0.57
  includes several MSPs
• Even high-altitude radio polar-cap models (e.g.,
  Kastergiou Johnston 2007) are unlikely to give
  fr gt fg 1
• Therefore

42
Radio g-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
  g-ray emission (Abdo et al. 2010)
• Fraction of G and R samples with Edot gt 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 g-ray band

(Ravi, Manchester Hobbs 2010)
43
Vela Pulsar Gamma-Ray Spectrum

• Integrated spectrum from Fermi LAT
• Power-law with exponential cutoff

• Power-law index G 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
44
Modelling of g-ray pulse profiles

• Two main models
• Outer-Gap model
• Slot-Gap or Two-Pole Caustic model

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

(Watters et al. 2009)
45
Blind Detection of PSR J1022-5746

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

(Abdo et al. 2009)
46
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

47
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

48
Fermi Observations of Known Pulsars

• In pre-Fermi era, seven pulsars known to emit
  g-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 4p2IP/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

.
.
.
49
Fermi Detections of Young Radio Pulsars

• PSR J1048-5832
• P 123.7 ms
• tc 20.3 kyr
• E 2x1036 erg/s
• Marginal EGRET detection

.

• PSR J22296114
• P 51.6 ms
• tc 10.5 kyr
• E 2x1037 erg/s
• X-ray profile double but single at g-ray

.
(Abdo et al. 2009)
Now 30 previously known young radio pulsars have
g-ray pulse detections
50
Gravitational Waves

• Prediction of general relativity and other
  theories of gravity
• Generated by acceleration of massive objects
• Propagate at the speed of light

• 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

(K. Thorne, T. Carnahan, LISA Gallery)
These sources create a stochastic GW background
in the Galaxy
51
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

LISA
LIGO

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

• 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

52
Timing Stability of MSPs

• 10-year data span for 20 PPTA MSPs
• Includes 1-bit f/b, Caltech FPTM and CPSR2 data
• sz frequency stability at different timescales
  t
• For white timing residuals, expect sz t-3/2
• Most pulsars roughly consistent with this out to
  10 years
• Good news for PTA projects!

10 ms
100 ns
(Verbiest et al. 2009)
53
Single-source Detection
Localisation with PPTA
Sensitivity
(Yardley et al. 2010)

• First realistic sensitivity curve for a PTA
  system!
• Computed GW strains for SMBH binary systems in
  Virgo cluster
• PPTA cant expect to detect individual systems -
  but SKA will!

(Anholm et al. 2008)
Need better sky distribution of pulsars -
international PTA collaborations are important!
54
PTA Spin-offs
PTA projects have many secondary objectives

• 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