Transcript Document

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
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)
.
The P – P Diagram
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!
Galactic Disk pulsars
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 ms sampling (cf., PMPS 96 channels, 250 ms)
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
Mid-latitude survey ~30% complete
27 pulsars detected so far, including 5 MSPs
(Keith et al. 2011)
PSR J1622-4950:a 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)
HTRU RRATs Search
• HTRU survey data searched for isolated dispersed pulses
• Identified as Rotating Radio Transients (RRATs)
11 new RRATs discovered!
(Burke-Spolaor et al. 2011)
1451 sources!
~100 pulsars!!
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!!
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 SAS2 (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)
Fermi Detections of Known MSPs
• Many MSPs have
relatively
. 2 high values
of E/d
• 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)
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!
Fermi – CTA1 Pulsar
First gamma-ray pulsar
found in a blind search!
PSR J0007+7303
(Abdo et al. 2008)
Fermi Blind-search Pulsars
• 25 mostly
. young,
high-E pulsars
• Have pulse profiles
very similar to radioselected sample
• Three have been
detected as faint radio
pulsars
• PSR J1907+0602
detected at Arecibo,
only 3 mJy!
• Most have low upper
limits on S1400
(Abdo et al. 2009, Saz Parkinson et al. 2010 )
17
(2010)
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!
.
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
• g-ray pulses detected: red dot
• g-ray point source: green triangle
(Abdo et al., 2009)
Radio – g Beaming
J0034-0534
• Two thirds of g-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 g-ray pulse profiles
.
• Some high-E/d2 radio
pulsars are not (yet)
detected by Fermi
.
• Radio beams for high-E pulsars are wide!
(Abdo et al. 2010)
.
• 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)
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)
Two Giant Glitches
PSR B2334+61:
• 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)
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 preglitch 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
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)
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
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)
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 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
The Parkes Pulsar Timing Array
• Using the Parkes 64-m radio telescope to observe 20 MSPs
Project
• ~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 ms
• 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 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
Before DM Correction
20cm post-fit
20cm
50cm
After DM Correction
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:
DMJupiter = 5 x 10-10 MSun
 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:
Best published value: (9.547919 ± 8) × 10-4 Msun
(Champion et al., 2010)
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!
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
SKA will detect GWs!
(Wen et al. 2010)
J06
J06
JB03
S0809
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 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
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)
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 >= Qc
(Ravi, Manchester & Hobbs 2010)
Radio – g-ray Beaming
• For the highest Edot pulsars, Qr >~ 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 >~ 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 …
 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 g-ray
profiles with caustic effects important
(Manchester 2005, Ravi et al. 2010)
Radio – g-ray Beaming
• Two samples:
 G: All pulsars found (or that
could be found) in the Fermi 6month 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 > 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)
.
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
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)
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 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
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 J2229+6114
• P = 51.6 ms
• tc = 10.5 kyr
• E = 2x1037 erg/s
• X-ray profile double
but single at g-ray
.
Now 30 previously known young radio
pulsars have g-ray pulse detections
(Abdo et al. 2009)
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
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
LIGO
LISA
• 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
Timing Stability
of MSPs
• 10-year data span for 20
PPTA MSPs
• Includes 1-bit f/b, Caltech
FPTM and CPSR2 data
10 ms
• z: frequency stability at
different timescales t
• For “white” timing residuals,
expect z ~ t-3/2
• Most pulsars roughly
consistent with this out to 10
years
• Good news for PTA projects!
(Verbiest et al. 2009)
100 ns
Single-source Detection
Sensitivity
Localisation with PPTA
(Yardley et al. 2010)
• 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!
(Anholm et al. 2008)
Need better sky distribution of pulsars international PTA collaborations are
important!
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
(Yan et al. 2010)