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Transcript
Radio Pulsars
R. N. Manchester
Australia Telescope National Facility, CSIRO
Sydney, Australia
Summary
• Introduction to pulsar basics
• Multibeam searches at Parkes
• Supernova remnants and globular clusters
• Applications of pulsar timing
The sound of a pulsar
The Vela pulsar
Located in the Vela
supernova remnant
Pulse period = 89 ms
Age = 11,000 years
What are pulsars?
• Pulsars are rotating neutron stars.
• Neutron stars are tiny stars with the mass of the
sun but a diameter of about 20 km.
• They rotate tens or even hundreds of times
every second and send out a beam of emission.
• If we lie in the path of the beam, we see a pulse
every revolution - the ‘light-house model’.
How are pulsars formed?
• Pulsars are formed at the end of the life of a
massive star.
• The inner core of the star collapses to form a
rapidly rotating, highly magnetised neutron star.
• The outer layers of the star are blown off in a
supernova explosion.
• We are left with a pulsar in the centre of an
expanding supernova remnant.
The Crab Nebula and its Pulsar
• Exploded in
1054 AD observed by the
Chinese.
• Pulsar at centre
spins 30 times a
second.
• Pulses from
radio band to
gamma-rays
Distribution of Pulsar Periods
Total number known ~ 1500
• ‘Normal’ pulsars:
0.1 - 8.5 seconds
• ‘Millisecond’
pulsars: 1.5 - 25 ms.
About 80 known.
Formation of millisecond pulsars
• MSPs are very old (~109 years).
• They have been ‘recycled’ by accretion from an
evolving binary companion.
• This accretion spins
up the neutron star to
millisecond periods.
• During the accretion
phase the system may
be detectable as an
X-ray pulsar.
Where are pulsars found?
• Most known pulsars are in the disk of our Galaxy The Milky Way.
• Twenty are in our nearest neighbour galaxies, the
Magellanic Clouds.
• About 30 young pulsars are associated with supernova
remnants.
• More than one third of the known millisecond pulsars
are in globular clusters.
Interstellar Dispersion
Ionised gas in the interstellar
medium causes lower radio
frequencies to arrive at the
Earth with a small delay
compared to higher
frequencies.
Given a model for the
distribution of ionised gas in
the Galaxy, the amount of
delay can be used to estimate
the distance to the pulsar.
Pulsars as Clocks
• Pulsar periods are generally very stable.
• However, they are not constant - all pulsars are slowing
down.
.
• The ratio of period P to slowdown rate P gives an
estimate of the pulsar age - typically 106 years.
• Young pulsars have unpredictable changes in period glitches and period noise.
• Millisecond pulsars have extremely stable periods.
Binary pulsars
• Some pulsars are in orbit around another star.
Orbital periods range from 1.6 hours to several years.
• Only a few percent of normal pulsars, but more than
half of all millisecond pulsars, are binary.
• Pulsar companion stars range from very low-mass
white dwarfs (~0.01 solar masses) to heavy normal
stars (10 - 15 solar masses).
• Seven pulsars have neutron-star companions.
• One pulsar has three planets in orbit around it.
The Parkes radio telescope has found
more than twice as many pulsars as the
rest of the world’s telescopes put together.
Parkes Multibeam
Pulsar Surveys
• Multibeam receiver installed
in mid-1997.
• Discovered more than 800
pulsars, more than 50% of all
known pulsars.
• The Parkes Multibeam
Pulsar Survey has found more
than 700 of these.
•High-latitude surveys have
found about 100 pulsars
including 12 millisecond
pulsars.
Parkes multibeam pulsar surveys
Distribution of Pulsar Periods
Distribution of Dispersion Measures
Distribution of pulsars on the Galactic Plane
.
P- P
Diagram
• Young pulsars have rapid
slow-down rates:
t = P/(2P)
.
• High-B pulsars also slow
.
down rapidly: Bs ~ (PP)1/2
• Most millisecond pulsars
are binary
Multibeam surveys:
• New sample of young &
high-B pulsars
• Several mildly recycled
binary pulsars, filling gap
between MSPs and ‘normal’
pulsars
PSR J1119-6127
PSR J1119-6127 - G292.2-0.5
ATCA 1.4 GHz
• P = 407 ms
• Age = 1.7 kyr
• No catalogued SNR
• Faint ring on MOST GPS
• Deep ATCA observation
revealed shell SNR exactly
centred on pulsar!
New SNR!
New Association!
PSR: Camilo et al. (2000) SNR: Crawford et al. (2001)
Pulsar – SNR Associations
Cumulative Distribution by Year of Discovery
Pulsars in 47 Tucanae
• 11 millisecond pulsars
discovered 1991-1995. All
but two single (non-binary)
• 12 more discovered since
1998 using multibeam
receiver. All but two binary.
(Camilo et al. 2000)
Positions of 47 Tuc Pulsars
• Positions from pulse
timing observations typical uncertainty
< 1 milliarcsecond
• All pulsars lie
within central region
of cluster
Camilo et al. (2000)
Proper motion of 47 Tuc pulsars
• Timing measurements
over ~10 years
• Proper motion due to
motion of 47 Tuc through
halo at ~150 km s-1
• Motion of individual
pulsars within cluster too
small to detect
• Mean proper motion of
pulsars more accurate than
and marginally
inconsistent with
Hipparchos value.
Hipparchos
Freire et al. (2003)
Ionized gas in 47 Tucanae
.
• Correlation of DM and P
.
• P due to acceleration in
cluster potential
• Pulsars on far side of
cluster have higher DM
• Gas density ~ 0.07 cm-3,
about 100 times local
density
• Total mass of gas in
cluster ~ 0.1 Msun
(Freire et al. 2001)
Millisecond pulsars in other clusters
NGC 6266
PSR J1701-30
P 5.24 ms
Pb 3.81 d
Mc >0.19 Msun
d
6.7 kpc
NGC 6397
PSR J1740-53
NGC6544
PSR J1807-24
NGC 6752
PSR J1910-59
3.65 ms
3.06 ms
1.35 d (eclipse)
0.071 d (1.7 h)
>0.18 Msun
>0.009 Msun (10 MJup)
2.2 kpc
2.5 kpc
3.27 ms
0.86 d
>0.19 Msun
3.9 kpc
D’Amico et al. (2001)
The Binary Pulsar PSR B1913+16
Discovered by Hulse & Taylor in 1975
Pulse period: 59 ms
Orbital Period:
7h 45m
Double neutron-star
system
Velocity at
periastron:
~ 0.001 of
velocity of light
Orbit Parameters for PSR B1913+16
Keplerian:
Semi-major axis
2.3417592(19) s
Eccentricity
0.6171308(4)
Orbital period
0.322997462736(7) days
Longitude of periastron 226.57528(6) degrees
Time of periastron
46443.99588319(3) (MJD)
Post-Keplerian (or relativistic):
Periastron advance
4.2226621(11) deg/year
Grav. redshift + Transverse Doppler
Orbital period decay
4.295(2) ms
-2.422(6) x 10-12
Neutron-star masses
PSR B1913+16:
• Periastron advance
• Grav. Redshift
• Orbit decay
First two measurements
determine the masses of
the two stars -
Both neutron stars!
(Diagram from C.M. Will, 2001)
PRS B1913+16 Orbit Decay
• Prediction based on
measured Keplerian
parameters and Einstein’s
general relativity
• Corrected for
acceleration in
gravitational field of
Galaxy
.
.
• Pb(pred)/Pb(obs) =
1.0023 +/- 0.0046
(Damour & Taylor 1991,1992)
PSR B1913+16
• First discovery of a binary pulsar
• First observational evidence for gravity waves
• First accurate determinations of neutron star masses
• Confirmation of general relativity as an accurate
description of strong-field gravitational interactions
Nobel Prize for Taylor & Hulse in 1993
Einstein
was right!
Parkes High Latitude Pulsar Survey
• Uses multibeam receiver
• Survey region: 220o < gl < 260o, -60o < gb < 60o
• Optimised for MSPs: tobs = 4 min, tsamp = 125 ms.
• 14 pulsars discovered so far, including 4 MSPs
• PSR J0737-3039: P = 22.7 ms, Pb = 2.4 h,
e = 0.088, min. companion mass = 1.25 Msun
=> double-neutron-star system!
PSR J0737-3039
PSR J0737-3039
• Most highly
relativistic binary
pulsar known!
• GR precession of
periastron = 16.86 +/0.05 deg/yr, four times
as large as for PSR
B1913+16!
• Other GR parameters
measurable in ~ 1 year
Implications for Gravitational
Wave Detectors
• Coalescence time 85 Myr – about 1/3 1913+16 value
• Luminosity ~1/6 value for 1913+16
=> many similar systems in Galaxy
• Implies an increase in the Galactic merger rate by about
factor of eight
• Increases predicted detection rate for LIGO from about
one per century to one every few years.
(Burgay et al., Nature, in press.)
Precision timing of PSR J0437-4715
PSR J0437-4715 is a binary millisecond pulsar discovered at
Parkes in 1993. It is the closest and strongest MSP known.
Timing observations at
Parkes over the past two
years using a baseband
recording system have
given the best-ever
pulsar timing precision.
Collaborative project: Swinburne University, Caltech and ATNF
PSR J0437-4715
Parameters from timing observations
R.A. (2000)
04h 37m 15.s7865145(7)
Dec. (2000)
-47o 15’ 08.”461584(8)
P
5.757451831072007(8) ms
Pb
5.741046(3) days
Eccentricity
0.000019186(5)
Proper motion
140.892(9) mas/year
Parallax
7.19(14) mas
Orbit inclination
42.75 deg.
(van Straten et al., Nature, July 12 2001)
Shapiro delay of PSR J0437-4715
Shape predicted from
measured parameters
Mcompanion =
0.236 +/- 0.017 Msun
Mpulsar =
1.58 +/- 0.18 Msun
RMS Residual = 35 ns!
Independent test of predictions of general relativity!
(van Straten et al., Nature, July 12 2001)
A Pulsar Timing Array
• Combine precision timing observations of many
millisecond pulsars widely distributed on celestial sphere
• Solve for all pulsar parameters as well as global terms
which affect all pulsars
• Can in principle detect effects of gravitational waves
passing over the Earth – could be first direct detection of
gravitational waves!
• Can detect long-term irregularities in terrestrial timescale
– establish a pulsar timescale!
• Can improve knowledge of Solar system properties, e.g.
masses and orbits of outer planets and asteroids.
Clock error
Same for pulsars in all directions
Earth
Error in Earth Velocity
Opposite sign in opposite directions - Dipole
Earth
Gravitational Wave passing over Earth
Opposite sign in orthogonal directions - Quadrupole
Earth
Summary
• Recent large-scale radio pulsar searches at Parkes have more
than doubled the number of known pulsars
• New population of high-B pulsars and new SNR associations
• Globular clusters contain many millisecond pulsars
• Precision timing of binary millisecond pulsars measures
many properties of binary stars and tests general relativity.
• Discovery of highly relativistic binary pulsar significantly
increases predicted rate of LIGO detections of merger events.
• A millisecond pulsar timing array can establish a pulsar
timescale and may detect gravitational waves.
Thank you!