pulsar properties - national radio astronomy observatory · 2010-11-17 · pulsar properties for...

Pulsar PropertiesFor additional information about pulsars, see the books Pulsar Astronomy by Andrew Lyne and FrancisGraham-Smith and Handbook of Pulsar Astronomy by Duncan Lorimer and Michael Kramer.

Known radio pulsars appear to emit short pulses of radio radiation with pulse periods between 1.4 ms and8.5 seconds. Even though the word pulsar is a combination of "pulse" and "star," pulsars are not pulsatingstars. Their radio emission is actually continuous but beamed, so any one observer sees a pulse of radiationeach time the beam sweeps across his line-of-sight. Since the pulse periods equal the rotation periods ofspinning neutron stars, they are quite stable. Even though the radio emission mechanism is not wellunderstood, radio observations of pulsars have yielded a number of important results because:

(1) Neutron stars are physics laboratories providing extreme conditions (deep gravitational potentials,densities exceeding nuclear densities, magnetic field strengths as high as or even 10 gauss) notavailable on Earth.

(2) Pulse periods can be measured with accuracies approaching 1 part in 10 , permitting exquisitely sensitivemeasurements of small quantities such as the power of gravitational radiation emitted by a binary pulsarsystem or the gravitational perturbations from planetary-mass objects orbiting a pulsar.

The radical proposal that neutron stars exist was made with trepidation by Baade & Zwicky in 1934: "With allreserve we advance the view that a supernova represents the transition of an ordinary star into a new formof star, the neutron star, which would be the end point of stellar evolution. Such a star may possess a verysmall radius and an extremely high density." Pulsars provided the first evidence that neutron stars really doexist. They tell us about the strong nuclear force and the nuclear equation of state in new ranges of pressureand density, test general relativity and alternative theories of gravitation in both shallow and relativisiticallydeep ( ) potentials, and led to the discovery of the first extrasolar planets.

Discovery and Basic Properties

Pulsars were discovered serendipidously in 1967 on chart-recorder records obtained during a low-frequency(· 1 MHz) survey of extragalactic radio sources that scintillate in the interplanetary plasma, just as starstwinkle in the Earth's atmosphere. This important discovery remains a warning against overprocessing databefore looking at them, ignoring unexpected signals, and failing to explore observational "parameter space"(here the relevant parameter being time). As radio instrumentation and data-taking computer programsbecome more sophisticated, signals are "cleaned up" before they reach the astronomer and optimal"matched filtering" tends to suppress the unexpected. Thus clipping circuits are used to remove the strongimpulses that are usually caused by terrestrial interference, and integrators smooth out fluctuations shorterthan the integration time. Pulsar signals "had been recorded but not recognized" several years earlier with the250-foot Jodrell Bank telescope. Most pulses seen by radio astronomers are just artificial interference fromradar, electric cattle fences, etc., and short pulses from sources at astronomical distances implyunexpectedly high brightness temperatures – K, the upper limit for incoherentelectron-synchrotron radiation set by inverse-Compton scattering. However, Cambridge University graduatestudent Jocelyn Bell noticed pulsars in her scintillation survey data because the pulses appeared earlier byabout 4 minutes every solar day, so they appeared exactly once per sidereal day and thus came fromoutside the solar system.

B 0 Ø 1 14 15

16

GM=(rc ) 2 µ 0

= 8

T 0 b Ø 1 25 10 K 0 30 µ 1 12

Pulsars http://www.cv.nrao.edu/course/astr534/Pulsars.html

1 of 16 11/17/2010 11:05 AM

Figure 1. "High-speed" chart recording of the first known pulsar, CP1919. This confirmation observationshowed that the "scruffy" signals observed previously were periodic.

The sources and emission mechanism were originally unknown, and even intelligent transmissions by LGM("little green men") were seriously suggested as explanations for pulsars. Astronomers were used to slowlyvarying or pulsating emission from stars, but the natural period of a radially pulsating star depends on itsmean density Ú and is typically days, not seconds. Likewise there is a lower limit to the rotation period P of agravitationally bound star, set by the requirement that the centrifugal acceleration at its equator not exceedthe gravitational acceleration. If a star of mass M and radius R rotates with angular velocity Ê Ù=P ,

Ê R

M

In terms of the mean density

or

Ú

= 2

2 <R2

GM

P 2

4Ù R2 3

< G

P 2 >

Ò

3

4ÙR3Ó 3Ù

GM

Ú ; =M

Ò

3

4ÙR3ÓÀ1

P >

Ò3Ù

GÚ

Ó1=2

>3Ù

GP 2(6A1)


2 of 16 11/17/2010 11:05 AM

This is actually a very conservative lower limit to Ú because a rapidly spinning star becomes oblate, increasingthe centrifugal acceleration and decreasing the gravitational acceleration at its equator.

Example: The first pulsar discovered (CP 1919+21, where the "CP" stands for Cambridge pulsar) has aperiod P :3 s. What is its minimum mean density?

Ú 0 g cm

This density limit is just consistent with the densities of white-dwarf stars. But soon the faster (P :033 s)pulsar in the Crab Nebula was discovered, and its period implied a density too high for any stable white dwarf.The Crab nebula is the remnant of a known supernova recorded by ancient Chinese astronomers as a "gueststar" in 1054 AD, so the discovery of this pulsar also confirmed the Baade & Zwicky suggestion that neutronstars are the compact remnants of supernovae. The fastest known pulsar has s implyingÚ 0 g cm , the density of nuclear matter. For a star of mass greater than the Chandrasekhar mass

(compact stars less massive than this are stable as white dwarfs), the maximum radius is

In the case of the pulsar with Ú 0 g cm ,

The canonical neutron star has and km, depending on the equation-of-state ofextremely dense matter composed of neutrons, quarks, etc. The extreme density and pressure turns mostof the star into a neutron superfluid that is a superconductor up to temperatures K. Any star ofsignificantly higher mass ( in standard models) must collapse and become a black hole. Themasses of several neutron stars have been measured with varying degrees of accuracy, and all turn out tobe very close to .

The Sun and many other stars are known to possess roughly dipolar magnetic fields. Stellar interiors aremostly ionized gas and hence good electrical conductors. Charged particles are constrained to move alongmagnetic field lines and, conversely, field lines are tied to the particle mass distribution. When the core of a

star collapses from a size cm to cm, its magnetic flux is conserved and the

initial magnetic field strength is multiplied by , the factor by which the cross-sectional area a falls. An

initial magnetic field strength of G becomes G after collapse, so young neutron starsshould have very strong dipolar fields. The best models of the core-collapse process show that a dynamoeffect may generate an even larger magnetic field. Such dynamos are thought to be able to produce the

G fields in magnetars, neutron stars having such strong magnetic fields that their radiation ispowered by magnetic field decay. Conservation of angular momentum during collapse increases the rotationrate by about the same factor, 10 , yielding initial periods in the millisecond range. Thus young neutron starsshould drive rapidly rotating magnetic dipoles.

= 1

>3Ù

GP 2=

3Ù

6:67 0 dyne cm gm (1:3 s)Â 1 À8 2 À2 2Ù 1 8 À3

= 0

P :4 0 = 1 Â 1 À3

> 1 14 À3

M :4M Ch ÙÒhc

2ÙG

Ó3=21

m2p

Ù 1 Ì

R <

Ò3M

4ÙÚ

Ó1=3

P :4 0 = 1 Â 1 À3 > 1 14 À3

R 0 cm 0 km <

ï

4Ù 0 g cmÂ 1 14 À33 :4 :0 0 gÂ 1 Â 2 Â 1 33

!1=3

Ù 2Â 1 6 Ù 2

M :4M Ù 1 Ì R 0 Ù 1

T 0 Ø 1 9

M M Ø 3 Ì

1:4M Ì

Ø 0 1 11 Ø 0 1 6 È a Ñ RB~ Á n~ d

Ø 0 1 10

B 00 Ø 1 B 0 Ø 1 12

10 0 14 À 1 15

10


3 of 16 11/17/2010 11:05 AM

Figure 2: A Pulsar. (left or top): A diagram of the traditional magnetic dipole model of a pulsar. (right orbottom) Diagram of a simple dipole magnetic field near the polar caps. The inset figure shows a schematicof the electon-positron cascade which is required by many models of coherent pulsar radio emission (Bothfigures are from the Handbook of Pulsar Astronomy by Lorimer and Kramer).

Energetics

If the magnetic dipole is inclined by some angle Ë from the rotation axis, it emits low-frequencyelectromagnetic radiation. Recall the Larmor formula for radiation from a rotating electric dipole:

P ;

where is the perpendicular component of the electric dipole moment. By analogy, the power ofthe magnetic dipole radiation from an inclined magnetic dipole is

> 0

rad =3c32q v2 2_

=3

2

c3(qr )ÿsin Ë 2

=3

2

c3(p )?ÿ

2

p ?


4 of 16 11/17/2010 11:05 AM

P

where is the perpendicular component of the magnetic dipole moment. For a uniformly magnetizedsphere with radius R and surface magnetic field strength B, the magnetic dipole moment is (see Jackson'sClassical Electrodynamics)

m R :

If the inclined magnetic dipole rotates with angular velocity Ê,

so

where P is the pulsar period. This electromagnetic radiation will appear at the very low frequency

kHz, so low that it cannot be observed, or even propagate through the ionized ISM. The hugepower radiated is responsible for pulsar slowdown as it extracts rotational kinetic energy from the neutronstar. The absorbed radiation can also light up a surrounding nebula, the Crab nebula for example.

The rotational kinetic energy E is related to the moment of inertia I by

E IÊ :

Example: The moment of inertia of the "canonical" neutron star (uniform-density sphere with

and km) is

Therefore the rotational energy of the Crab pulsar (P :033 s) is

E :8 0 ergs

rad =3

2

c3(m )?ÿ

2

(6A2)

m ?

= B 3

m (ÀiÊt) = m0 exp

m iÊm (ÀiÊt) _ = À 0 exp

m m (ÀiÊt) m ÿ = Ê20 exp = Ê2

P (BR ) ; rad =3

2

c3

m Ê2?

4

=3c3

2m2?Ò

P

2ÙÓ4

=2

3c33 sin Ë 2

Ò

P

2ÙÓ4

· = P À1 < 1

rot

rot = 2

1 2 =P 2

2Ù I2

M :4M Ù 1 ÌR 0 Ù 1

I MR 0 gm cm =5

2 2 Ù5

2 :4 :0 0 g 10 cm)Á 1 Á 2 Â 1 33 Á ( 6 2

Ù 1 45 2

= 0

rot =P 2

2Ù I2 Ù(0:033 s)2

2Ù 0 g cm2 Á 1 45 2

Ù 1 Â 1 49


5 of 16 11/17/2010 11:05 AM

Pulsars are observed to slow down gradually:

Note that P is dimensionless (e.g., seconds per second). From the observed period P and period derivative

P we can estimate the rate at which the rotational energy is decreasing.

and

Example: The Crab pulsar has P :033 s and . Its rotational energy is changing at the rate

4 0 erg s

Thus the low-frequency (30 Hz) magnetic dipole radiation from the Crab pulsar radiates a huge power, comparable with the entire radio output of our Galaxy. It exceeds the Eddington

limit, but that is not a problem because the energy source is not accretion. It greatly exceeds the averageradio pulse luminosity of the Crab pulsar, erg s . The long-wavelength magnetic dipole radiationenergy is absorbed by and powers the Crab nebula (a "megawave oven").

P _ Ñdt

dP> 0

_

_

ÊÊ dt

dErot =d

dtIÊ

Ò

2

1 2

Ó= I _

Ê Ù(ÀP P ) =P

2Ùso Ê_ = 2 À2 _

ÊÊ dt

dErot = I _ = IP

2Ù

P 2

2Ù(ÀP )_

dt

dErot =P 3

À4Ù IP2 _(6A3)

= 0 P 0 _ = 1 À12:4

dt

dErot =P 3

À4Ù IP2 _=

(0:033 s)3À4Ù 0 g cm 0 s s2 Á 1 45 2 Á 1 À12:4 À1

Ù À Â 1 38 À1

P dE =dt 0 L rad Ù À rot Ù 1 5Ì

Ø 0 1 30 À1


6 of 16 11/17/2010 11:05 AM

Figure 3: Composite image of the Crab nebula. Blue indicates X-rays (from Chandra), green is optical (fromthe HST), and red is radio (from the VLA). Image credit


7 of 16 11/17/2010 11:05 AM

If we use to estimate P , we can invert Larmor's formula for magnetic dipole radiation to find

and get a lower limit to the surface magnetic field strength B , since we don'tgenerally know the inclination angle Ë.

Evaluating the constants for the canonical pulsar in cgs units, we get

so the minimum magnetic field strength at the pulsar surface is

Example: What is the minimum magnetic field strength of the Crab pulsar (P :033 s, )?

This is an amazingly strong magnetic field. Its energy density is

Just one cm of this magnetic field contains over of energy,the annual output of a large nuclear power plant. A cubic meter contains more energy than has ever beengenerated by mankind.

ÀdE =dt rot rad

B ? = B sin Ë > B sin Ë

P rad = Àdt

dErot

(BR ) 2

3c33 sin Ë 2

Ò

P 2

4Ù2Ó2

=P 3

4Ù IP2 _

B 2 =3c IPP3 _

2 Ù RÁ 4 2 6 sin2 Ë

B (PP ) >

Ò3c I3

8Ù R2 6

Ó1=2

_ 1=2

:2 0 Ô

8Ù (10 cm)2 6 6

3 3 0 cm s ) 0 g cmÁ ( Â 1 10 À1 3 Á 1 45 2 Õ1=2Ù 3 Â 1 19

:2 0 Ò

B

Gauss

Ó> 3 Â 1 19

Ò

s

PP_Ó1=2

(6A4)

= 0 P 0 _ = 1 À12:4

:2 0 0 Ò

B

Gauss

Ó> 3 Â 1 19

Ò

s

0:033 s 0Á 1 À12:4Ó= 4Â 1 12

U 0 erg cm B =8Ù

B2

> 5Â 1 23 À3

3 5 0 J 0 W s :6 0 W yr Â 1 16 = 5Â 1 16 = 1 Â 1 9


8 of 16 11/17/2010 11:05 AM

If (B ) doesn't change significantly with time, we can estimate a pulsar's age Ü from PP by assumingthat the pulsar's initial period P was much shorter than the current period. Starting with

B

we find that

PP

doesn't change with time. Rewriting the identity PP P as PdP Pdt and integrating over thepulsar's lifetime Ü gives

since PP is assumed to be constant over time.

If , the characteristic age of the pulsar is

Ü

Note that the characteristic age is not affected by uncertainties in the radius R, moment of inertia I, or

B ; the only assumptions in its derivation are that and that PP (i.e. B) is constant.

Example: What is the characteristic age of the Crab pulsar (P :033 s, )?

Ü :1 0 s 300 yr

Its actual age is about 950 years.

sin Ë _

0

2 =3c IPP3 _

8Ù R2 6 sin2 Ë

_ =3c I3

8Ù R (B )2 6 sin Ë 2

_ = P _ = P _

dP (PP ) t P t Z

P0

P

P =

Z

0

Ü_ d = P _

Z

0

Ü

d

_

P Ü 2

P 2 À P02

= P _

P 02 Ü P 2

Ñ P

2P_(6A5)

sin Ë P 0 Ü P _

= 0 P 0 _ = 1 À12:4

=P

2P_=

0:033 s2 0Á 1 À12:4 Ù 4 Â 1 10 Ù

10 s yr7:5 À14:1 0 sÂ 1 10

Ù 1


9 of 16 11/17/2010 11:05 AM

Figure 4: P-Pdot Diagram. The PP diagram is useful for following the lives of pulsars, playing a rolesimilar to the Hertzsprung-Russell diagram for ordinary stars. It encodes a tremendous amount ofinformation about the pulsar population and its properties, as determined and estimated from two of the

primary observables, P and P . Using those parameters, one can estimate the pulsar age, magnetic field

strength B, and spin-down power E. (From the Handbook of Pulsar Astronomy, by Lorimer and Kramer)

The Lives of Pulsars

Pulsars are born in supernovae and appear in the upper left corner of the PP diagram. If B is conserved andthey age as described above, they gradually move to the right and down, along lines of constant B andcrossing lines of constant characteristic age. Pulsars with characteristic ages < 0 yr are often found in ornear recognizable supernova remnants. Older pulsars are not, either because their SNRs have faded toinvisibility or because the supernova explosions expelled the pulsars with enough speed that they have sinceescaped from their parent SNRs. The bulk of the pulsar population is older than 10 yr but much younger

than the Galaxy ( yr). The observed distribution of pulsars in the PP diagram indicates that somethingchanges as pulsars age. One controversial possibility is that the magnetic fields of old pulsars must decay on

time scales yr, causing old pulsars to move almost straight down in the PP diagram until they fallbelow into the graveyard below the death line and cease radiating radio pulses.

Almost all short-period pulsars below the spin-up line near are in binary systems, asevidenced by periodic (i.e. orbital) variations in their observed pulse periods. These recycled pulsars havebeen spun up by accreting mass and angular momentum from their companions, to the point that they emitradio pulses despite their relatively low magnetic field strengths G (the accretion causes asubstantial reduction in the magnetic field strength). The magnetic fields of neutron stars funnel ionizedaccreting material onto the magnetic polar caps, which become so hot that they emit X-rays. As the neutronstars rotate, the polar caps appear and disappear from view, causing periodic fluctuations in X-ray flux; many

_

_

_

_

1 5

5

Ø 0 1 10 _

Ø 0 1 7 _

log[P=P (sec)] 16 _ Ù À

B 0 Ø 1 8


10 of 16 11/17/2010 11:05 AM

are detectable as X-ray pulsars.

Millisecond pulsars (MSPs) with low-mass ( ) white-dwarf companions typically have orbitswith small eccentricities. Pulsars with extremely eccentric orbits usually have neutron-star companions,indicating that these companions also exploded as supernovae and nearly disrupted the binary system. Stellar interactions in globular clusters cause a much higher fraction of recycled pulsars per unit mass than inthe Galactic disk. These interactions can result in very strange systems such as pulsar–main-sequence-starbinaries and MSPs in highly eccentric orbits. In both cases, the original low-mass companion star thatrecycled the pulsar was ejected in an interaction and replaced by another star. (The eccentricity e of anelliptical orbit is defined as the ratio of the separation of the foci to the length of the major axis. It rangesbetween e for a circular orbit and e for a parabolic orbit.)

A few millisecond pulsars are isolated. They were probably recycled via the standard scenario in binarysystems, but the energetic millisecond pulsars eventually ablated their companions away.

Figure 5: Examples of Doppler variations observed in binary systems containing pulsars. (left or top) TheDoppler variations of the globular cluster MSP J1748 2446N in Terzan 5. This pulsar is in a nearly circular

M :1 M Ø 0 À 1 Ì

= 0 = 1

À


11 of 16 11/17/2010 11:05 AM

orbit (eccentricity ) with a companion of minimum mass 0.47 M . The difference between

the semi-major and semi-minor axes for this orbit is only 51 4 cm! The thick red lines show the periods asmeasured during GBT observations. (right or bottom) Similar Doppler variations from the highly eccentricbinary MSP J0514 4002A in the globular cluster NGC 1851. This pulsar has one of the most eccentricorbits known (e :888) and a massive white dwarf or neutron-star companion.

Emission Mechanisms

The radio pulses originate in the pulsar magnetosphere. Because the neutron star is a spinning magneticdipole, it acts as a unipolar generator. The total Lorentz force acting on a charged particle is

Charges in the magnetic equatorial region redistribute themselves by moving along closed field lines until they

build up an electrostatic field large enough to cancel the magnetic force and give . The voltage inducedis about 10 V in MKS units. However, the co-rotating field lines emerging from the polar caps cross thelight cylinder (the cylinder centered on the pulsar and aligned with the rotation axis at whose radius theco-rotating speed equals the speed of light) and these field lines cannot close. Electrons in the polar cap aremagnetically accelerated to very high energies along the open but curved field lines, where the accelerationresulting from the curvature causes them to emit curvature radiation that is strongly polarized in the planeof curvature. As the radio beam sweeps across the line-of-sight, the plane of polarization is observed torotate by up to 180 degrees, a purely geometrical effect. High-energy photons produced by curvatureradiation interact with the magnetic field and lower-energy photons to produce electron-positron pairs thatradiate more high-energy photons. The final results of this cascade process are bunches of charged particles

that emit at radio wavelengths. The death line in the PP diagram corresponds to neutron stars withsufficiently low B and high P that the curvature radiation near the polar surface is no longer capable ofgenerating particle cascades. The extremely high brightness temperatures are explained by coherentradiation. The electrons do not radiate as independent charges e; instead bunches of N electrons involumes whose dimensions are less than a wavelength emit in phase as charges Ne. Since Larmor's formulaindicates that the power radiated by a chage q is proportional to q , the radiation intensity can be N timesbrighter than incoherent radiation from the same total number N of electrons. Because the coherent volumeis smaller at shorter wavelengths, most pulsars have extremely steep radio spectra. Typical (negative)pulsar spectral indices are 1.7 ( ), although some can be much steeper (Ë ) and a handfulare almost flat (Ë :5).

Pulsars and the Interstellar Medium

(Note: the following closely follows the discussion in the Handbook of Pulsar Astronomy by Lorimer andKramer)With their sharp and short-duration pulse profiles and very high brightness temperatures, pulsars are uniqueprobes of the interstellar medium (ISM). The electrons in the ISM make up a cold plasma having a refractiveindex

where · is the frequency of the radio waves, · is the plasma frequency

e :6 0 = 4 Â 1 À5 ÌÆ

À = 0

F E : ~ = q

Ò~ +

c

v~ ÂB~ Ó

jF j ~ = 016

_

2

Ë Ø S / ·À1:7 > 3< 0

Ö 1 ; =

ÔÀ°·

·pÑ2Õ1=2

p


12 of 16 11/17/2010 11:05 AM

and n is the electron number density. For a typical ISM value cm , kHz. If · then Ö is imaginary and radio waves cannot propagate through the plasma.

For propagating radio waves, Ö and the group velocity v c of pulses is less than the vacuum speed

of light. For most radio observations so

A broadband pulse moves through a plasma more slowly at lower frequencies than at higher frequencies. Ifthe distance to the source is d, the dispersion delay t at frequency · is

In astronomically convenient units this becomes

where

in units of pc cm is called the dispersion measure.

· :97 kHz p =

Ò

Ùme

e n2 eÓ1=2

Ù 8 ÂÒ

ne

cmÀ3

Ó1=2

(6A6)

e n :03 e Ø 0 À3 · :5 p Ø 1 < ·p

< 1 g = Ö

· p Ü ·

v 1 g Ù c

ÒÀ

·p2

2·2

Ó(6A7)

t dl 1 dl =

Z

0

d

vgÀ1 À

c

d=c

1Z

0

dÒ+

·p2

2·2

ÓÀc

d

= : e2

2Ùm ce ·2

dlR0

dne

:149 0 Òt

sec

ÓÙ 4 Â 1 3

ÒDM

pc cmÀ3

ÓÒ·

MHz

ÓÀ2(6A8)

DM dl ÑZ

0

d

ne (6A9)

À3


13 of 16 11/17/2010 11:05 AM

Figure 6: Pulsar dispersion. Uncorrected dispersive delays for a pulsar observation over a bandwidth of 288MHz (96 channels of 3 MHz width each), centered at 1380 MHz. The delays wrap since the data are folded(i.e. averaged) modulo the pulse period. (From the Handbook of Pulsar Astronomy, by Lorimer and Kramer)

Measurements of the dispersion measure can provide distance estimates to pulsars. Crude estimates can bemade for pulsars near the Galactic plane assuming that cm . However, several sophisticatedmodels of the Galactic electron-density distribution now exist (e.g. NE2001; Cordes & Lazio 2002, astro-ph/0207156) that can provide much better ( ) distance estimates.

Since pulsar observations almost always cover a wide bandwidth, uncorrected differential delays across theband will cause dispersive smearing of the pulse profile. For pulsar searches, the DM is unknown andbecomes a search parameter much like the pulsar spin frequency. This extra search dimension is one of theprimary reasons that pulsar searches are computationally intensive.

Besides directly determining the integrated electron density along the line of site, observations of pulsars canbe used to probe the ISM via absorption by spectral lines of HI or molecules (which can be used to estimatethe pulsar distance as well), scintillation (allowing estimates of the pulsar transverse velocity), and pulsebroadening.

n :03 e Ø 0 À3

Ád=d 0% Ø 3


14 of 16 11/17/2010 11:05 AM

Figure 7: Pulsar HI Absorption Measurement. With a model for the Galactic rotation, such absorptionmeasurements can provide pulsar distance estimates or constraints. (From the Handbook of PulsarAstronomy, by Lorimer and Kramer)

Figure 8: Thin Screen Diffraction/Scattering model. Inhomogeneities in the ISM cause small-angledeviations in the paths of the radio waves. These deviations result in time (and therefore phase) delays thatcan interfere to create a diffraction pattern, broaden the pulses in time, and make a larger image of thepulsar on the sky. (From the Handbook of Pulsar Astronomy, by Lorimer and Kramer)


15 of 16 11/17/2010 11:05 AM

Figure 9: Pulse broadening caused by scattering. Scattering of the pulsed signal by ISM inhomogeneitiesresults in delays that cause a scattering tail. This scatter-broadening can greatly decrease both theobservational sensitivity and the timing precision for such pulsars. (From the Handbook of PulsarAstronomy, by Lorimer and Kramer)

Figure 10: Diffractive Scintillation of a Pulsar. The top plots show dynamic spectra of the bright pulsarB0355+54 taken on two consecutive days with the GBT. The bottom plots show the so-called secondaryspectra (the Fourier transforms of the dynamic spectra) and the so-called scintillation arcs (and movingarclets). (Figure provided by Dan Stinebring)


16 of 16 11/17/2010 11:05 AM

pulsar properties - national radio astronomy observatory · 2010-11-17 · pulsar properties for...

Documents