Neutron Stars 2: Phenomenology

Andreas ReiseneggerESO Visiting ScientistAssociate Professor,

Pontificia Universidad Católica de Chile

Chandra x-ray images of the PWNs surrounding the (A) Crab and (B) Vela pulsars. [Credit: NASA/CXC/Smithsonian Astrophysical Observatory, NASA/Pennsylvania State University, and G. Pavlov]

Outline• “Radio” pulsars:

– Classical pulsars– Millisecond pulsars– Binary radio pulsars & General Relativity

• X-ray binaries: high & low mass• Evolution, connections of pulsars & XRBs.• Magnetars• Thermal emitters: isolated & in SNRs• RRATs

BibliographyRadio pulsars:• Lyne & Graham-Smith, Pulsar Astronomy, 2nd ed.,

Cambridge Univ. Press (1998)• Lorimer & Kramer, Handbook of Pulsar Astronomy,

Cambridge Univ. Press (2005)• Manchester, Observational Properties of Pulsars, Science,

304, 542 (2004)Binary systems:• Stairs, Pulsars in Binary Systems: Probing Binary Stellar

Evolution & General Relativity, Science, 304, 547 (2004)• Lorimer, Binary & Millisecond Pulsars, Living Reviews in

Relativity, 8, 7 (2005)Others: See below.

NS Phenomenology

• The structure of a NS is almost entirely determined by its mass.

• The observable phenomenology, however, depends much more on several kinds of “hair”:

– Rotation ()

– Magnetic field (B)

– Accretion ( )M

Pulsars

PSR B0329+54http://www.jb.man.ac.uk/~pulsar/Education/Sounds/sounds.html

“Radio” pulsars

• Very wide range of photon energies

• Mostly non-thermal

• Thermal X-ray bump cooling

• UV/soft X-ray “hole” from interstellar absorption

D. J. Thompson, astro-ph/0312272

Dispersion measure• Dispersion relation for EM waves

in a plasma:

• Pulses travel more slowly at lower frequencies (and not at all below the plasma frequency).

• Progressively delayed arrival times of radio pulses observed at lower frequencies.

• Effect is

– proportional to the traversed column density of free electrons (measure of distance), and

– inversely proportional to 2

(check).

ccdkdv

menkc

g

e

e

2

2p

2

p222

p2

1

4with

Distribution of pulsars on the Galactic plane

Spin-down(magnetic dipole model)

Spin-down time (age?):

Lyne 2000, http://online.kitp.ucsb.edu/online/neustars_c00/lyne/oh/03.html

422

2

2

332

Bdtd

cI

Magnetic field:

PPB

3

||

PPts 2||2

Spin-down time vs. age

The spin-down time generally agrees (roughly) with independent ages from:

• historic SNe (Crab)• expansion of SNRs• travel time from Galactic disk• cooling of white-dwarf companions

0

2

020

2

3

andconstant if

12

1121

constant,withIf

Ktt

Kt

KK

s

Problem: “Braking index”

K involves the dipole moment (strength & orientation) & the moment of inertia of the star.

can only be measured in cases when is large & rapidly changing: young pulsars

When measured, n 2.0 - 2.8 (< 3):

– The dipole spin-down model is wrong, or

– the dipole moment is increasing with time.

constantif3

3

2

23

Kn

KK

Kaspi et al. 1999

“Magnetars”

Classical pulsars

Millisecond pulsars

PPts 2

PPB

Manchester et al. 2002

“Magnetars”

Classical pulsars

Millisecond pulsars circled: binary systems

2 populations of radio pulsars“Classical”• P ~ 8 s – 16 ms

• ts P/(2P’) ~ 103-8 yr• B (PP’)1/2 ~ 1011-13 G• Very few binaries.• Many of the youngest are

associated to supernova remnants (SNRs).

• Galactic disk. “Population I”

Millisecond• P ~ 20 ms – 1.4 ms

• ts P/(2P’) ~ 108-10 yr• B (PP’)1/2 ~ 108-9 G• Most in binaries, esp.

with cool white dwarfs.• No associations with

SNRs.• Many in globular clusters. “Population II”

“The Sounds of Pulsars”: Jodrell Bank obs. Web page: http://www.jb.man.ac.uk/~pulsar/Education/Sounds/sounds.html

X-ray binaries

High-mass companion (HMXB):• Young• X-ray pulsars: magnetic

chanelling of accretion flow• Cyclotron resonance features

B=(1-4)1012G

Low-mass companion (LMXB):• Likely old (low-mass

companions, globular cluster environment)

• Mostly non-pulsating (but QPOs, ms pulsations): weak magnetic field

http://wwwastro.msfc.nasa.gov/xray/openhouse/ns/

Origin & evolution of pulsars: the standard paradigm

“Classical” radio pulsars

• born in core-collapse supernovae

• evolve to longer P, with B const.

• eventually turn off (“death line”)

Millisecond pulsars descend from low-mass X-ray binaries.

Mass transfer in LMXBs produces

• spin-up• magnetic field

decay?

Classical pulsars

Millisecond pulsars

Pulsar glitches

• Sudden increase in the observed rotation rate of a pulsar, / < 10-5, followed by “relaxation” over weeks or months.

• Has been seen in many pulsars.

Interpretation:• Neutrons and protons are expected to form “Cooper pairs” & be in a superfluid

state (like He at low temperatures, o electrons in a superconducting solid).• Superfluids can only rotate by forming quantized vortex lines.• In the NS crust, these vortices can be “pinned” to the solid lattice, preventing

the neutrons from changing their rotation rate.• Only when the rest of the star has spun down significantly, the vortices move &

the neutrons transfer angular momentum to the rest of the star.

The binary

pulsar & GR

Kramer et al. 2006, Science, 314, 97

Magnetars: Brief history- 1• Strongest magnetic field that could possibly be contained in a NS:

• Woltjer (1964): Flux conservation from progenitor star could lead to NSs with B~1014-15G.

• Mazets & Golenetskii (1981): Multiple soft gamma-ray bursts from a single source (SGR 1806-20) detected by Venera spacecraft since Jan 1979.

• Mazets et al. (1979): “March 5 event”: Giant flare (highly super-Eddington) from SGR 0526-66 in LMC (possibly associated w. SNR N49).

• Fahlman & Gregory (1981): First “Anomalous X-ray Pulsar” (AXP): soft spectrum, at center of SNR, no optical counterpart.

• Koyama et al. (1987): AXP is spinning down, but X-ray luminosity much too high to attribute to rotational energy loss of a NS.

G104~~~8

18max4

22max B

RGMPB

Magnetars: Brief history- 2• Thompson & Duncan (TD 1993): Dynamo action just after formation of

a rapidly spinning NS can lead to B~1016G.

• DT (1992), Paczynski (1992), TD (1995, 1996): Strong, decaying field could explain super-Eddington bursts and persistent emission of SGRs & AXPs. TD 1996 predict slow pulsations and fast spin-down.

• Kouveliotou et al. (1998) measure P=7.5 s & B~1015G in SGR 1806-20.

• Gavriil et al. (2002); Kaspi et al. (2003): Several bursts detected from 2 different AXPs.

• SGRs & AXPs share

– fairly long periods ~5-12 s,

– persistent X-ray luminosities ~1035-36 erg/s (BB T ~ 0.4-0.7 keV + high-energy tail), too high to be explained from rotation,

– strong spin-down (inferred B~ 1014-15 G).

Woods & Thompson,astro-ph/0406133

Woods & Thompson,astro-ph/0406133

Woods & Thompson,astro-ph/0406133

Woods & Thompson,astro-ph/0406133

Woods & Thompson,astro-ph/0406133

astro-ph/0609066

Isolated, dim, thermal X-ray emitters

Isolated, dim, thermal X-ray

emitters

Spectra thermal, but with broad absorption lines of unclear origin: atomic? proton cyclotron transitions?

Haberl, astro-ph/0609066

Haberl, astro-ph/0609066

Compact Central Objects (CCOs)

• Near center of SNRs• No radio or gamma-ray emission• No pulsar wind nebula• Thermal X-ray spectrum:

temperature & luminosity intermediate between magnetars and dim isolated neutron stars

Pavlov et al., astro-ph/0311526

• “Rotating RAdio Transients” (RRATs; McLaughlin et al. 2006, Nature, 439, 817) emit occasional, bright radio bursts of 2-30 ms duration

• Intervals 4 min – 3 hr are multiples of a period P ~ 0.4 - 7 s, like slow radio pulsars or magnetars

• Hard to detect (visible ~ 1 s/day): True number should be much larger than for radio pulsars.

McLaughlin et al. 2006; Nature, 439, 817

RRATs vs. pulsars & magnetars

• pulsars (dots)• magnetars (squares)• the 1 radio-quiet isolated

neutron star with a measured period and period derivative (diamond)

• the 3 RRATs having measured periods and period derivatives (stars)

• vertical lines at the top of the plot mark the periods of the other 7 RRATs

McLaughlin et al. 2006; Nature, 439, 817

X-rays from a RRAT

X-ray spectrum of CXOU J181934.1–145804, fitted with an absorbed blackbody model (T=0.12 keV).

2006, ApJ, 639, L71