Neutron Stars 2: Phenomenology

Andreas ReiseneggerESO Visiting Scientist

Associate 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).

ccdk

dv

m

enkc

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

42

2

2

2

33

2 B

dt

d

cI

Magnetic field:

PPB

3

||

P

Pts 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

11

2

1

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

P

Pts 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

R

GMP

B

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