Millisecond pulsar population and related high energy phenomena

王 伟(NAOC)

July 2009, Pulsar Summer School Beijing

Contents

• Introduction to millisecond pulsars (MSPs);• High energy emissions in MSPs: observations

versus theories;• Millisecond pulsar population in the Galactic

Center (GC)• High energy contributions by MSPs toward the

GC

Formation scenario of MSPs

msMMMMkmRBP eddeq7/37/57/187/69 )/()4.1/()10/()10/(0.3

The spin equilibrium of neutron stars in low-mass X-ray binaries:

What are MSPs (?): Observations (in radio and X-rays): compact objects with very fast spin, P<30 ms, i,e, pulsars, derived dipole magnetic fields < 1010 G;

Theories (formation channel): old neutron stars in the low mass X-ray binaries (LMXB) are recycled to millisecond periods through accretions. Their current population likely consists of neutron stars in isolation (having evaporated their companions) and in LMXBs. - Very old (>108 yr) and fast-spin (P<30 ms) pulsars -

Observed spin period (fastest up to now ): PSR B1957+20 0.0016074 s PSR B1937+21 0.0015578 s PSR J1748-2446ad 0.0013966 s (Terzan 5 )

P – P diagram (circles: binaries)

What is the realistic formation channel of MSPs from normal neutron stars in LMXBs?

No confident evidence for the transition-state sources before 2008

A number of mildly recycled binary pulsars possibly found.

This channel confirmed by recent discovery: a radio MSP in a LMXB (Archibald et al. 2009): PSR J1023+0038, P=1.69 ms

.

Two populations of MSPsfrom the present observations: Globular clusters Galactic field

47Tuc

Ter5

NGC6266

Two globular clusters in the bulge

MSP distribution in the Galaxy

Soft X-ray emission properties of MSPs (0.1 - 10 keV)

• X-ray emission detected from ~35 MSPs (including Galactic disk and globular clusters);

• Six of these have pulsed emission; For three (J0437-4715, J2124-3358, J0030+0451) the emission appears

thermal dominated; Non-thermal emission dominated for J0218+4232, B1821-24 (in M28),

B1937+21.

• X-ray pulsar wind nebulae of MSPs are also detected recently (PSR B1957+20, Stappers et al. 2003, Huang et al. 2007)

Two origins of PWNe:• The interaction between the pulsar winds with the interstellar medium (ISM) – bow shock;• interaction between pulsar winds and the stellar winds of donor stars – intrabinary shock.

X-ray behavior of MSPs in globular clusters (e.g. Tuc 47 )

Slope =1

Slope =0.2

Observations showvery soft X-ray thermal spectra, kT ~ 0.1 keV

Previously thought to have no X-ray tails extended to 10 keV.

Only thermal emission

Recent Chandra deep observations(Bogdanov et al. 2006 …):Non-thermal emissions in some MSPs are also detected !

Origin:PWNe due to the intrabinary shock

Gas density of ISM in globular clusters is very low. So PWNe only are obvious in binary systems. Most are still thermal dominated.

Hard X-rays to Gamma-rays of MSPs (20 keV – GeV)

Three hard X-ray candidates

GeV candidate for PSRJ0218+4232 is reported by EGRET (Kuiper et al. 2004); confirmed by Fermi (Abdo et al. 2009b).

Fermi is a new advanced gamma-ray telescope, launched mid-2008, discovered near 30 gamma-ray pulsars, including 8 MSPs .

PSR J0030+0451 : P = 4.87 ms (Abdo et al. 2009a)

>100 MeV

0.3 -2.5 keV

1.4 GHz

Energy Cutoff :1- 2 GeV

Two populations of MSPsfrom the present observations: Globular clusters Galactic field

Other populations (?):an unresolved millisecond pulsar population in the Galactic center (GC) region?

The Galactic bugle is similar to globular clusters.

Millisecond pulsar populations in the Galaxy

Some Motivations

Chandra deep survey ofthe Galactic center (17’X17’, by Muno et al. 2003) discovered 2000weak unidentified X-ray sources (L>3x1030 erg/s)

Possible source populations: cataclysmic variables X-ray binaries young stars supernova ejecta pulsars/pulsar wind nebulae

Sharp turnover round GeVsimilar to the EGRET spectrum of some pulsars, e.g. Vela, Geminga, and gamma-ray spectrum of MSPs (PSR J0030+0451 by Fermi)

Why millisecond pulsars in the GC?

Normal Pulsars: Mass ratio of Muno’s field to the whole Galaxy: 10-3 birth rate: 10-5-10-4 per yr, active timescale: several Myr average birth velocity: 500 km/s (Arzoumanian, Chernoff, & Cordes 2002), few of them

will stay inside the GC region (escape speed around 200 km/s) Less than 10 normal active pulsars stay in the Muno’s region. Thus, normal pulsars cannot be the main contributor to pulsar population in the GC.

Millisecond Pulsars: MSPs can remain active nearly in the Hubble timescale after their birth (much longer

lifetime than normal pulsars); The average birth velocity of MSPs is around 130 km/s (Lyne et al. 1998), so they could

stay in the GC region throughout their life; A population analysis of Lyne et al. (1998) suggested around 3x105 MSPs in the whole

Galaxy.; The binary population synthesis in the GC (Taam 2005) shows about 200 MSPs are

produced through recycle scenario and stay in the region observed by Muno et al. (2003) if assume the present star-formation rate in the Galaxy.

Possible high energy contributions by unresolved MSPs toward the Galactic Center

The unidentified weak Chandra X-ray sources GeV spectrum from the GC observed by EGRET 511 keV annihilation line from the GC observed by INTEGRAL/SPI

Unidentified weak x-ray sources in the GC

Contributions by a millisecond pulsar population (Cheng et al. 2006):non-thermal hard x-rays from synchrotron radiation from compact wind nebulae in MSPsFor bow shocks: Lx ∝ n (p-2)/4 Lsd

p/2 (p>2, electron energy index)X-ray luminosity (2-10 keV) typically around 1030-33 erg/sPhoton index in x-rays : Γ=(p+1)/2 or (p+2)/2In general, 2<p<3, then Γ varies from 1.5-2.5

(Muno et al. 2004)

G 359.95-0.04 near Sgr A*(Wang, Lu & Gotthelf 2006)

Wind nebulae formed through bow shocks of high speed MSPs (>100 km/s) may contribute to the elongated x-ray features (x-ray tails, pulsar wind nebula candidates)

G 0.13-0.11(Wang, Lu & Lang 2002)

Sgr A*

N=6000

Contribution of millisecond pulsar population in the Galactic center to the GeV spectrum (predicted from outer-gap models)

Wang et al. 2005

INTEGRAL observations of 511 keV line:(Knodlseder et al. 2003, 2005; Churazov et al. 2005 )

(Knodlseder et al. 2005)

• morphology (with size of radius 6o-8o): diffuse, bulge-like, weak/ no disk component ;• high line luminosity and strong positronium continuum: line intensity implies the positron injection/ annihilation rate up to 1043 /s .

The possible positron sources in the present theoretical models• hypernovae/gamma-ray bursts in the GC (Casse et al. 2004; Bertone et al. 2006)• light dark matter annihilation (mass <100 MeV, Boehm et al. 2004; Casse & Fayet 2005)• winds of a millisecond pulsar population in the GC/bugle (Wang et al. 2006)• Sgr A*: continuous capture of stars by supermassive black hole (Cheng et al. 2006), p-p interactions

(Churazov et al. 2005)

Properties of 511keV emission

Previously, positrons in the Galaxy dominated by nucleosynthesis in supernovae;

But supernovae should distribute along the Galacic disk; inconsistent with 511 keV morphology

Millisecond pulsars can be the continuous positron injection sources because of their long life time.

Positrons produce through pair cascades near the surface of neutron stars ,

escaped from neutron stars as wind particles.For P=3ms, B=3x108 G, the positron injection rate for a MSP: ~ 5x1037 e+/s (Wang et al. 2006)

How many MSPs contribute to the annihilation line?Assume , N~ 6000x(6o/1.5o)2 ~105

Then the total positron injection rate from MSPs : 5x1042 e+/s Significant contribution to positrons in the GC, produce observed 511 keV luminosity

Possible discrimination between positron source models

Assume positrons diffuse in the magnetic field in the GC:The Larmor radius rL ~ E/eBThe diffuse timescale is estimated:We change the form λ ~ (rL ct)1/2

Take B ~ 10-5 G in the GC (Uchida & Gusten 1995; LaRosa et al. 2005), the mean lifetime of positrons is about 106 yr, the characteristic diffusion scale of positrons is ~ 1 pc. (but observational resolution >> 1pc)

The line intensity distribution may be similar to that of the positron sources. Supernovae, hypernovae/ GRBs: 511 keV emission may follow the distribution of molecular clouds in the GC; MSP population in the GC: line emission may follow the mass (e.g. stars) distribution of the GC; Light dark matter annihilation: line emission may follow the dark matter density profile.

Summary & Perspective Three possible MSP populations: globular clusters; Galactic

field; Galactic Center (GC). The population of MSPs in the GC is an assumption, but it seems reasonable.

MSP population can contribute to the weak unidentified Chandra sources in the GC , specially to the elongated x-ray features.

Unresolved MSP population can significantly contribute to the gamma-ray spectrum detected by EGRET in the GC;

These MSPs could be detected or resolved by Fermi; GeV gamma-ray emission from total MSPs in globular

clusters detected by Fermi (Tuc 47 has been a GeV source). MSPs in the GC/bulge could be the potential positron sources. Because the electron density in the direction of the GC is very

high, it is difficult to detect MSPs using the present radio telescopes. X-ray or Gamma-ray studies in the GC would probably be a feasible method to find MSP (candidates) .