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 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 < 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 (>10 8 yr) and fast-spin (P<30 ms) pulsars - Observed spin period (fastest up to now ): PSR B s PSR B s PSR J ad 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 J , P=1.69 ms.

Two populations of MSPs from 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 ( keV) X-ray emission detected from ~35 MSPs (including Galactic disk and globular clusters); Six of these have pulsed emission; For three (J , J , J ) the emission appears thermal dominated; Non-thermal emission dominated for J , B (in M28), B X-ray pulsar wind nebulae of MSPs are also detected recently ( PSR B , Stappers et al. 2003, Huang et al ) 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 show very 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 …): 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 PSR J 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 J : P = 4.87 ms (Abdo et al. 2009a) >100 MeV keV 1.4 GHz Energy Cutoff :1- 2 GeV

Two populations of MSPs from 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 of the Galactic center (17 ’ X17 ’, by Muno et al. 2003) discovered 2000 weak unidentified X-ray sources (L>3x10 30 erg/s) Possible source populations:  cataclysmic variables  X-ray binaries  young stars  supernova ejecta  pulsars/pulsar wind nebulae

Sharp turnover round GeV similar to the EGRET spectrum of some pulsars, e.g. Vela, Geminga, and gamma-ray spectrum of MSPs (PSR J by Fermi)

Why millisecond pulsars in the GC? Normal Pulsars: Mass ratio of Muno’s field to the whole Galaxy: birth rate: 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 3x10 5 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 MSPs For bow shocks: Lx ∝ n (p-2)/4 L sd p/2 (p>2, electron energy index) X-ray luminosity (2-10 keV) typically around erg/s Photon index in x-rays : Γ=(p+1)/2 or (p+2)/2 In general, 2<p<3, then Γ varies from (Muno et al. 2004)

G 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 (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 ) (Knodlseder et al. 2005)

morphology (with size of radius 6 o -8 o ): diffuse, bulge-like, weak/ no disk component ; high line luminosity and strong positronium continuum: line intensity implies the positron injection/ annihilation rate up to /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=3x10 8 G, the positron injection rate for a MSP: ~ 5x10 37 e + /s (Wang et al. 2006) How many MSPs contribute to the annihilation line? Assume, N~ 6000x(6 o /1.5 o ) 2 ~10 5 Then the total positron injection rate from MSPs : 5x10 42 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 r L ~ E/eB The diffuse timescale is estimated: We change the form λ ~ (r L ct) 1/2 Take B ~ G in the GC (Uchida & Gusten 1995; LaRosa et al. 2005), the mean lifetime of positrons is about 10 6 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).