Kick of neutron stars as a possible mechanism for

gamma-ray bursts

Yong-Feng Huang

Department of Astronomy, Nanjing University

Contents

1. Background --- GRBs

2. Central engines of GRBs

3. GRBs from NS kicks

Fishman et al. 1994

1. Background --- GRBs

Short bursts of -rays from the sky (Klebesadel et al. 1973) 。

Durations: ms --- ks

Basic features of GRBs

-- spatial features

-- detection rate:

-- temporal features

-- spectral features

• Profiles Complicated

• Durations 5 ms ─ 10 s ─ 5×103 s

• Variability 0.1ms ─1ms ,

• No repetition

1-2 events per day by CGRO/BATSE

• Photon Energy 0.1 ─ 1 MeV ─ 20 GeV

• Non-thermal:

N(E) E∝ -α

• High Energy Tail:

no cutoff above 1 MeV

• Fluence:

10-5 ─ 10-7 ergs /cm2

logIsotropicLack of weak GRBs

Breakthrough in 1997: afterglows discovered

X-ray afterglow of GRB 970228: discovered by BeppoSAX 。Optical, radio afterglow also observed

GRBs are at cosmological distances

Optical afterglow Groot et al., IAUC 6584 X-ray afterglow Costa et al. 1997, Nature

• Hundreds of GRBs localized • Optical afterglows: ~40 ％ • Redshift measured: ~20 ％ • Radio afterglows: ~10 ％ （ Error bars: 0.001’’ ）

Basic features of afterglows

GRBs located:960720 970111 970228 970402 970508 970616 970815 970828 971024 971214 971227 980302 980326 980329 980425 980515 980519 980613 980703 981220 981226 990123 990217 990506 990510 990712 991208 991216 000301c 000630 000630 000418 … …

F t ∝ –1 ─ t –2

Kann 2008

the Fireball Model

Inter-StellarMedium

初始火球

~108 kmγ~1000

internalshocks

External shocks

~1011 kmγ>>1R~10 km

E>1052 ergsM<10-5 Msun

Afterglows

However, what is the central engine?

GRB主暴

2. Central engines of GRBs

Observational constraints:1. Isotropic energy release: 1049 --- 1054 ergs 2. Event rate: ~10-6/galaxy/yr3. Low baryon contamination

~10-5Msun per 1052 ergs, gamma ~ 100 -- 10004. Duration: short ms -- ks5. Fast variability variation timescale as small as: ms6. Long-term activities Energy injections, flares: 1000 s – 10000 s

Central enginesCollapse of massive stars Collision of Compact stars Phase transition of NS to Strange star Kick of neutron stars (Cen 1999; Dar 1999; Huang et al. 2003) Other mechanisms

3. GRBs from NS kicks

Jet energy:

Momentum conservation:NSNSflow / VMcE

Radiation energy:

ergs104~05.0,3.0 54iso E

Assume: NS kick is associated with the launch of a jet.

And the jet is ultra-relativistic.

(Cen 1999; Dar 1999; Huang et al. 2003)

Kick mechanism

(i) Hydrodynamically driven kick mechanism kick time scale: ~0.1 s (Lai et al. 2001)

difficult to produce high speed NS (> 500km/s) (Janka & Muller 1994)

(ii) Neutrino-driven kick mechanism asymmetric neutrino emission induced by strong B difficult to produce high speed NS (need B > 1016 G) (Lai et al. 2001)

(iii) Electromagnetic radiation-driven kicks EM radiation from a rotating off-centered magnetic dipole

(Lai et al. 2001)

Event rate

Observational constraints:GRB event rate: ~10-6/galaxy/yr

SN rate: 1/30 – 1/50 /yr/galaxy

NS birth rate: ~10-2 /yr /galaxy

High speed NS birth rate: ~10-4 – 10-3 /yr/galaxy

Beam corrected GRB rate: ~10-7 – 10-6 /yr/galaxy

Baryon contamination

Observational constraints:Low baryon contamination

~10-5Msun per 1052 ergs, gamma ~ 100 – 1000

Naturally expected in the EM radiation-driven kicks

emitted particles are mainly electrons/positrons

Duration

Observational constraints:Duration: short

ms -- ks

Electromagnetic radiation-driven kicks

millisecond magnetar can do the job

(Usov 1992)

Fast variability

Observational constraints:Fast variability

variation timescale as small as: ms

Guaranteed by NS spin

may produce a precessing jet

Long-term activities

Observational constraints:Long-term activities

Energy injections, flares: 1000 s – 10000 s

Dipole radiation of the pulsar

Dai 2004

Short GRBs

Long GRBs: duration ~ 20 s

Short GRBs: duration ~ 0.2 s

For a NS, when P < Pcr ~ 0.5 – 1.6 ms, instability arises inside the star, and gravitational radiation plays the major role in slowing down the NS. Then the spin-down timescale is:

Conclusions

Observational constraints:1. Isotropic energy release: 1049 --- 1054 ergs 2. Event rate: ~10-6/galaxy/yr3. Low baryon contamination

~10-5Msun per 1052 ergs, gamma ~ 100 -- 10004. Duration: short ms -- ks5. Fast variability variation timescale as small as: ms6. Long-term activities Energy injections, flares: 1000 s – 10000 s

Thank you ！