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 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 ─ 5×10 3 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: ─ ergs /cm 2 log Isotropic Lack of weak GRBs

How far are GRBs from us? The answer is unknown for ~30 years! Poor localization ability of - ray detectors  Large error bars in position. ≥ 1 o ─2 o GRBs could not be detected in other bands: X-ray counterparts ? optical counterparts ? are missing radio counterparts ?

Breakthrough in 1997: afterglows discovered  X-ray afterglow of GRB : 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: c … … F ∝ t –1 ─ t –2 Kann 2008

Fireball model Large energy release: ergs Small volume: R ~ km

the Fireball Model Inter- Stellar Medium 初始火球 ~10 8 km γ~1000 internal shocks External shocks ~10 11 km γ>>1 R~10 km E>10 52 ergs M<10 -5 M sun Afterglows However, what is the central engine? GRB 主暴

2. Central engines of GRBs Observational constraints: 1. Isotropic energy release: ergs 2. Event rate: ~10 -6 /galaxy/yr 3. Low baryon contamination ~10 -5 M sun per ergs, gamma ~ Duration: short ms -- ks 5. Fast variability variation timescale as small as: ms 6. Long-term activities Energy injections, flares: 1000 s – s

Central engines Collapse 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 ) Other mechanisms

3. GRBs from NS kicks Jet energy: Momentum conservation: Radiation energy: 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 > 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 – /yr/galaxy Beam corrected GRB rate: ~10 -7 – /yr/galaxy

Baryon contamination Observational constraints: Low baryon contamination ~10 -5 M sun per 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 – 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: ergs 2. Event rate: ~10 -6 /galaxy/yr 3. Low baryon contamination ~10 -5 M sun per ergs, gamma ~ Duration: short ms -- ks 5. Fast variability variation timescale as small as: ms 6. Long-term activities Energy injections, flares: 1000 s – s Thank you ！