1. Kick of neutron stars as a possible mechanism for gamma-ray bursts Yong-Feng Huang Department of Astronomy, Nanjing University

2. Contents 1. Background --- GRBs 2. Central engines of GRBs 3. GRBs from NS kicks

3. Fishman et al. 1994 1. Background --- GRBs  Short bursts of -rays from the sky ( Klebesadel et al. 1973) 。  Durations: ms --- ks

4. 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: 10 -5 ─ 10 -7 ergs /cm 2 log Isotropic Lack of weak GRBs

5. 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 ?

6. 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

7. 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

8. Fireball model Large energy release: 10 50 --- 10 54 ergs Small volume: R ~ 10 --- 100 km

9. 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

10. 2. Central engines of GRBs Observational constraints: 1. Isotropic energy release: 10 49 --- 10 54 ergs 2. Event rate: ~10 -6 /galaxy/yr 3. Low baryon contamination ~10 -5 M sun per 10 52 ergs, gamma ~ 100 -- 1000 4. Duration: short ms -- ks 5. Fast variability variation timescale as small as: ms 6. Long-term activities Energy injections, flares: 1000 s – 10000 s

11. 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. 2003 ) Other mechanisms

12. 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)

13. 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 > 10 16 G) (Lai et al. 2001) (iii) Electromagnetic radiation-driven kicks EM radiation from a rotating off-centered magnetic dipole (Lai et al. 2001)

14. 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

15. Baryon contamination Observational constraints: Low baryon contamination ~10 -5 M sun per 10 52 ergs, gamma ~ 100 – 1000 Naturally expected in the EM radiation-driven kicks emitted particles are mainly electrons/positrons

16. Duration Observational constraints: Duration: short ms -- ks Electromagnetic radiation-driven kicks millisecond magnetar can do the job (Usov 1992)

17. Fast variability Observational constraints: Fast variability variation timescale as small as: ms Guaranteed by NS spin may produce a precessing jet

18. Long-term activities Observational constraints: Long-term activities Energy injections, flares: 1000 s – 10000 s Dipole radiation of the pulsar Dai 2004

19. 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:

20. Conclusions Observational constraints: 1. Isotropic energy release: 10 49 --- 10 54 ergs 2. Event rate: ~10 -6 /galaxy/yr 3. Low baryon contamination ~10 -5 M sun per 10 52 ergs, gamma ~ 100 -- 1000 4. Duration: short ms -- ks 5. Fast variability variation timescale as small as: ms 6. Long-term activities Energy injections, flares: 1000 s – 10000 s Thank you ！