1. Fast Radio Burst: Collisions between Neutron Stars and Asteroids/Comets? Huang, Yong-Feng School of Astronomy and Space Science, Nanjing University

2. Outline 1. Introduction: Fast Radio Bursts 2. Model: NS-asteroid collisions 1.Impact Process 2.Emission Process 3.X-ray Emission 4.Event Rate 3. Conclusion

3. Outline 1. Introduction: Fast Radio Bursts 2. Model: NS-asteroid collisions 1.Impact Process 2.Emission Process 3.X-ray Emission 4.Event Rate 3. Conclusion

4. 1.Fast radio burst (FRB ) was first discovered by Lorimer (2007). FRB 010724 2.FRBs are further confirmed with a considerable sample (Thornton et al. 2013; Keane et al. 2011; Burke-Spolaor & Bannister 2014; Spitler et al. 2014; Ravi et al. 2015). Lorimer et al. (2007)

5. Thornton et al. (2013) Fig. 2. A dynamic spectrum showing the frequencydependent delay of FRB 110220. Time is measured relative to the time of arrival in the highest frequency channel. For clarity we have integrated 30 time samples, corresponding to the dispersion smearing in the lowest frequency channel. (Inset) The top, middle, and bottom 25-MHz-wide dedispersed subband used in the pulse-fitting analysis (2); the peaks of the pulses are aligned to time = 0. The data are shown as solid gray lines and the best-fit profiles by dashed black lines. FRB 110220

6. Thornton et al. (2013)

7. Keane et al. (2011) Burke-Spolaor & Bannister (2014) J1852-08---Detected by Parks FRB 011025---Detected by Parks

8. FRB 121102---Detected by Arecibo (Spitler et al. 2014)

9. FRB 131104---Detected by Parks (Ravi et al. 2015)

10. Characteristics Duration : ~ a few millisecond Flux density : ~ a few Jy Dispersion measure (DM): ~500-1000 Compact objects Coherent emission Propagating through the ionized ISM

11. Characteristics Distance 1.Cosmological origin: (Thornton et al. 2013) 2.Extragalatic origin: dense regions of host galaxies (Katz 2014; Luan & Goldreich 2014) Event rate: (Keane 2014) 1.Event rate of gamma-ray burst: 2.Event rate of core-collapse supernova: No counterparts in other wavelengths have been detected yet (Petroff et al. 2015)

12. Petroff et al. (2015)

13. Scenarios involving neutron stars (NSs) Magnetosphere of NS is a proper energy reservoir, 1.Magnetar giant flares (Popov & Postnov 2007; Kulkarni et al. 2014; Lyubarsky 2014; Pen & Connor 2015) 2.Collapse of hypermassive NSs into black holes (Falcke & Rezzolla 2014; Zhang 2014; Ravi & Lasky 2014) 3.Binary NS mergers (Totani 2013) 4.Planetary companions around NSs (Mottez & Zarka 2014)

14. Other models Binary white dwarf mergers (Kashiyama et al. 2013) Flare stars (Loeb et al. 2014) Evaporation of primordial black holes (Barrau et al. 2014)

15. Outline 1. Introduction: Fast Radio Bursts 2. Model: NS-asteroid collisions 1.Impact Process 2.Emission Process 3.X-ray Emission 4.Event Rate 3. Conclusion

16. Motivations Pulsar timing observations have revealed that there may be planets (Wolszczan & Frail 1992) or asteroid belts (Shannon et al. 2013) around NSs. Previously, it has been suggested in the literature that small solid bodies such as asteroids or comets can impact NSs occasionally (Colgate & Petschek 1981; Mitrofanov & Sagdeev 1990; Katzet al. 1994; Huang & Geng 2014). Recent study shows transient, supergiant pulses from active or dormant pulsars may be triggered by debris entering their magnetospheres (Cordes & Wasserman 2015).

17. Assumptions FRBs are at cosmological distances with redshifts, i.e., The asteroids are of low angular momentum and possess ballistic trajectories. The mass of the asteroid is relatively large (typically ). The asteroid is assumed to be of Fe-Ni composition and the shear strength will be larger. The evaporation and electrodynamic effects imposed by NS can be ignored.

18. Colgate & Petschek (1981) Breakup radius Velocities of the leading and lagging fragments The difference of arrival time

19. Schematic picture Geng & Huang, arXiv:1502.05171, ApJ accepted

20. Duration Emission volume Characteristic frequency Luminosity (Kashiyama et al. 2013)

21. Coherent emission The coherent radiation mechanism may be effective only at a certain distance (Benford & Buschauer 1977), near which Using we can obtain If all the energy released is contributed by potential energy, i.e., then the mass needed is

22. Remnant thermal emission While some electrons are accelerated to move to the top of the fan, most of the matter collapsed would remain in a column on the NS surface. In our model, we assume that the radiation from the heated matter is thermal and the cooling obeys the decaying law as (Lyubarsky et al. 2002).Thus, the remnant X-ray light curve is

23. Predicted remnant X-ray light curves

24. Multi-band afterglows from forward/reverse shocks (Yi, Gao, Zhang 2014)

25. Event rate: a rough estimate In previous studies, the recurrence time ( ) of strong direct impacts (with ) in a typical NS planetary system is estimated to be years (Tremaine & Zytkow 1986; Mitrofanov & Sagdeev 1990; Katz et al. 1994; Litwin & Rosner 2001). Note that the co-moving volume of contains late-type galaxies (Thornton et al. 2013),and suppose the number of NSs in a typical galaxy is (Timmes et al. 1996). The observable event rate of the impact is

26. Summary Impacts between NSs and asteroids/comets may be a promising mechanism for FRBs. Typical mass: millisecond duration The consequent emitting shell at,containing electrons/positrons with, will emit in radio wavelength coherently. Generally no repeating bursts in our scenario. The event rate can be satisfactorily explained. We hope FAST can come out with a much larger sample! Thank You!