A Tidal Disruption model for gamma-ray burst of GRB 060614

1. A Tidal Disruption model for gamma-ray burst of GRB 060614 YE LU National Astronomical Observatories, Chinese Academy of Sciences June 22-27, 2008 Nanjing GRB Conference

2. Outline • Classification scheme for GRBs • Evidence for GRB060614 • Tidal disruption Model • Conclusion June 22-27, 2008 Nanjing GRB Conference

3. Traditional classification of GRBs Short hard GRBs Long soft GRBs 2秒 Kouveliotou et al., 1993 June 22-27, 2008 Nanjing GRB Conference

4. A New Classification Schem (Zhang et al. 2007; Zhang 2006)

5. GRB 060614 1. A short hard GRB Spectral lag as a function of peak luminosity showing GRB 060614 in the region of short-duration GRBs. (Gehrels et al., 2006, Nature)

6. T90=102 s 2. A Long duration GRB Gehrels et al., 2006, Nature • A first short, hard-spectrum lasting 4 s + an extended and softer episode lasting 100 s • A 9 s periodicity exists between 7 and 50 s in the -ray light curve The short episode is actually composed of about 5 mini-pulses, each pulse has a timescale of ~0.6s

7. 3. a nearby GRB (z=0.125) The red shift of GRB060614 (Johan et al. 2006, Nature).

8. 4. No Supernova associated The different colours correspond to different contributions from the supernova: no contribution (blue), a supernova fainter by 5.6 mag (green), and a supernova fainter by 4 mag (yellow) (Valle et al. 2006, Nature)

9. Main features of GRB060614 • A long GRB with duration of 102 s • A low red shift of z=0.125, and not associated with any supernova • Interesting substructures: the light curve of BAT reveals a first short episode of emission (lasting 4s) followed by an extended and some softer episode (lasting 100s) . And the short episode is actually composed of about 5 mini-pulses • There is a 9s periodicity between 7 and 50 s in the -ray light curve • an isotropic gamma-ray energy of 1.08 1050erg releases in 1keV ---10MeV range in the GRB rest frame Geherls et al., 2006; Gal-Yam et al. 2006; Fynbo et al. 2006; Jakobsson et al. 2007

10. Challenges and Question • GRB060614 is a very special event. It is neither a short/hard burst (compact star mergers origin), or a long/soft burst (collapsars origin) • What made GRB060614? June 22-27, 2008 Nanjing GRB Conference

11. GRB060614 might be produced through a tidal disruption of a star by an intermediate mass black hole (IMBH) We propose … Y. Lu, Y.F. Huang, & S.N. Zhang, 2008, ApJ, 684 June 22-27, 2008 Nanjing GRB Conference

12. General Picture The inner region of the debris disk dominated by radiation pressure (Shakura & Sunyaev 1973) Disk dominated by radiation pressure A debris disk formed by tidal disruption a mini-burst Thermal instability A GRB mini-bursts The ordered poloidal magnetic field threading the black hole with each block extracts energy via BZ processes to launch a powerful jet, giving birth to a mini-burst The thermal instability breaks the material into blocks The seed fields anchored in blocks are amplified creating ordered strong poloidal magnetic fields (Bp )

13. Model descriptions • A debris disk: formed through the tidal disruption of a star by an IMBH • Thermal Instability: the debris disk dominated by a radiation pressure is a thermal unstable. This breaks the disk material into many blocks, making the seed field be amplified and creating an ordered strong poloidal field (Bp) • A mini-burst: once a block reaches Rms and be dragged into the BH, Bpextractsa huge amount of energy via the BZ process (Blandford & Znajek 1977), giving birth to a mini-burst.Each mini-burst corresponds to a mini-pulse in the GRB light curve • A GRB: add all mini-bursts together to form a GRB

14. Energy of mini-bursts • The energy of a mini-burst extracted via BZ process: • The mean energy of a mini-burst:  is a viscous parameter of the disk, M 5=Mbh/105Msolar, is the thermal unstable region of the inner disk dominated by radiation pressure

15. Energy of a GRB • The total energy of a GRB is : Ntot is the number of mini-bursts, determined by the mass of a BH (=0.1 for a massive black hole ) and the ratio of the gas pressure to the total pressure of the disk (10-4<<1) (Cheng & Lu 2001) June 22-27, 2008 Nanjing GRB Conference

16. Timescales of bursts • The duration of mini-bursts: determined by blocks are dragged into the BH at Rms(Cheng & Lu 2001) • The total duration of a GRB: determined by the timescale of the thermal instability • The periodicity: given by the Kepler period

17. Application to GRB060614 Assuming a main sequence star (m*=1, r*=1) is tidally disrupted by an IMBH , we obtain • The black hole mass:consider the first short episode of emission lasting for ~4 s is actually composed of 5 mini-pulses. Setting tpulse=0.6s, we have M5=0.2 • The GRB duration:adopting =0.1, and M5=0.2, we have tduration100s, and tK=10s • The GRB energy: setting , =0.1, and M5=0.2 we have Epulse,ave=6.881048ergs, and Etot=2.061050ergs by considering Ntot=30

18. Conclusions • The tidal disruption of a solar type star by an IMBH with a mass of 2104Msoalr is proposed for the special event of GRB 060614, which is nearby long burst but is not associated with a supernova • The powerful energy extracted via the BZ process is enough to trigger a GRB, when the black hole is accreting at the Eddington rate • The basic observed features of GRB 060614 can all be reasonably explained

19. Acknowledgements • We would like to thank B. Zhang , K.S. Cheng, Z.G. Dai, X.D. Li, and Y.Z. Fang for their helpful comments and discussions in this work. • This research was supported by the National Natural Science Foundation of China (Grants 10273011, 10573021, 10433010, 10625313, 10521001, 10733010, 10725313 and 10221001), and by Chinese Academy of Science through project No. KJCX2-YW-T03

20. Thank you!