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

Outline  Classification scheme for GRBs  Evidence for GRB  Tidal disruption Model  Conclusion June 22-27, 2008 Nanjing GRB Conference

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

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

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

2. A Long duration GRB T 90 =102 s  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 Gehrels et al., 2006, Nature

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

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) 4. No Supernova associated

Main features of GRB  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  erg 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

Challenges and Question GRB 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

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

General Picture A GRB a mini-burst 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 Disk dominated by radiation pressure Thermal instability The inner region of the debris disk dominated by radiation pressure (Shakura & Sunyaev 1973) A debris disk formed by tidal disruption The thermal instability breaks the material into blocks The seed fields anchored in blocks are amplified creating ordered strong poloidal magnetic fields (B p ) mini-bursts

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 (B p )  A mini-burst: once a block reaches R ms and be dragged into the BH, B p extracts a 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

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 =M bh /10 5 M solar, is the thermal unstable region of the inner disk dominated by radiation pressure

Energy of a GRB The total energy of a GRB is : N tot 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

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

Application to GRB 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. S etting t pulse =0.6s, we have M 5 =0.2 The GRB duration: adopting  =0.1, and M 5 =0.2, we have t duration  100s, and t K =10s The GRB energy: setting,  =0.1, and M 5 =0.2 we have E pulse,ave =6.88  ergs, and E tot =2.06  ergs by considering N tot =30

Conclusions The tidal disruption of a solar type star by an IMBH with a mass of 2  10 4 M soalr is proposed for the special event of GRB , 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 can all be reasonably explained

 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 , , , , , , and ), and by Chinese Academy of Science through project No. KJCX2-YW-T03 Acknowledgements

Thank you!