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

Outline

Classification scheme for GRBs

Evidence for GRB060614

Tidal disruption Model

Conclusion

June 22-27, 2008 Nanjing GRB Conference

Traditional classification of GRBs

Short hard GRBs Long

soft GRBs

2秒Kouveliotou et al., 1993

June 22-27, 2008 Nanjing GRB Conference

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

2006)

GRB 060614

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

1. A short hard GRB

2. A Long duration GRB T90=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 GRB060614 (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 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

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

GRB060614 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 (Bp )

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 (Bp) A mini-burst: once a block reaches Rms and be

dragged into the BH, Bp 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:

Eddin

inpulse

MMmmr

ergsrME

/,52.21

1038.5 64/575

148

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

inr

Energy of a GRB

The total energy of a GRB is :

avepulsetottot ENE ,

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

11.0

2/1330

totN

Timescales of bursts

sMtt ffpulse 53

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

sMtduration 5150

The periodicity: given by the Kepler period

sMtK 550

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 tduration100s, and tK=10s

The GRB energy: setting , =0.1, and M5=0.2

we have Epulse,ave=6.881048ergs, and Etot=2.061050ergs by

considering Ntot=30

1m

Conclusions The tidal disruption of a solar type star by an IMBH

with a mass of 2104Msoalr 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

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

Acknowledgements

Thank you!