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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
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)
2. A Long duration GRB T90=102 s
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
3. a nearby GRB (z=0.125)
The red shift of GRB060614 (Johan et al. 2006, Nature).
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)
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
50
 an isotropic gamma-ray energy of 1.08 10 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
• 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
We propose …
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
June 22-27, 2008 Nanjing GRB Conference
General Picture
A debris disk formed
by tidal disruption
The inner region of the debris disk
Diskby
dominated
by
dominated
radiation pressure
pressure
(Shakuraradiation
& Sunyaev 1973)
a mini-burst
Thermal
instability
mini-bursts
A GRB
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
blocks are amplified
The seed fields anchored in
creating ordered strong poloidal magnetic fields (Bp )
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:
 57 / 64
E pulse  5.38 10  M 5 rin
ergs

 , m
  M / M Edd
1  rin  2.52m
48

1
The mean energy of a mini-burst:

 is a viscous parameter of the disk, M 5=Mbh/105Msolar, rin is the thermal
unstable region of the inner disk dominated by radiation pressure
Energy of a GRB

The total energy of a GRB is :
Etot  N tot E pulse,ave
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)
N tot  30
1/ 2
3

1
0.1
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 (Cheng & Lu 2001)
ms
t pulse  t ff  3M 5 s

The total duration of a GRB: determined by the
timescale of the thermal instability
t duration  50 1M 5 s

The periodicity: given by the Kepler period
t K  50M 5 s
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 minipulses. Setting t =0.6s, we have M5=0.2
pulse
The GRB duration: adopting =0.1, and M =0.2,
we have t 100s, and t =10s

5
duration

K
The GRB energy: setting m  1 , =0.1, and M =0.2
5
we have Epulse,ave=6.881048ergs, and Etot=2.061050ergs by
considering Ntot=30
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
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
Thank you!