12008GRB_Nanjing

Hyperaccretion disks around Neutron stars

Dong Zhang & Zigao Dai

Nanjing University

2008GRB_Nanjing 2

Outline

• Models of GRB inner engines

• Neutrino-cooled accretion disks

• X-ray flares

• Center objects to be Neutron Stars

• Hyperaccretion disks around Neutron stars

• Future work

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Models of GRB inner engines(from Nakar’s PPT, 2007)

LMXB NS-NSNS-BH

WD - WDNS Quark star

AIC Merger

Accretion disk +Black Hole

msecmagnetar

>1016 Gmagnetar

Merger(AIC)

Phasetransition

Quark star

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Neutrino-cooled accretion disks• High accretion rate ~0.01-10

Msun/s

• Short accreting timescale ~ 0.1-1s

• High density, temperature and pressure 109- 1011 g cm-3 , 1010 -1011K

• Optically thick• Neutrino-cooled ~1051 -1053erg/s

electron-positron capture, electron-position pair annihilation, nucleon bremsstrahlung, plasmma decay

• Different types of flows ADAFs, CDAFs, NDAFs

Ref.Eichler et al. 1989Paczynski 1991Narayan et al. 1992Popham et al. 1999Narayan et al. 2001Kohri & Mineshige 2002Di Matteo et al. 2002Lee et al. 2005Gu et al. 2006Chen & Beloborodov 2007Liu et al. 2007Janiuk et al. 2007

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X-ray flares• Fragmentation of a rapidly

rotating core (King, 2005)

• Magnetic regulation of the accretion flow (Proga, Zhang, 2006)

• Fragmentation of the accretion disk (Perna et al. 2005)

• Differential rotation in a post-merger pulsar (Dai et al. 2006)

• Infalling tidal tail of material from NS (Lee, Ramirez-Ruiz, 2007)

Lazzati et al. (2008)

Metzger et al. (2008)

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Center objects to be Neutron Stars

X-ray flares of short GRBs are due to magnetic reconnection-driven events from highly magnetized millisecond pulsars.

core

crust

B-field (poloidal)

B-field (toroidal)

Dai, Wang, Wu & Zhang, 2006, Science, 311, 1127 ( 动画)

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• X-ray flares after some GRBs may be due to a series of magnetic activities of highly-magnetized, millisecond-period pulsars.

• The GRBs themselves may originate from transient hyperaccretion disks surrounding the neutron stars via neutrino or magnetic processes.

• Hyperaccretion disks could also occur in type-II supernovae if fall-back matter has angular momentum.

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Hyperaccretion disks around Neutron stars

• Similar to BH-disk－ high accretion rate, short timescale－ high density, temperature and pressure－ optically thick, neutrino-cooled－ ADAFs, NDAFs

• Different to BH-disk－ the neutron star surface prevent heat energy from being advected inward

－ the inner region to be hotter and denser－ shock wave or not

1/( 1)

/( 1)

(3 2 ) /( 1)r

r

P r

v r

Outer disk－ similar to the BH-disk Inner disk－ the entropy conservation self-similar structure（ Cheavlier 1989; Medvedev 2001, 2004）

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Chen & Beloborodov 2007

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Two region steady disk model

Advection-dominated outer diskSelf-similar inner disk

NS

Shock wave ??

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• Thermodynamics and microphysics— mass continue equation— angular momentum equation — energy conservation equation--- pressure and charge equation (EOS)--- neutrino cooling rate

--- beta-equilibrium

• Two models a simple model and a more elaborate model

Size of the inner disk Structure distribution Efficiency of neutrino cooling Comparing with the BH-disk

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1. When the accretion rate is sufficiently low, most of the disk is advection-dominated, the energy is advected inward to heat the inner disk, and eventually released via neutrino emission in the inner disk.

2. When the accretion rate is moderate, the size of the inner disk reachs its minmium value, since the outer disk flow is mainly NDAFs.

3. If the accretion rate is large enough to make neutrino emission optically thick, then the effect of neutrino opacity becomes important.

Zhang & Dai 2008, ApJ, in press

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The “equivalent” adiabatic index and the electron fraction Ye in the elaborate model as a function of radius. The “equivalent” adiabatic index can be expressed by

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Luminosity and accretion rate solid line corresponds to the whole disk of a neutron star, dashed line to the inner disk, dotted line to the outer disk, and dash-dotted line to the black hole disk.

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Prospect 展望• Neutrino annihilation

Neutrinos from a hyperaccretion disk around a neutron star will be possibly annihilated to electron/positron pairs, which could further produce a jet. This could be helpful to draw the conclusion that some GRBs originate from neutrino annihilation rather than magnetic effects such as the Blandford-Znajek effect.

• Ultra-strongly magnetic fieldFor magnetars, the magnetic fields could play a significant role in the global structure of hyperaccretion disks as well as underlying microphysical processes, e.g., the quantum effect (Landau levels) on the electron distribution and magnetic pressure in the disks could become important.

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Neutrino annihilation (1)

10-1 100 101 1021046

1048

1050

1052

1054

Lv

(erg

s s-1

)

Accretion Rate (0.01MSUN

/s)

BH ann NS total1 NS ann1 NS ann2 NS total2 BH total

Neutrino luminosity and neutrino annihilation luminosity

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Neutrino annihilation (2)

10-1 100 101 1021044

1046

1048

1050

1052

1054

Lv

(erg

s s-1

)

Accretion Rate (0.01 Msun/s)

total total

Neutrino annihilation luminosity with different alpha and boundary condition.

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Neutrino annihilation (3)

10 20 30 40 501033

1035

1037

1039

10412

l vvdz

(er

gs s

-1cm

-1)

Radius (R/Rg)

md=1,BH

md=1,NS

md=1,NS+BB

The total integrated luminosity out to each radius for BH-disk and NS-disk.

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Ultra-strongly magnetic field

5 10 15 20 25 3010-2

10-1

100

101

102

103

104

10^15 10^16 10^17 none

De

nsi

ty

(10

11g

cm

-3)

Radius (106 cm)

5 10 15 20 25 30

10-1

100

101

102

103

104

10^15 10^16 10^17 none

Pre

ssur

e (1

029

erg

s cm

-3)

Radius (106 cm)

5 10 15 20 25 30

0.5

1.0

1.5

2.0

Tem

pera

ture

(1

01

1K

)

Radius (106 cm)

10^15 10^16 10^17 none

Density, pressure and temperature as functions of radius for the NS surface magnetic field to be 10^15, 10^16, 10^17 Gauss.

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Summary• Newborn neutron stars have been invoked to be central

engines of GRBs in some origin/afterglow models.• Hyperaccretion disks surrounding pulsars could provide an

energy source for some explosions via neutrino/magnetic processes.

• Compared with the black-hole disk, the neutron star disk can cool more efficiently and produce a much higher neutrino luminosity.

• Some GRBs may originate from neutrino annihilation.• The ultra-highly magnetic fields for magnetars could play

a significant role in the global structure of hyperaccretion disks

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Thank You !