Hyperaccreting Disks around Neutrons Stars and Magnetars for GRBs:

Neutrino Annihilation and Strong Magnetic Fields

Dong Zhang (Ohio State)

Zi-Gao Dai (Nanjing Univ)

張 棟

尾張の大うつけ

Outline

• Why Neutron Star Disks?

• Differences between Neutron Star Disks and Black Hole Disks

• Effects of Strong Magnetic Fields from the Center Star (Magnetar)

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

LMXB NS-NSNS-BH

WD - WDNS Quark star

AIC Merger

Accretion disk +Black whole

msecmagnetar

>1016 Gmagnetar

Merger(AIC)

Phasetransition

Quark star

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

Why Neutron Star Disks?• A neutron star can survive after collapses and merge

rs

• X-ray flares after some GRBs may be due to a series of magnetic activities of pulsars (Dai et al. 2006 ).

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

• Hyperaccreting disks could also provide the material and energy for SN associated with GRB.

• Magnetar (Thompson’s talk). We focus on the stellar-disk system.

Two region steady disk model

Advection-dominated outer diskSelf-similar inner disk

NS

Shock wave ??

NS vs BH ： Neutrino Luminosity

Zhang & Dai (2008)

Neutrino Annihilation

Zhang & Dai (2009a)

Zhang & Dai (2009a)

Relativistic Jet?

• More powerful neutrino annihilation rate along the disk polar region

• But heavier mass loss rate for ~ 100ms

• >100 ms: depending on the accretion rate and boundary condition

Dessart et al. 2009

Strong Magnetic Fields

Lovelace et al. (1995)

Strong Magnetic Fields (>10^15 G)

• Microphysics

• Disk Dynamic Equations Angular Momentum Joule Heating

• Funnel Accretion

Magnetic Fields: Open Configuration

Accretion Rate:0.1 M_0/s

Zhang & Dai (2009b)

Funnel AccretionRomanova (2007)

Long (2008)

Energy from…

Summary• Newborn neutron stars have been invoked to be cent

ral engines of GRBs in some origin/afterglow models.• Hyperaccreting disks surrounding pulsars could provi

de 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 to change the disk properties and produce a energetic relativisitic jet.

Thank You !