Hyperaccretion disks around Neutron stars

1. 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 2008GRB_Nanjing

3. Models of GRB inner engines (from Nakar’s PPT, 2007) NS Quark star NS-NS NS-BH WD - WD LMXB Phase transition Merger (AIC) Merger AIC >1016 G magnetar msec magnetar Accretion disk + Black Hole Quark star 2008GRB_Nanjing

4. Neutrino-cooled accretion disks Ref. Eichler et al. 1989 Paczynski 1991 Narayan et al. 1992 Popham et al. 1999 Narayan et al. 2001 Kohri & Mineshige 2002 Di Matteo et al. 2002 Lee et al. 2005 Gu et al. 2006 Chen & Beloborodov 2007 Liu et al. 2007 Janiuk et al. 2007 • High accretion rate~0.01-10 Msun/s • Short accreting timescale~ 0.1-1s • High density, temperature and pressure109- 1011 g cm-3 , 1010 -1011K • Optically thick • Neutrino-cooled ~1051 -1053erg/selectron-positron capture, electron-position pair annihilation, nucleon bremsstrahlung, plasmma decay • Different types of flowsADAFs, CDAFs, NDAFs 2008GRB_Nanjing

5. 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) 2008GRB_Nanjing

6. Center objects to be Neutron Stars X-ray flares of short GRBs are due to magnetic reconnection-driven events from highly magnetized millisecond pulsars. B-field (poloidal) core B-field (toroidal) crust Dai, Wang, Wu & Zhang, 2006, Science, 311, 1127 (动画) 2008GRB_Nanjing

7. 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. 2008GRB_Nanjing

8. Hyperaccretion disks around Neutron stars • Outer disk－similar to the BH-disk • Inner disk－the entropy conservation self-similar structure （Cheavlier 1989; Medvedev 2001, 2004） • 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 2008GRB_Nanjing

9. Chen & Beloborodov 2007 2008GRB_Nanjing

10. Two region steady disk model Advection-dominated outer disk Self-similar inner disk NS Shock wave ?? 2008GRB_Nanjing

11. Thermodynamics and microphysics— mass continue equation— angular momentum equation — energy conservation equation--- pressure and charge equation (EOS)--- neutrino cooling rate--- beta-equilibrium • Two modelsa simple model and a more elaborate model • Size of the inner disk • Structure distribution • Efficiency of neutrino cooling • Comparing with the BH-disk 2008GRB_Nanjing

12. Zhang & Dai 2008, ApJ, in press • 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. • When the accretion rate is moderate, the size of the inner disk reachs its minmium value, since the outer disk flow is mainly NDAFs. • If the accretion rate is large enough to make neutrino emission optically thick, then the effect of neutrino opacity becomes important. 2008GRB_Nanjing

13. 2008GRB_Nanjing

14. 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 2008GRB_Nanjing

15. Luminosity and accretion ratesolid 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. 2008GRB_Nanjing

16. Prospect 展望 • Neutrino annihilationNeutrinos 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. 2008GRB_Nanjing

17. Neutrino annihilation (1) Neutrino luminosity and neutrino annihilation luminosity 2008GRB_Nanjing

18. Neutrino annihilation (2) Neutrino annihilation luminosity with different alpha and boundary condition. 2008GRB_Nanjing

19. Neutrino annihilation (3) The total integrated luminosity out to each radius for BH-disk and NS-disk. 2008GRB_Nanjing

20. Ultra-strongly magnetic field Density, pressure and temperature as functions of radius for the NS surface magnetic field to be 10^15, 10^16, 10^17 Gauss. 2008GRB_Nanjing

21. 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 2008GRB_Nanjing

22. Thank You ! 2008GRB_Nanjing