2008GRB_Nanjing1 Hyperaccretion disks around Neutron stars Dong Zhang & Zigao Dai Nanjing University

2008GRB_Nanjing2 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_Nanjing3 Models of GRB inner engines (from Nakar’s PPT, 2007) LMXB NS-NS NS-BH WD - WD NS Quark star AIC Merger Accretion disk + Black Hole msec magnetar >10 16 G magnetar Merger (AIC) Phase transition Quark star

2008GRB_Nanjing4 Neutrino-cooled accretion disks High accretion rate ~ Msun/s Short accreting timescale ~ 0.1-1s High density, temperature and pressure g cm -3, K Optically thick Neutrino-cooled ~ erg/s electron-positron capture, electron- position pair annihilation, nucleon bremsstrahlung, plasmma decay Different types of flows ADAFs, CDAFs, NDAFs Ref. Eichler et al Paczynski 1991 Narayan et al Popham et al Narayan et al Kohri & Mineshige 2002 Di Matteo et al Lee et al Gu et al Chen & Beloborodov 2007 Liu et al Janiuk et al. 2007

2008GRB_Nanjing5 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_Nanjing6 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 ) 动画

2008GRB_Nanjing7 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_Nanjing8 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  Outer disk － similar to the BH-disk  Inner disk － the entropy conservation self-similar structure （ Cheavlier 1989; Medvedev 2001, 2004 ）

2008GRB_Nanjing9 Chen & Beloborodov 2007

2008GRB_Nanjing10 Two region steady disk model Advection-dominated outer disk Self-similar inner disk NS Shock wave ??

2008GRB_Nanjing11 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

2008GRB_Nanjing12 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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2008GRB_Nanjing14 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_Nanjing15 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.

2008GRB_Nanjing16 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 field For 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_Nanjing17 Neutrino annihilation (1) Neutrino luminosity and neutrino annihilation luminosity

2008GRB_Nanjing18 Neutrino annihilation (2) Neutrino annihilation luminosity with different alpha and boundary condition.

2008GRB_Nanjing19 Neutrino annihilation (3) The total integrated luminosity out to each radius for BH- disk and NS-disk.

2008GRB_Nanjing20 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_Nanjing21 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_Nanjing22 Thank You !