Radiation-hydrodynamic Models for ULXs and ULX-pulsars

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1 Radiation-hydrodynamic Models for ULXs and ULX-pulsars Tomohisa KAWASHIMA Division of Theoretical Astrophysics, NAOJ in collaboration with Ken OHSUGA, Hiroyuki TAKAHASHI (NAOJ) Shin MINESHIGE, Takumi OGAWA (Kyoto U.) Tessei YOSHIDA(Ehime U.), Ryoji MATSUMOTO (Chiba U.) ULX

2 Ultra-luminous X-Ray Sources (ULXs) before 2014 Off-centered, extragalactic X-ray sources with luminosities exceeding Eddington limit for stellar-mass BHs. L X = [erg s 1 ] for L E ' (M/10M )[erg s 1 ] (A) Sub-critically accreting intermediate-mass BHs (e.g., Colbert & Mushotzky 1999, Makishima et al. 2000) or (B) Super-critically accreting stellar-mass BHs (e.g., King et al. 2001, Watarai et al. 2001, Vieryadanti et al. 2006, Kawashima et al. 2009, 2012) It is still an open question

3 Discoveries of ULX-pulsars after 2014 image of ULX-pulsars Bachetti et al. (2014) discovered pulsed X- ray emissions in ULX M82 X-2 (period 1.37 sec, X-ray variety ~10%). Recently, pulsed emissions have been also observed in the other 2 ULXs (Israel et al. 2017). NAOJ These indicate anisotropic X-ray emissions from NS polar caps! Bachetti et al LX ~ erg s -1 (LE ~ erg s -1 for NSs) Super-critical accretion with LX/LE ~ 100! We can interpret that super-critical accretion onto NSs occur in ULX-pulsars.

4 Why do we consider super-critical accretion in ULXs? Now, we have 3 possible models for ULXs. Pulsed emissions have been observed in 3ULXs. This indicates that NSs are harbored in some ULXs. In general, super-critical accretion onto NSs and BHs may occur in ULXs! possible models for ULXs (A)sub-critically accreting IMBH or (B)super-critically accreting stellar-mass BH or (C) super-critical accreting NS Dynamics and radiative spectra of super-critical accretion is not well understood. We perform radiation-hydrodynamic simulations to study dynamics and spectra of super-critically accreting black holes and neutron stars.

5 Super-critical accretion onto BHs focusing on radiative spectra Kawashima et al. (2009, 2012)

6 ULXs spectra curious spectra 0.01 NGC1313X2 XMM-Newton data (Gladstone et al. 2009) 0.01 IC342-X1 XMM-Newton and Suzaku (Yoshida et al. 2013) EF_E EF_E NGC1313 IC342 X Energy (kev) 1 10 Energy (kev) ULX shows (1) moderately soft spectra with rollovers at ~5 kev, (2) hard spectra with power-law extending upto ~20 kev. ULXs present spectra with rollover at relatively higher photon energy, which are not seen in sub-critically accreting stellar-mass or supermassive BHs. (Gladstone et al. 2009, Yoshida et al. 2013, Walton et al. 2014)

7 Dynamics of super-critical accretion flows onto BHs Super-critical accretion onto BHs is feasible (e.g., Ohsuga et al. 2005, 2007) reason 1: anisotropic radiative fields reason 2 photon trapping effects (Begelman 1978) Recent GR-RMHD simulations have confirmed the results of nonrelativistic RHD simulations (McKinney et al. 2014; Sadowski et al. 2014, 2015; Takahashi et al. 2016) Ohsuga et al Ohsuga et al. 2007

8 Compton cooling effects on super-critically accreting BHs (Kawashima et al. 2009) axisymmetric 2D RHD simulations radiative transfer Flux-Limited Diffusion app. with gray app., hydro Godunov method PPM-LR) Effects of Compton cooling/heating are incorporated. wind jet accretion flow BH Simulated region Gas temperature in outflows decreases by 2 orders of magnitudes due to Compton cooling effects!

9 A structure of super-critical accretion flows and outflows (a) (b) < 10 2 c 10 8 K 10 8 K

10 Radiative spectra of super-critical accretion flows onto BHs (Kawashima et al. 2012) RHD simulation data SEDs Monte-Carlo radiative transfer (Bremsstrahlung emission/absorption, Compton/inverse Compton scatterings)

11 Effects of Comptonization on SEDs

12 Dependence of SEDs on mass accretion rate PL with photon index ULX spectra observed by XMM-Newton and Suzaku (Yoshida et al. 2013) ~5 kev rollover lower mass accretion rate : hard PL SEDs extending upto ~10 kev higher mass accretion rate : soft SEDs with rollover at ~5 kev. These features are similar to ULX spectra.

13 How is the rollover at ~ 5keV formed? Compton downscatterings by cool electrons in outflow Artificially, Compton upscatterings are not allowed in outflows higher mass accretion rate or higher viewing angle softening by Compton downscatteirngs in outflows Effects of Compton downscatterings in the cool outflows become more important. SEDs are softened and rollovers at ~5 kev appear.

14 Schematic view of super-critical accretion flows onto BHs high luminosity hard spectrum low luminosity soft spectrum centrifugal barrier bulk Compton (2) (3) sub-relativistic, mildly hot funnel jet cool, dense, and slow outflow absorption reflected shock downscattering BH thermal Compton (4) shock-heated region upscattering (1) radiation pressure dominant disk photon trapping Disk photons are Compton upscattered in inflowing mildly hot coronae. Escaping photons are Compton downscattered in outflowing mildly cool coronae.

15 Comparing with ULX spectra NGC1313 X-2 color: calculated SED black points: XMM-Newton data with absorption correction (provided by Dr. Gladstone) IC342 X-1 NGC5204 X

16 short summary (super-critically accreting BHs) super-critical accretion onto BHs is feasible. ( anisotropic radiation + photon trapping effects) Compton effects are important on radiative spectra of super-critical accretion flows and outflows around BHs. ULX spectra can be explained by super-critically accreting black holes.

17 Super-critical accreting NSs with mildly strong B-fields ( ) kawashima et al. (2016) NAOJ

18 Basic Equations of Radiation-Hydrodynamics mass: momentum: gas energy: radiation energy: We employ Flux-Limited Diffusion approximation, and pseudo-newtonian potential to mimic the effects of GR. We solve bremsstrahlung (free-free) emission/absorption and Thomson scattering.

19 simulation domain simulation box schematic image of ULX-pulsar polar accretion NS matnetic field super-critical accretion flow

20 simulation set-up simulatio box grid number boundary conditions (a) free boundary (b) hydro. absorption boundary rad. injection boundary: injecting radiative flux = absorbed enthalpy flux of gas (c) (d) reflection boundary hydro. reflection rad. free streaming (Ohsuga 2007) initially, quantities are uniformly mapped: (c) (a) (d) (b) NS

21 movie: a simulated accretion column onto a NS mass density radiation energy density

22 structure of a super-critical accretion column flux A radiative shock is formed at r~ 13km. circular motions appear under the shock. copious photons escape from the column wall.

23 θ-averaged structure of an accretion column free-falling region A shock at ~13 km separates an accretion column into two region: free-falling region and settling region. Dissipated energy is immediately converted to the radiation energy.

24 energy conversion from grav. energy to rad. energy (a) gas gravitational energy kinetic energy dissipated at ~13km radiation energy (b) radiation almost all the energy escape sideways The luminosity is luminous as much as M82 X-2. The emerged photons do not prevent mass accretion because the photons escape sideways.

25 Schematic picture of super-critical accretion columns onto NSs. NAOJ image CG of ULX-pulsars image CG of (radio) pulsars NAOJ

26 Super-critically accreting NSs with strong B-fields ( ) kawashima et al. in prep.

27 Basic Equations of Radiation Hydrodynamics mass: momentum: We do not solve this eq. to mimic the strong magnetic fields of NSs. gas energy: radiation energy: We employ Flux-Limited Diffusion approximation, and pseudo-newtonian potential to mimic the effects of GR. We solve bremsstrahlung (free-free) emission/absorption and Thomson scattering.

28 Time evolution of accretion columns color: mass density arrows fluid velocity color rad. energy density arrows rad. flux (lab. frame) A finger-like structure, which seems to be driven by the photon bubble instability (Klein & Arons1989, Arons 1991), appears.

29 Accreting region From the each side wall of the accreting region, the photons escape and the finger-like outflows is formed. radiative force ~ gravitational force in accreting regions.

30 Lside Lradial L side (r) = Z rmax r F (r, max )2 r sin max dr L radial (r) = Z max 0 F r (r, )2 r 2 sin d Side wall is still bright in the highly magnetized accretion columns.

31 Discussion: How will the accretion column become? polar accretion NS super-critical accretion flow matnetic field Will Column accretion is suspend except the side wall? If so, the the structure suggested by Basko & Sunyaev (1976) would be realized. If not, a lot of very narrow accretion columns (which may look like hair ) may appear inside the side wall. We may need global simulations in the near future.

32 short summary (super-critically accreting NSs) We have carried out RHD simulations of super-critical accretion columns onto NSs. Escape of enormous photons from the side wall enables continual super-critical column accretion. If we assume strong magnetic fields, a finger-like structure, which seems to be driven by photon bubble instability, appears. The inner structure of the columns differs from the weak B-field model, but the side wall is still bright. future works: RHD simulations with employing the scattering cross section corrected by strong B-fields Calculations of SEDs and Light curves of accretion columns Global simulations including accretion disks

33 What is the central engine of ULXs?? sub-critical IMBH super-critical stellar-mass BH super-critical NS Luminosity SED pulse Super-critical accretion would be preferred to explain ULXs [IMBHs may exist in Hyper-luminous X-ray sources (HLXs)] We need more theoretical works on super-critical accretion! It would be better to consider new diagnostics in addition to pulse observations (X-ray polarimetry?)

34 summary We have carried out RHD simulations of super-critical accretion columns onto BHs and NSs. Comptonized spectra of super-critically accreting BHs can explain the curious SEDs of ULXs. On the other hand, the X-ray pulse would be explained by the super-critical column accretion onto NSs. The super-critical accretion can occur in accreting column onto NSs because the photos can escape from the side wall of the column. Much more theoretical studies are needed to constrain the central engine of ULXs.