v Characteristics v Possible Interpretations L X = erg s -1

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1 Ultra Luminous X-ray sources v Characteristics L X = erg s -1 >> M black hole(bh) L Edd = 39 erg s -1. Usually seen in star forming regions. On arms of spiral galaxies (right). In starburst or irregular galaxies. v Possible Interpretations Stellar-mass ( M ) BHs shining at L/L Edd >>1, with super-critical low emissivity (η << 1) accretion flows (Mineshige+07). Intermediate-mass (0-00 M ) BHs shining at L/L Edd 1 with sub-critical and η~1 accretion flows (Makishima+00). LIGO has revealed the existence of massive (30~60 M ) BHs. Strong support to the hypothesis. 1

2 Spectral Characteristics of ULXs Featureless continua that extending over kev. Several types of continuum shapes. Broadened Disk (BD). Hard Power-Law (HPL). Soft Ultra Luminous (SUL). Some sources change their spectral types on months-year timescales. Stronger variability in > 1 kev. Spectra of ULXs in NGC1313 X-1 (HUL) XMM-Newton pn v Constrains on the picture of ULX (A) Parameterization of the spectral continuum. (B) Environments suggested by the lack of local spectral features. 2

3 (A) (A.1) Parameterization of Broadband Spectra v Modeling Multi-color disk (MCD) + thermal Comptonization (THC) The same model as used in BHB studies and generally successful on the ULX spectra (Gladstone+09) v Parameters Luminosity L Inner-disk temperature T in Coronal temperature T e Compton optical depth τ τ ~ ~12 ~20 T in (kev) 0.40± ± ±0.02 T e (kev) 1.6± ± ± 0.2 3

4 (A) (A.1) Parameterization of Broadband Spectra v Modeling Multi-color disk (MCD) + thermal Comptonization (THC) v New Characterization (Kobayashi+16) The same model as used in BHB studies and generally successful on the ULX spectra (Gladstone+09) v Parameters Luminosity L Inner-disk temperature T in Coronal temperature T e Compton optical depth τ Introduce a new parameter Q T e /T in. It represents the balance between the Compton cooling and gravitational heating of the coronal electrons. T e /T in =4 T e /T in =11.3 T e /T in =4.1 Q T e /T in 4

5 kev 2 (Photons cm 2 s 1 kev 1 ) (A.2) Spectral Transition Luminosity M33 X-8 BD Suzaku 20 Q=T e /T in Energy (kev) L C Hol IX X-1 NGC1313 X-1 Hol II X-1 IC342 X-1 M33 X-8 M33 X L x ( 39 erg/sec) BD HUL Holmberg IX X-1 The transition from BD to HPL is represented as Q=2. L C Energy (kev) The critical luminosity (L c ) of the spectral transition scatters by about an order of magnitude among ULXs. Assuming that the transition takes place at similar L/L Edd values, the mass of ULXs should also scatter by a similar factor. The argument holds regardless of their values of L/L Edd (< 1 or >> 1). L C kev 2 (Photons cm 2 s 1 kev 1 ) HUL BD XMM kobayashi 16

6 (B) Environments Suggested by the Featureless Spectra (B.1)Photoelectric Absorption 6 21 galactic N H subtracted NGC1313 X-1 N H (cm -2 ) Luminosity (erg sec -1 ) kev 2 (Photons cm s kev ) νfν -3-4 ratio Ratio /blue L bol (erg/sec) Energy (kev) Energy (kev) Energy (kev) Absorption column density is N H 21 cm -2 in many ULXs. Constant spectral ratio < 1keV, i.e. little change in N H. 6

7 Counts sec -1 kev χ (B.2) Lack of Line Features Equivalent Width (ev) Walton+13 ~500 ks exposure Holmberg IX X Energy (kev) Although emission and absorption lines were detected (Pinto+16) from some ULX, they are still very weak. Strong upper limits (< 30 ev) for the Fe line features (Walton +13).The limits are much more stringent than the EWs observed those in other accreting objects Rest frame energy (kev) 7

8 v From Observations Discussion v Suggested Condition of the System Masses of ULXs scatter over a factor of ~20. The ULX spectra are not strongly absorbed. The spectra lack strong emission/absorption features that are widely seen among other accreting objects. νfν -4-5 If the accretion is more super-critical (η<<1), one emitted photon would encounter a larger amount of accreting mater. The surrounding matter would be then less ionized. We would observe strong spectral features, in contradiction to the observation. Sub-critical accretion onto M BH scenario is suggested. χ Hagino et al H Energy (kev) 8

9 Conclusions We successfully quantified the ULX continua by introducing a new parameter Q T e /T in. From the critical luminosity where the spectral transition takes place, ULXs are suggested to scatter by a factor of ~20 in the mass range. These facts argue against the super-critical accretion scenario, which inevitably demands η << 1. ULXs are likely to be massive ( 0 M ) BHs, characterized by sub-critical accretion flows with η~1. 9