Black hole shadow images with partially-ssa thick accretion flows: possible evidence of a highly spinning black hole

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1 Black hole shadow images with partially-ssa thick accretion flows: possible evidence of a highly spinning black hole Tomohisa KAWASHIMA (NAOJ) In collaboration with Motoki KINO (Kogakuin U./NAOJ) Dawn of a new era for black hole jets in active galaxies Tohoku U. 2018, 1/27

2 BH shadow Pu et al. (2016) From the BH shadow observation by EHT, it is expected that we can obtain important informations of BH mass BH spin Physics of accretion flow Physics of Jet

3 A bit of worry determining BH-spin by using BH-shadow a=0 BH shadow a=0.998 a = 0, 0.5, BH event horizon analytic solution of shadow-surface If the accretion flow is optically-thin, the photon ring appears. Diameter of photon ring of non- and extremely-spinning BHs are ~10.4 rg and ~9.6 rg, respectively (i.e., the difference is only less than10%) If another feature of highly-spinning BH exist, it will help us to determine the BH spin by comparing the theoretical and near-future observational image of BH-shadow.

4 Partially SSA-thick scenario in M87 at 230GHz M87 is one of the most important target for detecting BH-shadow by EHT (Event Horizon Telescope) [expected diameter of BH-shadow (~35μac) is less than spatial resolution of EHT (~20μac)]. Kino et al. (2015) proposed that partially SSA-thick region exists around the SMBH in M87 at 230GHz. However, studies of BH-shadow have been focused on the condition in which SSAthin in the viscidity of BHs at ~230GHz. We explore the shape of BH-shadows of M87 surrounded by partially SSA-thick accretion flows. Kino et al. (2015)

5 Numerical Method For simplicity, we consider accretion flows only (i.e., we ignore the jet). We employ Keplarian shall model (i.e., simplified power-law solution model of radiatively inefficient flow by Broderick and Loeb 2006) except that we assume that the accretion disk extends down to the innermost stable orbit of matter rather than down to the event horizon. Due to the possible Synchrotron cooling in the M87, we assume that the accretion flow is moderately thin (H/R ~ 0.1) and low-beta plasma (P_gas/P_mag ~ 0.1); T_e ~ 10^{10} K and B ~ 100GHz at ISCO, which is consistent with the observations of M87 (Kino et al. 2015). We solve the GR ray-tracing radiative transfer taking into account the cyclosynchrotron emission/absorption due to thermal electrons (Mahadevan et al. 1996) from the observer-to-emitter. Solve the time-reversing GR radiative transfer equations observer screen (located at 10^4 gravitational radius) BH Accretion flow

6 Shadow image of highly spinning BH (a=0.998) Double bright ring structure appears in the image when the accretion flow is partially SSA-thick! SSA-thick innermost disk dark crescent Photon ring

7 Optical depth of the accretion flow SSA-thick innermost disk photon ring SSA-thick SSA-thin We can see that the SSA-thick innermost disk form the bright inner ring (not only 230 GHz but also 350 HGz). Photon rings are also SSA-thick when it is bright.

8 Dependence of the shape of BH-shadow on the BH-spin SSA-thick innermost disk dark crescent photon ring photon ring SSA-thick innermost disk If the BH-spin is high, then (i) the SSA-thick region appears inside the photon ring (ii) the center of the photon ring shift so that the bright region of the photon ring tends to overlap that of SSA-thick innermost disk image and dark crescent appears. These feature can help us to determine the BH-spin in M87.

9 Dependence of the shadow image on the viewing angle SSA-thick innermost disk dark crescent photon ring If we assume that the rotation axis of the accretion flow is parallel to the propagation direction of jet, viewing angle will be less than 30 deg from the observations of superluminal motion of knots in jets in M87. The dark crescent also appears if we assume viewing angle = 15 deg.

10 Dependence of the dark crescent size on BH spin solid lines: i=30deg dashed lines: i = 15deg SSA-thick innermost disk dark crescent photon ring If the BH-spin is high > 0.8, the dark crescent in M87 may be observed by EHT!

11 What can we expect when a jet exists? GR radiative transfer postprocessing GRMHD simulation by Moscibrodzka et al. (2016) RH1 RH5 RH10 230GHz RH20 RH100 RH200 From SEDs, we speculate that model RH1is SSA-thin, while model RH20,40, 100 are partially SSA-thick. The double-ring structure with dark crescent appears! The foot point of the jet, whose position will be related to ISCO, may be SSA-thick in their GRMHD simulation model. We expect that the dark crescent will essentially appears if the plasmas around highly spinning BH are partially SSA-thick.

12 Summary We have calculated BH-shadow of a SMBH surrounded by partially SSA-thick accretion flow. We have found that the double bright ring structure with dark crescent appears in highly spinning BH case. The width of dark crescent is marginally larger than the expected size detected by EHT w/ sparse modeling The detection of the dark crescent would be a new diagnostic method to determine the BH-spin. The model with relativistic jets will be expected to show the dark crescent if the plasma around highly spinning BH is partially SSA-thick.