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/72 X-ray Astronomy in our Galactic Center region Hironori Matsumoto Kobayashi-Maskawa Institute, Nagoya Univ. 1

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/72 Outline General introduction: What is X-ray? Introduction to X-ray astronomy in our Galactic center region. The 6.7 and 6.9 keV lines. Thermal phenomena The 6.4 keV line. Non-thermal phenomena 2

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/72 Optical Astronomy (Virgo Cluster of Galaxies) ~1deg 3

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/72 X-ray Astronomy 4

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/72 Optical and X-ray view Cluster of galaxies huge hot gas SDSSRASS We would overlook truths without X-rays. Mgal < Mgas 5

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/72 http://mc2.gulf-pixels.com/ 6

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/72 What is X-ray? X-ray = high-energy photon o h = 0.1 keV -- 100 keV o = 0.1 100 ( 1keV = 12.4 ) o kT = 10 6 10 10 K ( kT 1keV = 10 7 K ) We can see o Extremely high temperature o Non-thermal particle acceleration o Atomic process 7

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/72 X-ray cannot penetrate atmosphere http://mc2.gulf-pixels.com/ X-ray 8

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/72 Current X-ray Observatory USA: Chandra X-ray Observatory Europe: XMM-Newton High angular resolution (~0.5) High throughput (large effective area) And Rossi X-ray Timing Explorer Swift INTEGRAL etc. 9

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/72 Japanese X-ray satellite: Suzaku Launched on July 10, 2005 Small or medium size satellite 10

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/72 Characteristics of Suzaku Wide energy range (0.4 600keV) X-ray CCD: 0.412keV Semiconductor device + Scintillator : 10keV 600keV Good energy resolution Low and stable background, High throughput Moderate angular resolution. Very good at spectroscopy of the Galactic center diffuse emission. Introduce X-ray astronomy in the Galactic center region mainly with Suzaku results. 11

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/72 Imaging Spectroscopy 12 X-ray CCD X-ray mirror Total reflection (incident angle very small) Obtain image and spectrum simultaneously. Cf. grating superior energy resolution, but cannot obtain image. Suzaku XMM-Newton Chandra Swift etc. Measuring energy of each photon.

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/72 Extinction to the Galactic Center (GC) Cannot observe GC IR Radio X-ray Gamma-ray NH=10 22 ~10 23 cm -2 13

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/72 X-ray image of the GC region Red: 1 3 keV Green: 3 5 keV Blue: 5 8 keV Wang et al. 2002, Nature, 415, 148 Many point sources (X-ray binaries) Diffuse emission 20arcmin ~ 60pc Chandra image 14

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/72 Point sources: X-ray binaries Normal star X-ray Compact object White dwarf (Cataclysmic Variable) Neutron star Black hole 15

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/72 Distribution of diffuse X-ray 16 Galactic Center component |l|

/72 Bulge component 42 Galactic Center component |l|

/72 Galactic Ridge X-ray Emission (GRXE) 44 Galactic Center component |l|

/72 Beyond the GC region (|l|>1deg, b~0deg) Iron line exists outside the GC region. = Galactic Ridge X-ray Emission (GRXE) Yamauchi et al. 2009, PASJ, 61,295 Iron line from GRXE Uchiyama 2010, PhD thesis 45

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/72 6.9keV/6.7keV Intensity ratio of GRXE I(6.9keV)/I(6.7keV) Temperature of GRXE may be lower than the GC hot gas. Origin of GRXE may be different from that of the GC hot gas. 46

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/72 What is the origin of GRXE? Diffuse hot gas? The temperature may be lower than the GC hot gas. Problem of energy supply must be severe. Unresovled point sources? EW of the 6.7 keV line : 300 900 eV Too large as CVs Not yet resolved 47

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/72 Summary of highly-ionized Fe line Galactic Center (l~0deg, b~0deg) Origin: Hot gas (kT ~ 6keV) With some contributions from point sources. Distribution scale: ~150pc Bulge (l~0, b~1deg) Origin: Point sources CV and coronally active stars Galactic Ridge (|l|>8deg, b~0deg) Origin: unknown Diffuse hot gas? Unresolved point sources? 48

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/72 Origin of the 6.4 keV line Neutral (=cold) iron atoms emit X-rays!! 6.4keV line: neutral iron 49

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/72 Neutral iron distribution Suzaku 6.4keV image CS 6.4 keV line distributes widely in the GC region. 6.4 keV line traces the CS line (=molecular cloud) Tsuboi et al. 1999 50

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/72 6.4 keV line: non-thermal process Photo ionizationElectron impact What excites the neutral iron? X-ray E>7.1keV is required. Electrons E=10100keV is required. (Cross section for inner shell ionization is large) 51

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/72 Sgr B2 region Suzaku 6.4keV line image Sgr B2 region Strong 6.4keV line emitter 52

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/72 Sgr B2 region for the past decades Suzaku 6.4keV image 6.4keV line in the Sgr B2 region gradually faded Inui et al. 2009, PASJ, 61, S241 53

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/72 Time variability Decay time ~ 10 years Sgr B2 scale length (Chandra)~ 10 light years If SgrB2 is ionized by electrons, electrons must travel with ~light speed However, such high energy electrons have too small cross section of the inner shell ionization. X-ray photo ionization can explain the phenomena. Sgr B2 region must be an X-ray Reflection Nebula (XRN). 54

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/72 X-ray spectrum of Sgr B2 Strong 6.4 keV line EW 1.6keV Strong absorption edge at 7.1 keV Photo electric absorption by iron. NH ~ 10 24 cm-2 Suzaku X-ray spectrum Large EW and deep edge Fe I K Fe I K Power-law continuum 55

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/72 Power-law continuum Distribution of electrons is non-thermal (not Maxwell-Boltzman distribution.) In many case, we assume N(E)dE ~ E -s dE X-ray spectrum: I(E) ~ E - ph/s/cm2 : photon index In case of synchrotron emission, = (s+1)/2 Cf. radio band I(E) ~ E - erg/s/cm2 is used. : spectral index = - 1 56

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/72 EW and edge X-ray (E>7.1keV) Electron (E=10100keV) X-ray Photo ionization Electron impact Photo ionizationElectron impact Cross section of inner shell ionization Large (~10 -20 cm2)Small (~10 -22 cm2) continuumThomson scatteringBremsstrahlung EW of 6.4keV lineLarge (~1200 eV)Small (~300eV) EdgeDeep (N H ~10 24 cm -2 )Small (N H ~10 21 10 22 cm -2 ) Sgr B2 57

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/72 Spectrum also supports photo ionization 6.4keV line EW = 1600 eV NH = 10 24 cm-2 Photo ionizationElectron impact Cross section of inner shell ionization Large (~10 -20 cm2)Small (~10 -22 cm2) continuumThomson scatteringBremsstrahlung EW of 6.4keV lineLarge (~1200 eV)Small (~300eV) EdgeDeep (N H ~10 24 cm -2 )Small (N H ~10 21 10 22 cm -2 ) 58

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/72 Where is the X-ray source? Required luminosity: L(2-10keV) = 10 39 x (d/100pc) 2 erg/s No such X-ray object around Sgr B2 Candidate: Sgr A* d~109 pc, L(2-10keV) ~ 10 39 erg/s cf. current L(2-10keV) ~ 10 33 erg/s (Chandra: Baganoff et al. 2001) Sgr B2 Irradiating source d Sgr A* was bright 300 years ago! 59

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/72 Study another 6.4 keV clump Bright 6.4 keV clump 60

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/72 Suzaku X-ray spectrum Other neutral lines are weak. (Ratio to Fe is < 0.010.1) => The brightest neutral clump in the GC Neutral Fe He, H-like lines; mainly due to hot gas Si S Ar Ca Fe kT~6keV hot gas Suggesting low-kT gas (kT~1-2keV) 61

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/72 Spectrum fitting 2-kT hot gas model kT=0.99 +/-0.02keV kT=7.02+/-0.03keV NH=(7.1+/-0.3)e22cm-2 Neutral iron (6.4keV) + non-thermal continuum (power-law) = 1.85+/-0.15 NH=(1.7+/-0.3)e23 cm-2 These residuals correspond to neutral lines from S, Ar, Ca, Cr and Mn. 62

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/72 Discovery of neutral lines other than the 6.4keV line Without neutral lines With neutral lines Nobukawa et al. 2010, PASJ, 62, 423 63

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/72 Black: X-ray photo ionization Red: electron impact Solid line: solar abundance To explain the observation, Photo Ionization 1.6 solar Electron Impact 4.0 solar (dashed lines) EW of the neutral lines Supporting X-ray photo ionization. 64

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/72 Low-kT plasma (kT~1keV) and 6/4 keV line E=2-3keV image = low kT plasma 6.4keV image = neutral iron bright dim bright 65

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/72 E=2-3keV image = low kT plasma 6.4keV image = neutral iron bright dim bright kT=1keV plasma Absorbed by MC Observer MC We can determine the distance to MC by measuring the absorption on the low kT plasma. 66

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/72 X-ray tomography of MC Ryu et al. 2009, PASJ, 61, 751 67

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/72 If irradating source is Sgr A* 68

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/72 Sgr A* light curve Current Lx~10 33 erg/s 69

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/72 All 6.4keV line regions are photoionized? Many bright 6.4 keV clumps are photoionized. Irradiating source: Sgr A* However, the 6.4 keV line extends far beyond the GC center region. At least, we can see the 6.4keV line in the GRXE emission (l>10deg). Origin has not been clarified. l=28deg 70

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/72 Hint: TeV gamma-ray emission Color: diffuse TeV gamma-ray emission Contour: CS Diffuse TeV gamma-rays trace molecular cloud. This suggets that the origin of TeV may be cosmic-ray protons. p + p p + p + 0, 0 + 71

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/72 Similarity between 6.4keV and TeV Color: 6.4keV Contour: diffuse TeV Ionization by cosmic-ray (electron, proton) may contribute to some of the 6.4 keV line. 72

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/72 Summary of the 6.4 keV line 6.4 keV line: neutral iron Distribution of the line traces molecular clouds. Characteristic X-rays from neutral Si, S, Ar, Ca, Mn and Cr are discovered. EW of the neutral lines from bright 6.4keV clumps favor photoionization. Irradiating source: Sgr A* Sgr A* was 10 6 times brighter 300 years ago. Absorption measurement enables X-ray tomography 73