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

2. /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

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

4. /72 X-ray Astronomy 4

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

6. /72 ©http://mc2.gulf-pixels.com/ 6

7. /72 What is X-ray? X-ray = high-energy photon o hν = 0.1 keV -- 100 keV o λ = 0.1 – 100 Å ( λ 1keV = 12.4 Å ) 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

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

9. /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

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

11. /72 Characteristics of Suzaku Wide energy range (0.4 – 600keV) –X-ray CCD: 0.4—12keV –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

12. /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.

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

14. /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

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

16. /72 Distribution of diffuse X-ray 16 Galactic Center component |l|<1deg, |b|<1deg Bulge component |l| 1deg Ridge component |l|>10deg, |b|<1deg

17. /72 Typical X-ray spectrum of the GC diffuse Suzaku X-ray CCD Koyama et al. 2007, PASJ, 59, 245 What are these X-ray lines? 17 ΔE~130eV

18. /72 Characteristic X-rays We can know what atom emitting the X-ray line by measuring the line energy. Also we can know its ionization state. 18

19. /72 Three iron lines 6.4keV Kα from Neutral iron (Fe I) 6.7keV Kα from He-like ion of iron (Fe XXV) 6.9keV Kα from H-like ion of iron (Fe XXVI) Extremely important. You should memorize. 19

20. /72 Other lines Fe I Kβ Ni I Kα Ni XXVII Kα & Fe XXV Kβ Fe XXVI Kβ & Fe XXV Kγ Fe XXVI Kγ 20

21. /72 Iron distribution Fe I (neutral) Fe XXV (He-like) Fe XXVI (H-like) Suzaku image 21

22. /72 Iron distribution Highly-ionized iron –6.7 keV and 6.9 keV lines: similar to each other Neutral iron: –6.4 keV line –Different from the highly-ionized iron line. 6.4keV 6.7keV 6.9keV Origin of the highly-ionized iron lines is different from that of the neutral iron line. First, let’s consider the origin of the highly-ionized iron line. 22

23. /72 Origin of highly-ionized iron line Hot gas? Charge exchange between cosmic-rays and neutral matter? How to distinguish them? Key is the fine structure of the He-like iron. 23

24. /72 Charge exchange Fe +26 Interstellar matter (mainly H, He) Fe +25 Fe +24 H-like Fe line (6.9keV) He-like Fe line (6.7keV) Cosmic ray Highly-ionized iron atoms in cosmic-ray take electrons away from H or He in interstellar matter. 24

25. /72 Electron distribution of He-like ion 1s1s … ground state – 1 S 0 1s2s – 1 S 0, 3 S 1 1s2p – 1 P 1 – 3 P 0, 3 P 1, 3 P 2 2S+1 L J notation S: spin L: orbital angular momentum J: total angular momentum 25

26. /72 He-like iron fine structure Energy W x y z Highly resolved 6.7 keV line Center energy of the 6.7 keV line depends on the ratio of the fine structures. 26

27. /72 Charge exchange case Electron enters into a high- energy level of Fe. –Energy conservation. Then the electron cascades into the n=2 level. n=2 levels are statistically populated. E=0 n=1 13.6eV n~25 n=1 n=2 6.7keV line Fe XXV H 27

28. /72 Charge exchange case Statistically populated x, y, z lines are comparable to w line. The center energy of the 6.7 keV line is ~6666 eV. Z: 6637eV Y: 6668eV X:6682eV W: 6701eV E Suzaku CCD resolution center~6666eV 28

29. /72 Hot gas case Electron in the ground state is excited by collision –Electric dipole transition. –No flip of spin 1 P 1 level is mainly populated. W line is strong. Center energy ~6685eV Mainly populated w x y z Suzaku CCD resolution center~6685eV 29

30. /72 Suzaku measurement 2. Hot gas 1. Charge exchange 6666eV 6685eV z w w z Hi res CCD Hi res CCD 6680 +/- 1 eV Supporting the hot gas origin. 30

31. /72 Measuring the hot gas temperature He-like Fe K α(6.7keV) H-like Fe Kα (6.9keV) He-like Fe Kβ(7.9keV) I(6.9keV)/I(6.7keV): ionization temperature I(7.9keV)/I(6.7keV): electron temperature 31

32. /72 Temperature distribution kT = 5~7 keV Ionization temperature Ionization temperature ~ electron temperature ~ 5 – 7keV Constant from l = 0.2deg to -0.4 deg 32

33. /72 Measuring gas mass: Continuum Thermal bremsstrahlung Power-law (Discuss later) 33

34. /72 Thermal bremsstrahlung Electrons –Thermal distribution (Maxwell- Boltzman distribution) Interact with ions (atomic nuclei) Emitting X-rays Optically thin Emissivity = radiated power per unit volume Emissivity per unit frequency (~ X-ray spectrum) (erg s -1 cm -3 ) (erg s -1 cm -3 Hz -1 ) 34

35. /72 Thermal brems. emissivity n e n i v el : interacting frequency M-B distribution Curvature of spectrum (turn around at E~kT) We can measure Temperature (kT) (~6keV) Luminosity ∝ Emission Measure (EM = n e n i Volume) 35

36. /72 Energy and Mass of the hot gas Total emission measure (n e n i V)~ 10 60 cm -3 –Luminosity: L~2x10 36 erg/s The hot gas distibutes over 1 deg (~150pc). –Assuming uniform distribution, V ~ 10 62 cm 3 –n e ~ 0.1 cm -3 (n e ~ n i ) Total thermal energy E ~ 3/2 (ne + ni) kT V ~ 10 53 erg Total mass M ~ 8000 Msun 36

37. /72 Energetics The temperature (kT~6keV) is much greater than the gravitational potential of our Galaxy (kT~400eV). –Mgal ~ 2x10 11 Msun, Rgal ~ 20kpc  (G Mgal m p )/Rgal ~ 400eV Thus the gas must escape the Galaxy. –The hot gas extends over 1deg (~150pc) –The sound crossing time is ~ 10 5 years. Sound speed of the gas (kT~6keV) ~ 10 8 cm/s Total thermal energy ~10 53 erg Energy of ~10 48 erg/year must be supplied. 1SN/1000 year is required within the small GC region!! 37

38. /72 Energetics with filling factor Filling factor f: V  fV Density: n e ~ n i ~ 0.1 f -0.5 cm 3 Thermal energy: E = 10 53 f erg Energy input: 10 48 f erg/year 38 Small filling factor would resolve the energetic problem.

39. /72 Counterargument to the hot gas -- unresolved point sources -- Extremely small filling factor Should be faint, numerous, and hot. –Candidate: Cataclysmic Variable (white dwarf binary) Density ~ 3x10 -5 pc -3 secondary Accretion stream Magnetic white dwarf Surface of white dwarf Accretion stream kT=1—25keV 39

40. /72 X-ray spectrum of CV Example of CV spectrum CVGC hot gas EW of 6.7keV ~200eV~400eV EW of 6.9keV ~100eV~150eV Fe lines from CVs look different from those of the GC hot gas. Furthermore, only 40% of the diffuse X-ray are resolved into point sources with a ~1Msec Chandra observation (Revnivtsev and Sazanov 2007). 40

41. /72 Origin of the GC hot gas Multiple SNe? –But, typical temperature of SNRs: kT ~ 1keV Past Sgr A* activity? Magnetic reconnection? Not yet clarified. 41

42. /72 Bulge component 42 Galactic Center component |l|<1deg, |b|<1deg Bulge component |l| 1deg Ridge component |l|>10deg, |b|<1deg

43. /72 Diffuse X-rays in bulge is resolved into point sources. Chandra image at (l, b)=(0.113°, -1.424°) Chandra spectrum Revnivtsev et al. 2009, Nature, 458, 1142 Diffuse X-ray in bulge may be different from the GC diffuse. 43

44. /72 Galactic Ridge X-ray Emission (GRXE) 44 Galactic Center component |l|<1deg, |b|<1deg Bulge component |l| 1deg Ridge component |l|>10deg, |b|<1deg

45. /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

46. /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

47. /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

48. /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

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

50. /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

51. /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=10—100keV is required. (Cross section for inner shell ionization is large) 51

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

53. /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

54. /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

55. /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 Kα Fe I Kβ Power-law continuum 55

56. /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 -Γ ph/s/cm2 –Γ: photon index In case of synchrotron emission, Γ= (s+1)/2 –Cf. radio band I(E) ~ E -α erg/s/cm2 is used. α: spectral index α = Γ- 1 56

57. /72 EW and edge X-ray (E>7.1keV) Electron (E=10—100keV) X-ray Photo ionization Electron impact Photo ionizationElectron impact Cross section of inner shell ionization Large (σ~10 -20 cm2)Small (σ~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

58. /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 (σ~10 -20 cm2)Small (σ~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

59. /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

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

61. /72 Suzaku X-ray spectrum Other neutral lines are weak. (Ratio to Fe is < 0.01—0.1) => 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

62. /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) Γ = 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

63. /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

64. /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

65. /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

66. /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

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

68. /72 If irradating source is Sgr A*… 68

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

70. /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

71. /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 + π0, π0  γ + γ 71

72. /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

73. /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