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An Overview of Some Recent Results in the Galactic Centre

An Overview of Some Recent Results in the Galactic Centre. Frederick K. Baganoff MIT Kavli Institute for Astrophysics and Space Research. The X-ray Universe 2014 Trinity College, Dublin 2014 June 16-19. Outline. Traces of past Sgr A* activity Current Sgr A* activity Sgr A* flares

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An Overview of Some Recent Results in the Galactic Centre

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  1. An Overview of Some Recent Results in the Galactic Centre Frederick K. Baganoff MIT Kavli Institute for Astrophysics and Space Research The X-ray Universe 2014 Trinity College, Dublin 2014 June 16-19

  2. Outline • Traces of past Sgr A* activity • Current Sgr A* activity • Sgr A* flares • Sgr A*/G2 interaction monitoring • Magnetar SGR J1745-2900

  3. NuSTAR Detects Sgr A* Flares

  4. NuSTAR Detects Sgr A* Flares

  5. Sgr A* X-ray Visionary Project Frederick K. Baganoff, Michael A. Nowak (MIT Kavli Institute), Sera Markoff (API, University of Amsterdam) and Sgr A* XVP Collaboration www.sgra-star.com

  6. Science Goals • Obtain first high-resolution X-ray spectra of SgrA* and diffuse emission in the central pc • Spatially and spectrally resolve accretion flow within Sgr A*’s Bondi radius (~3”) • Measure energy and width of known Fe Kalpha line(s) at ~6.6 keV in ACIS-I Spectrum • Detect optically thin plasma emission lines, (e.g., Si, S, Fe), if present at levels predicted by RIAF models • Measure relative abundances of emitting plasmas and absorbing column along LOS

  7. Science Goals • Radio polarization measurements indicate most accreting matter does not reach Sgr A*’s event horizon --- dynamics and thermal structure of plasma will tell us how matter flows in and how much and where some of it flows out • Monitor X-ray flares of Sgr A* with increased cadence and minimal pile-up • Perform coordinated multi-wavelength monitoring of flares from radio up to gamma-rays, including mm VLBI • Constrain 3D GRMHD simulations of Sgr A* accretion flow using MW properties of flares and 1mm VLBI imaging

  8. Observational Overview • Completed 3Ms exposure of Sgr A* - 38 separate HETGS observations from 2012 February 6 to October 31 • Observing constraints very challenging: • Desired specific roll angles to minimize background: 76.4 (6), 76.6 (1), 92.2 (10), 268.7 (7), 270.7 (1) & 282.3 (13) degrees • Desired long exposures for multi-wavelength monitoring of Sgr A* flares: > 90ks (14)

  9. Brightest flare ~140x quiescence • LX ~ 2 x 1035 erg/s (2-8 keV) • Nowak et al. (2012) Sgr A* XVP Cumulative Light Curve

  10. Brightest Flare Light Curve Bayesian Blocks finds structure at peak Consistent w/ minimal pile-up

  11. Sgr A* Flare and Quiescent Spectra -- ACIS-I & HETGS • HETGS 1st Order • HETGS 0th + 1st Order • HETGS 1st Order • HETG 0th Order • HETGS 0th Order • ACIS-I Wang+2013: He-like Fe Kalpha line at 6.7 keV; No 6.4 keV line predicted by Sazonov+2012; but see Warwick talk

  12. Consistent Flare Properties for Brightest Chandra & XMM Flares ACIS-I Quies. HETGS Flare XMM 2002 F XMM 2007 F Photon index ~ 2 and NH ~ 15 x 1022 cm-2

  13. Properties of Brightest X-ray Flare Good agreement with two brightest XMM flares (Nowak et al. 2012, ApJ, 759, 95)

  14. 39 X-ray Flares in 3Ms Exposure: 1.12 +/- 0.18 flares/day Neilsen+2013

  15. Sgr A* Flare Science Topics • X-ray flares are non-thermal but mechanism is still undetermined: synchrotron with cooling break (SB), external Compton (EC) or synchrotron self-Compton (SSC)? • X-ray & NIR flares appear related (opt thin process) • What causes variable X-ray-to-NIR flux ratio? • mm/radio events are weaker & longer timescale (~opt thick process) • Are mm/radio events correlated with X-ray/NIR? • If so, do they lead, lag or sometimes lead other times lag? • Need sufficient sample of MWL flares to establish or refute correlation with X-ray/NIR flares

  16. Sgr A* Flare Science Topics • Origin(s) of flares undetermined: • magnetic reconnection? • shock in jet or inner accretion flow? • stochastic acceleration? • magnetic excitation by infalling asteroids? • What determines flare duty cycle and energy budgets? • Need broad-band flare spectra and time evolution observations to understand flares • What do flares tell us about innermost environment of Sgr A*’s accretion flow? • Does this phenomenon scale to other LLAGN? • Will G2 or similar events change Sgr A*’s rate of flaring and its accretion state?

  17. NuSTAR Detects Sgr A* Flare! Barriere+2014 NuSTAR PI: F. Harrison (Caltech)

  18. NuSTAR Detects Sgr A* Flares

  19. NuSTAR Detects Sgr A* Flares

  20. NuSTAR Detects Sgr A* Flares

  21. NuSTAR Spectra

  22. Flare Model Fits To NuSTAR Spectrum

  23. Where are Sgr A* Flares Located?

  24. 3Ms Chandra HETGS Image of Sgr A* • S2/S3 chip gap • Events projected to common tangent plane • Events outside ~250 pixels rotated to align grating arms • Moderate smearing of co-aligned spectra

  25. Ar XVII /S XVI • Fe XXV • S XVI • S XV • Ca XIX

  26. Fe XXV 1.8505 Angstroms

  27. No Detectable lines

  28. No Detectable lines

  29. Ca XIX 3.177 & 3.211 Angstroms

  30. Ar XVII 3.95 & Ar XVII/S XVI 3.995 Angstroms

  31. Ar XVII 3.95 & Ar XVII/S XVI 3.995 Angstroms

  32. S XVI 4.727 & 4.733 S XV 5.039 Angstroms

  33. S XV 5.039 & 5.101 Angstroms

  34. No Detectable Lines

  35. XVP Progress Summary • Number of X-ray flares tripled and essentially pile-up free: flare distributions may hint at multiple mechanisms • 0th-order quiescent spectrum (Fe Kalpha) disagrees with Sazonov et al. (2012) model for origin of extended emission vs RIAF • But see talk by Warwick: Low-luminosity X-ray sources and the Galactic ridge X-ray emission! • Brightest flare properties consistent with two brightest XMM flares: photon index ~ 2 and NH ~ 15 x 1022 cm-2 • Modeling background to produce cleanest gratings spectrum of Sgr A* in quiescence • Believe that we have good detection of He-like Fe line; working to reduce background to detect additional lines => possible relative abundance measurements

  36. Is G2 a Gas Cloud on Its Way Towards the Supermassive Black Hole at the Galactic Centre?

  37. Gillessen+2012 • ~3 earth-mass gas cloud approaching Sagittarius A* on a nearly radial orbit • Pericenter ~3100 RS, ~0.03” or ~36 light hr at 2013.5 • Cloud has begun to disrupt over past 3 yr, probably due to tidal shearing • Dynamical evolution and radiation of cloud will probe properties of accretion flow and feeding processes of Sgr A* • keV emission of Sgr A* may brighten significantly at closest approach • Hydrodynamic simulation predicts increased feeding of Sgr A* in a few years

  38. Infalling Dusty Gas Cloud in Galactic Center • Cloud detected at M and L’, not Ks or H • Dusty cloud Td ~ 550 K • Proper motion ~42 mas/yr or 1670 km/s • Br g radial velocity ~1650 km/s • e ~ 0.94 bound orbit • Orbital period ~137 (11) yr • Panel d shows orbits of cloud and star S2

  39. Velocity Shear in Gas Cloud • Integrated Br g maps vs stellar PSF ~21 mas E-W • Position-velocity maps of Br g emission show head-tail structure; ~62 mas for head • Tail spread ~200 mas downstream of head • Velocity gradient ~2 km/s/mas • 89 (30) km/s in 2003 increased to 350 (40) km/s in 2011

  40. Cloud Properties • LIR ~ 5 Lsun; LBr g ~ 2 x 10-3 Lsun • Case B recombination: ne ~ 2.6 x 105 fv-½cm-3 • Specific angular momentum ~50x less than other clouds • Current density ~300fv-½x greater than surrounding hot accretion flow; decrease to ~60fv-½x at peribothron

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