Presentation on theme: "An Overview of Some Recent Results in the Galactic Centre The X-ray Universe 2014 Trinity College, Dublin 2014 June 16-19 Frederick K. Baganoff MIT Kavli."— Presentation transcript:
An Overview of Some Recent Results in the Galactic Centre The X-ray Universe 2014 Trinity College, Dublin 2014 June 16-19 Frederick K. Baganoff MIT Kavli Institute for Astrophysics and Space Research
Outline Traces of past Sgr A* activity Current Sgr A* activity – Sgr A* flares – Sgr A*/G2 interaction monitoring Magnetar SGR J1745-2900
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
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 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 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 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 Science Goals
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)
Sgr A* XVP Cumulative Light Curve Brightest flare ~140x quiescence L X ~ 2 x 10 35 erg/s (2-8 keV) Nowak et al. (2012)
Sgr A* Flare and Quiescent Spectra -- ACIS-I & HETGS HETGS 1st Order HETG 0th Order ACIS-I HETGS 0th + 1st Order HETGS 1st Order HETGS 0th Order Wang+2013: He-like Fe Kalpha line at 6.7 keV; No 6.4 keV line predicted by Sazonov+2012; but see Warwick talk
Consistent Flare Properties for Brightest Chandra & XMM Flares ACIS-I Quies. HETGS Flare XMM 2002 F XMM 2007 F ~ 2 and N H ~ 15 x 10 22 cm -2 Photon index ~ 2 and N H ~ 15 x 10 22 cm -2
Properties of Brightest X-ray Flare Good agreement with two brightest XMM flares (Nowak et al. 2012, ApJ, 759, 95)
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
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?
NuSTAR Detects Sgr A* Flare! Barriere+2014 NuSTAR PI: F. Harrison (Caltech)
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 N H ~ 15 x 10 22 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
Is G2 a Gas Cloud on Its Way Towards the Supermassive Black Hole at the Galactic Centre?
Gillessen+2012 ~3 earth-mass gas cloud approaching Sagittarius A* on a nearly radial orbit Pericenter ~3100 R S, ~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
Infalling Dusty Gas Cloud in Galactic Center Cloud detected at M and L’, not K s or H Dusty cloud T d ~ 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
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
Cloud Properties L IR ~ 5 L sun ; L Br g ~ 2 x 10 -3 L sun Case B recombination: n e ~ 2.6 x 10 5 f v -½ cm -3 Specific angular momentum ~50x less than other clouds Current density ~300f v -½ x greater than surrounding hot accretion flow; decrease to ~60f v -½ x at peribothron
X-ray Emission as Probe of Accretion Flow Profile & BH Feeding Cloud remains cold until just before peribothron Post-shock T c ~ 6-10 x 10 6 K L x <~ 10 34 erg/s (2-8 keV); possibly variable Stronger X-ray emission for steeper radial profiles of accretion flow density & temperature and higher f v May release up to 10 48 erg over decade => ~10 39-40 erg/s Sufficient to produce Fe K a reflection features seen in Galactic Center (e.g., Sgr B2) => possible light echoes from previous clouds accreting onto Sgr A*?
Sgr A*/G2 Cloud Predictions Gillessen+2012: RJ & KH instabilities compress & heat G2 near pericenter causing sustained increase in NIR & X-ray fluxes on ~4- month dynamical timescale Sadowski+2013: MHD simulations predict radio bow shock visible 7-9 months prior to CM pericenter Shcherbakov 2013: magnetically arrested cloud model resists compression near pericenter but predicts radio bow shock and X- ray enhancement ~18 months prior to CM pericenter (i.e., 2012); original model ruled out by Chandra Sgr A* XVP monitoring – no X- ray enhancement detected Shcherbakov 2014: revised best-fit model to data predict enhancements below quiescent Sgr A* level => could see nothing if G2 is gas cloud
Alternative Interpretation – G2 a Star Phifer+2013: G2 is dusty, windy young star similar to other features with similar NIR colors in central 2’ Report Br_gamma line position offset from L’ position => suggest VLT slit pulling in two separate red clumps to produce apparently tidally sheared feature in position-velocity diagram MPE group disagrees; no consensus between groups yet If G2 a star => will remain intact through pericenter passage and move away from Sgr A* with constant Br_gamma flux However some wind and envelope material may fall onto Sgr A* accretion flow over next few years => keep monitoring
Chandra Monitoring NIR-derived orbit => pericenter ~120 AU 2014 late March +/- 3 weeks (Gillessen+2013; Meyer+2014) Chandra/ACIS-S3 monitored Sgr A*/G2 6 times between 2014 Feb 21 & June 3 SGR J1745-2900 located 2.4” (~0.1 pc) in projection from Sgr A* Extract light curves & spectra within 2” radius of magnetar and SgrA* products within 1.25” radius; Sgr A* contaminated ~0.6-0.9% by magnetar flux Avg unabsorbed quiescent 2-8 keV flux of Sgr A* in 3Ms XVP project 4.5e-13 erg cm 2 s -1 (Nowak+2012, Nielsen+2013) Measured fluxes corrected for magnetar contamination in range (3.5-4.1)e-13 erg cm 2 s -1 with uncertainties of +/-0.6e-13 erg cm 2 s -1 (90% confidence) Conclusion: Chandra finds no evidence for sustained enhanced emission at Sgr A* so far See ATel #6242 (Haggard+2014) for details
NIR & Radio Monitoring ATel #6110 - May 2 - Keck NIRC2 detects G2 at 3.8 um with de- reddened flux density 1.7 ± 0.2 mJy (or equivalently an observed L’ magnitude of 14.1 ± 0.2), consistent with 2002-2013 measurements (Ghez+2014) Conclude G2 currently undergoing closest approach & still intact ATel #5969 – March 10 – NRAO VLA Service monitoring at multiple freqencies & dates find no enhancements (Chandler+2014) ATel #6083 - April 21- JVN monitoring at 22 GHz finds avg fluxes consistent with previous levels (Tsuboi+2014)
SGR 1745-2900 : a magnetar within grasp of our SMBH The angular separation from Sgr A* is 2.4"+/-0.3" (within the PSF of all X-ray instruments beside Chandra). (Rea+2013)
SGR 1745-2900: Timing properties P = 3.7635537(2) s (epoch TJD 16424.5509871)871), Pdot =6.61(4)x10 -12 s/s B dip =1.6x10 14 G Edot = 5x10 33 erg/s tau c = 9 kyr Large timing noise! Pdot changes or glitches… (Mori et al. 2013; Kennea et al. 2013; Rea et al. 2013, Kaspi et al. 2014, Coti Zelati et al. 2014, in prep)
SGR 1745-2900: Spectral evolution (Coti-Zelati, Rea, Papitto et al. 2014, in prep)
Is SGR 1745-2900 bound to Sgr A*? SGR 1745-2900 has, on average, a 90% probability of being bound to the SMBH with an orbital period between 500 yr to several kyrs. (Rea+2013)
Is SGR 1745-2900 so close to SgrA* ? Using the Cordes & Lazio (2002) H distribution we estimate that a DM=1750 pc cm -3 results in a distance of 8.3 kpc (same as SgrA*). If we assume this distance, a 2.4" projected distance translates in: d = 0.09+/-0.02 pc (90% error). Distance determination from DM of the radio pulsar: (Rea+2013)
Chance coincidence probability - neutron star density in the Galactic disc 10 -4 pc -3 we expect ~0.3 neutron stars in a narrow cone of aperture 2.4", regardless of the age. - with an age < 10 kyr, the probability of SGR J1745–2900 being a neutron star wandering across the line of sight reduces to <3x10 -6. No foreground/background object. - it has been estimated that in the 1pc around the Galactic center there are ~80 pulsar with an average age of 10 7 yr, and we expect one or two pulsars with < 10kyr. (see e.g. Freitag et al. 2006; Wharton et al. 2012)
Why is the young pulsar close to Sgr A* a magnetar? (Schoedel+2009; Lu+2009; Rea+2013) We know of about a hundred young radio pulsars, but only 12 young magnetars. Why ts the only young PSR expected within 1pc from Sgr A* a magnetar? 1. The birth rate of magnetars is comparable with that of "normal" pulsars" 2. The Galactic center is more inclined to form magnetars than normal pulsars....probably both are true! 2.4” SgrA* SGR J1745-2900 VLT-NACO SGR J1745-2900
Summary Our normal Milky Way Galaxy has an active Galactic Center Weak AGN activity currently Possibly much stronger in the past century (see Ponti talk) X-ray Transients Magnetars & Pulsars in central pc => one day find aPSR close enough to Sgr A* to test GR?
Your consent to our cookies if you continue to use this website.