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
– Sgr A*/G2 interaction monitoring
• Magnetar SGR J1745-2900

NuSTAR Detects Sgr A* Flares

NuSTAR Detects Sgr A* Flares

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

• 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

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)

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

Sgr A* Flare and Quiescent Spectra -- ACIS-I &
HETGS
•
• HETGS 1st Order
ACIS-I
• HETG 0th Order
• 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
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)

39 X-ray Flares in 3Ms Exposure:
1.12 +/- 0.18 flares/day
Time (s)
1.5×10 6 0 5×10 5
Observing Gaps
Removed
10 6 2×10 6 2.5×10 6 3×10 6
13854 13846
0 5 10
Time (ks)
15 20 0
Neilsen+2013
10 20 30
Time (ks)
40 50

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)

NuSTAR Detects Sgr A* Flares

NuSTAR Detects Sgr A* Flares

NuSTAR Detects Sgr A* Flares

NuSTAR Spectra

Flare Model Fits To NuSTAR Spectrum

Where are Sgr A* Flares Located?

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

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

Fe XXV 1.8505 Angstroms
6
4
2
6
4
0
5
4
3
2
1
0
8
2
0
1.4
1.45
1.5
1.55
1.6
1.65
1.7
1.75
1.8
Wavelength [Angstroms]
1.85
1.9
1.95
H EG-1
H EG +1
H EG-1 + H EG+1
2 2.05
2.1

No Detectable lines
3
2
1
5
4
0
4
2
0
8
6
4
2
0
1.9
1.95
H EG-1
H EG +1
H EG-1 + H EG+1
2 2.05
2.1
2.15
2.2
2.25
2.3
Wavelength [Angstroms]
2.35
2.4
2.45
2.5
2.55
2.6

No Detectable lines
2
0
6
4
H EG-1
H EG +1
4
2
4
2
0
0
8
6
2.4
2.45
H EG-1 + H EG+1
2.5
2.55
2.6
2.65
2.7
2.75
2.8
Wavelength [Angstroms]
2.85
2.9
2.95
3 3.05
3.1

Ca XIX 3.177 & 3.211 Angstroms
6
4
2
0
4
2
0
10
8
6
4
2
0
2.9
2.95
H EG-1
H EG+1
H EG-1 + H EG +1
3 3.05
3.1
3.15
3.2
3.25
3.3
Wavelength [Angstroms]
3.35
3.4
3.45
3.5
3.55
3.6

Ar XVII 3.95 & Ar XVII/S XVI 3.995
Angstroms
2
0
4
2
8
6
0
2
1
0
6
5
4
3
4
3.4
3.45
3.5
3.55
3.6
3.65
3.7
3.75
3.8
Wavelength [Angstroms]
3.85
3.9
3.95
H EG-1
H EG +1
H EG-1 + H EG+1
4 4.05
4.1

Ar XVII 3.95 & Ar XVII/S XVI 3.995
Angstroms
H EG-1
2
0
4
2
8
6
0
2
1
0
6
4
3
4
3.9
3.95
H EG +1
H EG-1 + H EG+1
4 4.05
4.1
4.15
4.2
4.25
4.3
Wavelength [Angstroms]
4.35
4.4
4.45
4.5
4.55
4.6

S XVI 4.727 & 4.733 S XV 5.039
Angstroms
3
2
1
4
2
0
1
0
3
2
0
5
4
6
4.4
4.45
4.5
4.55
4.6
4.65
4.7
4.75
4.8
Wavelength [Angstroms]
4.85
4.9
4.95
H EG-1
H EG +1
H EG-1 + H EG+1
5 5.05
5.1

S XV 5.039 & 5.101 Angstroms
2
0
3
2
1
4
3
0
5
2
1
0
6
4
4.9
4.95
H EG-1
H EG +1
H EG-1 + H EG+1
5 5.05
5.1
5.15
5.2
5.25
5.3
Wavelength [Angstroms]
5.35
5.4
5.45
5.5
5.55
5.6

No Detectable Lines
H EG-1
2
1.5
1
0.5
0
2
1.5
1
0.5
0
3
2
1
0
5.4
5.45
5.5
5.55
5.6
5.65
5.7
5.75
Wavelength [Angstroms]
5.8
H EG+1
H EG-1 + H EG+1
5.85
5.9
5.95
6

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
Dusty cloud T or H 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 headtail 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 erg/s v
• May release up to 10 48 erg over decade => <Lx> ~10 39-40
• 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 ~4month 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 Xray enhancement ~18 months prior to CM pericenter (i.e., 2012); original model ruled out by Chandra Sgr A* XVP monitoring – no Xray 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 View of Central Parsec –
Pre Magnetar

Chandra Monitoring of Sgr A*/G2 &
SGR J1745-2900 in 2014

Chandra Avg Quiescent Count Rates of Sgr A*/G2 & SGR J1745-2900
Exponential decay ~ 146 d

Chandra Light Curve of Sgr A*/G2

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
(90% confidence) s -1
• 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 dereddened 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
Chandra HRC-I: 2005-2008 (25ks) Chandra HRC-S: 29/04/2013 (10ks)
SgrA*
2.4
” 2.4
”
SgrA*
SGR J1745-2900 SGR J1745-2900
(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…

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).
Chandra HRC-I: 2005-2008 (25ks) Chandra HRC-S: 29/04/2013 (10ks)
SgrA*
2.4
” 2.4
”
SgrA*
SGR J1745-2900 SGR J1745-2900
(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)
Chandra HRC-I: 2005-2008 (25ks)
SgrA*
2.4
”
SGR J1745-2900
Chandra HRC-S: 29/04/2013 (10ks)
SGR J1745-2900
2.4
”
SgrA*

Why is the young pulsar close to Sgr A* a magnetar?
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?
VLT-NACO
•SgrA*
•2.4”
•SGR J1745-2900
• SGR J1745-2900
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!
(Schoedel+2009; Lu+2009; Rea+2013)

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?