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
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
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
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
•LX ~ 2 x 1035 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
•HETGS 0th + 1st Order
•HETGS 1st Order
•HETG 0th Order
•ACIS-I
•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 NH ~ 15 x 1022 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
Count Rate (cts/s)
0.1
0.2
0
5×10
5
10
Time (s)
1.5×106
6
2×106
2.5×106
3×106
Count Rate (cts/s)
0.02 0.04 0.06 0.08
0
Observing Gaps
Removed
13846
0
13854
0
5
10
15
20
0
Time (ks)
10
20
30
Time (ks)
Neilsen+2013
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
Counts/bin
6
HEG-1
4
2
0
5
HEG+1
Counts/bin
4
3
2
1
Counts/bin
0
8
HEG-1 + HEG+1
6
4
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
2
2.05
2.1
No Detectable lines
HEG-1
Counts/bin
5
4
3
2
1
0
Counts/bin
HEG+1
4
2
Counts/bin
0
8
HEG-1 + HEG+1
6
4
2
0
1.9
1.95
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
Counts/bin
6
HEG-1
4
2
0
Counts/bin
HEG+1
4
2
Counts/bin
0
8
HEG-1 + HEG+1
6
4
2
0
2.4
2.45
2.5
2.55
2.6
2.65
2.8
2.75
2.7
Wavelength [Angstroms]
2.85
2.9
2.95
3
3.05
3.1
Ca XIX 3.177 & 3.211 Angstroms
Counts/bin
6
HEG-1
4
2
0
Counts/bin
HEG+1
4
2
0
10
HEG-1 + HEG+1
Counts/bin
8
6
4
2
0
2.9
2.95
3
3.05
3.1
3.15
3.3
3.25
3.2
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
Counts/bin
5
HEG-1
4
3
2
1
0
Counts/bin
6
HEG+1
4
2
0
HEG-1 + HEG+1
Counts/bin
8
6
4
2
0
3.4
3.45
3.5
3.55
3.6
3.65
3.8
3.75
3.7
Wavelength [Angstroms]
3.85
3.9
3.95
4
4.05
4.1
Ar XVII 3.95 & Ar XVII/S XVI 3.995
Angstroms
HEG-1
Counts/bin
4
3
2
1
0
Counts/bin
6
HEG+1
4
2
0
HEG-1 + HEG+1
Counts/bin
8
6
4
2
0
3.9
3.95
4
4.05
4.1
4.15
4.3
4.25
4.2
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
HEG-1
Counts/bin
3
2
1
0
HEG+1
Counts/bin
5
4
3
2
1
0
HEG-1 + HEG+1
Counts/bin
6
4
2
0
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
5
5.05
5.1
S XV 5.039 & 5.101 Angstroms
HEG-1
Counts/bin
3
2
1
Counts/bin
0
5
HEG+1
4
3
2
1
Counts/bin
0
6
HEG-1 + HEG+1
4
2
0
4.9
4.95
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
HEG-1
Counts/bin
2
1.5
1
0.5
Counts/bin
0
2
HEG+1
1.5
1
0.5
0
HEG-1 + HEG+1
Counts/bin
3
2
1
0
5.4
5.45
5.5
5.55
5.6
5.65
5.7
5.75
Wavelength [Angstroms]
5.8
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 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
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 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
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
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
•
•
•
•
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
X-ray Emission as Probe of Accretion Flow
Profile & BH Feeding
Cloud remains cold until just before peribothron
Post-shock Tc ~ 6-10 x 106 K
Lx <~ 1034 erg/s (2-8 keV); possibly variable
Stronger X-ray emission for steeper radial profiles of
accretion flow density & temperature and higher fv
• May release up to 1048 erg over decade => <Lx> ~1039-40
erg/s
• Sufficient to produce Fe Ka 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 cm2 s-1 (Nowak+2012, Nielsen+2013)
• Measured fluxes corrected for magnetar contamination in range
(3.5-4.1)e-13 erg cm2 s-1 with uncertainties of +/-0.6e-13 erg cm2 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 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*
SgrA*
2.4”
SGR J1745-2900
2.4”
SGR J1745-2900
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
Bdip =1.6x1014 G
Edot = 5x1033 erg/s
tauc = 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* ?
Distance determination from DM of the
radio pulsar:
Chandra HRC-I: 2005-2008 (25ks)
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:
Chandra HRC-S: 29/04/2013 (10ks)
SgrA*
SgrA*
2.4”
2.4”
d = 0.09+/-0.02 pc (90% error).
SGR J1745-2900
(Rea+2013)
SGR J1745-2900
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.
Chandra HRC-I: 2005-2008 (25ks)
- 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.
Chandra HRC-S: 29/04/2013 (10ks)
SgrA*
SgrA*
2.4”
2.4”
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 107 yr,
and we expect one or two pulsars with <
10kyr. (see e.g. Freitag et al. 2006; Wharton et al.
2012)
SGR J1745-2900
SGR J1745-2900
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?