Possible high energy phenomena related tothe stellar capture by the galactic
supermassive black holes
K S Cheng
University of Hong Kong
China

Outline Introduction
 Swift J1644+57

 Positron Annihilation in GC Fermi Bubbles
 Cosmic Rays with energy >1015eV
 Summary

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Galactic centers: some are active, most aredormant

NGC 3115

M87

Sgr A*Tidal disruption event (TDE)
can light dormant SMBH.
So TDE is promising tool toprobe galactic center BHs.
Ghez et al. 2005
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X-ray/UV flare produced by tidal disruption of stellar object(Rees 1988):
When a star’s orbit in tidal radius (tidal
1/3
force=self gravity)
MRt ~  BH  R*
 M* 

Initially radiating with Eddintong
luminosity:
 M
LEdd  1.3 10  7
 10 M ergs/s

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Thermal spectrum of effective temperature
1/4

Teff

 LEdd ~2 4 rt  r 
 2.4 105  * 
r 

1/2

Fallbacktime (mostboundmaterial):
tf ~ days toweeks.

1/6 m* 
M 

M 81/12 K3/2

1RX J 1242.6-1119A (Komossa et al. 2004)

Decaying after peak as power-law with5/3
timef   t t flare 
on a timescale:r t flare ~1.1yr  * r  m* 
1/2
 M8
M 

Rate of TDEs~10-5-10-3 yr-1gal-1(e.g.2015/8/18
Wang & Merritt 2004)

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Extra-galactic Events – direct evidence
𝑡𝑐𝑖𝑟 ~2𝑡𝑝𝑒𝑎𝑘t^-5/3Jet (t<t_crit)

Halpern et al. 2004

𝑡𝑝𝑒𝑎𝑘 ~0.2

1𝑀𝐵𝐻 2106𝑀𝑀𝑠𝑦𝑟

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Two recent super-Eddington events - Swift 1644+573
and Swift J2058.4 + 0516
Sw1644+57
M~10^6 solar masses

Sw2058+05
M<10^8 solarmasses

Swift J1644+57: first TDE with a jet(Levan et al. 2011; Bloom et al. 2011; Burrows et al. 2011; Zauderder et al. 2011)• Triggered Swift BAT on
March 28, 2011

/X-ray• Triggered BAT 3 more times
over next few days
• Remains bright in X-rays• IR and Radio Brightening

IR-Optical• Host galaxy at z = 0.35
• NOT a (normal) GRB- low luminosity
Radio- duration ~ months•NOT a normal AGN

Levan et al. 2011

- no evidence for AGN orpast activity
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8Host Galaxy at z = 0.35

Levan et al. 2011

Not an AGN• Within < 150 pc of galactic center SMBH origin•LX > 1048 erg s-1 > 10000 LEdd of 106M⊙
black holesuper-Eddaccretion and/or beaming

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Blazar model for Swift J1644+57
Emission from theaccretion disk is
Compton-upscattered,giving rise to theobserved x-rays.

Bloom et al. 2011

Fermi LAT

radiot ~ 3 days

X-rays
Av=3-5• synchrotron self-absorption  Rradio > 1016 cm  Г~20  external shock from ISM
interaction (Giannios & Metzger 2011)• X-ray variability  RX ~ c tX 2 ~ 3x1014 (/20)2 cm  “internal” process (e.g. shocks,reconnection)
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Internal model for X-ray flares

Levan et al. 2011, Science

X-ray flux increases 10 times in 200
seconds, difficult for internal shocks!
Many flares in the X-ray band!

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Internal-shock model for X-ray flares
Two shocks structure:
Forward shock

Reverse shock
L14Lγ4γ14

unshocked material

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shocked material

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unshocked material13t=31 hrs

t=3 days

Opticalextinction
regionWang & Cheng 20122015/8/18

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Internal shock

Our model predicts that theexternal shock (dashed line) will
dominate the X-ray
emission when the internal shock has
ended, i.e. no more ejecta coming out
from the BH.

external shock
Wang & Cheng 2012

external shock
External shock model fixed by radio(Zauderder et al. 2013)2015/8/18

Chandra observation at 630 days
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Radio fitting with external shock model (Wang et al. 2014)

Surprise ! Recent X-ray observations by Swift and Chandra(1)Internal shock emission- theradio emission must come fromother model(2)Hadronic collisions?

• The bulge emission is centered on the Galactic Center with an•

extension of ~6 ° - 8° (FWHM)
Emissions appears to be diffuse. No evidence were found forsignificant emission from point sources in the Galactic Centere+/ e- 511 keV Line from the Galactic CenterRegion
 First reported more than 30 years ago as a 476

keV feature (Jonhson et al. 1972) Identified with a narrow 511 keV line in 1978
(Leventhal et al. 1978)
 Observed by SMM, OSSE, INTEGRAL etc.
 The brightest gamma-ray line in the Galaxy(~ 10-3ph/s/cm2) is indeed from the GC
 511keV  1037 erg / s  Fe  10 43 e  / sPositron Annihilation

Interstellar cloud properties and e+/e- annihilation lines

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NCU, Jung-Li

FWHM
2.47keVAnnihilationin warm gas
region with T=8000K(Churazov etal 2005)

Observations indicate that the intensity ratio between continuouscomponent and the line component is 3.65+/-0.82 for a freebackground

This suggests that most annihilation photons are mainly produced
via formation of Positroniums.

TeV Gamma-Ray Emission from theGalactic Center (Aharonian et al., 2004)
Chandra GC survey
NASA/UMass/D.Wang et al.
CANGAROO (80%)
H.E.S.S. (95%)
Whipple(95%)

Contours from Hooper et al. 2004

F ( E ) ~ E

2.2

Q  1035 erg / s~ 10 pc

Implications of annihilation data
 It is important to notice that the implied input energy for e+ ishuge if they are produced by proton-proton collisions.
Because it takes at least 1GeV to produce positron and theimplied input power is ~1040 erg/s. However, p-p collisions
also produce gamma-rays via the decay of neutral pions andthe observed gamma-rays should be ~3x1038 erg/s, which ismuch higher than the current observed value. Furthermore, if
protons are ejected by a source, it takes 107 years to diffuse
to 500pc region and the implied total input energy is 1054 erg!

p  p   ,0 0 2 eModels of positron production (Prantzos etal. 2011)
 Radioactivity from stellar nucleosynthesis – e.g.
56Co in SNIa, 26Al and 44Ti from massive star

explosions (Hypernovae/gamma-ray bursts)
 High energy processes in cosmic rays andcompact objects – e.g. p-p collisions (energysources could be pulsars, MSPs, magnetars etc.
 Positron production by the Galactic black hole –e.g. Tidal disruption of nearby stars andsubsequent accretion of their material
 Dark matter and ”non-standard” models

Tidal Stellar Capture as Energy Input
Jet ~RX(Komossaet al. 2004) tidal disruption events takenWeJ 1242.6-1119A
suggest thatthe periodic

place in the galactic center could be the energy injection forthe positrons. In each capture event roughly >10^52 ergs
accretion energy can be carried away by relativistic protons.
The subsequent hadronic collision processes can produce
positrons.

Quasi-Stationary model Main assumption: the positron flux from theBulge (~ 1043 e  / s ) and the gamma-rayflux( < 2 10 erg / s ) are produced in the sameeruption event, also assuming positrons
annihilate once they are produced Result: The maximum production of thermal
positrons is only 2 1041 e / s because eachneutral pion has energy ~100MeV
 Conclusion: The model fails to explain theseobserved fluxes
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Repeated capture model Main assumption:

τcool > τcapture> τcollision
Stellar capture takes place in a time scale less than thepositron cooling time scale τcool ~1013s (E/100MeV)/(n/10cm-3 ).
If this assumption is satisfied, the gamma-ray flux must
decrease substantially between two capture events, which
implies the p-p collision time scale (τcollision ~1012s /(n/103cm-3)
must be shorter than the capture time scale. This implies thatthe gas density for p-p collision and e+ thermalization must bedifferent! (we will see the justification later) Result: The production of thermal positrons is almost constantand gamma-ray intensity will be only about 2-3 order ofmagnitude smaller than that implied by the annihilation
intensity.
 Conclusion: The energy input of primary protons per capture is
only ~1052 ergs

Repeated Stellar Capture model (np =1000 cm-3)

PositronannihilationGammarays

Parameters of the Interstellar Medium in theGalactic Bulge Region (Jean et al. 2005)
 The budge (region inside the radius ~230 pc andheight 45 pc) contains 7 107 M 
 90% of this mass is trapped in small high density
clouds (as high as 103 cm-3)while the remaining 10%
is homogeneously distributed with the averagedensity ~10 cm-3.
74
10M  and it is equally The HI mass is about
distribute between cold and warm neutral gas
 The remaining mass is in warm and hot ionized gas:
about 90% of this mass is in the warm phase and
10% is in the hot phase.

Propagation of> GeV protons

andTens MeV positrons

Morfill’s model (1982)

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Particle escape from the Central region

Central region filled by molecular gas
Dense clouds
Surrounding region filled by warm neutral and ionizedfractions of the interstellar gas
Flux of relativistic protons
Flux of tens MeV positrons
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Over-production of gamma-rays in tens
MeV range (Beacom and Yüksel 2006)

Rapid cooling by magnetic field?
(Chernyshov et al. 2010)

(Su et al. 2010;Dobler et al. 2010)

Planck intermediate results

30 -44 GHz.
Good correlation between radio and gamma-rays
Spectrum is harder than in the Galaxy ( -2.1)

Ade et al, 2013

Multi-wavelength Observationsof Fermi Bubble
1997 - the ROSAT All-Sky Survey at 1.5 keV(Snowden et al. 1997) revealed a biconical X-ray
structure over the inner tens of degrees around theGalactic center (GC). This requires 10^55 ergs
injection energy in a time scale of 10Myrs.
 2004 - A microwave residual excess (named “themicrowave haze”) with spherical (non-disklike)morphology about ∼ 4 kpc in radius toward the GC(Finkbeiner 2004) 2010 – A pair of giant gamma-ray bubble in GeV wasfound The injected power to maintain these radiations~10^40-10^41 erg/s is required.
Fermi Observations

After the backgroundsubtraction

Spectra (Su et al. 2010)

Origin of the bubble?
 Since this pair of bubbles are locatedsymmetrically around the GC, it must relate
to activities around GC
 Other activities in GC – electron/positronannihilation lines, diffuse gamma-rays,
several TeV sources, SNe etc.
 Most important- hot plasma with kT~10KeV
is found in GC, which cannot be confined inGC and has to be maintained by constantinjected rate into halo of >10^40 erg/sThis injected energy is similar to that to maintain the radiation in FB

Possible sources of energy injection forFB
 Exceptional frequent SNe rate in GC in

past 10^7 yr – but the line emission ofradioactive 26^Al is missing
 Past accretion events onto the central
supermassive black hole (SMBH) in thelast ~ 10 Myr similar to NGC3081
 Annihilation of dark matter – the bubble upto 4-8kpc. It is unclear why the dark matter
distributes like a pair of bubbles
 In next few slides I offer our view about theorigin of FB, which could be wrong !!!!!!!

Blackat atthethe
centerof Milky
Way Waywith MassBlackHole
Hole
centerof Milky–
6 solar masses!
3.7x10Mass = 3x106 solar masses!

MPE / R. Genzel et al.

Tidal Stellar Capture as Energy
Input
Jet ~J 1242.6-1119A(Komossaevents
et al. 2004)
The RX
periodictidal disruption
taken place in the galactic center could beenergy injection to the FB. In each capture event roughly >10^52 ergs can bereleased via relativistic and non-relativistic particles, which can heat up the gas
in the GC. This hot gas can expand to the galactic halo with velocity ~10^8cm/s,
which is faster than the local sound speed in the halo, therefore strong shock
can be formed and accelerate electrons in the halo to relativistic energies. The
subsequent IC and synchrotron radiation processes produce the observed
gamma-rays and radio waves respectively.

Model fitting

Injected spectrum from the shock front is E^-2, then evolves according toFlokker-Planck eq. with maximum energy determined by the balance
between acceleration and cooling and the energy break due to diffusion loss.
The actual spectrum is nearly -3. Here 50GeV is the break energy in theelectron spectrum, which corresponds to the escape time scale of 10Myr

Numerical model with adiabatic losses

Models for Fermi Bubble
 Alternative acceleration mechanisms

and electron injection models can befound in Cheng et al. 2014-2015(Stochastic acceleration frombackground plasma;Stochastic reacceleration of electrons injected bySNRs;Hadronic model of the Bubbles)

Could we find Fermi Bubble-likecases in other galaxies?
 Fermi bubbles are FAINT structures(signal << background) It is difficult to observe the exact copy ofthem from other distance galaxies
except the local group (3Mpc indiameter) The angular resolution of gamma-rays islow (GAMMA-400 ~0.01º (Eγ > 100GeV) )Maximum energy of the accelerated particle
 The SNRs has a limited age and particle acc.

Time is Equating these two time scales and in theBohm approximation Emax ~ 1014 - 1015 eV
 Magnetic field amplification in young SNRs?
 Multiple acceleration?
 More long-lived objects?

Acceleration in FB using SNe particles
as injected spectrum
𝜕2𝑓 1 𝜕𝜕𝑓1 𝜕𝜕𝑓
4−𝐷 2 −
ρ𝐷− 2𝜅𝑝= 𝑄 𝑝, ρ, 𝑧𝜕𝑧ρ 𝜕ρ𝜕ρ𝑝 𝜕𝑝𝜕𝑝𝜅 = 𝜅𝐵 𝜃 ρ𝐵 − ρ𝑧 2 −4.7𝑝
𝑄 𝑝, ρ, 𝑧 = 𝑁𝑆𝑁 ρ exp − 2 𝑝
exp −𝑝𝑘𝑧𝑑

 Particles injected in disk according to SNR density andp_k ~10^15eV/c
 Acceleration is only inside the bubble

The CR spectrum near theEarthUltra-high energy neutrinos produced bycosmic rays accelerated by FB
Icecube has detected 45 out of 28 events above
30TeV, which may
originate from cosmicrays with energies>10^15eV from FB(Lunardini et al. 2014).
However the spatial
distribution of CRs withenergies >10^15eV does
not show any
concentration in thedirection of FB!

Summary







Although the tidal stellar capture rate is low ~10-3 yr-1 to 10-5 yr-1 ,these processes are common in other normal galaxies, e g. Swift1644+57.
But each of these capture events can release ~1052 ergs. Perhaps
these tidal captures may be responsible for many high energy
phenomena around GC including positron annihilation, FB andCosmic rays with energy >1015 eV etc.
However all these phenomena are indirect evidence of tidal
capture events and there are alternative explanations
In our models there are still a lot of short falls needed to be fixed.
For examples how to avoid over-production of tens MeV gammarays, how the jet energy produced by accretion can be efficiently
transferred to heat up the gas in tens pc region. It is unclear
whether successive star captures produce only a single shock at
the border of expending envelope of hot plasma or series ofshocks going one after other. The Cosmic ray model requires too
many fine tune parameters etc.
More careful studies are necessary!