A Review on the Fermi Bubbles

A <strong>Review</strong> on the Fermi
Bubbles
Benny Tsz Ho Tsang (HKU)

Outline of the FB
γ-ray
X-ray Galactic
Center (GC)
Radio
Milky Way ~20 kpc
Radio
γ-ray
Luminosities: X-ray > Gamma > Radio
Outline of the talk
X-ray Radio ϒ-ray
(Snowden et al. 1997) (Finkbeiner 2004) (Su et al. 2010)
(i) Reveal & Analysis
(ii) Models

X-ray Observation
In 1997: (Snowden et al. 1997) ROSAT all-sky survey discovered
X-ray edge feature in 1.5 keV band, first evidence for the existence
the bubbles.
Proposed origin: fast winds that drove a shock into the halo gas
with the velocity 10 8 cm s −1 .
Estimated energy release at GC: 10 55 erg
should also be periodic on a timescale of 10–15 Myr

(WMAP) Microwave Haze
In 2004: WMAP “microwave haze”
was discovered (Finkbeiner 2004)
~4 kpc in radius
Revealed after subtracting the thermal
dust, free-free, and soft CR electron
synchrotron

Possible Mechanisms
Inverse Compton (IC) scattering of cosmic ray (CR)
electrons with interstellar radiation field (ISRF)
Same electron population can also produces microwave
synchrotron radiation (WMAP Haze).
A harder electron component then the diffused population
accelerated by SN shocks required. Also, electrons have shorter
cooling time
Pion decay as a product of CR proton/heavy ion-ISM
interaction pushed by strong winds.

Fermi Observation
In 2010, Su et al. (2010) made use of 1.6-year Fermi data to
generate the all-sky maps in different energy bins.
Major step to reveal the bubbles:
Diffuse galactic emission models as foreground for removal.

Template-based model: foreground subtraction
−
−
SFD galactic dust
map
pion gamma template
Disk model
IC gamma by CR
electrons, mainly
injected by SN shocks
=
=

Gamma-ray Morphology
Profiles of weight
averaged radial intensity
in two energy bands
Defined 0° at the bubble
edge
Sharp reduction in
observed intensity
Even before subtraction
in case of South bubble
North-South similarity
with NO limb
brightening
Red: before subtraction Blue: after subtraction
(Su et al. 2010)
1-5 GeV
5-20 GeV

Gamma-ray Energy Spectra
Consider the entire bubble
Spectra obtained by templates
fitting: SFD dust, simple disk
model
Much harder
dN/dE ~ E -2
with spatial distinct suggest a
separate electron component
Cross-checked with other
templates: spectra very similar.
(Su et al. 2010)
Bubbles

Spectral Variation
By splitting the template for
entire bubble into:
Interior vs shell
North vs South
The resulting spectral variation is
small
Conclusion:
Spatial & spectrally uniform
North-South symmetric
Harder spectrum than other
galactic emission
Largely independent of choice
of templates
(Su et al. 2010)

Multi-wavelength detections
Green: FB Purple: donut-like structure
Red: Loop I Blue: Northern arc
(Su et al. 2010)

IC γ-ray spectra
IC Picture: Power-law electron
spectrum
Steady-state electron spectrum of dN/dE ~ E γ at energy range
[0.1, 1000]GeV, different power-law indices γ(Su et al. 2010)
Synchrotron
10 μG 5 μG
γ = 1.8 – 2.4
The same population of electron naturally generate WMAP
haze, problems with persistency of electron spectrum

IC Picture: Star Capture
Star capture model summary
Shock wave acceleration of electron due to periodic
star capture by Sgr A* (Cheng et al. 2011)
Once every ~3× 10 4 yr, about 50% energy ejected
Initial electron spectrum of powerlaw, accelerated by
shocks, modified by energy loss and escapes
IC gamma spectra by
CMB, IR, optical photons
Expected IC ϒ-ray flux with
different maximum energy of
accelerated electrons

Pion Decay Picture: Proton
Long timescale star formation (SF) near GC (Crocker &
Aharonian 2011), associated CR injected to the bubble
Assuming protons trapped for timescales of 10 10 yr
Protons accelerated quickly
In saturation
ϒ-ray luminosity independent
of local gas density
Explains sharp edge
Near steady-state protons
Uniform intensity
~Ep -2.7
~Ep -2.1
~Ep -2.3

Gamma-ray properties
Sharp edges
Uniform intensity
Summary
Hard spectrum: distinct component of CR electrons
North-South symmetry
Detection both at X-ray (edge) and microwave (haze)
ϒ-ray production by electron IC or pion decay
Tidal eruption of star near Sgr A*
Star Formation near Sgr A*
Accretion on Sgr A*