Olaf Reimer (Institute for Astro- and Particle Physics)

Institute for Cosmic Ray Research, Kashiwa

Feb 20, 2019

Dictionary:“Diffuse” : = wide spread; not localized or confined; with no distinct margin

Astronomer:“Diffuse” : = all emission that cannot be resolved into individual sources
Careful, this depends decisively on instrument characteristics.

Astrophysicist:“Diffuse” : = all emission processes that are related to interstellar,interplanetary, and/or intergalactic matter

Now let‘s start looking at the high-energy γ-ray sky!

Fermi-LAT E>100 MeV by 3FGL[LAT collaboration 2015]~ 70% of all observed photons are attributed to the diffuse Galactic emission

Galactic Diffuse

YourScience CaseHere

- Cosmic Rays present throughout our Galaxy- B-fields (evident via synchrotron radio maps)- Interstellar radiation fields (CMB, IR, OPT/UV)Inverse Compton

Bremsstrahlung

π0-decay

Fermi-LAT 0.6 < E < 307 GeV
by D3PO algorithm [Selig ea 2015]

taken from Thorsten Enßlintaken from Thorsten Enßlintaken from Thorsten Enßlintaken from Thorsten Enßlintaken from Thorsten Enßlin

CLOUD-LIKE

BUBBLE-LIKE D4PO upcoming [Pumpe ea 2018, A&A 619, A119 ]Fermi-LAT E > 50 GeV by 2FHL
[LAT collaboration 2015]

 median location uncertainty of 1.8 arcmin! (68%)

 Multi-TeV: MILAGRO; ARGO-YBJ
Abdo ea 2008: A MEASUREMENT OF THE SPATIAL DISTRIBUTION OF DIFFUSE TeVGAMMA-RAY EMISSION FROM THE GALACTIC PLANE WITH MILAGRO(median energy 15 TeV) Multi-TeV: MILAGRO; ARGO-YBJ
no-interarm region

interarm region

Abdo ea 2009: MILAGRO OBSERVATIONS OF MULTI-TeV EMISSION FROM GALACTICSOURCES IN THE FERMI BRIGHT SOURCE LIST
…measurement of the diffuse emission (Abdo et al. 2008) at its highest value, in the inner Galaxy (30< l < 65, |b| < 2).
Using this value, we expect 5.3×10−17 TeV−1 s−1 cm−2 in a 1 bin at 35 TeV, which is only about ∼15% contamination for
the weakest sources …. The GALPROP conventional model, for comparison, would only constitute∼3% contamination.

Discrepancy between measurement and spectral model is factor 5!

 Multi-TeV: MILAGRO; ARGO-YBJ
25° < l < 100°, |b| < 5°25° < l < 100°

ARGO-YBJ

65° < l < 85°, |b| < 5°

MILAGROResults:
 no discrepancy to LAT diffuse model no MILAGRO-like multi-TeV excess

ARGO-YBJ[Bartoli ea 2015, ApJ 806:20]

H.E.S.S. collaboration
A&A Special Issue 2018

H.E.S.S. collaboration
A&A Special Issue 2018

Diffuse Galactic TeV-emission has been assessed, too:•••

Galactic Center Ridge emission [Nature 2006, 2016]
Diffuse Galactic γ-ray emission with H.E.S.S. [PRD 2014]
HGPS: b=0 centered 1D-Gaussian [A&A 2018]→H.E.S.S. collaboration
A&A Special Issue 2018

1% Crab

10% Crab

HAWC collaboration 2017

Sufficient power and ability to particle
Acceleration up to Knee (PeV)
According to current understanding there
are three candidate classes:
 Supernova Remnants
 Pulsar/PWNs
 Stellar winds

CRs move at relativistic speed but are affectedby magnetic fields in our Galaxy
Circular motion around field lines if B-field regular.
Turbulence can alter B-field structure substantially,leading to diffusive processes.If random it will beisotropized.
Expect conditions in ISM somewhere between
these extrema.

Observables
 Composition
 Energy dependence

However:Arrival directions are isotropized-> no CR sources, only proxies or EM
Low-energy flux is solar modulated
Flux at Earth typical for Milkyway?

Observables
 Secondary-to-Primary ratios
Interactions between CR primaries and ISM result in secondary CR particles (charged particles, nuclei,neutral particles).
A ratio between secondary to primary CR nuclei or isotopes is indicative for particle transport physics
To study secondary/primary ratios one needs accurate knowledge about the interaction kinematics(x-sections, multiplicities etc.)

Any matter besides stars in our Galaxy iscondiered ISM.
It will include gas, dust, radiation as well
as CRs. Their energy density is roughly
similar.
ISM is dynamic and features structure on
all scales.
No single ISM constituent dominates
the dynamics of our Galaxy!

Gas and dust in our Galaxy provide the target for
production of secondary CRs.
Gas and dust split into a ratio of approx. 100,
meaning most of the mass is in the gas phase.
Its composition is mostly hydrogen (~70% massfraction) and helium (~28 mass fraction).
Whereas hydrogen is comparably easy to observe,
helium is not. Similary between their distributions
is therefore only assumed.
Prime tracer of HI is the 21-cm line emission, for
H2 a proxy in the 2.5-cm line emission of CO isused. It can be converted to deduce the massfraction of molecular hydrogen.

Stars, dust and the Cosmic Microwave Background constitute the principal components of the Galactic
Radiation field.
As we observe at position Earth and the star and dust components differ at Galactic locations, a modelof the radiation field needs to be inferred. It should deduce the contributions from stellar distributions,from dust and connect observed properties at Earth via radiative transport.
Over decades, the radiation field provided with the GALPROP code provided the most credible model forradiation fields in our Galaxy.
The most decisive imprint of radiation fields will be made out via Inverse Compton scattering (IC). The
process is anisotropic and one needs to calculate IC contributions throughout the (3D)-Galaxy.
There is sufficient simplicity in such global models, and many parameters are weakly constrained. This isparticularly problematic for the Galactic Center region and the inner Galaxy.

Example for the frequency- andlocation dependence of the radiation
fields as of the model put forward with
the GALPROP code
[Strong Moskalenko Reimer 2000]

Loop I:

Haslam 408 MHz

Fermi E > 300 MeV↑?
↓Fermi diffuse model
WMAP polarized emission 23 GHz

There appears to exist arc-like excesses against the diffuse model:
Fainter than pion production and bremsstrahlung as calculated from HI tracer,
fainter than IC as templated in diffuse model.  The realm of diffuse templates!

Nearby molecular clouds: Orion (d ~ 400 pc)
LAT collaboration ´12
Mono R2

Orion B

Orion AE > 200 MeVHICOAlternatives?
E(B-V) ?

A more closer lookon the XCO :Xco: 1.63 × 1020 cm-2 K-1 km-1 s

1.35 - 2.34 × 1020 cm-2 K-1 km-1 s

Nearby molecular clouds: Orion (d ~ 400 pc)
Consequently, spectral extraction of relative emission components differs:
LAT collaboration ´12

Xco static

Xco variable

Xco partily compensatedby E(B-V)

 Nonlinear conversion between H2 and CO in diffuse molecular gas?
 Unseen part in velocity integrated CO intensity (aka WCO) ?

 consistent with LIS spectrum,comparable in clouds with 103 < M < 8 ×106 M⨀

LAT collaboration ´11 little arm/interarm contrast→ loose coupling with the kpc-scale surface density of gas or star formation
 shallow emissivity gradient in the outer Galaxy:
too shallow even for a large halo size !
? large amounts of missing gas / badly understood tracers ?
? non-uniform diffusion ?
? simplistic diffuse emission model ?

compare two targets:(i) gas at (10º < |b| < 70º)(= local within ~ 1 kpc)

[Casandjian 2015](ii) individual nearby clouds(within a few 100 pc)[Grenier ea 2015]

results: < 30% spatial/spectral variations

 no large effects from local injection/propagation effects

100 MeV – 10 GeV

LAT collaboration ´09→ standard CR interaction models adequate (which do justice to locallymeasured CR abundances, CR sec/prim ratios, long/lat distr.)→ Fermi/LAT errors are systematics dominatedsince then: quality of LAT data exceeds progressively realismof CR propagation model / diffuse emission templates!

→ “analysis model“ based on templated emission components (IC, ISO)+ a ring-emissivity model for HI and CO (for H2)+ an extinction E(B-V) template following the spirit of unseen “dark“ gas model grid of 0.125°
 interstellar radiation fields via GALPROP templates cube of 30 energy planes from 50 MeV to 600 GeV
 GALPROP-derived template for Inverse Compton
 dedicated templates for large-scale regions of excess emission← Loop I / NPS

Galactic Lobes→
Galactic Plane excess regions →Result: Fermi diffuse model became a point-source analysis model!
Aim to minimize residuals goes on the expense of consistent physics !
Almost impossible to interpret when interesting physics shows up !

→“propagation- model“ basedon CR propagation physics that fit CR
data, and allow predictions for γ-rayemissivities→ thus far, GALPROP in axial-symmetriccylindrical geometry commonly used
→ normalizations (scaling) introduced
here & there:

LAT collaboration ‘12

Olaf Reimer - LHCski2016 Obergurgl Apr 13, 201640

[Fermi-LAT Ajello ea 2016]

[Fermi-LAT Ajello ea 2016]

from simple slab and halo approximation
to full 3D propagation, matter & source distributions inspiral arms, realistic B-field models, stochastic
sources & energy losses on local scales (TeV!) improvements on math-numerical, geometry, & physics side
 still need to solve the beastly transport equation:

This is only an artists impression.

We have remarkably contrasting
views about the geometry of ourGalaxy!
As soon as we leave 2D axisymmetry,a full new parameter space isbeing introduced to CR transport!
[Werner ea. 2015][Johannesson ea. 2018]

3D gas distributions using kinematic distances[Johannesson ea. 2018]

Renaud ea 2013
Pfrommer ea 2017

We don’t know how our Milkyway
looks like, precisely!
 PICARD: axisymmetric,Steiman 4-arm,Dame 2-arm,
Cordes-Lazio NE2001
Pfrommer AREPOMHD
e.g. CRp distribution by PICARD in 4-arm model:
1 GeV
10 GeV
100 GeV1 TeV

γ-ray predictions by PICARD: total intensity @ 100 GeV
axisymmetric
4-arm

2-arm

γ-ray predictions by PICARD: Inverse Compton @100GeV

difference (residuals) between
axisymmetric and 4-arm model(still using identical set ofpropagation parameters)
 major differences in
3D model predictions!
[Kissmann ea 2013, 2014, 2015, 2017]an observation-driven Interstellar Radiation Field[Popescu ea 2017, Niederwanger ea 2019]
Galactic Centersolid: PICARD
dashed: GALPROP

Earthsolid: PICARD
dashed: GALPROP

up to 25% difference in IC

up to ∼10 % difference total

an observation-driven Interstellar Radiation Field[Popescu ea 2017, Niederwanger ea 2019]4-arm model axisymmetric

IC predictions from PICARD
10 GeV

1 TeV

100 TeV3D B-field geometry

an observation-driven source CR source modelspiral arms?

?

Carlson ea 2016an observation-driven source term for the transport equation

Step 1:
 Source distribution MC[Egberts ea 2018]
 Projected onto
3D-Galaxy modelan observation-driven source term for the transport equation

Step 2:
 H.E.S.S. catalog sources placed at l,b and according to observed Lγ
 categorized into suspected hadronic or leptonic emission dominance Galaxy filled to completeness limit according to log N-log S

Electrons (TeV-scale) in Galactic planean observation-driven source term for the transport equation

Step 2:
 H.E.S.S. catalog sources placed at l,b and according to observed Lγ
 categorized into suspected hadronic or leptonic emission dominance Galaxy filled to completeness limit according to log N-log S

Electrons(TeV-scale)ininGalactic
Galacticplane
planeCarbon (TeV-scale) preditctions for the VHE diffuse emission (e.g. neutral pion or IC @1.2 TeV) preditctions for the VHE diffuse emission: total diffuse @ 1.2 TeVThis is currently being compared to H.E.S.S. data analyzed withing thenewly developed Run-Wise simulation analysis framework [Holler ea 2018].

 stay tuned for measurement of the VHE diffuse emission throughout
the HGPS survey region later this year?

Galactic Diffuse Emission models do not predict GeV intensity correctly [f(l,b,E)]-> scaling of predictions vs. consistent set of propagation parameters?

Galactic Center Excess in GeV * source of DM annihilation or sub-threshold sources?

?

There is the GC bulge emission in TeV * stochastic particle injection at PeV or Dxx as function of galacticentric radius?

?

There is indication for non-uniform diffusion  Differencey among 2ndaries, Geminga *?

High-energy electron spectrum is subject of alternative interpretations *?

Alternative proxies for 3D CR source distribution models?

Contribution of unresolved (sub-threshold) sources in different Galactic source classes?

Fermi bubbles *?

Large-scale anisotropy in the CR flux * There is rich physics in the diffuse Galactic Gamma-ray emission.
 Understanding the Galactic foreground opens access to precision measuremnts inthe VHE, e.g. source morphology, extension, anisotropies …
 Propagation scales at TeV energies require 3D models and decapc grid resolution.
 Diffuse emission is guaranteed to exist at detectable level in the H.E.S.S. data
 PICARD predictions ready to be tested
 Analysis of IACT data usually supresses large-scale gradients(small camera FoV vs. background maker)
 RWS analysis framework in joint French-Austrian project developed
Expected CTA sensitivity will elevate diffuse emission problem and unresolved
source contribution to a new level (or menace)!
We started to prepare for it.
→ simplyfied PICARD predictions were part of CTA sky model for the data challenge→ diffuse emission measurement with H.E.S.S. well before CTA (PICARD & RWS)