Unresolved sources in the Galactic diffuse gamma-ray emission at TeV - - PowerPoint PPT Presentation

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Unresolved sources in the Galactic diffuse gamma-ray emission at TeV - - PowerPoint PPT Presentation

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Unresolved sources in the Galactic diffuse gamma-ray emission at TeV energies Kathrin Egberts, University of Potsdam ICRC 2017, July 19, Busan Galactic diffuse gamma-ray emission Diffuse Galactic gamma-rays can be used to probe cosmic-ray

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Kathrin Egberts, University of Potsdam ICRC 2017, July 19, Busan

Unresolved sources in the Galactic diffuse gamma-ray emission at TeV energies

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Kathrin Egberts . Unresolved sources in the Galactic diffuse gamma-ray emission at TeV energies . ICRC 2017

Galactic diffuse gamma-ray emission

■ Diffuse Galactic gamma-rays can be used to probe cosmic-ray propagation ■ At MeV-GeV energies, diffuse gamma-ray emission dominates the gamma-ray

sky

■ Complexity because of several competing mechanisms 


(bremsstrahlung, pion production and decay, inverse
 Compton scattering) and 
 insufficient knowledge of 
 boundary conditions

■ Increase lever arm to TeV


energies?

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NASA/DOE/Fermi LAT Collaboration

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Kathrin Egberts . Unresolved sources in the Galactic diffuse gamma-ray emission at TeV energies . ICRC 2017

Galactic diffuse gamma-ray emission

■ At TeV energies, gamma-ray sky

is source dominated

■ H.E.S.S. Galactic Plane Survey

(HGPS):

▪ 2800 hours of high-quality data,

taken in the years 2004 to 2013

▪ Longitude l = 250° to 65°, 


latitude |b| < 3.5°

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H.E.S.S. Source of the Month Jan 2016

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Kathrin Egberts . Unresolved sources in the Galactic diffuse gamma-ray emission at TeV energies . ICRC 2017

H.E.S.S. measurement of Galactic diffuse emission

■ Observation of large-scale diffuse

emission signal outside the source regions (cumulative by the use of projections)

■ Minimum contribution of hadronic

diffuse emission 17% (36% when considering heavier nuclei)

■ Additional possible contributions:

diffuse emission from inverse Compton scattering and unresolved gamma-ray sources

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significantly detected γ-ray sources

H.E.S.S. Collaboration, PRD 2014

full signal signal outside the detected source regions

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Kathrin Egberts . Unresolved sources in the Galactic diffuse gamma-ray emission at TeV energies . ICRC 2017

Determination of the unresolved sources in the HGPS

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Carrigan for the H.E.S.S. Collaboration ICRC 2013

■ Only small fraction of Galactic

gamma-ray sources is resolved (significantly detected) in the HGPS

■ Infer information about the

population from our knowledge

  • f detected sources

■ In the simplest case consider

  • nly gamma-ray flux (1D)
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Kathrin Egberts . Unresolved sources in the Galactic diffuse gamma-ray emission at TeV energies . ICRC 2017

Determination of the unresolved sources in the HGPS

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Renaud, Recontres de Moriond 2009

■ Representation of the flux

distribution → log N - log S

■ Completeness threshold: minimum

flux that can be significantly detected throughout the observed region (limited by instrument sensitivity and observation time)

■ Need for a prescription of how to

extrapolate to unresolved low fluxes

→ Simulation of 
 VHE source population

HGPS log N - log S from 2009, no exact match with the diffuse measurement

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Kathrin Egberts . Unresolved sources in the Galactic diffuse gamma-ray emission at TeV energies . ICRC 2017

Simulation of a very-high-energy source population in the Milky Way

■ Simplistic approach: characterise

gamma-ray sources by position and luminosity at 1 TeV

■ Assume the sources to follow the

matter density in Milky Way 
 → spiral arm structure (4-arm model based on CII COBE data) with a bar (Spitzer data)

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Steiman-Cameron, Astrophys. J. 2010

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Kathrin Egberts . Unresolved sources in the Galactic diffuse gamma-ray emission at TeV energies . ICRC 2017

Simulation of a very-high-energy source population in the Milky Way

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X [kpc]

  • 15
  • 10
  • 5

5 10 15 Y [kpc]

  • 15
  • 10
  • 5

5 10 15

Earth

X [kpc]

  • 15
  • 10
  • 5

5 10 15 Y [kpc]

  • 15
  • 10
  • 5

5 10 15

Earth

Simulation of 10,000 sources

■ Simplistic approach: characterise

gamma-ray sources by position and luminosity at 1 TeV

■ Assume the sources to follow the

matter density in Milky Way 
 → spiral arm structure (4-arm model based on CII COBE data) with a bar (Spitzer data)

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Kathrin Egberts . Unresolved sources in the Galactic diffuse gamma-ray emission at TeV energies . ICRC 2017

Simulation of a very-high-energy source population in the Milky Way

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X [kpc]

  • 15
  • 10
  • 5

5 10 15 Y [kpc]

  • 15
  • 10
  • 5

5 10 15

Earth

Simulation of 10,000 sources inside and outside the HGPS /: Simulation of 500 sources 
 inside/outside of the HGPS

■ Restrict the viewing angle to the

region where diffuse emission is

  • bserved in the HGPS: 

  • 75° < l < 60°, -1° < b < 1°

■ Reduce the number of sources to a

reasonable number (500?) to correctly reproduce statistical scatter

■ Determine the spread by simulating

many realisations

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Kathrin Egberts . Unresolved sources in the Galactic diffuse gamma-ray emission at TeV energies . ICRC 2017

log N - log S of the source simulations

■ log N - log S distribution of

realisations does not yet necessarily describe the existing data of know sources

■ Fit of the individual realisations to

the measured log N - log S in the range of completeness:

▪ fit in x and y: scaling of source

luminosity and total number of sources

▪ disregard distributions that fail to

describe the data

▪ use 68% containment in each flux bin

to determine scatter of simulations

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Log(Flux [arbitrary units])

  • 5
  • 4.5
  • 4
  • 3.5
  • 3
  • 2.5
  • 2
  • 1.5
  • 1
  • 0.5

Number of sources (F>Flux) 1 10

2

10

3

10

1,000 realisations with 500 sources each

Renaud, Recontres de Moriond 2009

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Kathrin Egberts . Unresolved sources in the Galactic diffuse gamma-ray emission at TeV energies . ICRC 2017

Fitting the simulated log N - log S to measured one

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Number of sources (F>Flux)

1 10

2

10

3

10

Flux (%Crab)

  • 3

10

  • 2

10

  • 1

10 1

Data Simulations

3 example realisations

■ Difference at low fluxes

corresponds to the amount of unresolved sources

■ Average of ~500 VHE

gamma-ray sources with a spread of 120-2500

■ Comparison with the

H.E.S.S. measured diffuse flux: median

  • vershoots the total

signal

■ According to lower bound, at least 2/3 of the signal are unresolved sources

HGPS data according to Renaud, Recontres de Moriond 2009 completeness threshold

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Kathrin Egberts . Unresolved sources in the Galactic diffuse gamma-ray emission at TeV energies . ICRC 2017

Summary & Conclusion

■ The TeV gamma-ray sky is dominated by sources. This is also true for the diffuse

emission along the Galactic plane

■ log N - log S distribution allows to determine the amount of unresolved sources

  • n the basis of the observed sources

■ At least 2/3 of the H.E.S.S. Galactic diffuse emission consists of unresolved

sources

■ Limitations most likely due to the limited amount of observational data,

improvement by increased sophistication likely a subdominant effect

■ The future:

▪ More accurate with data of the upcoming H.E.S.S. source catalog ▪ Detailed parameter studies to investigate the span of the simulations ▪ Inclusion of extension likely to improve the accuracy, additional consideration of 2D

information

▪ More sophisticated analyses of diffuse TeV gamma-ray emission are based on template

fitting rather than simple background subtraction. There, an unresolved sources term is needed in the physics modelling

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