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

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 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? NASA/DOE/Fermi LAT Collaboration Kathrin Egberts . Unresolved sources in the Galactic diffuse gamma-ray emission at TeV energies . ICRC 2017 2

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° H.E.S.S. Source of the Month Jan 2016 Kathrin Egberts . Unresolved sources in the Galactic diffuse gamma-ray emission at TeV energies . ICRC 2017 3

H.E.S.S. measurement of Galactic diffuse emission significantly detected γ -ray sources full signal H.E.S.S. Collaboration, PRD 2014 ■ 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 signal outside the detected source regions considering heavier nuclei) ■ Additional possible contributions: diffuse emission from inverse Compton scattering and unresolved gamma-ray sources Kathrin Egberts . Unresolved sources in the Galactic diffuse gamma-ray emission at TeV energies . ICRC 2017 4

Determination of the unresolved sources in the HGPS ■ Only small fraction of Galactic gamma-ray sources is resolved (significantly detected) in the HGPS ■ Infer information about the population from our knowledge of detected sources ■ In the simplest case consider only gamma-ray flux (1D) Carrigan for the H.E.S.S. Collaboration ICRC 2013 Kathrin Egberts . Unresolved sources in the Galactic diffuse gamma-ray emission at TeV energies . ICRC 2017 5

Determination of the unresolved sources in the HGPS HGPS log N - log S from 2009, no exact ■ Representation of the flux match with the diffuse measurement 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 Renaud, Recontres de Moriond 2009 Kathrin Egberts . Unresolved sources in the Galactic diffuse gamma-ray emission at TeV energies . ICRC 2017 6

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) Steiman-Cameron, Astrophys. J. 2010 Kathrin Egberts . Unresolved sources in the Galactic diffuse gamma-ray emission at TeV energies . ICRC 2017 7

Simulation of a very-high-energy source population in the Milky Way Simulation of 10,000 sources 15 15 ■ Simplistic approach: characterise Y [kpc] Y [kpc] gamma-ray sources by position and Earth Earth luminosity at 1 TeV 10 10 ■ Assume the sources to follow the 5 5 matter density in Milky Way 
 → spiral arm structure (4-arm model based on CII COBE data) with a bar 0 0 (Spitzer data) -5 -5 -10 -10 -15 -15 -15 -15 -10 -10 -5 -5 0 0 5 5 10 10 15 15 X [kpc] X [kpc] Kathrin Egberts . Unresolved sources in the Galactic diffuse gamma-ray emission at TeV energies . ICRC 2017 8

Simulation of a very-high-energy source population in the Milky Way Simulation of 10,000 sources inside and outside the HGPS � / � : Simulation of 500 sources 
 inside/outside of the HGPS 15 ■ Restrict the viewing angle to the Y [kpc] region where diffuse emission is Earth observed in the HGPS: 
 10 -75° < l < 60°, -1° < b < 1° ■ Reduce the number of sources to a 5 reasonable number (500?) to correctly reproduce statistical scatter 0 ■ Determine the spread by simulating many realisations -5 -10 -15 -15 -10 -5 0 5 10 15 X [kpc] Kathrin Egberts . Unresolved sources in the Galactic diffuse gamma-ray emission at TeV energies . ICRC 2017 9

log N - log S of the source simulations 1,000 realisations with 500 sources each ■ log N - log S distribution of realisations does not yet necessarily Number of sources (F>Flux) 3 10 describe the existing data of know sources ■ Fit of the individual realisations to 2 10 the measured log N - log S in the range of completeness : 10 ▪ fit in x and y: scaling of source luminosity and total number of sources 1 ▪ disregard distributions that fail to -5 -4.5 -4 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 Log(Flux [arbitrary units]) describe the data ▪ use 68% containment in each flux bin to determine scatter of simulations Renaud, Recontres de Moriond 2009 Kathrin Egberts . Unresolved sources in the Galactic diffuse gamma-ray emission at TeV energies . ICRC 2017 10

Fitting the simulated log N - log S to measured one 3 example realisations ■ Difference at low fluxes Number of sources (F>Flux) Data corresponds to the Simulations amount of unresolved 3 sources 10 ■ Average of ~500 VHE gamma-ray sources 2 10 with a spread of 120-2500 HGPS data according to Renaud, Recontres de Moriond 2009 ■ Comparison with the 10 H.E.S.S. measured diffuse flux: median completeness threshold overshoots the total 1 signal -3 -2 -1 10 10 10 1 Flux (%Crab) ■ According to lower bound, at least 2/3 of the signal are unresolved sources Kathrin Egberts . Unresolved sources in the Galactic diffuse gamma-ray emission at TeV energies . ICRC 2017 11

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 on 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 Kathrin Egberts . Unresolved sources in the Galactic diffuse gamma-ray emission at TeV energies . ICRC 2017 12