The extragalactic gamma-ray background
Markus Ackermann
2nd ASTROGAM Workshop
26.03.2015
The gamma-ray sky at MeV and GeV
COMPTEL 1-30 MeV
Fermi LAT E > 1 GeV
Markus Ackermann | ASTROGAM workshop | 26/03/2015 | Page 2
The gamma-ray sky at MeV and GeV
COMPTEL 1-30 MeV
Fermi LAT E > 1 GeV
Resolved sources
Markus Ackermann | ASTROGAM workshop | 26/03/2015 | Page 3
The gamma-ray sky at MeV and GeV
COMPTEL 1-30 MeV
Galactic diffuse emission
(CR interactions with the interstellar medium)
Inverse Compton
π0-decay
Bremsstrahlung
Fermi LAT E > 1 GeV
Resolved sources
Markus Ackermann | ASTROGAM workshop | 26/03/2015 | Page 3
The gamma-ray sky at MeV and GeV
COMPTEL 1-30 MeV
Galactic diffuse emission
(CR interactions with the interstellar medium)
Inverse Compton
π0-decay
Bremsstrahlung
Fermi LAT E > 1 GeV
Resolved sources
Isotropic diffuse
emission (IGRB)
Markus Ackermann | ASTROGAM workshop | 26/03/2015 | Page 3
7, Inoue&Totani09)
Blazars
ms
Diffuse processes
Intergalactic
shocks
Emission due
to
Emission from star
forming galaxies (e.g.
▪ Widely varying predictions of
the annihilation of
▪ DominantEmission
class offrom
LAT Pavlidou&Fields02)
extra-
EGB contribution ranging from
Cosmological Dark
1% to 100%.
Matter (eg. Jungman+96)
particle accelerated
galactic sources.
EGB
a-ray Background mayRadio
encryptgalaxies
the signature of the
(Stecker&Salomon96,
Mücke&Pohl00,
Narumoto&Totani04,Dermer0
7, Inoue&Totani09)
in astrophysics
▪ 27 sources listed in 2FGL.
Markevitch+05
Dark matter annihilation
4
ark
D
%
23 tter
Ma
ms
contribute
sBlazars
this
important ?
20-100% of the
73% Dark Energy
▪ Potential signal dependent on
A to
in Intergalactic
shocks (Loeb&Waxmann00)
nature of DM, cross-section and
structure of DM distribution.
Emission due to
the annihilation of
Emission from
Cosmological Dark
particle accelerated Pavlidou&Fields02)
Star-forming galaxies Matter (eg. Jungman+96)
in Intergalactic
outside the
shocks (Loeb&Waxmann00) ▪ Several galaxies
Markevitch+05
local group resolved by LAT.
Emission from star
forming galaxies (e.g.
73% Dark Energy
4%
GRBs +
pulsars
A to
ms
rk
a
%D r
3
2 tte
High-latitude
Ma
Emission
due to
▪ Small
contributions
expected.
the annihilation of
Cosmological Dark
Matter (eg. Jungman+96)
rk
a
%D r
3
2 tte
Ma
A to
Undetected sources
4%
EGB (Stecker&Salomon96,
The Extragalactic Gamma-ray Background
may encrypt the signature of the
Mücke&Pohl00,
most powerful processes in astrophysics
Narumoto&Totani04,Dermer0
4%
!
The origin of the isotropic
background
at
MeV
and
73%above
Dark Energy
Blazars
contribute
Why is this
important ?
20-100% of the
Interactions of UHE cosmic
rays with the EBL
▪ Strongly dependent on evolution
4
of UHECR sources.
▪ 1% - 100% of EGB emission.
Isotropic Galactic
contributions
▪ Contributions from an extremely
large Galactic electron halo.
▪ CR interaction in small solar
system bodies.
Markus Ackermann | ASTROGAM workshop | 26/03/2015 | Page 4
The isotropic and the total extragalactic background
Intensity that can be resolved into
sources depends on:
Resolved sources
▪
the sensitivity of the instrument.
▪
the exposure of the observation.
➞ The isotropic γ-ray background
depends on the sensitivity to
identify sources.
Isotropic γ-ray background (IGRB)
+
➞ Important as an upper limit on
diffuse processes.
➞ The total extragalactic γ-ray
background is instrument and
observation independent.
➞ Useful for comparisons with
source
population
models.
Total extragalactic γ-ray background (EGB)
Markus Ackermann | ASTROGAM workshop | 26/03/2015 | Page 5
Fermi LAT analysis of the isotropic gamma-ray background
Masked regions:
> Galactic plane
> Regions with dense molecular clouds
> Regions with non-local atomic hydrogen clouds
=
Interstellar gas
Solar disk and IC
+
Isotropic emission
Inverse Compton (IC)
Isotropic
γ-ray
background
(IGRB)
+
Loop I / Local Loop
Resolved sources (2FGL)
+
Galactic diffuse emission
Contamination from
CR induced
background
Markus Ackermann | ASTROGAM workshop | 26/03/2015 | Page 6
Cosmic-ray induced background in the LAT orbit
> CR intensity up to 106 times higher than EGB in the LAT orbit.
> Two energy regimes:
>
Primary CR dominate at high energies.
>
Secondaries from CR interactions in the atmosphere dominate at low energies.
Low-energy analysis
High-energy analysis
Ackermann et al., ApJS, 2012
!Ackermann!et!al.!2012,!ApJS,!203,!4!
Markus Ackermann | ASTROGAM workshop | 26/03/2015 | Page 7
High-purity event classification for LAT data
Low-energy analysis
High-energy analysis
> Publicly available LAT event
classes (P7ULTRACLEAN)
have insufficient background
rejection for EGB study.
> New high-purity event
classes developed for EGB
analysis.
> P7REP_IGRB_LO
▪
Optimized to reject secondary
CR background at low energies
> P7REP_IGRB_HI
▪
Optimized to reject primary CR
background while retaining high
statistics
Ackermann et al., ApJ, 2015
Markus Ackermann | ASTROGAM workshop | 26/03/2015 | Page 8
Cosmic-ray contamination in high-purity event class
> Effective contamination from mis-classified CRs in comparison to IGRB intensity
Ackermann et al., ApJ, 2015
IGRB
Positrons annihilating
in the micro-meteorite
shield of the LAT
Residual
contamination
from cosmic
rays.
Markus Ackermann | ASTROGAM workshop | 26/03/2015 | Page 9
IGRB spectrum between 100 MeV and 820 GeV
> IGRB spectrum can be parametrized by single power-law + exponential cutoff.
> Spectral index ~ 2.3 , cutoff energy ~ 250 GeV.
Ackermann et al., ApJ, 2015
IGRB
Markus Ackermann
| ASTROGAM workshop | 26/03/2015 | Page 10
> It is not compatible with a simple power-law
(χ2 > 85).
IGRB spectrum systematics
Low-energy analysis
High-energy analysis
> Error bars:
statistical error
different models
for Galactic
foreground.
+
syst. error from effective
area parametrization
+
syst. error from CR
background subtraction
> Yellow band:
systematic uncertainties
from foreground model
variations.
Ackermann et al., ApJ, 2015
> Modeling of the Galactic diffuse foreground dominates systematic
uncertainties of the measurements
> Measurements of Galactic diffuse emission in 1 MeV to 100 MeV range would
help to understand foreground better
Markus Ackermann | ASTROGAM workshop | 26/03/2015 | Page 11
Comparison of IGRB and EGB measurements above 100 MeV
Ackermann et al., ApJ, 2015
> Total EGB = isotropic gamma-ray background + intensity of detected sources
Markus Ackermann | ASTROGAM workshop | 26/03/2015 | Page 12
Comparison to other measurements
Ackermann et al., ApJ, 2015
> Cosmic x-ray and gamma-ray background now measured over 9 orders of
magnitude in energy.
> Largest uncertainties in the 1 MeV - 100 MeV range.
Markus Ackermann | ASTROGAM workshop | 26/03/2015 | Page 13
Source population contributions to the EGB
Ajello et al., ApJL, 2015
> Observed extragalactic LAT source populations can account for the EGB intensity.
> Significant uncertainties in modeling contributions.
Markus Ackermann | ASTROGAM workshop | 26/03/2015 | Page 14
Contributions of star-forming galaxies
Ackermann et al., ApJ 755, 164, 2012
> Different population models predict different contributions to the EGB.
> Measurement of spectral features in the 10 MeV - 200 MeV range would help to
constrain these population models.
Markus Ackermann | ASTROGAM workshop | 26/03/2015 | Page 15
ASTROGAM and the EGB measurement
Ackermann et al., ApJ, 2015
ASTROGAM
Fermi LAT
PASS8
analysis
Markus Ackermann | ASTROGAM workshop | 26/03/2015 | Page 16
EGB measurement and angular resolution
Improved angular resolution will
help to:
COMPTEL 1-30 MeV
> Identify extragalactic sources
> Calculate the contribution of
the corresponding populations
to the MeV EGB
?
ASTROGAM
Markus Ackermann | ASTROGAM workshop | 26/03/2015 | Page 17
Summary
> Cosmic x-ray and gamma-ray backgrounds have
been measured over 9 orders of magnitude in energy.
> Fermi LAT data enabled the most accurate
Fermi LAT
measurement between 100 MeV and 820 GeV.
> Detected extragalactic sources allow an estimate of
the contributions of the corresponding populations to
the EGB.
> ASTROGAM would deliver a significant improvement
of the EGB measurement precision between 0.3 MeV
and 100 MeV.
> Potential spectral features would give important clues
on the origin of the EGB.
> Good angular resolution will allow to detect more MeV
ASTROGAM
sources and study the corresponding populations.
Markus Ackermann | ASTROGAM workshop | 26/03/2015 | Page 18