The Fermi -LAT Sky – with focus on interstellar emission – Guðlaugur Jóhannesson gudlaugu@hi.is Dark Matter with Machine Learning, Trieste, April 8 2018 LAT collaboration, Troy Porter & Igor Moskalenko Gulli Johannesson HI & NORDITA The Fermi -LAT Sky
Talk overview This talk is not about DM. This talk is not about ML. The focus of the talk will be high-energy interstellar emission: An overview of the required “ingredients”. Specifjc issues that may possibly be solved with new analysis techniques. These issues impact searches for DM signal in the Fermi -LAT sky. Gulli Johannesson HI & NORDITA The Fermi -LAT Sky
Talk overview This talk is not about DM. This talk is not about ML. The focus of the talk will be high-energy interstellar emission: An overview of the required “ingredients”. Specifjc issues that may possibly be solved with new analysis techniques. These issues impact searches for DM signal in the Fermi -LAT sky. Gulli Johannesson HI & NORDITA The Fermi -LAT Sky
Talk overview This talk is not about DM. This talk is not about ML. The focus of the talk will be high-energy interstellar emission: An overview of the required “ingredients”. Specifjc issues that may possibly be solved with new analysis techniques. These issues impact searches for DM signal in the Fermi -LAT sky. Gulli Johannesson HI & NORDITA The Fermi -LAT Sky
Talk overview This talk is not about DM. This talk is not about ML. The focus of the talk will be high-energy interstellar emission: An overview of the required “ingredients”. Specifjc issues that may possibly be solved with new analysis techniques. These issues impact searches for DM signal in the Fermi -LAT sky. Gulli Johannesson HI & NORDITA The Fermi -LAT Sky
Talk overview This talk is not about DM. This talk is not about ML. The focus of the talk will be high-energy interstellar emission: An overview of the required “ingredients”. Specifjc issues that may possibly be solved with new analysis techniques. These issues impact searches for DM signal in the Fermi -LAT sky. Gulli Johannesson HI & NORDITA The Fermi -LAT Sky
Gulli Johannesson HI & NORDITA The Fermi -LAT Sky
8 years of LAT data above 1 GeV (P8 PSF3) A simple question How many point sources are there in this image? Gulli Johannesson HI & NORDITA The Fermi -LAT Sky
8 years of LAT data above 1 GeV (P8 PSF3) A simple question How many point sources are there in this image? Gulli Johannesson HI & NORDITA The Fermi -LAT Sky
The fourth Fermi -LAT source catalog (4FGL) https://fermi.gsfc.nasa.gov/ssc/data/access/lat/8yr_ catalog/ 5098 sources 75 spatially extended 3009 AGNs (2938 Blazars) 564 Other associations 1525 unassociated No DM sources There is no confjrmed DM source at the moment. Gulli Johannesson HI & NORDITA The Fermi -LAT Sky
The fourth Fermi -LAT source catalog (4FGL) https://fermi.gsfc.nasa.gov/ssc/data/access/lat/8yr_ catalog/ 5098 sources 75 spatially extended 3009 AGNs (2938 Blazars) 564 Other associations 1525 unassociated No DM sources There is no confjrmed DM source at the moment. Gulli Johannesson HI & NORDITA The Fermi -LAT Sky
Pulling the sky apart Usually modeled in terms of interstellar emission, sources, and isotropic background = + Gulli Johannesson HI & NORDITA The Fermi -LAT Sky
What is high-energy interstellar emission? Emission processes HI & NORDITA Gulli Johannesson “Only” need to know the distribution of CRs, the targets, and the interaction processes. Very simple and useful with radiation. Inverse Compton (IC) from interactions gas. Bremsstrahlung from interactions with CR nuclei: radiation). and the interstellar medium (gas and interactions between cosmic-rays (CRs) Interstellar emission arises from Typical defjnition Stars e e p Gas The Fermi -LAT Sky γ π 0 γ γ γ π 0 –decay from interactions with gas. CR electrons ( e + and e − ):
What is high-energy interstellar emission? Emission processes HI & NORDITA Gulli Johannesson “Only” need to know the distribution of CRs, the targets, and the interaction processes. Very simple and useful with radiation. Inverse Compton (IC) from interactions gas. Bremsstrahlung from interactions with CR nuclei: radiation). and the interstellar medium (gas and interactions between cosmic-rays (CRs) Interstellar emission arises from Typical defjnition Stars e e p Gas The Fermi -LAT Sky γ π 0 γ γ γ π 0 –decay from interactions with gas. CR electrons ( e + and e − ):
Interstellar Matter CO to constrain the distribution. HI & NORDITA Gulli Johannesson Metals (1.5%) He (28%) H (70%) Dust (1%) Gas (99%) Components by mass Use 21-cm line emission of H i and 2.6-cm line of Gas and Dust hydrogen. Assumed to follow the same distribution as Helium is really diffjcult to observe. The dust is more important in terms of dynamics. The gas provides most of the mass. Split into dust and gas phase with a gas-to-dust The Fermi -LAT Sky ratio of ∼ 100. Interstellar gas is mostly hydrogen ( ∼ 70% of mass) and helium ( ∼ 28% of mass).
The Interstellar Gas as hydrogen. HI & NORDITA Gulli Johannesson Moskalenko et al. 2002, ApJ 565 Radial distribution in and near the plane distribution of Hydrogen. provide important information on the interesting as targets for CRs, but it can Rest of the interstellar medium is not Helium assumed to have the same distribution Trivia height approximately 1 kpc. component with a large scale height. Scale Ionized (H ii ): The least signifjcant clumpy. Scale height approximately 100 pc. Molecular (H 2 ): The densest phase, very 200 pc. large fjlling factor. Scale height approximately Atomic (H i ): The most massive phase with a Hydrogen observed in three phases: The Fermi -LAT Sky
Atomic Hydrogen Hyperfjne splitting of the lowest energy state causes line emission at 21 cm that can be HI & NORDITA Gulli Johannesson Signifjcant issues in the plane where optical depth is larger, especially the inner Galaxy. Works very well at high latitudes where optical depth is small. This is the “easy” component! The Fermi -LAT Sky Not optically thin along the plane so we need to correct for optical depth Usually done using the approximation of a homogeneous line of sight used to estimate its column density. � T ( v ) � N H i ( v ) = − log T S ( v ) C 1 − T S ( v ) − T bg where v is the observed Doppler velocity, T S ( v ) is the spin temperature, T ( v ) is the brightness of the emission expressed as temperature, T bg ≈ 2 . 7 K, and C is a constant. Need to know T S ( v ) for all lines of sight but usually assume a single value for the entire sky.
Atomic Hydrogen Hyperfjne splitting of the lowest energy state causes line emission at 21 cm that can be HI & NORDITA Gulli Johannesson Signifjcant issues in the plane where optical depth is larger, especially the inner Galaxy. Works very well at high latitudes where optical depth is small. This is the “easy” component! The Fermi -LAT Sky Not optically thin along the plane so we need to correct for optical depth Usually done using the approximation of a homogeneous line of sight used to estimate its column density. � T ( v ) � N H i ( v ) = − log T S ( v ) C 1 − T S ( v ) − T bg where v is the observed Doppler velocity, T S ( v ) is the spin temperature, T ( v ) is the brightness of the emission expressed as temperature, T bg ≈ 2 . 7 K, and C is a constant. Need to know T S ( v ) for all lines of sight but usually assume a single value for the entire sky.
HI4PI survey (Ben Bekhti, N. et al. 2016, A&A 594) Gulli Johannesson HI & NORDITA The Fermi -LAT Sky
Efgects of T S The fjgure before used the optically thin signifjcant impact on the derived column density. The efgect is not uniform across the sky. Map below shows ratio of HI column using the optically thin assumption. Gulli Johannesson HI & NORDITA The Fermi -LAT Sky assumption with T S ≫ T B . Getting the value of T S correct can have a density assuming T S = 125 K over that
Measuring T S seen in both absorption and emission. Observed range from few 10 K up to thousands of K. Limited coverage and strong variations from LOS to LOS. LOS separated by less than half degree. Gulli Johannesson HI & NORDITA The Fermi -LAT Sky T S can be measured if the 21–cm line is Example distribution of T B and T S for two T S values from Strasser & Taylor 2004.
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