Recent claims of a gamma-ray excess in the diffuse galactic emission detected by the Fermi Large Area Telescope with a morphology similar to the WMAP haze were based on the assumption that spatial templates of the interstellar medium (ISM) column density and the 408 Mhz sky are good proxies for neutral pion and inverse Compton (IC) gamma-ray emission, respectively. We identify significant systematic effects in this procedure that can artificially induce an additional diffuse component with a morphology strikingly similar to the claimed gamma-ray haze. To quantitatively illustrate this point we calculate sky-maps of the ratio of the gamma-ray emission from neutral pions to the ISM column density, and of IC to synchrotron emission, using detailed galactic cosmic-ray models and simulations. In the region above and below the galactic center, the ISM template underestimates the gamma-ray emission due to neutral pion decay by approximately 20%. Additionally, the synchrotron template tends to under-estimate the IC emission at low energies (few GeV) and to over-estimate it at higher energies (tens of GeV) by potentially large factors that depend crucially on the assumed magnetic field structure of the Galaxy. The size of the systematic effects we find are comparable to the size of the claimed "Fermi haze" signal. We thus conclude that a detailed model for the galactic diffuse emission is necessary in order to conclusively assess the presence of a gamma-ray excess possibly associated to the WMAP haze morphology.
Deep Dive into Systematic Effects in Extracting a "Gamma-Ray Haze" from Spatial Templates.
Recent claims of a gamma-ray excess in the diffuse galactic emission detected by the Fermi Large Area Telescope with a morphology similar to the WMAP haze were based on the assumption that spatial templates of the interstellar medium (ISM) column density and the 408 Mhz sky are good proxies for neutral pion and inverse Compton (IC) gamma-ray emission, respectively. We identify significant systematic effects in this procedure that can artificially induce an additional diffuse component with a morphology strikingly similar to the claimed gamma-ray haze. To quantitatively illustrate this point we calculate sky-maps of the ratio of the gamma-ray emission from neutral pions to the ISM column density, and of IC to synchrotron emission, using detailed galactic cosmic-ray models and simulations. In the region above and below the galactic center, the ISM template underestimates the gamma-ray emission due to neutral pion decay by approximately 20%. Additionally, the synchrotron template tends to
One of the most exciting yet observationally challenging scientific objectives of the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope (Atwood et al. 2009), is the indirect detection of particle dark matter (Baltz et al. 2008). However, limited gamma-ray statistics make diffuse signals arising from the pair-annihilation of dark matter difficult to differentiate from astrophysical processes. The limitation of using a diffuse signal to search for non-standard emission stems from difficulties in controlling the instrumental background and formulating a rigorous model for the astrophysical diffuse foregrounds.
An intriguing excess of microwave radiation in the WMAP data has been uncovered by Finkbeiner (2004) and Dobler & Finkbeiner (2008).
The morphology and spectrum of the WMAP haze indicates a hard electron-positron injection spectrum spherically distributed around the galactic center. While the origin of this haze need not be related to new particle physics, the possibility that the WMAP haze corresponds to synchrotron radiation of stable leptons produced by dark matter has been explored in several studies (see e.g. Hooper et al. 2007). A potentially conclusive way to determine whether the WMAP haze originates from a population of energetic leptons is to observe gamma-rays produced by inverse Compton up-scattering (IC) of photons in the interstellar galactic radiation field (ISRF).
Recently, Dobler et al. (2009) (hereafter D09) examined the LAT gamma-ray sky and reported an excess emission morphologically similar to the WMAP haze. D09’s observations suggest a confirmation of the haze hypothesis: that excess microwave emission stems from relativistic electron synchrotron with a spherical source distribution and a hard injection spectrum. In the “Type 2” and “Type 3” fits of D09, the excess was claimed over a best-fit background using spatial templates which employed the gas map of Schlegel et al. (1998) (SFD) to trace gamma-ray emission from π 0 decay, and the 408 Mhz Haslam synchrotron map (Haslam et al. 1982) to trace IC emission from galactic cosmic ray electrons. The spatial templates (plus an isotropic component obtained by mean-subtracting the residual skymap) were used to fit the observed gamma-ray sky in energy bins spanning 2-100 GeV. This analysis uncovered a residual gamma-ray emission above and below the galactic center with a morphology and spectrum similar to that found in the WMAP dataset (Finkbeiner 2004).
In this Letter, we test the following assumptions used in D09 for the removal of astrophysical foregrounds at gamma-ray energies:
(1) that line of sight ISM maps are adequate tracers for the morphology of π 0 emission, and
(2) that the 408 Mhz synchrotron map (Haslam et al. 1982) is an adequate tracer for the morphology of the galactic IC emission.
Assumption (1) entails neglecting the morphology of galactic cosmic-ray sources, since the observed π 0 emission results from the line-of-sight integral of the gas density (“target”) times the cosmic-ray density (“beam”). Assumption (2) neglects the difference between the morphology of the ISRF and the galactic magnetic fields.
On theoretical grounds, we expect that any detailed galactic cosmic-ray model would predict systematic deviations from the templates used in D09. Utilizing the galactic cosmic-ray propagation code Galprop, we find that the procedure based on spatial templates creates deviations comparable to the amplitude of the D09 residual. Furthermore, we find that these deviations are morphologically similar to the Fermi haze. We thus conclude that the determination of an excess gamma-ray diffuse emission cannot reliably be assessed from the spatial template proxies used in the “Type 2” and “Type 3” fits of D09. We stress that our results do not claim that there is no “haze” in the Fermi data. In particular, the systematic effects we study here are not relavent to explain the puzzling excess emission in the “Type 1” fit of D09, which employes Fermi-LAT data in the 1-2 GeV range as a proxy for the morphology of the π 0 component. We comment on this “Type 1” approach in Section 5.
Employing the cosmic ray propagation code Galprop (v 50.1p) (Strong & Moskalenko 1998;Strong et al. 2009), we compute the line-of-sight emission for galactic synchrotron, IC and π 0 decay predicted by a Galprop model that is consistent with all cosmic ray and photon observations (see Strong et al. 2009, for further detail). Except where noted, we employ standard parameters given by the GALDEF file 599278 1 throughout this work.
A large uncertainty in the propagation of cosmic rays relates to the intensity and orientation of galactic magnetic fields (Broadbent et al. 1990;Heiles 1996;Vallee 1996) as the intensity of synchrotron radiation varies with the square of the local magnetic field intensity. In our default simulation we assume a magnetic field of random orientation and an intensity that exponentially decays in both r and z
…(Full text truncated)…
This content is AI-processed based on ArXiv data.
