An accurate determination of the local dark matter (DM) density is crucial to interpreting data from direct detection and certain indirect detection experiments, as it is degenerate with the DM-nucleon interaction strength. Here I give an update to our ongoing project to make a determination of the local DM density. Our method uses the positions and velocities of a set of tracer stars extending upwards out of the Milky Way disc, to which we fit a baryon and DM mass model using Bayesian nested sampling. The framework we have set up holds the promise of allowing us to minimise the number of assumptions needed, and thus determine the local DM density accurately and with a full quantification of its uncertainty.
Mass/orbit modeling of spherical systems: from orbits of galaxies in clusters to the nature of dark matter using dwarf spheroidals
I will present a brief review of techniques to extract the radial profiles of total mass, from which one can infer the dark matter (DM) profile and of velocity anisotropy, related to orbital shapes and formation history of structures. This will allow me to compare the shapes of the anisotropy profiles of elliptical, S0 and spiral galaxies in clusters. I will show how accurate is our knowledge of the inner slope of the DM profile (related to the nature of DM) as inferred from mass/orbit modeling of DM-dominated dwarf spheroidal galaxies, and how much this accuracy can be improved with the advent of accurate proper motion data as is expected with the proposed Theia satellite.
Updates on Galactic dwarf galaxies frequentist J-factors
Dwarf spheroidal satellite galaxies (dSphs) of the Milky Way appear to
be the most dark matter (DM) dominated objects in the near Universe.
Their proximity coupled with their low astrophysical background make
them ideal targets for DM indirect detection, which can be achieved by
searching for DM decay or annihilation into, e.g., γ-rays. The latter
approach requires the calculation of the J-factor, which quantifies the
amount of DM along the line of observation. This quantity has been previously derived with Bayesian techniques, thereby subjecting the results
to the effects of priors. We present here the progresses in the development
of a fully frequentist approach, namely its validation on the simulations
generated by the Gaia Challenge team and the results on the kinematic
data from 20 dSphs. The former imply satisfactory statistical properties
of the method and provide a good indication of its reliability. The latter
are in most cases compatible with results previously obtained by other
groups; the largest deviations are observed in the faintest systems. We
find that the error bugdet is dominated by model systematic effects for
dSphs with more than 100 stars. We conclude with an overview of the
possible improvements, extensions and applications of our technique.
(oskar klein center, stockholm university)
Model-independent interpretation of the 2016 Fermi-LAT measurement of gamma-ray anisotropies
Over the summer Fermi LAT will publish a new measurement of the angular power spectrum of anisotropies in the diffuse gamma-ray emission. The new data indicate the detection of significant Poissonian angular power over the energy range between 0.5 and 500 GeV. Such an observable provides valuable insight on the nature of the Diffuse Gamma-Ray Background (DGRB), i.e. the radiation produced by gamma-ray emitters not bright enough to be resolved individually by Fermi-LAT. Normally, any measurement of the DGRB is explained by building a model of the contributing unresolved sources, e.g. parametrising their abundance, energy spectrum or redshift evolution. Instead, in this talk I will interpret the new anisotropy data in a model independent way. Without relying to any specific parametrization, I will derive the composition of the DGRB in terms of multiple classes of unresolved sources and I will determine how many classes are need to best fit the data.
The proton and helium anomalies in the light of the Myriad model
A hardening of the proton and helium fluxes is observed above a few hundreds of GeV/nuc. The actual distribution of the local sources of primary cosmic rays has been suggested as a potential solution to this puzzling behavior. Some authors even claim that a single source is responsible for the proton and helium anomalies. But how probable are such explanations ? To answer that question, I will discuss the Myriad model and the probabilistic nature of the predictions on primary cosmic ray fluxes. I will show that at any given energy, these fluxes are distributed according to a stable law well-known to financial analysts
Astrophysical and Dark Matter Models for the Galactic Center Gamma-Ray Excess
Fermi-LAT observations have discovered a gamma-ray excess emanating from the Galactic center of the Milky Way. While the existence of this excess is now certain, its origin is not. Three distinct classes of models have been posited to explain its key features: dark matter annihilation, a population of sub-threshold gamma-ray pulsars, and diffuse emission from the intense Galactic center environment. In this talk, I will describe the successes and failures of each model as an explanation for the Galactic center excess — outlining several significant advances in modeling pulsars and diffuse emission properties near the Galactic center. Finally, I will describe smoking gun tests of each scenario which may rule out, or lend credence to each model within the next five years.
Understanding uncertainties in modeling the Galactic diffuse gamma-ray emission
The nature of the Galactic diffuse gamma-ray emission as measured by the Fermi Gamma-ray Space Telescope has remained an active area of research for the last several years. In particular, the discovery of a GeV excess towards the Galactic center has generated enormous interest in trying to understand its origins, whether astrophysical or more exotic. While most analyses of the GeV excess confirm its existence, its morphology is not well-constrained, which limits our ability to understand the origin of this excess.
We therefore introduce a new template-fitting approach to study the various components of the Galactic diffuse gamma-ray emission, and their correlations and uncertainties. One application will be to characterize the morphology and the spectrum of the excesses in the inner Galaxy. Rather than starting from fixed predictions from cosmic-ray propagation codes and examining the residuals to understand the quality of fits and the presence of excesses, we introduce additional fine-grained variations in the templates that account for uncertainties in gas tracers and the small scale variations in the density of cosmic rays. This approach results in ~100,000 free parameters for analysis of the Galactic disk, which we fit with an algorithm borrowed from positron emission tomography imaging.
I will present first results from applying this template-fitting approach to the Galactic diffuse gamma-ray emission, including a characterization of the GeV excess in the Galactic center as well as other distinct excesses along the disk.
Population synthesis of Fermi LAT sources: A Bayesian analysis using posterior predictive distributions
The Fermi-LAT has provided an unprecedented view of the gamma-ray sky and in particular has found a host of previously unknown point sources, i.e. the 3FGL. Of the 3033 objects in the 3FGL, 1010 remain unassociated to a particular source class. Additionally, the origin of the GeV-scale excess emission towards the galactic centre remains unknown. Recent statistical studies have shown that the emission could be due to a population of unresolved point sources, in particular millisecond pulsars (MSPs). The ability of MSPs to make up this excess whilst remaining below the detection threshold of Fermi is highly dependent on their gamma-ray luminosity function. We present a statistically rigorous method of analysing the entire 3FGL data set to provide constraints on spatial distributions, luminosity functions, and spectral shapes whilst also providing posteriors for the association of a source to different classes of objects. We do this by combining the power of an unbinned likelihood analysis and generation of the posterior predictive distribution in a Bayesian framework. In this talk I will present our method and discuss its results in the context of the gamma-ray luminosity function of MSPs.
The Primary Importance of Secondaries: Gamma-Ray Detectability of MeV Dark Matter
The past two decades have seen a rapid development of $\gamma$-ray astronomy, in particular at energies above a few hundred MeV where Fermi-LAT has revolutionised the field. As a result, extensive studies have been undertaken to characterise gamma-ray annihilation spectra of dark matter with masses above ~1 GeV. However, due to the lacking sensitivity of current experiments at lower energies, the so-called MeV gap, MeV dark matter has been much less studied. At these mass scales the main annihilation channels are to either neutrinos, electrons, pions or directly to photons. The electron channel has been extensively studied in the context of the 511 keV line. In this work, we study the general prospects for detecting MeV dark matter annihilating predominantly to electrons and positrons. We emphasise the importance of the often overlooked bremsstrahlung and in-flight annihilation spectral features, which in many cases provide the dominant $\gamma$-ray signal in this regime.
Self-consistent calculation of Earth-Shadowing effects in Dark Matter direct detection
Direct detection experiments aim to detect the interaction of Galactic Dark Matter (DM) with terrestrial nuclei. But many of these DM particles will pass through the Earth before reaching the detector. During this transit, they may interact and scatter, altering their distribution at the Earth's surface. I will sketch the first fully self-consistent calculation of this ‘Earth-Shadowing’ effect, taking into account DM particle deflection and assuming the most general DM-nucleus interactions. Remarkably, in some scenarios, Earth-Shadowing can actually increase the DM flux, while in others the flux is depleted. I will explore the impact this has on current constraints on light DM and on strongly interacting DM, as well as hinting at some interesting modulation and directional signatures.
This talk will discuss results from "Supernova neutrino physics with xenon dark matter detectors: A timely perspective", arXiv:1606.09243. Work in collaboration with Rafael Lang, Shayne Reichard, Marco Selvi and Irene Tamborra.
High energy neutrinos from cosmic ray interactions in the sun
When cosmic rays hit the sun high energy neutrinos, along with many other particles, are produced as a result of a chain of interactions and decays in a process similar to how particles are produced in the Earth's atmosphere. The high energy neutrinos produced in such a fashion will propagate and oscillate from the Sun to the Earth and can in principle be detected by modern neutrino detectors such as IceCube. It is important to estimate this flux as it would act as a background for the hypothesized neutrinos coming from WIMP annihilations in the solar interior. In addition a detection can provide additional astrophysical information about neutrino oscillation parameters. We perform an updated calculation of the neutrino flux from cosmic ray interactions in the sun, including full cascade evolution in the solar atmosphere in a modern fashion and neutrino propagation and oscillation from the Sun to the Earth.