Ilias Cholis

Background model systematics for the Fermi GeV excess

The possible gamma-ray excess in the inner Galaxy and the Galactic center (GC) suggested by Fermi-LAT observations has triggered a large number of studies. It has been interpreted as a variety of dierent phenomena such as a signal from WIMP dark matter annihilation, gamma-ray emission from a population of millisecond pulsars, or emission from cosmic rays injected in a sequence of burst-like events or continuously at the GC. We present the rst comprehensive study of model systematics coming from the Galactic diuse emission in the inner part of our Galaxy and their impact on the inferred properties of the excess emission at Galactic latitudes 2 < jbj < 20 and 300 MeV to 500 GeV. We study both theoretical and empirical model systematics, which we deduce from a large range of Galactic diuse emission models and a principal component analysis of residuals in numerous test regions along the Galactic plane. We show that the hypothesis of an extended spherical excess emission with a uniform energy spectrum is compatible with the Fermi-LAT data in our region of interest at 95% CL. Assuming that this excess is the extended counterpart of the one seen in the inner few degrees of the Galaxy, we derive a lower limit of 10:0 (95% CL) on its extension away from the GC. We show that, in light of the large correlated uncertainties that aect

The Fermi Haze: A Gamma-Ray Counterpart to the Microwave Haze

The Fermi Gamma-ray Space Telescope reveals a diffuse inverse Compton (IC) signal in the inner Galaxy with a similar spatial morphology to the microwave haze observed by WMAP, supporting the synchrotron interpretation of the microwave signal. Using spatial templates, we regress out π0 gammas, as well as IC and bremsstrahlung components associated with known soft-synchrotron counterparts. We find a significant gamma-ray excess toward the Galactic center with a spectrum that is significantly harder than other sky components and is most consistent with IC from a hard population of electrons. The morphology and spectrum are consistent with it being the IC counterpart to the electrons which generate the microwave haze seen at WMAP frequencies. In addition, the implied electron spectrum is hard; electrons accelerated in supernova shocks in the disk which then diffuse a few kpc to the haze region would have a softer spectrum. We describe the full-sky Fermi maps used in this analysis and make them available for download.

Did LIGO detect dark matter

We consider the possibility that the black-hole (BH) binary detected by LIGO may be a signature of dark matter. Interestingly enough, there remains a window for masses 20M_{⊙}≲M_{bh}≲100M_{⊙} where primordial black holes (PBHs) may constitute the dark matter. If two BHs in a galactic halo pass sufficiently close, they radiate enough energy in gravitational waves to become gravitationally bound. The bound BHs will rapidly spiral inward due to the emission of gravitational radiation and ultimately will merge. Uncertainties in the rate for such events arise from our imprecise knowledge of the phase-space structure of galactic halos on the smallest scales. Still, reasonable estimates span a range that overlaps the 2-53  Gpc^{-3} yr^{-1} rate estimated from GW150914, thus raising the possibility that LIGO has detected PBH dark matter. PBH mergers are likely to be distributed spatially more like dark matter than luminous matter and have neither optical nor neutrino counterparts. They may be distinguished from mergers of BHs from more traditional astrophysical sources through the observed mass spectrum, their high ellipticities, or their stochastic gravitational wave background. Next-generation experiments will be invaluable in performing these tests.

A tale of tails: Dark matter interpretations of the Fermi GeV excess in light of background model systematics

Several groups have identified an extended excess of gamma rays over the modeled foreground and background emissions towards the Galactic center (GC) based on observations with the Fermi Large Area Telescope. This excess emission is compatible in morphology and spectrum with a telltale sign from dark matter (DM) annihilation. Here, we present a critical reassessment of DM interpretations of the GC signal in light of the foreground and background uncertainties that some of us recently outlaid in Calore et al. (2014). We find that a much larger number of DM models fits the gamma-ray data than previously noted. In particular: (1) In the case of DM annihilation into (b) over barb, we find that even large DM masses up to m(chi) similar or equal to 74 GeV are allowed at p-value > 0.05. (2) Surprisingly, annihilation into nonrelativistic hh gives a good fit to the data. (3) The inverse Compton emission from mu(+)mu(-) with m(chi) similar to 60-70 GeV can also account for the excess at higher latitudes, vertical bar b vertical bar > 2 degrees, both in its spectrum and morphology. We also present novel constraints on a large number of mixed annihilation channels, including cascade annihilation involving hidden sector mediators. Finally, we show that the current limits from dwarf spheroidal observations are not in tension with a DM interpretation when uncertainties on the DM halo profile are accounted for.

The PAMELA positron excess from annihilations into a light boson

Recently published results from the PAMELA experiment have shown conclusive evidence for an excess of positrons at high ( ~ 10–100 GeV) energies, confirming earlier indications from HEAT and AMS-01. Such a signal is generally expected from dark matter annihilations. However, the hard positron spectrum and large amplitude are difficult to achieve in most conventional WIMP models. The absence of any associated excess in anti-protons is highly constraining on models with hadronic annihilation modes. We revisit an earlier proposal, wherein the dark matter annihilates into a new light (GeV) boson , which is kinematically constrained to go to hard leptonic states, without anti-protons or π0s. We find this provides a very good fit to the data. The light boson naturally provides a mechanism by which large cross sections can be achieved through the Sommerfeld enhancement, as was recently proposed. Depending on the mass of the WIMP, the rise may continue above 300 GeV, the extent of PAMELAs ability to discriminate between electrons and positrons.

New Limits on Dark Matter Annihilation from Alpha Magnetic Spectrometer Cosmic Ray Positron Data

The Alpha Magnetic Spectrometer experiment onboard the International Space Station has recently provided cosmic ray electron and positron data with unprecedented precision in the range from 0.5 to 350 GeV. The observed rise in the positron fraction at energies above 10 GeV remains unexplained, with proposed solutions ranging from local pulsars to TeV-scale dark matter. Here, we make use of this high quality data to place stringent limits on dark matter with masses below ~300 GeV, annihilating or decaying to leptonic final states, essentially independent of the origin of this rise. We significantly improve on existing constraints, in some cases by up to 2 orders of magnitude.

Antiprotons from dark matter annihilation in the Galaxy: astrophysical uncertainties

The latest years have seen steady progresses in WIMP dark matter (DM) searches, with hints of possible signals suggested both in direct and indirect detection. Antiprotons play a key role in this context, since WIMP annihilations can be a copious source of antiprotons, and the antiproton flux from conventional astrophysical sources is predicted with fair accuracy and matches the measured cosmic ray (CR) spectrum very well. Using the publicly available numerical DRAGON code, we reconsider antiprotons as a tool to set constraints on DM models; we compare against the most upto-date ¯ p measurements, taking also into account the latest spectral information on the p and He CR fluxes. In particular, we probe carefully the uncertainties associated to both standard astrophysical and DM originated antiprotons, by using a variety of distinctively different assumptions for the propagation of CRs and for the DM distribution in the Galaxy. We find that the impact of the astrophysical uncertainties on constraining the DM properties of a wide class of annihilating DM models can be much stronger, up to a factor of ∼ 50, than the one due to uncertainties on the DM distribution (∼ 2−6). Remarkably, even reducing the uncertainties on the propagation parameters derived by local observables, non-local effects can change our predictions for the constraints even by 50%. Nevertheless, current ¯ p data can place tight constraints on DM models, excluding some of those suggested in connection with indirect and direct searches. Finally we discuss the impact of upcoming CR spectral data from the AMS-02 instrument on DM model constraints.

Dark Matter and Pulsar Origins of the Rising Cosmic Ray Positron Fraction in Light of New Data From AMS

The rise of the cosmic ray positron fraction with energy, as first observed with high confidence by PAMELA, implies that a large flux of high energy positrons has been recently (or is being currently) injected into the local volume of the Milky Way. With the new and much more precise measurement of the positron fraction recently provided by AMS, we revisit the question of the origin of these high energy positrons. We find that while some dark matter models (annihilating directly to electrons or muons) no longer appear to be capable of accommodating these data, other models in which ∼1-3 TeV dark matter particles annihilate to unstable intermediate states could still be responsible for the observed signal. Nearby pulsars also remain capable of explaining the observed positron fraction. Future measurements of the positron fraction by AMS (using a larger data set), combined with their anticipated measurements of various cosmic ray secondary-to-primary ratios, may enable us to further discriminate between these remaining scenarios.

Cosmic neutrino pevatrons: A brand new pathway to astronomy, astrophysics, and particle physics

Abstract The announcement by the IceCube Collaboration of the observation of 28 cosmic neutrino candidates has been greeted with a great deal of justified excitement. The data reported so far depart by 4.3 σ from the expected atmospheric neutrino background, which raises the obvious question: “Where in the Cosmos are these neutrinos coming from?” We review the many possibilities which have been explored in the literature to address this question, including origins at either Galactic or extragalactic celestial objects. For completeness, we also briefly discuss new physical processes which may either explain or be constrained by IceCube data.

Millisecond pulsars cannot account for the inner Galaxy’s GeV excess

Using data from the Fermi Gamma-Ray Space Telescope, a spatially extended component of gamma rays has been identified from the direction of the Galactic center, peaking at energies of ∼2–3  GeV. More recently, it has been shown that this signal is not confined to the innermost hundreds of parsecs of the Galaxy, but instead extends to at least ∼3  kpc from the Galactic center. While the spectrum, intensity, and angular distribution of this signal is in good agreement with predictions from annihilating dark matter, it has also been suggested that a population of unresolved millisecond pulsars could be responsible for this excess GeV emission from the inner Galaxy. In this paper, we consider this later possibility in detail. Comparing the observed spectral shape of the inner Galaxy’s GeV excess to the spectrum measured from 37 millisecond pulsars by Fermi, we find that these sources exhibit a spectral shape that is much too soft at sub-GeV energies to accommodate this signal. We also construct population models to describe the spatial distribution and luminosity function of the Milky Way’s millisecond pulsars. After taking into account constraints from the observed distribution of Fermi sources (including both sources known to be millisecond pulsars, and unidentified sources which could be pulsars), we find that millisecond pulsars can account for no more than ∼10% of the inner Galaxy’s GeV excess. Each of these arguments strongly disfavor millisecond pulsars as the source of this signal.