arXiv subjects

Ilias Cholis

Publications and source records attributed to Ilias Cholis.

85 records · Page 5Linked to original sources

arXiv2011

New Constraints from PAMELA anti-proton data on Annihilating and Decaying Dark Matter

Recently the PAMELA experiment has released its updated anti-proton flux and anti-proton to proton flux ratio data up to energies of ~200GeV. With no clear excess of cosmic ray anti-protons at high energies, one can extend constraints on the production of anti-protons from dark matter. In this letter, we consider both the cases of dark matter annihilating and decaying into standard model particles that produce significant numbers of anti-protons. We provide two sets of constraints on the annihilation cross-sections/decay lifetimes. In the one set of constraints we ignore any source of anti-protons other than dark matter, which give the highest allowed cross-sections/inverse lifetimes. In the other set we include also anti-protons produced in collisions of cosmic rays with interstellar medium nuclei, getting tighter but more realistic constraints on the annihilation cross-sections/decay lifetimes.

astro-ph.HE↗

arXiv2011

Fermi Gamma-ray Haze via Dark Matter and Millisecond Pulsars

We study possible astrophysical and dark matter (DM) explanations for the Fermi gamma-ray haze in the Milky Way halo. As representatives of various DM models, we consider DM particles annihilating into W+W-, b-bbar, and e+e-. In the first two cases, the prompt gamma-ray emission from DM annihilations is significant or even dominant at E > 10 GeV, while inverse Compton scattering (ICS) from annihilating DM products is insignificant. For the e+e- annihilation mode, we require a boost factor of order 100 to get significant contribution to the gamma-ray haze from ICS photons. Possible astrophysical sources of high energy particles at high latitudes include type Ia supernovae (SNe) and millisecond pulsars (MSPs). Based on our current understanding of Ia SNe rates, they do not contribute significantly to gamma-ray flux in the halo of the Milky Way. As the MSP population in the stellar halo of the Milky Way is not well constrained, MSPs may be a viable source of gamma-rays at high latitudes provided that there are ~ 20 000 - 60 000 of MSPs in the Milky Way stellar halo. In this case, pulsed gamma-ray emission from MSPs can contribute to gamma-rays around few GeV's while the ICS photons from MSP electrons and positrons may be significant at all energies in the gamma-ray haze. The plausibility of such a population of MSPs is discussed. Consistency with the Wilkinson Microwave Anisotropy Probe (WMAP) microwave haze requires that either a significant fraction of MSP spin-down energy is converted into e+e- flux or the DM annihilates predominantly into leptons with a boost factor of order 100.

astro-ph.HE↗

arXiv2011

Spherical harmonics analysis of Fermi gamma-ray data and the Galactic dark matter halo

We argue that the decomposition of gamma-ray maps in spherical harmonics is a sensitive tool to study dark matter (DM) annihilation or decay in the main Galactic halo of the Milky Way. Using the spherical harmonic decomposition in a window excluding the Galactic plane, we show for one year of Fermi data that adding a spherical template (such as a line-of-sight DM annihilation profile) to an astrophysical background significantly reduces chi^2 of the fit to the data. In some energy bins the significance of this DM-like fraction is above three sigma. This can be viewed as a hint of DM annihilation signal, although astrophysical sources cannot be ruled out at this moment. We use the derived DM fraction as a conservative upper limit on DM annihilation signal. In the case of bb-bar annihilation channel the limits are about a factor of two less constraining than the limits from dwarf galaxies. The uncertainty of our method is dominated by systematics related to modeling the astrophysical background. We show that with one year of Fermi data the statistical sensitivity would be sufficient to detect DM annihilation with thermal freeze out cross section for masses below 100 GeV.

astro-ph.HE↗

arXiv2011

The Fermi Gamma-Ray Haze from Dark Matter Annihilations and Anisotropic Diffusion

Recent full-sky maps of the Galaxy from the Fermi Gamma-Ray Space Telescope have revealed a diffuse component of emission towards the Galactic center and extending up to roughly +/-50 degrees in latitude. This Fermi "haze" is the inverse Compton emission generated by the same electrons which generate the microwave synchrotron haze at WMAP wavelengths. The gamma-ray haze has two distinct characteristics: the spectrum is significantly harder than emission elsewhere in the Galaxy and the morphology is elongated in latitude with respect to longitude with an axis ratio ~2. If these electrons are generated through annihilations of dark matter particles in the Galactic halo, this morphology is difficult to realize with a standard spherical halo and isotropic cosmic-ray diffusion. However, we show that anisotropic diffusion along ordered magnetic field lines towards the center of the Galaxy coupled with a prolate dark matter halo can easily yield the required morphology without making unrealistic assumptions about diffusion parameters. Furthermore, a Sommerfeld enhancement to the self annihilation cross-section of ~30 yields a good fit to the morphology, amplitude, and spectrum of both the gamma-ray and microwave haze. The model is also consistent with local cosmic-ray measurements as well as CMB constraints.

astro-ph.HE↗

arXiv2010

Consequences of a Dark Disk for the Fermi and PAMELA Signals in Theories with a Sommerfeld Enhancement

Much attention has been given to dark matter explanations of the PAMELA positron fraction and Fermi electronic excesses. For those theories with a TeV-scale WIMP annihilating through a light force-carrier, the associated Sommerfeld enhancement provides a natural explanation of the large boost factor needed to explain the signals, and the light force-carrier naturally gives rise to hard cosmic ray spectra without excess pi0 gamma rays or anti-protons. The Sommerfeld enhancement of the annihilation rate, which at low relative velocities v scales as 1/v, relies on the comparatively low velocity dispersion of the dark matter particles in the smooth halo. Dark matter substructures in which the velocity dispersion is smaller than in the smooth halo have even larger annihilation rates. N-body simulations containing only dark matter predict the existence of such structures, for example subhalos and caustics, and the effects of these substructures on dark matter indirect detection signals have been studied extensively. The addition of baryons into cosmological simulations of disk-dominated galaxies gives rise to an additional substructure component, a dark disk. The disk has a lower velocity dispersion than the spherical halo component by a factor ~6, so the contributions to dark matter signals from the disk can be more significant in Sommerfeld models than for WIMPs without such low-velocity enhancements. We consider the consequences of a dark disk on the observed signals of cosmic rays as measured by Fermi and PAMELA in models where the WIMP annihilations are into a light boson. We find that both the PAMELA and Fermi results are easily accomodated by scenarios in which a disk signal is included with the standard spherical halo signal. Limits from extrapolations to the center of the galaxy can also be modified.

astro-ph.HE↗

arXiv2010

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

The Fermi Gamma-Ray Space Telescope reveals a diffuse inverse Compton 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 pi0 gammas, as well as IC and bremsstrahlung components associated with known soft-synchrotron counterparts. We find a significant gamma-ray excess towards 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.

astro-ph.HE↗

arXiv2009

MiXDM: Cosmic Ray Signals from Multiple States of Dark Matter

Recent data from cosmic ray experiments such as PAMELA, Fermi, ATIC and PPB-BETS all suggest the need for a new primary source of electrons and positrons at high (>~100 GeV) energies. Many proposals have been put forth to explain these data, usually relying on a single particle to annihilate or decay to produce e+e-. In this paper, we consider models with multiple species of WIMPs with significantly different masses. We show if such dark matter candidates chi_i annihilate into light bosons, they naturally produce equal annihilation rates, even as the available numbers of pairs for annihilation n_chi_i^2 differ by orders of magnitude. We argue that a consequence of these models can be to add additional signal naturally at lower (~100 GeV) versus higher (~ TeV) energies, changing the expected spectrum and even adding bumps at lower energies, which may alleviate some of the tension in the required annihilation rates between PAMELA and Fermi. These spectral changes may yield observable consequences in the microwave Haze signal observed at the upcoming Planck satellite. Such a model can connect to other observable signals such as DAMA and INTEGRAL by having the lighter (heavier) state be a pseudo-Dirac fermion with splitting 100 keV (1 MeV). We show that variations in the halo velocity dispersion can alleviate constraints from final state radiation in the galactic center and galactic ridge. If the lighter WIMP has a large self-interaction cross section, the light-WIMP halo might collapse, dramatically altering expectations for direct and indirect detection signatures.

astro-ph.HE↗

arXiv2009

Pulsars versus Dark Matter Interpretation of ATIC/PAMELA

In this paper, we study the flux of electrons and positrons injected by pulsars and by annihilating or decaying dark matter in the context of recent ATIC, PAMELA, Fermi, and HESS data. We review the flux from a single pulsar and derive the flux from a distribution of pulsars. We point out that the particle acceleration in the pulsar magnetosphere is insufficient to explain the observed excess of electrons and positrons with energy E ~ 1 TeV and one has to take into account an additional acceleration of electrons at the termination shock between the pulsar and its wind nebula. We show that at energies less than a few hundred GeV, the flux from a continuous distribution of pulsars provides a good approximation to the expected flux from pulsars in the Australia Telescope National Facility (ATNF) catalog. At higher energies, we demonstrate that the electron/positron flux measured at the Earth will be dominated by a few young nearby pulsars, and therefore the spectrum would contain bumplike features. We argue that the presence of such features at high energies would strongly suggest a pulsar origin of the anomalous contribution to electron and positron fluxes. The absence of features either points to a dark matter origin or constrains pulsar models in such a way that the fluctuations are suppressed. Also we derive that the features can be partially smeared due to spatial variation of the energy losses during propagation.

astro-ph.HE↗

arXiv2009

The Case for a 700+ GeV WIMP: Cosmic Ray Spectra from PAMELA, Fermi and ATIC

Multiple lines of evidence indicate an anomalous injection of high-energy e+- in the Galactic halo. The recent $e^+$ fraction spectrum from the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) shows a sharp rise up to 100 GeV. The Fermi Gamma-ray Space Telescope has found a significant hardening of the e+e- cosmic ray spectrum above 100 GeV, with a break, confirmed by HESS at around 1 TeV. The Advanced Thin Ionization Calorimeter (ATIC) has also detected detected a similar excess, falling back to the expected spectrum at 1 TeV and above. Excess microwaves towards the galactic center in the WMAP data are consistent with hard synchrotron radiation from a population of 10-100 GeV e+- (the WMAP ``Haze''). We argue that dark matter annihilations can provide a consistent explanation of all of these data, focusing on dominantly leptonic modes, either directly or through a new light boson. Normalizing the signal to the highest energy evidence (Fermi and HESS), we find that similar cross sections provide good fits to PAMELA and the Haze, and that both the required cross section and annihilation modes are achievable in models with Sommerfeld-enhanced annihilation. These models naturally predict significant production of gamma rays in the galactic center via a variety of mechanisms. Most notably, there is a robust inverse-Compton scattered (ICS) gamma-ray signal arising from the energetic electrons and positrons, detectable at Fermi/GLAST energies, which should provide smoking gun evidence for this production.

astro-ph↗

arXiv2009

The Fermi gamma-ray spectrum of the inner galaxy: Implications for annihilating dark matter

Recently, a preliminary spectrum from the Fermi Gamma-ray Space Telescope has been presented for the inner galaxy (-30 < l < 30, -5 < b < 5), as well as the galactic center (-1 < l < 1, -1< b < 1). We consider the implications of these data for dark matter annihilation models, especially models capable of producing the cosmic-ray excesses previously observed by PAMELA and Fermi. These local cosmic-ray excesses, when extrapolated to the inner galaxy, imply inverse Compton scattering (ICS) gamma-ray signals largely consistent with the preliminary Fermi gamma-ray spectrum. For specific halos and models, particularly those with prompt photons, the data have begun to constrain the allowed parameter space. Although significant modeling and background uncertainties remain, we explore how large a signal is permitted by the current data. Based upon this, we make predictions for what signal could be present in other regions of the sky where dark matter signals may be easier to isolate from the astrophysical backgrounds.

astro-ph.HE↗

arXiv2009

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 any model with hadronic annihilation modes. We revisit an earlier proposal, whereby the dark matter annihilates into a new light (<~GeV) boson phi, which is kinematically constrained to go to hard leptonic states, without anti-protons or pi0's. 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 PAMELA's ability to discriminate electrons and positrons.

astro-ph↗

arXiv2008

High Energy Positrons From Annihilating Dark Matter

Recent preliminary results from the PAMELA experiment indicate the presence of an excess of cosmic ray positrons above 10 GeV. In this letter, we consider possibility that this signal is the result of dark matter annihilations taking place in the halo of the Milky Way. Rather than focusing on a specific particle physics model, we take a phenomenological approach and consider a variety of masses and two-body annihilation modes, including W+W-, ZZ, b bbar, tau+ tau-, mu+ mu-, and e+e. We also consider a range of diffusion parameters consistent with current cosmic ray data. We find that a significant upturn in the positron fraction above 10 GeV is compatible with a wide range of dark matter annihilation modes, although very large annihilation cross sections and/or boost factors arising from inhomogeneities in the local dark matter distribution are required to produce the observed intensity of the signal. We comment on constraints from gamma rays, synchrotron emission, and cosmic ray antiproton measurements.

hep-ph↗

arXiv2008

High Energy Positrons and the WMAP Haze from Exciting Dark Matter

We consider the signals of positrons and electrons from "exciting" dark matter (XDM) annihilation. Because of the light (m_phi ~< 1 GeV) force carrier phi into which the dark matter states can annihilate, the electrons and positrons are generally very boosted, yielding a hard spectrum, in addition to the low energy positrons needed for INTEGRAL observations of the galactic center. We consider the relevance of this scenario for HEAT, PAMELA and the WMAP "haze," focusing on light (m_phi ~< 2 m_pi) phi bosons, and find that significant signals can be found for all three, although significant signals generally require high dark matter densities. We find that measurements of the positron fraction are generally insensitive to the halo model, but do suffer significant astrophysical uncertainties. We discuss the implications for upcoming PAMELA results.

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