arXiv subjects

Savvas M. Koushiappas

Publications and source records attributed to Savvas M. Koushiappas.

53 records · Page 3Linked to original sources

arXiv2009

Energetic neutrinos from the Sun and Earth and dark matter substructure

Dark matter halos contain a wealth of substructure in the form of subhalos and tidal streams. Enhancements in the dark matter density of these regions leads to enhanced rates in direct detection experiments, as well as enhanced dark matter capture rates in the Sun and the Earth. Direct detection experiments probe the present-day dark matter density, while energetic neutrinos probe the past history of the dark matter density along the solar system's orbit about the Galactic center. We discuss how an elevated energetic neutrino flux can be used to probe the level of substructure present at the Galactic radius of the solar system.

astro-ph.HE↗

arXiv2009

Galactic Substructure and Energetic Neutrinos from the Sun and the Earth

We consider the effects of Galactic substructure on energetic neutrinos from annihilation of weakly-interacting massive particles (WIMPs) that have been captured by the Sun and Earth. Substructure gives rise to a time-varying capture rate and thus to time variation in the annihilation rate and resulting energetic-neutrino flux. However, there may be a time lag between the capture and annihilation rates. The energetic-neutrino flux may then be determined by the density of dark matter in the Solar System's past trajectory, rather than the local density. The signature of such an effect may be sought in the ratio of the direct- to indirect-detection rates.

astro-ph.CO↗

arXiv2009

The detection of sub-solar mass dark matter halos

Dark matter halos of sub-solar mass are the first bound objects to form in cold dark matter theories. In this article, I discuss the present understanding of "microhalos'', their role in structure formation, and the implications of their potential presence, in the interpretation of dark matter experiments.

astro-ph.CO↗

arXiv2009

Strong gravitational lensing probes of the particle nature of dark matter

There is a vast menagerie of plausible candidates for the constituents of dark matter, both within and beyond extensions of the Standard Model of particle physics. Each of these candidates may have scattering (and other) cross section properties that are consistent with the dark matter abundance, BBN, and the most scales in the matter power spectrum; but which may have vastly different behavior at sub-galactic "cutoff" scales, below which dark matter density fluctuations are smoothed out. The only way to quantitatively measure the power spectrum behavior at sub-galactic scales at distances beyond the local universe, and indeed over cosmic time, is through probes available in multiply imaged strong gravitational lenses. Gravitational potential perturbations by dark matter substructure encode information in the observed relative magnifications, positions, and time delays in a strong lens. Each of these is sensitive to a different moment of the substructure mass function and to different effective mass ranges of the substructure. The time delay perturbations, in particular, are proving to be largely immune to the degeneracies and systematic uncertainties that have impacted exploitation of strong lenses for such studies. There is great potential for a coordinated theoretical and observational effort to enable a sophisticated exploitation of strong gravitational lenses as direct probes of dark matter properties. This opportunity motivates this white paper, and drives the need for: a) strong support of the theoretical work necessary to understand all astrophysical consequences for different dark matter candidates; and b) tailored observational campaigns, and even a fully dedicated mission, to obtain the requisite data.

astro-ph.CO↗

arXiv2008

Can proper motions of dark-matter subhalos be detected?

One of the goals of NASA's Fermi Gamma-ray Space Telescope (formerly GLAST) will be detection of gamma rays from dark-matter annihilation in the Galactic halo. Theoretical arguments suggest that dark matter may be bound into subhalos with masses as small as 10^{-4}--10^2 Earth mass. If so, it may be possible to detect individual subhalos as point sources in the Fermi Telescope. It has further been argued that some of these point sources may exhibit proper motions. Here we show that upper limits to the diffuse gamma-ray background constrain the number of subhalos close enough to exhibit proper motions to be less than one.

astro-ph↗

arXiv2008

Galactic Substructure and Direct Detection of Dark Matter

We study the effects of substructure in the Galactic halo on direct detection of dark matter, on searches for energetic neutrinos from WIMP annihilation in the Sun and Earth, and on the enhancement in the WIMP annihilation rate in the halo. Our central result is a probability distribution function (PDF) P(ρ) for the local dark-matter density. This distribution must be taken into account when using null dark-matter searches to constrain the properties of dark-matter candidates. We take two approaches to calculating the PDF. The first is an analytic model that capitalizes on the scale-invariant nature of the structure--formation hierarchy in order to address early stages in the hierarchy (very small scales; high densities). Our second approach uses simulation-inspired results to describe the PDF that arises from lower-density larger-scale substructures which formed in more recent stages in the merger hierarchy. The distributions are skew positive, and they peak at densities lower than the mean density. The local dark-matter density may be as small as 1/10th the canonical value of ~ 0.4 GeV/cm^3, but it is probably no less than 0.2 GeV/cm^3.

astro-ph↗

arXiv2007

The Most Dark Matter Dominated Galaxies: Predicted Gamma-ray Signals from the Faintest Milky Way Dwarfs

We use kinematic data from three new, nearby, extremely low-luminosity Milky Way dwarf galaxies (Ursa Major II, Willman 1, and Coma Berenices) to constrain the properties of their dark matter halos, and from these make predictions for the gamma-ray flux from annihilation of dark matter particles in these halos. We show that these 10^3 solar luminosity dwarfs are the most dark matter dominated galaxies in the Universe, with total masses within 100 pc in excess of 10^6 solar masses. Coupled with their relative proximity, their large masses imply that they should have mean gamma-ray fluxes comparable to or greater than any other known satellite galaxy of the Milky Way. Our results are robust to both variations of the inner slope of the density profile and the effect of tidal interactions. The fluxes could be boosted by up to two orders of magnitude if we include the density enhancements caused by surviving dark matter substructure.

astro-ph↗

arXiv2007

Angular Anisotropies in the Cosmic Gamma-ray Background as a Probe of its Origin

Notwithstanding the advent of the Gamma-ray Large Area Telescope, theoretical models predict that a significant fraction of the cosmic gamma-ray background (CGB), at the level of 20% of the currently measured value, will remain unresolved. The angular power spectrum of intensity fluctuations of the CGB contains information on its origin. We show that probing the latter from a few tens of arcmin to several degree scales, together with complementary GLAST observations of gamma-ray emission from galaxy clusters and the blazars luminosity function, can discriminate between a background that originates from unresolved blazars or cosmic rays accelerated at structure formation shocks.

astro-ph↗

arXiv2007

Precise constraints on the dark matter content of Milky Way dwarf galaxies for gamma-ray experiments

We examine the prospects for detecting gamma-rays from dark matter annihilation in the six most promising dwarf spheroidal (dSph) satellite galaxies of the Milky Way. We use recently-measured velocity dispersion profiles to provide a systematic investigation of the dark matter mass distribution of each galaxy, and show that the uncertainty in the gamma-ray flux from mass modeling is less than a factor of ~ 5 for each dSph if we assume a smooth NFW profile. We show that Ursa Minor and Draco are the most promising dSphs for gamma-ray detection with GLAST and other planned observatories. For each dSph, we investigate the flux enhancement resulting from halo substructure, and show that the enhancement factor relative to a smooth halo flux cannot be greater than about 100. This enhancement depends very weakly on the lower mass cut-off scale of the substructure mass function. While the amplitude of the expected flux from each dSph depends sensitively on the dark matter model, we show that the flux ratios between the six Sphs are known to within a factor of about 10. The flux ratios are also relatively insensitive to the current theoretical range of cold dark matter halo central slopes and substructure fractions.

astro-ph↗

arXiv2007

Detecting the dark matter via the proper motion of gamma-rays from microhalos

I discuss the prospects for detecting the dark matter via the proper motion of sub-solar mass dark matter halos in the vicinity of the solar neighbourhood. Microhalos that survive tidal disruption could exhibit proper motion of order few arcminutes per year. For dark matter particles that couple to photons, such as the lightest supersymmetric or Kaluza-Klein particles, microhalos could be detected via their Gamma-ray photon emission from annihilations. A detection of proper motion of a microhalo in the Gamma-ray part of the spectrum contains not only information about the particle physics properties of the dark matter particle, but also provides an insight into hierarchical structure formation at very early times.

astro-ph↗

arXiv2007

Limits on the Radiative Decay of Sterile Neutrino Dark Matter from the Unresolved Cosmic and Soft X-ray Backgrounds

We present upper limits on line emission in the Cosmic X-ray background (CXB) that would be produced by decay of sterile neutrino dark matter. We employ the spectra of the unresolved component of the CXB in the Chandra Deep Fields North and South obtained with the Chandra CCD detector in the E=0.8-9 keV band. The expected decay flux comes from the dark matter on the lines of sight through the Milky Way galactic halo. Our constraints on the sterile neutrino decay rate are sensitive to the modeling of the Milky Way halo. The highest halo mass estimates provide a limit on the sterile neutrino mass of m_s<2.9 keV in the Dodelson-Widrow production model, while the lowest halo mass estimates provide the conservative limit of m_s<5.7 keV (2-sigma). We also discuss constraints from a short observation of the softer (E<1 keV) X-ray background with a rocket-borne calorimeter by McCammon and collaborators.

astro-ph↗

arXiv2006

Proper motion of gamma-rays from microhalo sources

I discuss the prospects of detecting the smallest dark matter bound structures present in the Milky Way by searching for the proper motion of $γ$-ray sources in the upcoming GLAST all sky map. I show that for dark matter particle candidates that couple to photons the detection of at least one $γ$-ray microhalo source with proper motion places a constraint on the couplings and mass of the dark matter particle. For SUSY dark matter, proper motion detection implies that the mass of the particle is less than 500 GeV and the kinetic decoupling temperature is in the range of [4-100] MeV.

astro-ph↗

arXiv2006

Constraints on Sterile Neutrino Dark Matter

We present a comprehensive analysis of constraints on the sterile neutrino as a dark matter candidate. The minimal production scenario with a standard thermal history and negligible cosmological lepton number is in conflict with conservative radiative decay constraints from the cosmic X-ray background in combination with stringent small-scale structure limits from the Lyman-alpha forest. We show that entropy release through massive particle decay after production does not alleviate these constraints. We further show that radiative decay constraints from local group dwarf galaxies are subject to large uncertainties in the dark matter density profile of these systems. Within the strongest set of constraints, resonant production of cold sterile neutrino dark matter in non-zero lepton number cosmologies remains allowed.

astro-ph↗

arXiv2005

Testing models of supermassive black hole seed formation through gravity waves

We study the gravitational wave background produced from the formation and assembly of supermassive black holes within the cosmological paradigm of hierarchical structure formation. In particular, we focus on a supermassive black hole formation scenario in which the present-day population of supermassive black holes is built from high-mass seed black holes and we compute the concomitant spectrum of gravitational radiation produced by mergers of the seed black holes. We find that this scenario predicts a large, gravitational wave background that should be resolved into individual sources with space interferometers such as the proposed Laser Interferometric Space Antenna (LISA). The number of inspiral, merger and ringdown events above a signal to noise ratio of 5 that result from massive black hole seeds is of order 10^3. This prediction is robust and insensitive to several of the details of the model. We conclude that an interferometer such as LISA will be able to effectively rule out or confirm a class of models where supermassive black holes grow from high-mass seed black holes and may be able to place strong limits on the role of mergers as a channel for supermassive black hole growth. Supermassive black hole seeds likely form in the earliest proto-galactic structures at high redshift and the masses of supermassive black holes are known to be strongly correlated with the potentials of the spheroids in which they reside, therefore these results imply that space interferometers can be used as a powerful probe of the physics of galaxy formation and proto-galaxy formation at very high redshift.

astro-ph↗

arXiv2005

Dark Matter Halos: Shapes, The Substructure Crisis, and Indirect Detection

In this proceeding, we briefly review three recent results. First, we show that halos formed in simulations with gas cooling are significantly rounder than halos formed in dissipationless $N$-body simulations. The increase in principle axis ratios is $δ(c/a) ~ 0.2 - 0.4$ in the inner halo and remains significant at large radii. Second, we discuss the CDM substructure crisis and demonstrate the sensitivity of the crisis to the spectrum of primordial density fluctuations on small scales. Third, we assess the ability of experiments like VERITAS and GLAST to detect $γ$-rays from neutralino dark matter annihilation in dark subhalos about the MW.

astro-ph↗

arXiv2003

The observability of gamma-rays from neutralino annihilations in Milky Way substructure

We estimate the probability of detecting gamma-rays from the annihilation of neutralino dark matter in dense, central regions of Milky Way substructure. We characterize Galactic substructure statistically based on Monte Carlo realizations of the formation of a Milky Way-like halo using a semi-analytic method that has been calibrated against the results of high-resolution N-body simulations. We find that it may be possible for the upcoming experiments GLAST and VERITAS, working in concert, to detect gamma-rays from dark matter substructure if the neutralino is relatively light ($\Mchi \lsim 100 \GeV$), while for $\Mchi \gsim 500 \GeV$ such a detection would be unlikely. We perform most of our calculations within the framework of the standard $Λ$CDM cosmological model; however, we also investigate the robustness of our results to various assumptions and find that the probability of detection is sensitive to poorly-constrained input parameters, particularly those that characterize the primordial power spectrum. Specifically, the best-fitting power spectrum of the WMAP team, with a running spectral index, predicts roughly a factor of fifty fewer detectable subhalos compared to the standard $\LCDM$ cosmological model with scale-invariant power spectrum. We conclude that the lack of a detected gamma-ray signal gives very little information about the supersymmetric parameter space due to uncertainties associated with both the properties of substructure and cosmological parameters.

astro-ph↗

arXiv2003

Gamma-rays From Neutralino Annihilation in Milky Way Substructure: What Can We Learn?

We estimate the probability of detecting gamma-rays from the annihilation of neutralino dark matter in the substructure of the Milky Way. We characterize substructure statistically based on Monte Carlo realizations of the formation of a Milky Way-like halo using semi-analytic methods that have been calibrated against N-body simulations. We find that it may be possible for the upcoming GLAST and VERITAS experiments, working in concert, to detect gamma-rays from dark matter substructure if $\Mchi \lsim 100 \GeV$, while for $\Mchi \gsim 500 \GeV$ such a detection seems unlikely. We investigate the effects of the underlying cosmological model and find that the probability of detection is sensitive to the primordial power spectrum of density fluctuations on small (galactic and sub-galactic) scales. We conclude that the lack of such a detection reveals little about the supersymmetric parameter space due to the uncertainties associated with the properties of substructure and cosmological parameters.

astro-ph↗