arXiv subjects

Mattia Fornasa

Publications and source records attributed to Mattia Fornasa.

At least 19 recordsLinked to original sources

arXiv2018

MultiDark-Clusters: Galaxy Cluster Mock Light-Cones, eROSITA and the Cluster Power Spectrum

Cosmological simulations are fundamental tools to study structure formation and the astrophysics of evolving structures, in particular clusters of galaxies. While hydrodynamical simulations cannot sample efficiently large volumes and explore different cosmologies at the same time, N-body simulations lack the baryonic physics that is crucial to determine the observed properties of clusters. One solution is to use (semi-)analytical models to implement the needed baryonic physics. In this way, we can generate the many mock universes that will be required to fully exploit future large sky surveys, such as that from the upcoming eROSITA X-ray telescope. We developed a phenomenological model based on observations of clusters to implement gas density and temperature information on the dark-matter-only halos of the MultiDark simulations. We generate several full-sky mock light-cones of clusters for the WMAP and Planck cosmologies, adopting different parameters in our phenomenological model of the intra-cluster medium. For one of these simulations and models, we also generate 100 light-cones corresponding to 100 random observers and explore the variance among them in several quantities. In this first paper on MultiDark mock galaxy cluster light-cones, we focus on presenting our methodology and discuss predictions for eROSITA, in particular, exploring the potential of angular power spectrum analyses of its detected (and undetected) cluster population to study X-ray scaling relations, the intra-cluster medium, and the composition of the cosmic X-ray background. We make publicly available on-line more than 400 GB of light-cones, which include the expected eROSITA count rate, on Skies & Universes (http://www.skiesanduniverses.org).

astro-ph.CO↗

arXiv2017

How bright can the brightest neutrino source be?

After the discovery of extraterrestrial high-energy neutrinos, the next major goal of neutrino telescopes will be identifying astrophysical objects that produce them. The flux of the brightest source $F_{\rm max}$, however, cannot be probed by studying the diffuse neutrino intensity. We aim at constraining $F_{\rm max}$ by adopting a broken power-law flux distribution, a hypothesis supported by observed properties of any generic astrophysical sources. The first estimate of $F_{\rm max}$ comes from the fact that we can only observe one universe, and hence, the expected number of sources above $F_{\rm max}$ cannot be too small compared with one. For abundant source classes such as starburst galaxies, this one-source constraint yields a value of $F_{\rm max}$ that is an order of magnitude lower than the current upper limits from point-source searches. Then we derive upper limits on $F_{\rm max}$ assuming that the angular power spectrum is consistent with neutrino shot noise yet. We find that the limits obtained with upgoing muon neutrinos in IceCube can already be quite competitive, especially for rare but bright source populations such as blazars. The limits will improve nearly quadratically with exposure, and therefore be even more powerful for the next generation of neutrino telescopes.

astro-ph.HE↗

arXiv2017

Cross-correlation of weak lensing and gamma rays: implications for the nature of dark matter

We measure the cross-correlation between Fermi-LAT gamma-ray photons and over 1000 deg$^2$ of weak lensing data from the Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS), the Red Cluster Sequence Lensing Survey (RCSLenS), and the Kilo Degree Survey (KiDS). We present the first measurement of tomographic weak lensing cross-correlations and the first application of spectral binning to cross-correlations between gamma rays and weak lensing. The measurements are performed using an angular power spectrum estimator while the covariance is estimated using an analytical prescription. We verify the accuracy of our covariance estimate by comparing it to two internal covariance estimators. Based on the non-detection of a cross-correlation signal, we derive constraints on weakly interacting massive particle (WIMP) dark matter. We compute exclusion limits on the dark matter annihilation cross-section $\langleσ_\rm{ann} v \rangle$, decay rate $Γ_\rm{dec}$, and particle mass $m_\rm{DM}$. We find that in the absence of a cross-correlation signal, tomography does not significantly improve the constraining power of the analysis. Assuming a strong contribution to the gamma-ray flux due to small-scale clustering of dark matter and accounting for known astrophysical sources of gamma rays, we exclude the thermal relic cross-section for particle masses of $m_\rm{DM}\lesssim 20$ GeV.

astro-ph.CO↗

arXiv2017

Astrophysical interpretation of the anisotropies in the unresolved gamma-ray background

Recently, a new measurement of the auto- and cross-correlation angular power spectrum (APS) of the isotropic gamma-ray background was performed, based on 81 months of data of the Fermi Large-Area Telescope (LAT). Here, we fit, for the first time, the new APS data with a model describing the emission of unresolved blazars. These sources are expected to dominate the anisotropy signal. The model we employ in our analysis reproduces well the blazars resolved by Fermi LAT. When considering the APS obtained by masking the sources in the 3FGL catalogue, we find that unresolved blazars under-produce the measured APS below $\sim$1 GeV. Contrary to past results, this suggests the presence of a new contribution to the low-energy APS, with a significance of, at least, 5$σ$. The excess can be ascribed to a new class of faint gamma-ray emitters. If we consider the APS obtained by masking the sources in the 2FGL catalogue, there is no under-production of the APS below 1 GeV, but the new source class is still preferred over the blazars-only scenario (with a significance larger than 10$σ$). The properties of the new source class and the level of anisotropies induced in the isotropic gamma-ray background are the same, independent of the APS data used. In particular, the new gamma-ray emitters must have a soft energy spectrum, with a spectral index ranging, approximately, from 2.7 to 3.2. This complicates their interpretation in terms of known sources, since, normally, star-forming and radio galaxies are observed with a harder spectrum. The new source class identified here is also expected to contribute significantly to the intensity of the isotropic gamma-ray background.

astro-ph.HE↗

arXiv2016

The angular power spectrum of the diffuse gamma-ray emission as measured by the Fermi Large Area Telescope and constraints on its Dark Matter interpretation

The isotropic gamma-ray background arises from the contribution of unresolved sources, including members of confirmed source classes and proposed gamma-ray emitters such as the radiation induced by dark matter annihilation and decay. Clues about the properties of the contributing sources are imprinted in the anisotropy characteristics of the gamma-ray background. We use 81 months of Pass 7 Reprocessed data from the Fermi Large Area Telescope to perform a measurement of the anisotropy angular power spectrum of the gamma-ray background. We analyze energies between 0.5 and 500 GeV, extending the range considered in the previous measurement based on 22 months of data. We also compute, for the first time, the cross-correlation angular power spectrum between different energy bins. We find that the derived angular spectra are compatible with being Poissonian, i.e. constant in multipole. Moreover, the energy dependence of the anisotropy suggests that the signal is due to two populations of sources, contributing, respectively, below and above 2 GeV. Finally, using data from state-of-the-art numerical simulations to model the dark matter distribution, we constrain the contribution from dark matter annihilation and decay in Galactic and extragalactic structures to the measured anisotropy. These constraints are competitive with those that can be derived from the average intensity of the isotropic gamma-ray background. Data are available at https://www-glast.stanford.edu/pub_data/552.

astro-ph.HE↗

arXiv2016

How to calculate dark matter direct detection exclusion limits that are consistent with gamma rays from annihilation in the Milky Way halo

When comparing constraints on the Weakly Interacting Massive Particle (WIMP) properties from direct and indirect detection experiments it is crucial that the assumptions made about the dark matter (DM) distribution are realistic and consistent. For instance, if the Fermi-LAT Galactic centre GeV gamma-ray excess was due to WIMP annihilation, its morphology would be incompatible with the Standard Halo Model that is usually used to interpret data from direct detection experiments. In this article, we calculate exclusion limits from direct detection experiments using self-consistent velocity distributions, derived from mass models of the Milky Way where the DM halo has a generalized NFW profile. We use two different methods to make the mass model compatible with a DM interpretation of the Galactic centre gamma-ray excess. Firstly, we fix the inner slope of the DM density profile to the value that best fits the morphology of the excess. Secondly, we allow the inner slope to vary and include the morphology of the excess in the data sets used to constrain the gravitational potential of the Milky Way. The resulting direct detection limits differ significantly from those derived using the Standard Halo Model, in particular for light WIMPs, due to the differences in both the local DM density and velocity distribution.

astro-ph.CO↗

arXiv2015

Tomographic-spectral approach for dark matter detection in the cross-correlation between cosmic shear and diffuse gamma-ray emission

We recently proposed to cross-correlate the diffuse extragalactic gamma-ray background with the gravitational lensing signal of cosmic shear. This represents a novel and promising strategy to search for annihilating or decaying particle dark matter (DM) candidates. In the present work, we demonstrate the potential of a tomographic-spectral approach: measuring the cross-correlation in separate bins of redshift and energy significantly improves the sensitivity to a DM signal. Indeed, the technique proposed here takes advantage of the different scaling of the astrophysical and DM components with redshift and, simultaneously, of their different energy spectra and different angular extensions. The sensitivity to a particle DM signal is extremely promising even when the DM-induced emission is quite faint. We first quantify the prospects of detecting DM by cross-correlating the Fermi Large Area Telescope (LAT) diffuse gamma-ray background with the cosmic shear expected from the Dark Energy Survey. Under the hypothesis of a significant subhalo boost, such a measurement can deliver a 5-sigma detection of DM, if the DM particle is lighter than 300 GeV and has a thermal annihilation rate. We then forecast the capability of the European Space Agency Euclid satellite (whose launch is planned for 2020), in combination with an hypothetical future gamma-ray detector with slightly improved specifications compared to current telescopes. We predict that the cross-correlation of their data will allow a measurement of the DM mass with an uncertainty of a factor of 1.5-2, even for moderate subhalo boosts, for DM masses up to few hundreds of GeV and thermal annihilation rates.

astro-ph.CO↗

arXiv2015

The nature of the Diffuse Gamma-Ray Background

We review the current understanding of the diffuse gamma-ray background (DGRB). The DGRB is what remains of the total measured gamma-ray emission after the subtraction of the resolved sources and of the diffuse Galactic foregrounds. It is interpreted as the cumulative emission of sources that are not bright enough to be detected individually. Yet, its exact composition remains unveiled. Well-established astrophysical source populations (e.g. blazars, misaligned AGNs, star-forming galaxies and millisecond pulsars) all represent guaranteed contributors to the DGRB. More exotic scenarios, such as dark matter annihilation or decay, may contribute as well. In this review, we describe how these components have been modeled in the literature and how the DGRB can be used to provide valuable information on each of them. We summarize the observational information currently available on the DGRB, paying particular attention to the most recent measurement of its intensity energy spectrum by the Fermi LAT Collaboration. We also discuss the novel analyses of the auto-correlation angular power spectrum of the DGRB and of its cross-correlation with tracers of the large-scale structure of the Universe. New data sets already (or soon) available are expected to provide further insight on the nature of this emission. By summarizing where we stand on the current knowledge of the DGRB, this review is intended both as a useful reference for those interested in the topic and as a means to trigger new ideas for further research.

astro-ph.CO↗

arXiv2015

Probing WIMP particle physics and astrophysics with direct detection and neutrino telescope data

With positive signals from multiple direct detection experiments it will, in principle, be possible to measure the mass and cross sections of weakly-interacting massive particle (WIMP) dark matter. Recent work has shown that, with a polynomial parameterisation of the WIMP speed distribution, it is possible to make an unbiased measurement of the WIMP mass, without making any astrophysical assumptions. However, direct detection experiments are not sensitive to low-speed WIMPs and, therefore, any model-independent approach will lead to a bias in the cross section. This problem can be solved with the addition of measurements of the flux of neutrinos from the Sun. This is because the flux of neutrinos produced from the annihilation of WIMPs which have been gravitationally captured in the Sun is sensitive to low-speed WIMPs. Using mock data from next-generation direct detection experiments and from the IceCube neutrino telescope, we show that the complementary information from IceCube on low-speed WIMPs breaks the degeneracy between the cross section and the speed distribution. This allows unbiased determinations of the WIMP mass and spin-independent and spin-dependent cross sections to be made, and the speed distribution to be reconstructed. We use two parameterisations of the speed distribution: binned and polynomial. While the polynomial parameterisation can encompass a wider range of speed distributions, this leads to larger uncertainties in the particle physics parameters.

astro-ph.CO↗

arXiv2013

A self-consistent phase-space distribution function for the anisotropic Dark Matter halo of the Milky Way

Dark Matter (DM) direct detection experiments usually assume the simplest possible 'Standard Halo Model' for the Milky Way (MW) halo in which the velocity distribution is Maxwellian. This model assumes that the MW halo is an isotropic, isothermal sphere, hypotheses that are unlikely to be valid in reality. An alternative approach is to derive a self-consistent solution for a particular mass model of the MW (i.e. obtained from its gravitational potential) using the Eddington formalism, which assumes isotropy. In this paper we extend this approach to incorporate an anisotropic phase-space distribution function. We perform Bayesian scans over the parameters defining the mass model of the MW and parameterising the phase-space density, implementing constraints from a wide range of astronomical observations. The scans allow us to estimate the precision reached in the reconstruction of the velocity distribution (for different DM halo profiles). As expected, allowing for an anisotropic velocity tensor increases the uncertainty in the reconstruction of f(v) but the distribution can still be determined with a precision of a factor of 4-5. The mean velocity distribution resembles the isotropic case, however the amplitude of the high-velocity tail is up to a factor of 2 larger. Our results agree with the phenomenological parametrization proposed in Mao et al. (2013) as a good fit to N-body simulations (with or without baryons), since their velocity distribution is contained in our 68% credible interval.

astro-ph.CO↗

arXiv2012

Nuclear uncertainties in the spin-dependent structure functions for direct dark matter detection

We study the effect that uncertainties in the nuclear spin-dependent structure functions have in the determination of the dark matter (DM) parameters in a direct detection experiment. We show that different nuclear models that describe the spin-dependent structure function of specific target nuclei can lead to variations in the reconstructed values of the DM mass and scattering cross-section. We propose a parametrization of the spin structure functions that allows us to treat these uncertainties as variations of three parameters, with a central value and deviation that depend on the specific nucleus. The method is illustrated for germanium and xenon detectors with an exposure of 300 kg yr, assuming a hypothetical detection of DM and studying a series of benchmark points for the DM properties. We find that the effect of these uncertainties can be similar in amplitude to that of astrophysical uncertainties, especially in those cases where the spin-dependent contribution to the elastic scattering cross-section is sizable.

hep-ph↗

arXiv2012

Characterization of Dark-Matter-induced anisotropies in the diffuse gamma-ray background

The Fermi-LAT collaboration has recently reported the detection of angular power above the photon noise level in the diffuse gamma-ray background between 1 and 50 GeV. Such signal can be used to constrain a possible contribution from Dark-Matter-induced photons. We estimate the intensity and features of the angular power spectrum (APS) of this potential Dark Matter (DM) signal, for both decaying and annihilating DM candidates, by constructing template all-sky gamma-ray maps for the emission produced in the galactic halo and its substructures, as well as in extragalactic (sub)halos. The DM distribution is given by state-of-the-art N-body simulations of cosmic structure formation, namely Millennium-II for extragalactic (sub)halos, and Aquarius for the galactic halo and its subhalos. We use a hybrid method of extrapolation to account for (sub)structures that are below the resolution limit of the simulations, allowing us to estimate the total emission all the way down to the minimal self-bound halo mass. We describe in detail the features appearing in the APS of our template maps and we estimate the effect of various uncertainties such as the value of the minimal halo mass, the fraction of substructures hosted in a halo and the shape of the DM density profile. Our results indicate that the fluctuation APS of the DM-induced emission is of the same order as the Fermi-LAT APS, suggesting that one can constrain this hypothetical emission from the comparison with the measured anisotropy. We also quantify the uncertainties affecting our results, finding "theoretical error bands" spanning more than two orders of magnitude and dominated (for a given particle physics model) by our lack of knowledge of the abundance of low-mass (sub)halos.

astro-ph.HE↗

arXiv2011

Dark Matter implications of the Fermi-LAT measurement of anisotropies in the diffuse gamma-ray background: status report

For the first time, the Fermi-LAT measured the angular power spectrum (APS) of anisotropies in the diffuse gamma-ray background. The data is found to be broadly compatible with a model with contributions from the point sources in the 1-year catalog, the galactic diffuse background, and the extragalactic isotropic emission; however deviations are present at both large and small angular scales. In this study, we complement the model with a contribution from Dark Matter (DM) whose distribution is modeled exploiting the results of the most recent N-body simulations, considering the contribution of extragalactic halos and subhalos (from Millennium-II) and of galactic substructures (from Aquarius). With the use of the Fermi Science Tools, these simulations serve as templates to produce mock gamma-ray count maps for DM gamma-ray emission, both in the case of an annihilating and a decaying DM candidate. The APS will then be computed and compared with the Fermi-LAT results to derive constraints on the DM particle physics properties. The possible systematic due to an imperfect model of the galactic foreground is also studied and taken into account properly. The present paper reports on the status of the project.

astro-ph.CO↗

arXiv2011

A cross-correlation study of the Fermi-LAT $γ$-ray diffuse extragalactic signal

In this work, starting from 21 months of data from the Fermi-Large Area Telescope, we derive maps of the residual isotropic gamma-ray emission, a relevant fraction of which is expected to be contributed by the extragalactic diffuse gamma-ray background. We compute the angular two-point auto-correlation function of the residual Fermi-LAT maps at energies E>1GeV, E>3GeV and E>30GeV well above the Galactic plane and find no significant correlation signal. This is, indeed, what is expected if the EGB were contributed by BL Lacertae, Flat Spectrum Radio Quasars or star-forming galaxies, since, in this case, the predicted signal is very weak. Then, we search for the Integrated Sachs-Wolfe signature by cross-correlating the Fermi-LAT maps with the WMAP7-Cosmic Microwave Background map. We find a cross-correlation consistent with zero, even though the expected signal is larger than that of the EGB auto-correlation. Finally, in an attempt to constrain the nature of the gamma-ray background we cross-correlate the Fermi-LAT maps with the angular distributions of objects that may contribute to the EGB: QSOs in the SDSS-DR6 catalog, NVSS galaxies, 2MASS galaxies and LRG in the SDSS catalog. The cross-correlation is always consistent with zero, in agreement with theoretical expectations, but we find (with low statistical significance) some interesting features that may indicate that some specific classes of objects contribute to the EGB. A chi2 analysis confirms that the correlation properties of the 21-month data do not provide strong constraints of the EGB origin. However, the results suggest that the situation will significantly improve with the 5- and 10-year Fermi-LAT data. The future EGB analysis will then allow placing significant constraints on the nature of the EGB and might provide in addition a detection of the ISW signal. (Abridged)

astro-ph.CO↗

arXiv2010

Identification of Dark Matter particles with LHC and direct detection data

Dark matter (DM) is currently searched for with a variety of detection strategies. Accelerator searches are particularly promising, but even if Weakly Interacting Massive Particles (WIMPs) are found at the Large Hadron Collider (LHC), it will be difficult to prove that they constitute the bulk of the DM in the Universe. We show that a significantly better reconstruction of the DM properties can be obtained with a combined analysis of LHC and direct detection (DD) data, by making a simple Ansatz on the WIMP local density, i.e. by assuming that the local density scales with the cosmological relic abundance. We demonstrate this method in an explicit example in the context of a 24-parameter supersymmetric model, with a neutralino LSP in the stau co-annihilation region. Our results show that future ton-scale DD experiments will allow to break degeneracies in the SUSY parameter space and achieve a significantly better reconstruction of the neutralino composition and its relic density than with LHC data alone.

hep-ph↗

arXiv2009

Dark Matter Annihilation around Intermediate Mass Black Holes: an update

The formation and evolution of Black Holes inevitably affects the distribution of dark and baryonic matter in the neighborhood of the Black Hole. These effects may be particularly relevant around Supermassive and Intermediate Mass Black Holes (IMBHs), the formation of which can lead to large Dark Matter overdensities, called {\em spikes} and {\em mini-spikes} respectively. Despite being larger and more dense, spikes evolve at the very centers of galactic halos, in regions where numerous dynamical effects tend to destroy them. Mini-spikes may be more likely to survive, and they have been proposed as worthwhile targets for indirect Dark Matter searches. We review here the formation scenarios and the prospects for detection of mini-spikes, and we present new estimates for the abundances of mini-spikes to illustrate the sensitivity of such predictions to cosmological parameters and uncertainties regarding the astrophysics of Black Hole formation at high redshift. We also connect the IMBHs scenario to the recent measurements of cosmic-ray electron and positron spectra by the PAMELA, ATIC, H.E.S.S., and Fermi collaborations.

astro-ph.HE↗

arXiv2009

Dark Matter signals from Draco and Willman 1: Prospects for MAGIC II and CTA

The next generation of ground-based Imaging Air Cherenkov Telescopes (IACTs) will play an important role in indirect dark matter searches. In this article, we consider two particularly promising candidate sources for dark matter annihilation signals, the nearby dwarf galaxies Draco and Willman 1, and study the prospects of detecting such a signal for the soon-operating MAGIC II telescope system as well as for the planned installation of CTA, taking special care of describing the experimental features that affect the detectional prospects. For the first time in such a study, we fully take into account the effect of internal bremsstrahlung, which has recently been shown to considerably enhance, in some cases, the gamma-ray flux at the high energies where Atmospheric Cherenkov Telescopes operate, thus leading to significantly harder annihilation spectra than traditionally considered. While the detection of the spectral features introduced by internal bremsstrahlung would constitute a smoking gun signature for dark matter annihilation, we find that for most models the overall flux still remains at a level that will be challenging to detect unless one adopts rather (though by no means overly) optimistic astrophysical assumptions about the distribution of dark matter in the dwarfs.

astro-ph↗

arXiv2009

Anisotropy probe of galactic and extra-galactic Dark Matter annihilations

We study the flux and the angular power spectrum of gamma-rays produced by Dark Matter (DM) annihilations in the Milky Way (MW) and in extra-galactic halos. The annihilation signal receives contributions from: a) the smooth MW halo, b) resolved and unresolved substructures in the MW, c) external DM halos at all redshifts, including d) their substructures. Adopting a self-consistent description of local and extra-galactic substructures, we show that the annihilation flux from substructures in the MW dominates over all the other components for angles larger than O(1) degrees from the Galactic Center, unless an extreme prescription is adopted for the substructures concentration. We also compute the angular power spectrum of gamma-ray anisotropies and find that, for an optimistic choice of the particle physics parameters, an interesting signature of DM annihilations could soon be discovered by the Fermi LAT satellite at low multipoles, l<100, where the dominant contribution comes from MW substructures with mass M>10^4 solar masses. For the substructures models we have adopted, we find that the contribution of extra-galactic annihilations is instead negligible at all scales.

astro-ph.CO↗