ArticlePublisher preview available

Constraints on decaying dark matter from the isotropic gamma-ray background

IOP Publishing
Journal of Cosmology and Astroparticle Physics
Authors:
Carlos Blanco at Universidad Nacional del Altiplano, Puno
Carlos Blanco
Dan Hooper
Dan Hooper
Dan Hooper
  • This person is not on ResearchGate, or hasn't claimed this research yet.
Read publisher preview
Download citation
To read the full-text of this research, you can request a copy directly from the authors.

Abstract

If the dark matter is unstable, the decay of these particles throughout the universe and in the halo of the Milky Way could contribute significantly to the isotropic gamma-ray background (IGRB) as measured by Fermi. In this article, we calculate the high-latitude gamma-ray flux resulting from dark matter decay for a wide range of channels and masses, including all contributions from inverse Compton scattering and accounting for the production and full evolution of cosmological electromagnetic cascades. We also make use of recent multi-wavelength analyses that constrain the astrophysical contributions to the IGRB, enabling us to more strongly restrict the presence any component arising from decaying dark matter. Over a wide range of decay channels and masses (from GeV to EeV and above), we derive stringent lower limits on the dark matter's lifetime, generally in the range of τ ∼ (1-5)× 10 ²⁸ s.}.
A preview of this full-text is provided by IOP Publishing.
Learn more
Content available from Journal of Cosmology and Astroparticle Physics 
This content is subject to copyright. Terms and conditions apply.
JCAP03(2019)019
ournal of
Cosmology and Astroparticle Physics
An IOP and SISSA journal
J
Constraints on decaying dark matter
from the isotropic gamma-ray
background
Carlos Blancoa,b,1and Dan Hooperb,c,d,2
aUniversity of Chicago, Department of Physics,
Chicago, IL 60637, U.S.A.
bUniversity of Chicago, Kavli Institute for Cosmological Physics,
Chicago, IL 60637, U.S.A.
cFermi National Accelerator Laboratory, Center for Particle Astrophysics,
Batavia, IL 60510, U.S.A.
dUniversity of Chicago, Department of Astronomy and Astrophysics,
Chicago, IL 60637, U.S.A.
E-mail: carlosblanco2718@uchicago.edu,dhooper@fnal.gov
Received December 1, 2018
Revised February 1, 2019
Accepted February 27, 2019
Published March 11, 2019
Abstract. If the dark matter is unstable, the decay of these particles throughout the universe
and in the halo of the Milky Way could contribute significantly to the isotropic gamma-ray
background (IGRB) as measured by Fermi. In this article, we calculate the high-latitude
gamma-ray flux resulting from dark matter decay for a wide range of channels and masses,
including all contributions from inverse Compton scattering and accounting for the produc-
tion and full evolution of cosmological electromagnetic cascades. We also make use of recent
multi-wavelength analyses that constrain the astrophysical contributions to the IGRB, en-
abling us to more strongly restrict the presence any component arising from decaying dark
matter. Over a wide range of decay channels and masses (from GeV to EeV and above),
we derive stringent lower limits on the dark matter’s lifetime, generally in the range of
τ∼(1–5) ×1028 s.
Keywords: dark matter theory, gamma ray theory, semi-analytic modeling
ArXiv ePrint: 1811.05988
1ORCID: http://orcid.org/0000-0001-8971-834X.
2ORCID: http://orcid.org/0000-0001-8837-4127.
c
2019 IOP Publishing Ltd and Sissa Medialab https://doi.org/10.1088/1475-7516/2019/03/019
... Note that while Fermi is optimized to search for O(GeV) photons, it can be sensitive to much heavier DM, as the high energy photons, electrons, and positrons produced in the decay will interact with cosmic background radiation, generating a cascade process converting energy down to lower scales [130][131][132][133][134]. The lower bound on the lifetime of heavy DM becomes essentially mass independent for DM masses above a few PeV if any appreciable portion of the mass energy is deposited into electromagnetic channels, since leptons and photons at these energies rapidly produce electromagnetic cascades extending down to GeV energies which may become visible in the isotropic gamma-ray sky [134]. ...
... Note that while Fermi is optimized to search for O(GeV) photons, it can be sensitive to much heavier DM, as the high energy photons, electrons, and positrons produced in the decay will interact with cosmic background radiation, generating a cascade process converting energy down to lower scales [130][131][132][133][134]. The lower bound on the lifetime of heavy DM becomes essentially mass independent for DM masses above a few PeV if any appreciable portion of the mass energy is deposited into electromagnetic channels, since leptons and photons at these energies rapidly produce electromagnetic cascades extending down to GeV energies which may become visible in the isotropic gamma-ray sky [134]. If the direct decay products can be observed, however, the constraints are generally stronger as seen from the other constraints in Fig. 3. ...
... When considering photon final states, a careful accounting of the inverse-Compton scattering (ICS) contribution is also required [139]. Furthermore, the full development of electromagnetic cascades, from a cycle of ICS and pair-production, must be taken into account in order to predict diffuse and isotropic signals [131][132][133][134]140]. Before the propagation The results are chosen to highlight the complementarity between different search strategies for this single DM hypothesis, and in particular we show limits obtained using γ-ray [126], neutrino [127], and cosmic-ray [128] studies. ...
Snowmass2021 cosmic frontier white paper: Ultraheavy particle dark matter
Article
Full-text available
  • Nov 2023
We outline the unique opportunities and challenges in the search for “ultraheavy” dark matter candidates with masses between roughly 10 TeV and the Planck scale m_{\rm pl} ≈ 10^{16} TeV. This mass range presents a wide and relatively unexplored dark matter parameter space, with a rich space of possible models and cosmic histories. We emphasize that both current detectors and new, targeted search techniques, via both direct and indirect detection, are poised to contribute to searches for ultraheavy particle dark matter in the coming decade. We highlight the need for new developments in this space, including new analyses of current and imminent direct and indirect experiments targeting ultraheavy dark matter and development of new, ultra-sensitive detector technologies like next-generation liquid noble detectors, neutrino experiments, and specialized quantum sensing techniques.
View
... A significant fraction of the dark matter contribution to the IGRB comes from our own Galactic halo (e.g. [47]). Our halo has a dark matter distribution that we approximate with a spherically symmetric density profile ρ gal ðrÞ. ...
... The majority of the IGRB is known to originate from astrophysical sources (e.g. [47,56]). In order to model these contributions, we consider spectral components from blazar and nonblazar sources. ...
Limits on dark matter annihilation in prompt cusps from the isotropic gamma-ray background
Article
Full-text available
  • Apr 2024
Recent studies indicate that thermally produced dark matter will form highly concentrated, low-mass cusps in the early universe that often survive until the present. While these cusps contain a small fraction of the dark matter, their high density significantly increases the expected γ-ray flux from dark matter annihilation, particularly in searches of large angular regions. We utilize 14 years of Fermi-LAT data to set strong constraints on dark matter annihilation through a detailed study of the isotropic γ-ray background, excluding with 95% confidence dark matter annihilation to bb¯ final states for dark matter masses below 120 GeV.
View
... 12 Reference [115] constrained τ ≳ 1.3 × 10 27 s for DM decay to bb for m DM > 10 11 GeV; considering that the upper limit only depends on the energy injected into nonneutrino species, we may infer that the upper limit for hh or W þ W − final states would be the same to within 10%. Reference [116] used more aggressive modeling of the extragalactic gamma-ray background to constrain τ ≳ 10 28 s for m DM ≳ 10 9 GeV, for both W þ W − and hh final states, with the limits insensitive at the less than 10% level to m DM for m DM ≳ 10 9 GeV [115]. Since the energy injection is dominated by decays with z ≪ 1, we may translate these limits to limits on f a through the relation f a M pl ...
... where ξðz ¼ 0Þ is the value at z ¼ 0. Using the lifetime bound from [116], this then implies f a ≲ 9.2 × 10 11 GeV ...
Signatures of primordial energy injection from axion strings
Article
Full-text available
  • Mar 2024
Axion strings are horizon-size topological defects that may be produced in the early Universe. Ultralight axion-like particles may form strings that persist to temperatures below that of big bang nucleosynthesis. Such strings have been considered previously as sources of gravitational waves and cosmic microwave background (CMB) polarization rotation. In this work we show, through analytic arguments and dedicated adaptive mesh refinement cosmological simulations, that axion strings deposit a subdominant fraction of their energy into high-energy Standard Model (SM) final states, for example, by the direct production of heavy radial modes that subsequently decay to SM particles. This high-energy SM radiation is absorbed by the primordial plasma, leading to novel signatures in precision big bang nucleosynthesis, the CMB power spectrum, and gamma-ray surveys. In particular, we show that CMB power spectrum data constrains axion strings with decay constants fa≲1012 GeV, up to model dependence on the ultraviolet completion, for axion masses ma≲10−29 eV; future CMB surveys could find striking evidence of axion strings with lower decay constants.
View
... Here we define flux (Φ) in units of photons cm −2 s −1 . Generalizing the formulae in [54][55][56] for example, we immediately find: ...
Astrophysical constraints on decaying dark gravitons
Article
Full-text available
  • Jun 2024
  • J HIGH ENERGY PHYS
A bstract In the dark dimension scenario, which predicts an extra dimension of micron scale, dark gravitons (KK modes) are a natural dark matter candidate. In this paper, we study observable features of this model. In particular, their decay to standard matter fields can distort the CMB and impact other astrophysical signals. Using this we place bounds on the parameters of this model. In particular we find that the natural range of parameters in this scenario is consistent with these constraints and leads to the prediction that the mean mass of the dark matter today is close to a few hundred keV and the effective size of the extra dimension is around 1–30 μm.
View
... While future UHE neutrino detection are found to yield constraints comparable to that from UHECRs, UHE gamma-ray detection will lead to a few order of magnitudes improved constraints than that from UHECRs. Our constraints based on the TA Amaterasu event are consistent with those existing in the literature [31,40,41,50,[59][60][61][62][63][64][65][66][67][68][69][70][71][72][73][74][75][76]. These works are based on various observations of UHE-CRs, UHE gamma-rays, and UHE neutrinos, but not the recent TA Amaterasu event. ...
The Amaterasu particle: constraining the superheavy dark matter origin of UHECRs
Preprint
Full-text available
  • Jun 2024
Amaterasu, the second most energetic ($244$ EeV) cosmic ray particle has been recently detected by the Telescope Array (TA) surface detector. The origin of the TA Amaterasu event is puzzling, as its arrival direction points back to a void in the local Universe, lacking conventional astrophysical ultra-high-energy (UHE) cosmic ray (CR) sources. Hence, we explore the possibility if this TA Amaterasu event could have originated from the decay of superheavy dark matter (SHDM) in the Milky Way. Such an origin also opens up multi-messenger detection channels in both UHE gamma-rays and UHE neutrinos. In this present work, using the TA Amaterasu event and the multi-messenger limits/sensitivities from various UHE telescopes, we place stringent constraints on the lifetime and mass of the SHDM. We find that the non-detection of the corresponding gamma-rays at the Pierre Auger Observatory (PAO) and the TA is in severe tension with the SHDM parameter space required to explain the TA Amaterasu event. Additionally, we extend the multi-messenger analysis to the future UHE gamma-ray and UHE neutrino telescopes such as PAO upgrade, GRAND 200k and IceCube-Gen2. We find that the bounds from the future neutrino telescopes will be able to compete with the present UHECR bounds. However, compared to the existing UHE gamma-ray bounds, the future PAO upgrade and the GRAND 200k gamma-ray detectors will improve the bounds on SHDM lifetime by at least one order of magnitude.
View
... For this reason, high-energy astrophysical observation plays a crucial role because it provides a unique window for probing heavy DM. Many of current constraints on the annihilation cross sections and lifetime of heavy DM are based on observations of electromagnetic emission, which use various targets with distinct advantages to probe DM include the diffuse gamma-ray background [19][20][21][22][23][24][25][26][27], galaxy clusters [28][29][30][31][32][33][34][35][36][37][38][39], the Milky Way halo [40][41][42][43][44][45][46][47], and dwarf spheroidal galaxies (dSphs) [6,[48][49][50][51][52][53][54][55][56][57][58][59][60][61][62]. Multi-messenger approaches are also powerful, and other constraints on heavy DM include those from charged cosmic rays [63][64][65][66][67][68][69][70][71], neutrinos [33,43,59,69,[72][73][74], and the cosmic microwave background (CMB) [75,76]. ...
Search for heavy dark matter from dwarf spheroidal galaxies: leveraging cascades and subhalo models
Article
Full-text available
The Fermi Large Area Telescope (Fermi-LAT) has been widely used to search for Weakly Interacting Massive Particle (WIMP) dark matter signals due to its unparalleled sensitivity in the GeV energy band. The leading constraints for WIMP by Fermi-LAT are obtained from the analyses of dwarf spheroidal galaxies within the Local Group, which are compelling targets for dark matter searches due to their relatively low astrophysical backgrounds and high dark matter content. In the meantime, the search for heavy dark matter with masses above TeV remains a compelling and relatively unexplored frontier. In this study, we utilize 14-year Fermi-LAT data to search for dark matter annihilation and decay signals in 8 classical dwarf spheroidal galaxies within the Local Group. We consider secondary emission caused by electromagnetic cascades of prompt gamma rays and electrons/positrons from dark matter, which enables us to extend the search with Fermi-LAT to heavier dark matter cases. We also update the dark matter subhalo model with informative priors respecting the fact that they reside in subhalos of our Milky Way halo aiming to enhance the robustness of our results. We place constraints on dark matter annihilation cross section and decay lifetime for dark matter masses ranging from 10³ GeV to 10¹¹ GeV, where our limits are more stringent than those obtained by many other high-energy gamma-ray instruments.
View
... However, the high-energy gamma rays and electrons/positrons (e ± ) produced by dark matter decay interact with the radiation and magnetic fields inside galaxy clusters, causing electromagnetic cascades. The secondary gamma rays generated through inverse-Compton and synchrotron processes are cascaded down to the energy range that Fermi-LAT can detect [36,43,[67][68][69]. We search for gamma-ray signals from dark matter decay in galaxy clusters taking into account the electromagnetic cascades and place limits on the lifetime of dark matter. ...
Constraining decaying very heavy dark matter from galaxy clusters with 14 year Fermi-LAT data
Article
Full-text available
Galaxy clusters are promising targets for indirect detection of dark matter thanks to the large dark matter content. Using 14 years of Fermi-LAT data from seven nearby galaxy clusters, we obtain constraints on the lifetime of decaying very heavy dark matter particles with masses ranging from 10³ GeV to 10¹⁶ GeV. We consider a variety of decaying channels and calculate prompt gamma rays and electrons/positrons from the dark matter. Furthermore, we take into account electromagnetic cascades induced by the primary gamma rays and electrons/positrons, and search for the resulting gamma-ray signals from the directions of the galaxy clusters. We adopt a Navarro-Frenk-White profile of the dark matter halos, and use the profile likelihood method to set lower limits on the dark matter lifetime at a 95% confidence level. Our results are competitive with those obtained through other gamma-ray observations of galaxy clusters and provide complementary constraints to existing indirect searches for decaying very heavy dark matter.
View
... The Fermi LAT IGRB measurement provides one of the strongest lower limits on the lifetime τ of decaying dark matter. The limits for several decay channels exceed τ (1 − 5) × 10 28 over a DM particle mass range from a few GeV to EeV [79]. In contrast to the signal from annihilation, which is proportional to the square of the dark matter density, and, therefore, best visible in overdense regions such as the centre of the Milky Way, a signal from decaying dark matter is proportional to the density only. ...
Searches for beyond-standard-model physics with astroparticle physics instruments
Article
Full-text available
  • Dec 2023
Many instruments for astroparticle physics are primarily geared towards multi-messenger astrophysics, to study the origin of cosmic rays and to understand high-energy astrophysical processes. Since these instruments observe the Universe at extreme energies and in kinematic ranges not accessible at accelerators these experiments provide also unique and complementary opportunities to search for particles and physics beyond the standard model of particle physics. In particular, the reach of IceCube, Fermi and KATRIN to search for and constrain Dark Matter, Axions, heavy Big Bang relics, sterile neutrinos and Lorentz invariance violation will be discussed. The contents of this article are based on material presented at the Humboldt–Kolleg ‘Clues to a mysterious Universe—exploring the interface of particle, gravity and quantum physics’ in June 2022. This article is part of the theme issue ‘The particle-gravity frontier’.
View
... If the GCE is due to dark matter annihilation, then annihilation must account for a significant fraction of the entire 1 to 10 GeV background. This is in tension with claims that almost all of this background is produced by unresolved discrete extragalactic sources such as star-forming galaxies and active galactic nuclei [59]. ...
Prompt cusps and the dark matter annihilation signal
Article
Full-text available
As the first dark matter objects gravitationally condense, a density cusp forms immediately at every initial density maximum. Numerical simulations and theoretical arguments suggest that these prompt cusps can survive until the present day. We show that if dark matter is a thermally produced weakly interacting massive particle, many thousands of prompt cusps with individual masses similar to that of the Earth may be present in every solar mass of dark matter. This radically alters predictions for the amount and spatial distribution of dark matter annihilation radiation. The annihilation rate is boosted by at least an order of magnitude compared to previous predictions, both in the cosmological average and within galaxy-scale halos. Moreover, the signal is predominantly boosted outside of the centers of galactic halos, so alternative targets become significantly more attractive for indirect-detection searches. For example, prompt cusps present new opportunities to test the dark matter interpretation of the Galactic Center γ-ray excess by searching for similar spectral signatures in the isotropic γ-ray background and large-scale cosmic structure.
View
Sub-GeV millicharge dark matter from the U(1) X hidden sector
Article
We conduct a comprehensive study on the sub-GeV millicharge dark matter produced through the freeze-in mechanism. We discuss in general the mixing mechanism, encompassing both kinetic mixing and mass mixing, between the U(1) X hidden sector and the standard model, which can generate millicharge carried by the dark fermions from the hidden sector. We discuss in depth how such millicharge is generated, and clarify several misunderstandings regarding this subject in the literature. Without employing an effective field theory approach, where the photon field directly mixed with the additional U(1), we analyze a general renormalizable model and investigate the complete evolution of the hidden sector particles. Due to the substantial self-interactions among hidden sector particles, the evolution of the hidden sector temperature plays a crucial role, which is addressed concurrently with the number densities of hidden sector particles by solving a set of coupled Boltzmann equations. We thoroughly examine eight benchmark models from six distinct cases. Some of our key findings from the analysis of these benchmark models may be generalizable and applicable to broader freeze-in scenarios. We also explore the possibility that the 𝒪(keV) U(1) X dark photon is a viable dark matter candidate, even though it can contribute at most ∼ 5% to the total observed dark matter relic density.
View
{\gamma}-Cascade: A Simple Program to Compute Cosmological Gamma-Ray Propagation
Article
Full-text available
Modeling electromagnetic cascades during gamma-ray transport is important in many applications within astrophysics. This document introduces {\gamma}-Cascade, a publicly available Mathematica package which allows users to calculate observed gamma-ray fluxes from point sources as well as from a distribution of sources. {\gamma}-Cascade semi-analytically computes the effects of electromagnetic interactions during gamma-ray transport.
View
View
A search for dark matter in the Galactic halo with HAWC
Article
Full-text available
The High Altitude Water Cherenkov (HAWC) gamma-ray observatory is a wide field-of-view observatory sensitive to 500 GeV – 100 TeV gamma rays and cosmic rays. With its observations over 2/3 of the sky every day, the HAWC observatory is sensitive to a wide variety of astrophysical sources, including possible gamma rays from dark matter. Dark matter annihilation and decay in the Milky Way Galaxy should produce gamma-ray signals across many degrees on the sky. The HAWC instantaneous field-of-view of 2 sr enables observations of extended regions on the sky, such as those from dark matter in the Galactic halo. Here we show limits on the dark matter annihilation cross-section and decay lifetime from HAWC observations of the Galactic halo with 15 months of data. These are some of the most robust limits on TeV and PeV dark matter, largely insensitive to the dark matter morphology. These limits begin to constrain models in which PeV IceCube neutrinos are explained by dark matter which primarily decays into hadrons.
View
Novel Gamma-Ray Signatures of PeV-Scale Dark Matter
Article
Full-text available
The gamma-ray annihilation and decay products of very heavy dark matter particles can undergo attenuation through pair production, leading to the development of electromagnetic cascades. This has a significant impact not only on the spectral shape of the gamma-ray signal, but also on the angular distribution of the observed photons. Such phenomena are particularly important in light of the new HAWC experiment, which provides unprecedented sensitivity to multi-TeV photons and thus to very heavy dark matter particles. In this study, we focus on dark matter in the 100 TeV-100 PeV mass range, and calculate the spectral and angular distribution of gamma-rays from dwarf galaxies and from nearby galaxy clusters in this class of models.
View
Constraining dark matter lifetime with a deep gamma-ray survey of the Perseus galaxy cluster with MAGIC
Article
  • Aug 2018
Clusters of galaxies are the largest known gravitationally bound structures in the Universe, with masses around 10¹⁵ M⊙, most of it in the form of dark matter. The ground-based Imaging Atmospheric Cherenkov Telescope MAGIC made a deep survey of the Perseus cluster of galaxies using almost 400 h of data recorded between 2009 and 2017. This is the deepest observational campaign so far on a cluster of galaxies in the very high energy range. We search for gamma-ray signals from dark matter particles in the mass range between 200 GeV and 200 TeV decaying into standard model pairs. We apply an analysis optimized for the spectral and morphological features expected from dark matter decays and find no evidence of decaying dark matter. From this, we conclude that dark matter particles have a decay lifetime longer than ∼10²⁶ s in all considered channels. Our results improve previous lower limits found by MAGIC and represent the strongest limits on decaying dark matter particles from ground-based gamma-ray instruments.
View
Implications of a 21-cm signal for dark matter annihilation and decay
Article
  • Jul 2018
Measurements of the temperature of the baryons at the end of the cosmic dark ages can potentially set very precise constraints on energy injection from exotic sources, such as annihilation or decay of the dark matter. However, additional effects that lower the gas temperature can substantially weaken the expected constraints on exotic energy injection, whereas additional radiation backgrounds can conceal the effect of an increased gas temperature in measurements of the 21-cm hyperfine transition of neutral hydrogen. Motivated in part by recent claims of a detection of 21-cm absorption from z∼17 by the EDGES experiment, we derive the constraints on dark matter annihilation and decay that can be placed in the presence of extra radiation backgrounds or effects that modify the gas temperature, such as dark matter (DM)–baryon scattering and early baryon-photon decoupling. We find that if the EDGES observation is confirmed, then constraints on light DM decaying or annihilating to electrons will in most scenarios be stronger than existing state-of-the-art limits from the cosmic microwave background, potentially by several orders of magnitude. More generally, our results allow mapping any future measurement of the global 21-cm signal into constraints on dark matter annihilation and decay, within the broad range of scenarios we consider.
View
micrOMEGAs5.0 : freeze-in
Article
  • Jan 2018
  • COMPUT PHYS COMMUN
We present a major upgrade of the micrOMEGAs dark matter code to compute the abundance of feebly interacting dark matter candidates through the freeze-in mechanism in generic extensions of the Standard Model of particle physics. We develop the necessary formalism in order to solve the freeze-in Boltzmann equations while making as few simplifying assumptions as possible concerning the phase-space distributions of the particles involved in the dark matter production process. We further show that this formalism allows us to treat different freeze-in scenarios and discuss the way it is implemented in the code. We find that, depending on the New Physics scenario under consideration, the effect of a proper treatment of statistics on the predicted dark matter abundance can range from a few percent up to a factor of two, or more. We moreover illustrate the underlying physics, as well as the various novel functionalities of micrOMEGAs, by presenting several example results obtained for different dark matter models.
View
Measuring the Local Diffusion Coefficient with H.E.S.S. Observations of Very High-Energy Electrons
Article
  • Nov 2017
The HAWC Collaboration has recently reported the detection of bright and spatially extended multi-TeV gamma-ray emission from Geminga, Monogem, and a handful of other nearby, middle-aged pulsars. The angular profile of the emission observed from these pulsars is surprising, in that it implies that cosmic-ray diffusion is significantly inhibited within ~25 pc of these objects, compared to the expectations of standard Galactic diffusion models. This raises the important question of whether the diffusion coefficient in the local interstellar medium is also low, or whether it is instead better fit by the mean Galactic value. Here, we utilize recent observations of the cosmic-ray electron spectrum (extending up to ~20 TeV) by the H.E.S.S. Collaboration to show that the local diffusion coefficient cannot be as low as it is in the regions surrounding Geminga and Monogem. Instead, we conclude that cosmic rays efficiently diffuse through the bulk of the local interstellar medium. Among other implications, this further supports the conclusion that pulsars significantly contribute to the observed positron excess.
View
Gamma-ray Constraints on Decaying Dark Matter and Implications for IceCube
Article
  • Dec 2016
Utilizing the Fermi measurement of the gamma-ray spectrum toward the Galactic Center, we derive some of the strongest constraints to date on the dark matter (DM) lifetime in the mass range from hundreds of MeV to above an EeV. Our profile-likelihood based analysis relies on 413 weeks of Fermi Pass 8 data from 200 MeV to 2 TeV, along with up-to-date models for diffuse gamma-ray emission within the Milky Way. We model Galactic and extragalactic DM decay and include contributions to the DM-induced gamma-ray flux resulting from both primary emission and inverse-Compton scattering of primary electrons and positrons. For the extragalactic flux, we also calculate the spectrum associated with cascades of high-energy gamma-rays scattering off of the cosmic background radiation. We argue that a decaying DM interpretation for the 10 TeV-1 PeV neutrino flux observed by IceCube is disfavored by our constraints. Our results also challenge a decaying DM explanation of the AMS-02 positron flux. We interpret the results in terms of individual final states and in the context of simplified scenarios such as a hidden-sector glueball model.
View
Star-Forming Galaxies Significantly Contribute to the Isotropic Gamma-Ray Background
Article
  • Dec 2016
The origin of the isotropic gamma-ray background (IGRB) --- the portion of the extragalactic gamma-ray sky that is not resolvable into individual point sources --- provides a powerful probe into the evolution of the high-energy universe. Star-forming galaxies (SFGs) are among the most likely contributors to the IGRB, though their contribution is difficult to constrain because their flux distribution is dominated by numerous faint sources. We produce a novel joint-likelihood analysis of the $\gamma$-ray emission from 584 SFGs, utilizing advanced statistical techniques to compare the distribution of low-significance excesses against the non-Poissonian $\gamma$-ray background fluctuations. We first examine the theoretically well-motivated relationship between the far-IR and $\gamma$-ray luminosities of SFGs, utilizing a model where the $\gamma$-ray luminosity is given by log$_{10}$(L$_\gamma$/(erg s$^{-1}$)) = $\alpha$ log$_{10}$(L$_{IR}$/(10$^{10}$L$_\odot$)) + $\beta$. We calculate best-fit parameters $\alpha$ = 1.18 $\pm$ 0.15, $\beta$ = 38.49 $\pm$ 0.24, with a log-normal dispersion in this relationship given by $\sigma$ = 0.39 $\pm$ 0.12. The best-fit values of $\alpha$ and $\beta$ are consistent with previous studies. We find a larger dispersion in the far-IR to $\gamma$-ray correlation than previous studies. This dispersion is significant at the level of 5.7$\sigma$. These results imply that SFGs significantly contribute to the IGRB, producing between 61.0$^{+30.2}_{-18.3}\%$ of the total IGRB intensity above an energy of 1 GeV. Along with recent works, this strongly indicates that multiple source classes provide comparable contributions to the IGRB intensity. We discuss the implication of these results for the interpretation of the IceCube neutrinos.
View