arXiv subjects

Tim Linden

Publications and source records attributed to Tim Linden.

At least 73 records · Page 4Linked to original sources

arXiv2021

Celestial-Body Focused Dark Matter Annihilation Throughout the Galaxy

Indirect detection experiments typically measure the flux of annihilating dark matter (DM) particles propagating freely through galactic halos. We consider a new scenario where celestial bodies "focus" DM annihilation events, increasing the efficiency of halo annihilation. In this setup, DM is first captured by celestial bodies, such as neutron stars or brown dwarfs, and then annihilates within them. If DM annihilates to sufficiently long-lived particles, they can escape and subsequently decay into detectable radiation. This produces a distinctive annihilation morphology, which scales as the product of the DM and celestial body densities, rather than as DM density squared. We show that this signal can dominate over the halo annihilation rate in $γ$-ray observations in both the Milky Way Galactic center and globular clusters. We use \textit{Fermi} and H.E.S.S. data to constrain the DM-nucleon scattering cross section, setting powerful new limits down to $\sim10^{-39}~$cm$^2$ for sub-GeV DM using brown dwarfs, which is up to nine orders of magnitude stronger than existing limits. We demonstrate that neutron stars can set limits for TeV-scale DM down to about $10^{-47}~$cm$^2$.

astro-ph.HE↗

arXiv2020

First Observations of Solar Disk Gamma Rays over a Full Solar Cycle

The solar disk is among the brightest gamma-ray sources in the sky. It is also among the most mysterious. No existing model fully explains the luminosity, spectrum, time variability, and morphology of its emission. We perform the first analysis of solar-disk gamma rays over a full 11-year solar cycle, utilizing a powerful new method to differentiate solar signals from astrophysical backgrounds. We produce: (i) a robustly measured spectrum from 100 MeV to 100 GeV, reaching a precision of several percent in the 1-10 GeV range, (ii) new results on the anti-correlation between solar activity and gamma-ray emission, (iii) strong constraints on short-timescale variability, ranging from hours to years, and (iv) new detections of the equatorial and polar morphologies of high-energy gamma rays. Intriguingly, we find no significant energy dependence in the time variability of solar-disk emission, indicating that strong magnetic-field effects close to the solar surface, rather than modulation throughout the heliosphere, must primarily control the flux and morphology of solar-disk emission.

astro-ph.HE↗

arXiv2020

Anti-Deuterons and Anti-Helium Nuclei from Annihilating Dark Matter

Recent studies of the cosmic-ray antiproton-to-proton ratio have identified an excess of $\sim$10-20 GeV antiprotons relative to the predictions of standard astrophysical models. Intriguingly, the properties of this excess are consistent with the same range of dark matter models that can account for the long-standing excess of $γ$-rays observed from the Galactic Center. Such dark matter candidates can also produce significant fluxes of anti-deuterium and anti-helium nuclei. Here we study the production and transport of such particles, both from astrophysical processes as well as from dark matter annihilation. Importantly, in the case of AMS-02, we find that Alfvénic reacceleration (i.e., diffusion in momentum space) can boost the expected number of $\bar{\rm d}$ and ${}^{3}\overline{\textrm{He}}$ events from annihilating dark matter by an order of magnitude or more. For relatively large values of the Alfvén speed, and for dark matter candidates that are capable of producing the antiproton and $γ$-ray excesses, we expect annihilations to produce a few anti-deuteron events and about one anti-helium event in six years of AMS-02 data. This is particularly interesting in light of recent reports from the AMS-02 Collaboration describing the detection of a number of anti-helium candidate events.

astro-ph.HE↗

arXiv2020

Constraining the Charge-Sign and Rigidity-Dependence of Solar Modulation

Our ability to identify the sources of cosmic rays and understand how these particles propagate through the interstellar medium is hindered by the combined effects of the solar wind and its embedded magnetic field, collectively known as solar modulation. In this paper, we build upon our previous work to model and constrain the effects of solar modulation on the cosmic-ray spectrum, using data from AMS-02 and BESS Polar II collected between 2007 and 2012, during which the heliospheric magnetic field was in a state of negative polarity. Our model uses measurements of the heliospheric magnetic field and the tilt angle of the heliospheric current sheet to accurately predict the effects of solar modulation as a function of time, charge, and rigidity. By incorporating data from a period of negative polarity, we have been able to robustly observe and constrain the charge-dependent effects of solar modulation.

astro-ph.HE↗

arXiv2020

Dark Matter Annihilation Can Produce a Detectable Antihelium Flux through $\barΛ_b$ Decays

Recent observations by the Alpha Magnetic Spectrometer (AMS-02) have tentatively detected a handful of cosmic-ray antihelium events. Such events have long been considered as smoking-gun evidence for new physics, because astrophysical antihelium production is expected to be negligible. However, the dark-matter-induced antihelium flux is also expected to fall below current sensitivities, particularly in light of existing antiproton constraints. Here, we demonstrate that a previously neglected standard model process -- the production of antihelium through the displaced-vertex decay of $\barΛ_b$-baryons -- can significantly boost the dark matter induced antihelium flux. This process can triple the standard prompt-production of antihelium, and more importantly, entirely dominate the production of the high-energy antihelium nuclei reported by AMS-02.

hep-ph↗

arXiv2020

Breaking a dark degeneracy: The gamma-ray signature of early matter domination

The Universe's early thermal history is poorly constrained, and it is possible that it underwent a period of early matter domination driven by a heavy particle or an oscillating scalar field that decayed into radiation before the onset of Big Bang nucleosynthesis. The entropy sourced by this particle's decay reduces the cross section required for thermal-relic dark matter to achieve the observed abundance. This degeneracy between dark matter properties and the thermal history vastly widens the field of viable dark matter candidates, undermining efforts to constrain dark matter's identity. Fortunately, an early matter-dominated era also amplifies density fluctuations at small scales and leads to early microhalo formation, boosting the dark matter annihilation rate and bringing smaller cross sections into the view of existing indirect-detection probes. Employing several recently developed models of microhalo formation and evolution, we develop a procedure to derive indirect-detection constraints on dark matter annihilation in cosmologies with early matter domination. This procedure properly accounts for the unique morphology of microhalo-dominated signals. While constraints depend on dark matter's free-streaming scale, the microhalos make it possible to obtain upper bounds as small as $\langleσv\rangle \lesssim 10^{-32}$ cm$^3$s$^{-1}$ using Fermi-LAT observations of the isotropic gamma-ray background and the Draco dwarf galaxy.

astro-ph.CO↗

arXiv2019

TeV Halos are Everywhere: Prospects for New Discoveries

Milagro and HAWC have detected extended TeV gamma-ray emission around nearby pulsar wind nebulae (PWNe). Building on these discoveries, Linden et al. [1] identified a new source class -- TeV halos -- powered by the interactions of high-energy electrons and positrons that have escaped from the PWN, but which remain trapped in a larger region where diffusion is inhibited compared to the interstellar medium. Many theoretical properties of TeV halos remain mysterious, but empirical arguments suggest that they are ubiquitous. The key to progress is finding more halos. We outline prospects for new discoveries and calculate their expectations and uncertainties. We predict, using models normalized to current data, that future HAWC and CTA observations will detect in total $\sim$50--240 TeV halos, though we note that multiple systematic uncertainties still exist. Further, the existing HESS source catalog could contain $\sim$10--50 TeV halos that are presently classified as unidentified sources or PWN candidates. We quantify the importance of these detections for new probes of the evolution of TeV halos, pulsar properties, and the sources of high-energy gamma rays and cosmic rays.

astro-ph.HE↗

arXiv2019

Cosmic Rays and Magnetic Fields in the Core and Halo of the Starburst M82: Implications for Galactic Wind Physics

Cosmic rays (CRs) and magnetic fields may be dynamically important in driving large-scale galactic outflows from rapidly star-forming galaxies. We construct two-dimensional axisymmetric models of the local starburst and super-wind galaxy M82 using the CR propagation code GALPROP. Using prescribed gas density and magnetic field distributions, wind profiles, CR injection rates, and stellar radiation fields, we simultaneously fit both the integrated gamma-ray emission and the spatially-resolved multi-frequency radio emission extended along M82's minor axis. We explore the resulting constraints on the gas density, magnetic field strength, CR energy density, and the assumed CR advection profile. In accord with earlier one-zone studies, we generically find low central CR pressures, strong secondary electron/positron production, and an important role for relativistic bremsstrahlung losses in shaping the synchrotron spectrum. We find that the relatively low central CR density produces CR pressure gradients that are weak compared to gravity, strongly limiting the role of CRs in driving M82's fast and mass-loaded galactic outflow. Our models require strong magnetic fields and advection speeds of order ~1000 km/s on kpc scales along the minor axis in order to reproduce the extended radio emission. Degeneracies between the controlling physical parameters of the model and caveats to these findings are discussed.

astro-ph.GA↗

arXiv2019

A Robust Excess in the Cosmic-Ray Antiproton Spectrum: Implications for Annihilating Dark Matter

An excess of $\sim$10-20 GeV cosmic-ray antiprotons has been identified in the spectrum reported by the AMS-02 Collaboration. The systematic uncertainties associated with this signal, however, have made it difficult to interpret these results. In this paper, we revisit the uncertainties associated with the time, charge and energy-dependent effects of solar modulation, the antiproton production cross section, and interstellar cosmic-ray propagation. After accounting for these uncertainties, we confirm the presence of a 4.7$σ$ antiproton excess, consistent with that arising from a $m_χ \approx 64-88$ GeV dark matter particle annihilating to $b\bar{b}$ with a cross section of $σv \simeq (0.8-5.2) \times 10^{-26}$ cm$^{3}$/s. If we allow for the stochastic acceleration of secondary antiprotons in supernova remnants, the data continues to favor a similar range of dark matter models ($m_χ\approx 46-94$ GeV, $σv \approx (0.7-3.8)\times 10^{-26}$ cm$^3/$s) with a significance of 3.3$σ$. The same range of dark matter models that are favored to explain the antiproton excess can also accommodate the excess of GeV-scale gamma rays observed from the Galactic Center.

astro-ph.HE↗

arXiv2019

A Robust Method for Treating Astrophysical Mismodeling in Dark Matter Annihilation Searches of Dwarf Spheroidal Galaxies

Fermi-LAT observations have strongly constrained dark matter annihilation through the joint-likelihood analysis of dwarf spheroidal galaxies (dSphs). These constraints are expected to be robust because dSphs have measurable dark matter content and produce negligible astrophysical emission. However, each dSph is dim, with a predicted flux that typically falls below the accuracy of the background model. We show that this significantly diminishes the reliability of previous joint-likelihood algorithms, and develop an improved analysis that directly accounts for the effect of background mismodeling. This method produces more robust limits and detections of dark matter in both real and mock data. We calculate improved limits on the dark matter annihilation cross-section, which differ by nearly a factor of two from previous analyses - despite examining identical data.

astro-ph.HE↗

arXiv2019

Looking Under a Better Lamppost: MeV-scale Dark Matter Candidates

The era of precision cosmology has revealed that about 85% of the matter in the universe is dark matter. Two well-motivated candidates are weakly interacting massive particles (WIMPs) and weakly interacting sub-eV particles (WISPs) (e.g. axions). Both WIMPs and WISPs possess distinct γ-ray signatures. Over the last decade, data taken between 50 MeV to >300 GeV by the Fermi Large Area Telescope (Fermi-LAT) have provided stringent constraints on both classes of dark matter models. Thus far, there are no conclusive detections. However, there is an intriguing γ-ray excess associated with the Galactic center that could be explained by WIMP annihilation. At lower energies, the poor angular resolution of the Fermi-LAT makes source identification challenging, inhibiting our ability to more sensitively probe both the Galactic center excess, as well as lower-mass WIMP and WISP models. Additionally, targeted WISP searches (e.g., those probing supernovae and blazars) would greatly benefit from enhanced energy resolution and polarization measurements in the MeV range. To address these issues, a new telescope that is optimized for MeV observations is needed. Such an instrument would allow us to explore new areas of dark matter parameter space and provide unprecedented access to its particle nature.

astro-ph.HE↗

arXiv2019

Positron Annihilation in the Galaxy

The 511 keV line from positron annihilation in the Galaxy was the first $γ$-ray line detected to originate from outside our solar system. Going into the fifth decade since the discovery, the source of positrons is still unconfirmed and remains one of the enduring mysteries in $γ$-ray astronomy. With a large flux of $\sim$10$^{-3}$ $γ$/cm$^{2}$/s, after 15 years in operation INTEGRAL/SPI has detected the 511 keV line at $>50σ$ and has performed high-resolution spectral studies which conclude that Galactic positrons predominantly annihilate at low energies in warm phases of the interstellar medium. The results from imaging are less certain, but show a spatial distribution with a strong concentration in the center of the Galaxy. The observed emission from the Galactic disk has low surface brightness and the scale height is poorly constrained, therefore, the shear number of annihilating positrons in our Galaxy is still not well know. Positrons produced in $β^+$-decay of nucleosynthesis products, such as $^{26}$Al, can account for some of the annihilation emission in the disk, but the observed spatial distribution, in particular the excess in the Galactic bulge, remains difficult to explain. Additionally, one of the largest uncertainties in these studies is the unknown distance that positrons propagate before annihilation. In this paper, we will summarize the current knowledge base of Galactic positrons, and discuss how next-generation instruments could finally provide the answers.

astro-ph.HE↗

arXiv2018

Unexpected Dip in the Solar Gamma-Ray Spectrum

The solar disk is a bright source of multi-GeV gamma rays, due to the interactions of hadronic cosmic rays with the solar atmosphere. However, the underlying production mechanism is not understood, except that its efficiency must be greatly enhanced by magnetic fields that redirect some cosmic rays from ingoing to outgoing before they interact. To elucidate the nature of this emission, we perform a new analysis of solar atmospheric gamma rays with 9 years of Fermi-LAT data, which spans nearly the full 11-year solar cycle. We detect significant gamma-ray emission from the solar disk from 1 GeV up to $\gtrsim200$ GeV. The overall gamma-ray spectrum is much harder ($\sim E_γ^{-2.2}$) than the cosmic-ray spectrum ($\sim E_{\rm CR}^{-2.7}$). We find a clear anticorrelation between the solar cycle phase and the gamma-ray flux between 1-10 GeV. Surprisingly, we observe a spectral dip between $\sim$30-50 GeV in an otherwise power-law spectrum. This was not predicted, is not understood, and may provide crucial clues to the gamma-ray emission mechanism. The flux above 100 GeV, which is brightest during the solar minimum, poses exciting opportunities for HAWC, LHAASO, IceCube, and KM3NeT.

astro-ph.HE↗

arXiv2018

Active Galactic Nuclei and the Origin of IceCube's Diffuse Neutrino Flux

The excess of neutrino candidate events detected by IceCube from the direction of TXS 0506+056 has generated a great deal of interest in blazars as sources of high-energy neutrinos. In this study, we analyze the publicly available portion of the IceCube dataset, performing searches for neutrino point sources in spatial coincidence with the blazars and other active galactic nuclei contained in the Fermi 3LAC and the Roma BZCAT catalogs, as well as in spatial and temporal coincidence with flaring sources identified in the Fermi Collaboration's All-Sky Variability Analysis (FAVA). We find no evidence that blazars generate a significant flux of high-energy neutrinos, and conclude that no more than 5-15% of the diffuse flux measured by IceCube can originate from this class of objects. While we cannot rule out the possibility that TXS 0506+056 has at times generated significant neutrino emission, we find that such behavior cannot be common among blazars, requiring TXS 0506+056 to be a rather extreme outlier and not representative of the overall blazar population. The bulk of the diffuse high-energy neutrino flux must instead be generated by a significantly larger population of less-luminous sources, such as non-blazar active galactic nuclei.

astro-ph.HE↗

arXiv2018

Self-Generated Cosmic-Ray Confinement in TeV Halos: Implications for TeV Gamma-Ray Emission and the Positron Excess

Recent observations have detected extended TeV gamma-ray emission surrounding young and middle-aged pulsars. The morphology of these "TeV halos" requires cosmic-ray diffusion to be locally suppressed by a factor of ~100-1000 compared to the typical interstellar medium. No model currently explains this suppression. We show that cosmic-ray self-confinement can significantly inhibit diffusion near pulsars. The steep cosmic-ray gradient generates Alfven waves that resonantly scatter the same cosmic-ray population, suppressing diffusion within ~20 pc of pulsars younger than ~100 kyr. In this model, TeV halos evolve through two phases, a growth phase where Alfven waves are resonantly generated and cosmic-ray diffusion becomes increasingly suppressed, and a subsequent relaxation phase where the diffusion coefficient returns to the standard interstellar value. Intriguingly, cosmic-rays are not strongly confined early in the TeV halo evolution, allowing a significant fraction of injected e+e- to escape. If these e+e- also escape from the surrounding supernova remnant, they would provide a natural explanation for the positron excess observed by PAMELA and AMS-02. Recently created TeV cosmic-rays are confined in the TeV halo, matching observations by HAWC and H.E.S.S. While our default model relaxes too rapidly to explain the confinement of TeV cosmic rays around mature pulsars, such as Geminga, models utilizing a Kraichnan turbulence spectrum experience much slower relaxation. Thus, observations of TeV halos around mature pulsars may provide a probe into our understanding of interstellar turbulence.

astro-ph.HE↗

arXiv2018

Evidence for Cosmic-Ray Escape in the Small Magellanic Cloud using Fermi Gamma-rays

Galaxy formation simulations demonstrate that cosmic-ray (CR) feedback may be important in the launching of galactic-scale winds. CR protons dominate the bulk of the CR population, yet most observational constraints of CR feedback come from synchrotron emission of CR electrons. In this paper, we present an analysis of 105 months of Fermi Gamma-ray Space Telescope observations of the Small Magellanic Cloud (SMC), with the aim of exploring CR feedback and transport in an external galaxy. We produce maps of the 2-300 GeV emission and detect statistically significant, extended emission along the Bar and the Wing, where active star formation is occurring. Gamma-ray emission is not detected above 13 GeV, and we set stringent upper-limits on the flux above this energy. We find the best fit to the gamma-ray spectrum is a single-component model with a power-law of index $Γ=-2.11\pm0.06\pm0.06$ and an exponential cutoff energy of $E_{\rm c} =13.1\pm5.1\pm1.6$ GeV. We assess the relative contribution of pulsars and CRs to the emission, and we find that pulsars may produce up to 14$^{+4}_{-2}$% of the flux above 100 MeV. Thus, we attribute most of the gamma-ray emission (based on its spectrum and morphology) to CR interactions with the ISM. We show that the gamma-ray emissivity of the SMC is five times smaller than that of the Milky Way and that the SMC is far below the calorimetric limit, where all CR protons experience pion losses. We interpret these findings as evidence that CRs are escaping the SMC via advection and diffusion.

astro-ph.HE↗

arXiv2018

Millisecond Pulsars, TeV Halos, and Implications For The Galactic Center Gamma-Ray Excess

Observations by HAWC indicate that many young pulsars (including Geminga and Monogem) are surrounded by spatially extended, multi-TeV emitting regions. It is not currently known, however, whether TeV emission is also produced by recycled, millisecond pulsars (MSPs). In this study, we perform a stacked analysis of 24 MSPs within HAWC's field-of-view, finding between 2.6-3.2 sigma evidence that these sources are, in fact, surrounded by TeV halos. The efficiency with which these MSPs produce TeV halos is similar to that exhibited by young pulsars. This result suggests that several dozen MSPs will ultimately be detectable by HAWC, including many "invisible" pulsars without radio beams oriented in our direction. The TeV halos of unresolved MSPs could also dominate the TeV-scale diffuse emission observed at high galactic latitudes. We also discuss the possibility that TeV and radio observations could be used to constrain the population of MSPs that is present in the inner Milky Way, thereby providing us with a new way to test the hypothesis that MSPs are responsible for the Galactic Center GeV excess.

astro-ph.HE↗

arXiv2018

Searching for Dark Matter with Neutron Star Mergers and Quiet Kilonovae

We identify new astrophysical signatures of dark matter that implodes neutron stars (NSs), which could decisively test whether NS-imploding dark matter is responsible for missing pulsars in the Milky Way galactic center, the source of some $r$-process elements, and the origin of fast-radio bursts. First, NS-imploding dark matter forms $\sim 10^{-10}$ solar mass or smaller black holes inside neutron stars, which proceed to convert neutron stars into $\sim$1.5 solar mass BHs. This decreases the number of neutron star mergers seen by LIGO/Virgo (LV) and associated merger kilonovae seen by telescopes like DES, BlackGEM, and ZTF, and instead, producing a population of "black mergers" containing $\sim$1.5 solar mass black holes. Second, dark matter-induced neutron star implosions may create a new kind of kilonovae that lacks a detectable, accompanying gravitational signal, which we call "quiet kilonovae." Using DES data and the Milky Way's r-process abundance, we constrain quiet kilonovae. Third, the spatial distribution of neutron star merger kilonovae and quiet kilonovae in galaxies can be used to detect dark matter. NS-imploding dark matter destroys most neutron stars at the centers of disc galaxies, so that neutron star merger kilonovae would appear mostly in a donut at large radii. We find that as few as ten neutron star merger kilonova events, located to $\sim$1 kpc precision could validate or exclude dark matter-induced neutron star implosions at $2 σ$ confidence, exploring dark matter-nucleon cross-sections 4-10 orders of magnitude below current direct detection experimental limits. Similarly, NS-imploding dark matter as the source of fast radio bursts can be tested at $2 σ$ confidence once 20 bursts are located in host galaxies by radio arrays like CHIME and HIRAX.

hep-ph↗