Antihelium from Dark Matter

AMS-02 has reported the tentative detection of approximately a dozen anti-Helium 3 and anti-Helium 4 nuclei. Astrophysical interactions capable of making high-energy anti-nuclei are kinematically suppressed, making such a signal a potential smoking gun for dark matter annihilation. Unfortunately, it is also extremely difficult to explain such a signal with dark matter models either, due to the very small range of coalescence momenta that is capable of producing such particles. We present a new astrophysical method for enhancing the dark matter induced anti-Helium flux, by using Alfven waves to reaccelerate very low-energy anti-Helium particles to higher energies, where they may be more readily detected by AMS-02.

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Breaking a Dark Degeneracy

The standard picture of the "thermal WIMP annihilation cross-section" assumes a standard cosmology where radiation dominates the energy-density of the universe between the end of inflation and thermal freeze-out. However, this area is not probed by current cosmological constraints. If the universe instead includes a temporary period of matter domination between the end of inflation and the start of BBN, the standard thermal-freeze out picture changes considerably -- often producing much smaller annihialtion cross-sections that are difficult to probe. Fortunately, such models simultaneously lead to a significant increase in the dark matter substructure, allowing current gamma-ray experiments to constrain much smaller annihilation cross-sections. We compute current indirect detection constraints from gamma-ray observations of the isotropic gamma-ray background and the Draco dwarf, finding cross-sections which can lie as low as 1e-32 cm^3 s^-1.

Cosmic-Rays Don't Drive Winds in M82

We construct the most detailed models of cosmic-ray injection, propagation, and energy losses in the starburst galaxy M82, including the first two-dimensional model of cosmic-ray injection and propagation in the dense M82 nuclear core. We find that the high-gas density of of the starburst core quickly cools high-energy protons injected by supernovae in teh starburst region. By comparing these gamma-ray constraints with the morphology of observed radio emission, we place strong constraints on the steady state cosmic-ray density above and below the galactic plane. Because we find the cosmic-ray density (and its gradient) to be small, we can rule out models where cosmic-rays drive the galactic winds in galaxies like M82.

Robust Searches for Dark Matter in dSphs

We produce a new, and significantly more robust, methodology to study dim point-sources in the Fermi-LAT data. Previous methods face difficulties due to the inadequacy of the diffuse gamma-ray emission model. Low-significance excesses (with both positive and negative fluxes!) appear much more commonly in the data than predicted by Poisson fluctuations. We utilize blank sky positions, where no source is expected, to characterize the impact of diffuse model uncertainties on the putative flux of point sources, and marginalize over this flux profile to isolate the additional contribution of dwarf spheroidal galaxies on the gamma-ray flux. We find no gamma-ray excess consistent with the position of dSphs, and place strong constraints on the dark matter annihilation cross-section.

A Robust Detection of an Antiproton Excess

Previous studies by Cuoco et al (1610.03071) and Cui et al. (1610.03840) have uncovered evidence for an excess in cosmic-ray antiprotons in the AMS-02 data, centered at a rigidity of around 5 GeV. The resilience of this excess has been called into question by other analyses (e.g., Reinert & Winkler 1712.00002), which argued that uncertainties in the background modeling could decrease the significance of the signal. We utilize an improved method to constrain solar modulation and antiproton production uncertainties, and test number of diffuse emission models, finding that the statistical significance of the excess remains robust, providing an intriguing signal that remains consistent with dark matter annihilation.

Prospects for Future TeV Halo Detections

Observations by the HAWC telescope have detected a large population of ``TeV Halos", bright, diffuse, and hard spectrum, TeV sources surrounding young and middle aged pulsars. The cleanest TeV halo systems, observed at the largest scales surrounding older pulsars like Geminga and Monogem, were enabled by the large ROI of HAWC observations. However, young pulsars are expected to provide brighter, if more complex, emission sources. We study the observation of younger TeV halos with the HAWC, HESS, and CTA instruments, predicting that upcoming instruments will detect between 50-240 halos!

AGN Don't Produce the IceCube Neutrino Flux

Recent IceCube observations of the flaring blazar TXS 0506+056 have excited the community, potentially providing the first detection of a neutrino source. Additionally, the detection of neutrino emission from a blazar might indicate that blazars dominate the neutrino background. Here, however, we utilize detailed catalogs of optically and gamma-ray bright blazars, including a huge population of source that could potentially produce the diffuse IceCube neutrino emission. We find no evidence of neutrino hotspots associated with the known positions of blazars, and use this lack of correlation to rule out models where more than approximately 20% of the diffuse IceCube neutrino background is produced by blazar activity.

HAWC Constrains Annihilation to Long-Lived Mediators

Dark Matter Annihilation directly to neutrino final states has long been the worst-case scenario for indirect detection searches. One promising direction involves IceCube searches for dark matter annihilation in the Sun, as the neutrinos could easily escape through the dense solar material. A related search, utilizing gamma-ray telescopes, involves dark matter annihilation to meta-stable final states, which can propagate out of the solar interior before decaying to more common particles (e.g., gamma-rays, bottom quarks, etc.). We utilize HAWC observations of the Sun to examine on this unique channel, setting constraints that lie several orders of magnitude below previous experiments.

Upper Limits on TeV Gamma-Ray Flux from Sun

Recently, we made the unexpected detection of solar gamma-rays up to energies exceeing 100 GeV with the Fermi-LAT telescope. These observations showed that, contrary to previous expectations, the solar gamma-ray spectrum during solar minimum continues to much higher energies than expected. The much larger effective area of HAWC will potentially allow the coninued detection of solar gamma-rays up to TeV energies or beyond. While we don't yet have solar minimum observations from HAWC, we can evaluate the signal during solar maximum, when the Fermi-LAT predictions indicate the emission will be dim. We do not find evidence of TeV solar gamma-ray emission, and set the strongest constraints yet available on its flux. This same search strategy will soon be used for searches during solar minimum, which will be much more exciting!

Cosmic-Ray Self-Confinement Models and TeV halos

We build the first semi-analytic model of cosmic-ray propagation in a TeV halo, and provide the first theoretically motivated explanation for the low-diffusion constant found within these sources. The steep gradient in cosmic-ray energy produced by the bright central pulsar excites Alfven waves, which efficiently confine their parent cosmic-ray population. This feedback allows the cosmic rays to effective self-confine. We model the time evolution of these sources, finding that the maximum age of TeV halos depends sensitively on the turbulence model.

Full Publication List:

77. Anti-Deuterons and Anti-Helium Nuclei from Annihilating Dark Matter/a>
Ilias Cholis, Tim Linden, Dan Hooper
Submitted to PRL

76. Breaking a Dark Degeneracy: The gamma-ray signature of early matter domination
M. Sten Delos, Tim Linden, Adrienne Erickcek
Physical Review D 100 123546

75. Cosmic Rays and Magnetic Fields in the Core and Halo of the Starbust M82: Implications for Galactic Wind Physics
Benjamin Buckman, Tim Linden, Todd Thompson
Accepted by MNRAS

74. A Robust Method for Treating Astrophysical Mismodeling in Dark Matter Annihilation Searches of Dwarf Spheroidal Galaxies
Tim Linden
Physical Review D 043017

73. A Robust Excess in the Cosmic-Ray Antiproton Spectrum: Implications for Annihilating Dark Matter
Ilias Cholis, Tim Linden, Dan Hooper
Physical Review D 99 103026

72. TeV Halos are Everywhere: Prospects for New Discoveries
Takahiro Sudoh, Tim Linden, John Beacom
Submitted to PRD

71. Active Galactic Nuclei and the Origin of IceCube’s Diffuse Neutrino Flux
Dan Hooper, Tim Linden, Abby Vieregg
Accepted by JCAP

70. Constraints on Spin-Dependent Dark Matter Scattering with Long-Lived Mediators from TeV Observations of the Sun with HAWC
HAWC Collaboration
Physical Review D 98 123012

69. First HAWC Observations of the Sun Constrain Steady TeV Gamma-Ray Emission
HAWC Collaboration
Physical Review D 98 123011

68. Self-Generated Cosmic-Ray Confinement in TeV Halos: Implications for TeV γ-ray Emission and the Positron Excess
Carmelo Evoli, Tim Linden, Giovanni Morlino
Physical Review D 98 063017

67. Evidence for Cosmic-Ray Escape in the Small Magellanic Cloud using Fermi Gamma-rays
Laura Lopez, Katie Auchettl, Tim Linden, Alberto Bolatto, Todd Thompson, Enrico Ramirez-Ruiz
The Astrophysical Journal 867 44

66. An Unexpected Dip in the Solar Gamma-Ray Spectrum
Qing-Wen Tang, Kenny Ng, Tim Linden, Bei Zhou, John Beacom, Annika Peter
Physical Review D, 98 063019

65. Millisecond Pulsars, TeV Halos, and Implications for the Galactic Center Gamma-Ray Excess
Dan Hooper, Tim Linden
Physical Review D 98 043005

64. Evidence for a New Component of High-Energy Solar Gamma-Ray Production
Tim Linden, Bei Zhou, John Beacom, Annika Peter, Kenny Ng, Qing-Wen Tang
Physical Review Letters 121 131103

63. The Radio Synchrotron Background: Conference Summary and Report
Jack Singal, Jibran Haider et. al. (including Tim Linden)
Publications of the Astronomical Society of the Pacific, 130 985

62. Measuring the Local Diffusion Coefficient with HESS Observations of High-Energy Electrons
Dan Hooper, Tim Linden
Physical Review D 98 083009

61. Comment on "Characterizing the Population of Pulsars in the Galactic Bulge with the Fermi Large Area Telescope" [arXiv: 1705.00009v1]
Richard Bartels, Dan Hooper, Tim Linden, Siddharth Mishra-Sharma, Nick Rodd, Ben Safdi, Tracy Slatyer
Physics of the Dark Universe 20 88 2018

60. Pulsar TeV Halos Explain the TeV Excess Observed by Milagro
Tim Linden, Ben Buckman
Physical Review Letters 120 121101

59. Black Mergers, Quiet Kilonovae, and r-Process Afterglow Donuts From Dark Matter
Joe Bramante, Tim Linden, Yu-Dai Tsai
Physical Review D 97 055016

58. TeV Gamma Rays from Galactic Center Pulsars
Dan Hooper, Ilias Cholis, Tim Linden
Physics of the Dark Universe 21 40 2018

57. IceCube and HAWC Constraints on Very-High-Energy Emission from the Fermi Bubbles
Ke Fang, Meng Su, Tim Linden, Kohta Murase
Physical Review D 96 123007

56. Dark Kinetic Heating of Neutron Stars: An Infrared Window On WIMPs, SIMPs, and Higgsinos
Masha Baryakhtar, Joe Bramante, Shirley Li, Tim Linden, Nirmal Raj
Physical Review Letters 119 131801

55. Using HAWC to Detect Invisible Pulsars
Tim Linden, Katie Auchettl, Joseph Bramante, Ilias Cholis, Ke Fang, Dan Hooper, Tanvi Karwal, Shirley Li
Physical Review D 96 103016

54. HAWC Observations Strongly Favor Pulsar Interpretations of the Cosmic-Ray Positron Excess
Dan Hooper, Ilias Cholis, Tim Linden, Ke Fang
Physical Review D 96 103013

53. Evidence for the Stochastic Acceleration of Secondary Antiprotons by Supernova Remnants
Ilias Cholis, Dan Hooper, Tim Linden
Physical Review D 95 123007

52. Low Mass X-Ray Binaries in the Inner Galaxy: Implications for MSPs and the GeV Excess
Daryl Haggard, Craig Heinke, Dan Hooper, Tim Linden
Journal of Cosmology and Astroparticle Physics 1705 05 056

51. Star-Forming Galaxies Significantly Contribute to the Isotropic Gamma-Ray Background
Tim Linden
Physical Review D 96 083001

50. The Angular Power Spectrum of diffuse gamma-rays measured by Fermi and DM constraints
M. Fornasa, A. Cuoco, J. Zavala, J. Gaskins, M. Sanchez-Conde, G. Gomez-Vargas, E. Komatsu, Tim Linden, F. Prada, F. Zandenel, A. Morselli
Physical Review D 94, 123005

49. The Gamma-Ray Pulsar Population of Globular Clusters: Implications for the GeV Excess
Dan Hooper, Tim Linden
Journal of Cosmology and Astroparticle Physics 1608 08 018

48. Indirect Detection Constraints on s and t Channel Simplified Models of Dark Matter
Linda Carpenter, Russell Colburn, Jessica Goodman, Tim Linden
Physical Review D 94 055027

47. Radio Galaxies Dominate the High-Energy Diffuse Gamma-Ray Background
Dan Hooper, Tim Linden, Alejandro Lopez
Journal of Cosmology and Astroparticle Physics 1608 08 019

46. The High-Energy Tail of the Galactic Center Gamma-Ray Excess
Tim Linden, Nicholas Rodd, Benjamin Safdi, Tracy Slatyer
Physical Review D 94, 103013

45. Improved Cosmic-Ray Injection Models and the Galactic Center Gamma-Ray Excess
Eric Carlson, Tim Linden, Stefano Profumo
Physical Review D 94 063504

44. Is the Gamma-Ray Source J2212.5+0703 A Dark Matter Subhalo?
Bridget Bertoni, Dan Hooper, Tim Linden
Journal of Cosmology and Astroparticle Physics 1609 05 049

43. On the R-Process Enrichment of Dwarf Spheroidal Galaxies
Joseph Bramante, Tim Linden
The Astrophysical Journal 826 1 57

42. A Predictive Analytic Model for the Solar Modulation of Cosmic Rays
Ilias Cholis, Dan Hooper, Tim Linden
Physical Review D 93 4 043016

41. Putting Things Back Where They Belong: Tracing Cosmic-Ray Injection with H2
Eric Carlson, Tim Linden, Stefano Profumo
Physical Review Letters 117 111101

40. Known Radio Pulsars Do Not Contribute to the Galactic Center Gamma-Ray Excess
Tim Linden
Physical Review D 93 6 063003

39. Cluster Mergers and the Origin of the ARCADE-2 Excess
Ke Fang, Tim Linden
Journal of Cosmology and Astroparticle Physics 1610 10 004

38. The Galactic Center GeV Excess from a Series of Leptonic Cosmic-Ray Outbursts
Ilias Cholis, Carmelo Evoli, Francesca Calore, Tim Linden, Christoph Weniger, Dan Hooper
Journal of Cosmology and Astroparticle Physics 1512 12 005

37. Examining the Fermi-LAT Third Source Catalog in Search of Dark Matter Subhalos
Bridget Bertoni, Dan Hooper, Tim Linden
Journal of Cosmology and Astroparticle Physics 1512 12 035

36. On the Gamma-Ray Emission from Reticulum II and Other Dwarf Galaxies
Dan Hooper, Tim Linden
Journal of Cosmology and Astroparticle Physics 1509 09 016

35. The Galactic Center Excess in Gamma-Rays from Annihilation of Self-Interacting Dark Matter
Manoj Kaplinghat, Tim Linden, Haibo Yu
Physical Review Letters, 114 211303

34. On the Formation of Ultra-Luminous X-Ray Sources with NS Accretors: The Case of M82-X2
Tassos Fragos, Tim Linden, Vicky Kalogera, Panos Sklias
The Astrophysical Journal Letters, 802 2041

33. The Anisotropy of the Extragalactic Radio Background from Dark Matter
Ke Fang, Tim Linden
Physical Review D 91 083501 (2015)

32. What Does the PAMELA Antiproton Spectrum Tell Us About Dark Matter?
Dan Hooper, Tim Linden, Philipp Mertsch
Journal of Cosmology and Astroparticle Physics 03 021 (2015)

31. Improving the Sensitivity to Dark Matter Annihilation in Dwarf Spheroidal Galaxies
Eric Carlson, Dan Hooper, Tim Linden
Physical Review D, 91 061302 (2015)

30. A Critical Reevaluation of Radio Constraints on Annihilating Dark Matter
Ilias Cholis, Dan Hooper, Tim Linden
Physical Review D, 91 083507 (2015)

29. Challenges in Explaining the Galactic Center Gamma-Ray Excess with Millisecond Pulsars
Ilias Cholis, Dan Hooper, Tim Linden
Journal of Cosmology and Astroparticle Physics, 06 043 (2015)

28. A New Determination of the Spectrum and Luminosity Function of Millisecond Pulsars
Ilias Cholis, Dan Hooper, Tim Linden
Submitted to PRD

27. The Circular Polarization of Pulsar Wind Nebulae and the Cosmic-Ray Positron Excess
Tim Linden
The Astrophysical Journal 799 200 (2015)

26. Detecting Dark Matter with Imploding Pulsars in the Galactic Center
Joseph Bramante, Tim Linden
Physical Review Letters, 113 191301

25. Searching for Dark Matter Annihilation in the Smith High-Velocity Cloud
Alex Drlica-Wagner, German Gomez-Vargas, Jack Hewitt, Tim Linden, Luigi Tibaldo
The Astrophysical Journal, 790 24

24. Is the Ultra-High Energy Cosmic-Ray Excess Correlated with IceCube Neutrinos?
Ke Fang, Toshihiro Fujii, Tim Linden, Angela Olinto
The Astrophysical Journal, 794 126

23. The Characterization of the Gamma-Ray Signal from the Central Milky Way
Tansu Daylan, Doug Finkbeiner, Dan Hooper, Tim Linden, Stephen Portillo, Nicholas Rodd, Tracy Slatyer
Physics of the Dark Universe 12 2016 1

22. Antihelium from Dark Matter
Eric Carlson, Adam Coogan, Tim Linden, Stefano Profumo, Alejandro Ibarra, Sebastian Wild
Physical Review D, 89 076005

21. Tying Dark Matter to Baryons with Self-Interactions
Manoj Kaplinghat, Ryan Keeley, Tim Linden, Haibo Yu
Physical Review Letters, 113 021302 (2014)

20. Pulsars Cannot Account for the Inner Galaxy's GeV Excess
Dan Hooper, Ilias Cholis, Tim Linden, Jennifer Siegal-Gaskins, Tracy Slatyer
Physical Review D, 88 083009

19. A Clustering Analysis of the 130 GeV Gamma-Ray Feature
Eric Carlson, Tim Linden, Stefano Profumo, Christoph Weniger
Physical Review D, 88 043006

18. Probing the Pulsar Origin of the Positron Fraction with Atmospheric Cherenkov Telescopes
Tim Linden, Stefano Profumo
The Astrophysical Journal, 772 18

17. Ultraluminous X-Ray Sources in the Most Metal-Poor Galaxies
A. Prestwich, M. Tsantaki, A. Zezas, F. Jackson, T. Roberts, R. Foltz, Tim Linden, V. Kalogera
The Astrophysical Journal 769 2 92

16. Testing the WMAP-Planck Haze with Spiral Galaxies
Eric Carlson, Dan Hooper, Tim Linden, Stefano Profumo
Journal of Cosmology and Astroparticle Physics, 1307 026 (2013)

15. Chandra Observations of the Collisional Ring Galaxy NGC 922
A. Prestwich, J. Galache, Tim Linden, V. Kalogera, A. Zezas, T. Roberts, R. Kilgard, A. Wolter, G. Trinchieri
The Astrophysical Journal, 747 2 150

14. Are Lines from Unassociated Gamma-Ray Sources Evidence for Dark Matter Annihilation?
Dan Hooper, Tim Linden
Physical Review D, 86 8 083532

13. Exploring the Nature of the GC Gamma-Ray Source with the Cherenkov Telescope Array
Tim Linden, Stefano Profumo
The Astrophysical Journal, 760 23 7

12. Gamma-Rays in the Fermi-LAT Data: Is it a Bubble?
Stefano Profumo, Tim Linden
Journal of Cosmology and Astroparticle Physics, 007 011

11. The Morphology of Hadronic Emission Models for the Galactic Center
Tim Linden, Elizabeth Lovegrove, Stefano Profumo
The Astrophysical Journal, 753 1 41

10. The Isotropic Radio Background and Annihilating Dark Matter
Dan Hooper, Alexander Belikov, Tesla Jeltema, Tim Linden, Stefano Profumo, Tracy Slatyer
Physical Review D, 86 10 103003

9. Anisotropies in the Gamma-Ray Background Measured by the Fermi-LAT
The Fermi-LAT Collaboration: A. Cuoco, Tim Linden, N. Maziotta, J. Siegal-Gaskins, V. Vitale, E. Komatsu
Physical Review D, 85 8 083007

8. On The Rarity of X-Ray Binaries with Naked Helium Donors
Tim Linden, Francesca Valsecchi, Vicky Kalogera
The Astrophysical Journal, 748 2 114

7. Origin of Gamma-Rays from the Galactic Center
Dan Hooper, Tim Linden
Physical Review D, 84 12 123005

6. Dark Matter and Synchrotron Emission from Galactic Center Radio Filaments
Tim Linden, Dan Hooper, Farhad Yusef-Zadeh
The Astrophysical Journal, 741 2 95

5. Gamma-Rays from the Galactic Center and the WMAP Haze
Dan Hooper, Tim Linden
Physics Review D, 83 8 083517 (2011)

4. The Effect of Starburst Metallicity on Bright X-Ray Binary Formation Pathways
Tim Linden, Vicky Kalogera, Jeremy Sepinsky, Andrea Prestwich, Andreas Zezas, Jay Gallagher
The Astrophysical Journal, 725 2 1984

3. The Morphology of Dark Matter Synchrotron Emission with Self-Consistent Diffusion Models
Tim Linden, Stefano Profumo, Brandon Anderson
Physical Review D, 82 6 228 063529

2. Systematic Effects in Extracting a ``Gamma-Ray Haze" from Spatial Templates
Tim Linden, Stefano Profumo
The Astrophysical Journal Letters, 714 2 228

1. Probing Electron-Capture Supernovae: X-Ray Binaries in Starbursts
Tim Linden, Jeremy Sepinsky, Vicky Kalogera, Chris Belczynski
The Astrophysical Journal, 699 2 1573 (2009)



Tim Linden

Assistant Professor, Stockholm University

linden@fysik.su.se