Publications

Gamma Rays in a Halo Around the Sun

The Sun is one of the brightest gamma-ray sources in the sky. The gamma-ray emission comes from two processes: First, high-energy cosmic-ray protons hit the solar surface and produce outgoing gamma-rays via hadronic interactions. Second, high-energy electrons and positrons upscatter starlight to gamma-ray energies via inverse-Compton scattering. This forms a bright, diffuse halo of gamma-ray emission around the Sun. We use 15 years of Fermi-LAT data to observe the spectrum, morphology, time-variation, and azimuthal distribution of gamma-rays from the solar halo, detecting bright emission between 31.6 MeV and 100 GeV, and out to 45 degress from the Sun. The morphology and spectrum of the emission is primarily governed by solar modulation, and we set strong constraints on the solar modulation potential in regions near the Sun that are inaccessible to local cosmic-ray probes of solar dynamics.

Every Nearby Energetic Pulsar Is Surrounded by a Region of Inhibited Diffusion

H.E.S.S. has recently released the first measurements of the total e+e- spectrum up to energies of 40 TeV. The e+e- flux is found to be a smooth and steeply falling powerlaw. This is inconsistent with models where nearby pulsars convert ~10% of their spin-down power into e+e- pairs that then propagate into the generic interstellar medium. We show that there are 20 nearby ATNF pulsars that would individually overproduce the H.E.S.S. data. Given strong evidence that these pulsars do convert a significant fraction of their spindown power into e+e- pairs, we conclude that all of these pulsars must be surrounded by regions of inhibited diffusion that prevent these pairs from leaking into the interstellar medium. While these regions can be supernova remnants or pulsar wind nebula, the existnece of several ~500 kyr pulsars within this dataset indicates that TeV halos are also a common occurance.

Looking for the γ-Ray Cascades of the KM3-230213A Neutrino Source

The KM3NeT collaboration recently announced the detection of an extremely high energy neutrino, with a best fit energy near 200 PeV. Such a source challenges our models of particle acceleration. Generically, such models will produce a bright GeV-TeV gamma-ray flux which is produced by the attenuation and re-emission of PeV gamma-rays that accommpanies the neutrino event. We utilize 16 years of Fermi-LAT data to search for such a signal, but do not find a convincing corresponding gamma-ray source. We utilize this model to set strong lower limits on the intergalactic magnetic field strength, which controls the evolution of the attenuated gamma-ray signal.

SuperK Observations Strongly Constrain Leptophilic Dark Matter Scattering

Leptophilic dark matter models (which primarily couple to leptonic standard model particles), are a well-motivated class of dark matter models that can explain several anomalies in the neutrino and charged-lepton sectors (for example, g-2). However, these models are difficult to target with traditional direct detection, because they do not benefit from the A^4 enhancement typical of spin-independent nuclear scattering. The huge electron density in the Sun makes it a compelling target for these searches -- and fortunately, our efforts to search for dark matter capture in the Sun are highly enhanced in leptonic models, because any subsequent dark matter annihilation will produce a high-energy neutrino flux that escapes the Sun and is detectable from Earth. We use 10 years of SuperK observations, which have not detected any neutrino excess from the Sun above 100 MeV, to strongly constrain this class of models -- obtaining constraints that exceed terrestrial direct detection by an order of magnitude across many leptophilic annihilation states. These results (along with the promise of upcoming HyperK data) indicate that solar neutrino searches may provide the strongest constraints on leptophilic dark matter scattering.

Galaxy Clusters Provide World-Leading Constraints on Dark Matter in Prompt Cusps

Recent studies have found that dark matter can form prompt cusps in the very early universe, and that --- contrary to previous estimates --- these cusps can survive until the present day so long as they are not in regions with particularly large baryonic densities. The annihilation inside these cusps dominates the total annihilation rate despite their small size -- and makes the largest dark matter structures (which have the most cusps) the most important targets for dark matter indirect detection. We use 15 years of data to investigate dark matter signals from the prompt cusps in galaxy clusters. We find no excess and place strong limits on the dark matter annihilation cross-section, ruling out dark matter annihilating at the thermal cross-section below 200 GeV, and placing our result in tension with dark matter models aimed to explain the galactic center excess.

GRB Observations Strongly Constrain Heavy Axions

Heavy Axions can be very efficiently produced in extremely high temperature plasmas. One of the highest temperature plasmas in the universe is produced in the very early stages of GRB fireballs. These fireballs can form on distance scales of less than 10 km, and achieve temperatures as high as 100 MeV. Once produced, these axions can free-stream outside of the GRB acceleration region, and decay back to photons farther from the main GRB jet. This prevents them from being accelerated up to high energies, and disrupts the production of the high-energy beam that typifies blazar jets. Using observations that high-energy jets have been observed from many GRBs, we strongly constrain the axion parameter space, producing world leading limits on axions with GeV-scale masses.

Improved Measurements of the MSP Luminosity Function in Globular Clusters

The nature of the Galactic Center excess has been debated for over a decade -- with dark matter and millisecond pulsars as the two leading candidates. We use measurements of globular clustetrs, which have bright GeV gamma-ray fluxes which are thought to be powered by populations of MSPs, in order to calculate the luminosity function of MSPs. We produce a world-leading calculation for the luminosity function of these sources. If a similar luminosity function existed for detectable MSPs in the galactic center, we would expect more observed sources than have presently been found. Unless there is either a large missing cache of unassociated Fermi sources that are MSPs, or the MSP luminosity function is catagorically dimmer than in globular clusters -- this strongly limits the potential for MSPs to contribute to the Galactic center excess.

Full Publication List:

73. First Observations of Solar Halo Gamma Rays Over a Full Solar Cycle
Tim Linden, Jung-Tsung Li, Bei Zhou, Isabelle John, Milena Crnogorčević, Annika Peter, John Beacom
To Be Submitted

72. Every Nearby Energetic Pulsar Is Surrounded by a Region of Inhibited Diffusion
Isabelle John, Tim Linden
To Be Submitted

71. Looking for the γ-Ray Cascades of the KM3-230213A Neutrino Source
Milena Crnogorčević, Carlos Blanco, Tim Linden
Submitted to JCAP

70. Super-Kamiokande Strongly Constrains Leptophilic Dark Matter Capture in the Sun
Thong Nguyen, Tim Linden, Pierluca Carenza, Axel Widmark
To Be Submitted

69. Gamma-Ray Observations of Galaxy Clusters Strongly Constrain Dark Matter Annihilation in Prompt Cusps
Milena Crnogorčević, Sten Delos, Nadia Kuritzén, Tim Linden
To Be Submitted

68. Heavy Axions Can Disrupt Gamma-Ray Bursts
Oindrila Ghosh, Sunniva Jacobsen, Tim Linden
To Be Submitted

67. Millisecond Pulsars in Globular Clusters and Implications for the Galactic Center Gamma-Ray Excess
Aurelio Amerio, Dan Hooper, Tim Linden
To Be Submitted

66. Strong Constraints on Dark Photon and Scalar Dark Matter Decay from INTEGRAL and AMS-02
Thong Nugyen, Isabelle John, Tim Linden, Tim Tait
To Be Submitted

65. X-Ray Constraints on Dark Photon Tridents
Tim Linden, Thong Nguyen, Tim Tait
To Be Submitted

64. Gamma-ray Emission from a Young Star Cluster in the Star-Forming Region RCW 38
Paarmita Pandey, Laura Lopez, Anna Rosen, Todd Thompson, Tim Linden, Ian Blackstone, Lachlan Lancaster
Astrophysical Journal 976 1 98 (2024)

63. Indirect Searches for Dark Photon-Photon Tridents in Celestial Objects
Tim Linden, Thong Nguyen, Tim Tait
To Be Submitted

62. Strong Constraints on Dark Matter Annihilation in Ursa Major III/UNIONS 1
Milena Crnogorčević, Tim Linden
Physical Review D 109 8 083018 (2024)

61. Gamma-ray lines in 15 years of Fermi-LAT data: New constraints on Higgs portal dark matter
Pedro De la Torre Luque, Tim Linden, Juri Smirnov
Physical Review D 109 (2024) 4, L041301

60. Limits on dark matter annihilation in prompt cusps from the isotropic gamma-ray background
M. Sten Delos, Michael Korsmeier, Axel Widmark, Carlos Blanco, Tim Linden, Simon White
Physical Review D 109 8 083512 (2024)

59. On the Neutrino and Gamma-Ray Emission from NGC 1068
Carlos Blanco, Dan Hooper, Tim Linden, Elena Pinetti
To Be Submitted

58. Where are the Cascades from Blazar Jets? An Emerging Tension in the Gamma-Ray Sky
Carlos Blanco, Oindrila Ghosh, Sunniva Jacobsen, Tim Linden
Submitted to PRL

57. On the gamma-ray emission from the core of the Sagittarius dwarf galaxy
Addy Evans, Louis Strigari, Oskar Svenborn, Andrea Albert, Pat Harding, Dan Hooper, Tim Linden, Andrew Pace
Monthly Notices of the Royal Astronomical Society 524 3 4574

56. The Cherenkov Telescope Array Will Test Whether Pulsars Generate the Galactic Center Gamma-Ray Excess
Celeste Keith, Dan Hooper, Tim Linden
Physical Review D 107 10, 103001

55. The TeV Sun Rises: Discovery of Gamma rays from the Quiescent Sun with HAWC
HAWC Collaboration
Physical Review Letters 131, 051201 (2023)

54. White Dwarfs in Dwarf Spheroidal Galaxies: A New Class of Compact-Dark-Matter Detectors
Juri Smirnov, Ariel Goobar, Tim Linden, Edvard Mörtsell
Physical Review Letters 132 15 151401 (2024)

53. Weighing the Local Interstellar Medium using Gamma Rays and Dust
Axel Widmark, Michael Korsmeier, Tim Linden
Physical Review Letters 130 161002 (2023)

52. Pulsars Do Not Produce Sharp Features in the Cosmic-Ray Electron and Positron Spectra
Isabelle John, Tim Linden
Physical Review D 107 10, 103021 (2023)

51. Anisotropic diffusion cannot explain TeV halo observations
Pedro De la Torre Luque, Ottavio Fornieri, Tim Linden
Physical Review D 106 123033

50. Self-Generated Cosmic-Ray Turbulence Can Explain the Morphology of TeV Halos
Payel Mukhopadhyay, Tim Linden
Physical Review D 105, 123008

49. Gamma-Rays from Star Forming Activity Appear to Outshine Misaligned Active Galactic Nuclei
Carlos Blanco, Tim Linden
Journal of Cosmology and Astroparticle Physics 02 003

48. First Analysis of Jupiter in Gamma Rays and a New Search for Dark Matter
Rebecca Leane, Tim Linden
Physical Review Letters 131 7 071001

47. Evidence of TeV Halos Around Millisecond Pulsars
Dan Hooper, Tim Linden
Physical Review D 105, 103013

46. The Highest Energy HAWC Sources are Leptonic and Powered by Pulsars
Takahiro Sudoh, Tim Linden, Dan Hooper
Journal of Cosmology and Astroparticle Physics 08 (2021) 010

45. First Observations of Solar Disk Gamma Rays over a Full Solar Cycle
Tim Linden, John Beacom, Annika Peter, Benjamin Buckman, Bei Zhao, Guanying Zhu
Physical Review D 105 (2022) 6, 063013

44. Constraining the Charge-Sign and Rigidity-Dependence of Solar Modulation
Ilias Cholis, Dan Hooper, Tim Linden
Journal of Cosmology and Astroparticle Physics 10 051

43. Millisecond Pulsars Modify the Radio-SFR Correlation in Quiescent Galaxies
Takahiro Sudoh, Tim Linden, John Beacom
Physical Review D 103 083017

42. 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
Monthly Notices of the Royal Astronomical Society 494 2679

41. TeV Halos are Everywhere: Prospects for New Discoveries
Takahiro Sudoh, Tim Linden, John Beacom
Physical Review D 100 043016

40. Active Galactic Nuclei and the Origin of IceCube’s Diffuse Neutrino Flux
Dan Hooper, Tim Linden, Abby Vieregg
Journal of Cosmology and Astroparticle Physics 02 012

39. 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

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

37. 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

36. 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

35. 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

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

33. 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

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

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

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

29. 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

28. 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

27. 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

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

25. 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

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

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

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

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

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

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

18. 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

17. 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

16. 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)

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

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

13. 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

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

11. 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

10. 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)

9. 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

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

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

6. 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

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

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

All-sky Medium Energy Gamma-ray Observatory: Exploring the Extreme Multimessenger Universe

Pulsars in a Bubble? Following Electron Diffusion in the Galaxy with TeV Gamma Rays

The Sun at GeV--TeV Energies: A New Laboratory for Astroparticle Physics

Astrophysics

Gamma Rays in a Halo Around the Sun

Research Topics

Every Nearby Energetic Pulsar Is Surrounded by a Region of Inhibited Diffusion

Looking for the γ-Ray Cascades of the KM3-230213A Neutrino Source

SuperK Observations Strongly Constrain Leptophilic Dark Matter Scattering

Galaxy Clusters Provide World-Leading Constraints on Dark Matter in Prompt Cusps

GRB Observations Strongly Constrain Heavy Axions

Improved Measurements of the MSP Luminosity Function in Globular Clusters

Full Publication List:

Theses

Dark Matter Annihilation at the Galactic Center

Young X-Ray Binary Populations: Metallicity Effects and a Diagnostic for Electron-Capture Supernovae

Modeling the Spatial and Luminosity Distributions of X-Ray Binaries: An Introduction to the StarChart Code

White Papers

Tim Linden