Publications

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.

X-Ray Constraints on Dark Photon Tridents

We investigate models where keV dark photons compose the dark matter. Such photons can kinetically couple and decay into standard model particles. At masses below twice the electron mass, most standard model final states are kinematically inaccessible, making the dominant decay path the decay into a three-photon trident. This produces a unique spectral signature in x-ray observations. We utilize 16 years of INTEGRAL data to set world leading constraints on the dark photon kinetic coupling at energies between 90 keV up to 1 MeV. Our results exceed current direct detection bounds by orders of magnitude, especially at energies above 200 keV, and improve previous x-ray constraints by up to a factor of 100.

Detecting Gamma-rays from Young Stellar Clusters in the Milky Way

We detect GeV gamma-ray emission from the young star forming region RCW 38. Because this cluster is only 500 kyr old, it does not have any supernovae or pulsars capable of producing the gamma-ray emission. Thus, our analysis indicates that young stellar objects are themselves capable of efficiently accelerating cosmic-rays to GeV energies. By comparing gamma-ray and x-ray observations, we find that thermal pressure significantly exceeds cosmic-ray pressure in the dense cluster environment.

Using Celestial Bodies to Search for Dark Photon-Photon Tridents

We analyze a novel signal, the annihilation of dark matter through a low-mass dark photon mediator into a trident of three gamma-rays. This produces a hard spectral signal which is unique compared to other gamma-ray indirect detection targets. Moreover, the dark photon may be long lived, allowing it to escape from celestial bodies. We peform a detailed analysis of the dark photon-photon trident signal and then analyze gamma-ray data on both solar system and galactic center targets, placing strong limits on the dark matter/baryon coupling in such scenarios.

Full Publication List:

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

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

28. 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

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

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

25. 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)

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

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

22. 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)

21. 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

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

19. 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

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

17. 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

16. 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

15. 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

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

13. 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

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

11. 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

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

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

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

7. 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)

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

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

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

3. 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

2. 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

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

Theses

Dark Matter Annihilation at the Galactic Center

Doctoral Thesis
13 June 2013

White Papers

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

AMEGO Collaboration
17 July 2019

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

HAWC Collaboration
18 March 2019

Tim Linden

Assistant Professor, Stockholm University

linden@fysik.su.se