Publications

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.

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.

Ursa Major III Strongly Constrains Dark Matter Annihilation

Ursa Major III is a recently discovered stellar cluster that currently stands as the smallest Milky Way cluster that has ever been discovered. Observations indicate that the cluster would not be stable unless it was gravitationally supported by a large dark matter halo. This, in combination with its proximity, indicates that it might be the optimal target for dark matter indirect detection searches. We search for gamma-rays from Ursa Major III using 15 years of Fermi-LAT data and find no gamma-ray signal. This sets strong constraints on dark matter annihilation in Ursa Major III, which can be translated to very strong limits on the dark matter annihilation cross-section if the high dark matter density of the satellite is confirmed.

Future Constraints on Primordial Black Holes from MeV Telescopes

Black Holes that are formed promptly in the early universe are one potential component of the universe's dark matter. Such black holes can Hawking radiate, producing radiation with a temperature that is inversely proportional to their mass. For black holes in the mass range of about 10^15 - 10^17 grams, this emission is concentrated in the MeV band, where previous instrumental constraints have been relatively weak. In this paper, we analyze the capability of next-generation MeV telescopes, such as e-ASTROGAM and AMEGO, to observe or constrain such a dark matter signal, utilizing for the first time well-motivated astrophysical background templates extrapolated from Fermi-LAT observations. We show that if primordial black holes are responsible for the previously detected excess of 511 keV gamma-rays, such telescopes could definitively determine the characteristics of the underlying primordial black hole population.

First Analysis of Jupiter in Gamma Rays and a New Search for Dark Matter

Despite being observed at optical wavelengths since time immemorial, Jupiter has never been directly studied in GeV gamma-rays. This is primarily an instrumental challenge -- but also due to the fact that Jupiter is not expected to be a bright GeV gamma-ray source. However, the proximity of Jupiter, combined with the spectacular sensitivity of the Fermi-LAT, means that Jupiter observations may spark a new frontier in astrophysical studies. Additionally, we show that dark matter models which annihilate through a light mediator may produce a bright Jovian gamma-ray flux, allowing us to test dark matter models inaccessible to any other study. Using 11 years of Fermi-LAT data, and a detailed methodology for removing astrophysical backgorunds, we set strong limits on the Jupiter gamma-ray flux and thus on dark matter annihilation through light mediators, but potentially find exciting evidence for a signal below 15 MeV. The nature of this signal will require upcoming MeV instruments, like AMEGO or e-ASTROGAM to verify.

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.

Full Publication List:

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

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

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

13. The Sensitivity of Future Gamma-Ray Telescopes to Primordial Black Holes
Celeste Keith, Dan Hooper, Tim Linden, Rayne Liu
Physical Review D 106 (2022) 4, 043003

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

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

10. 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

9. 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

8. 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

7. 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

6. 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

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

4. 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

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

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

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

Theses

Dark Matter Annihilation at the Galactic Center

Doctoral Thesis
13 June 2013

Tim Linden

Associate Professor, Stockholm University

linden@fysik.su.se

Snowmass2021 Cosmic Frontier White Paper: Puzzling Excesses in Dark Matter Searches and How to Resolve Them

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

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

Positron Annihilation in the Galaxy

Dark Matter Searches with Gamma Rays