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.

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.

The Dark Main Sequence

Dark Matter can accumulate and annihilate within stars, depositing energy in the stellar core which travels outward and provides pressure support and eventually thermal radiation. This process is qualitatively identical to fusion, and in regions with very high dark matter densities (such as the galactic center) the dark matter induced power can exceed nuclear fusion to produce a dark matter powered star. We show that stars powered by dark matter are situated along a dark Hertzsprung-Russel diagram, with properties similar to, but observationally separable from the the standard Hertzsprung-Russel Diagram of fusion powered stars. Two important observables are: (1) an extremely top-heavy initial mass function, as the lowest mass stars are predominantly disrupted while heavier main sequence stars become effectively immortal due to the limitless dark matter fuel, (2) two new dark branches of stars situated along the Henyey and Hayashi tracks, which would not be normally observable in standard stellar evolution.

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.

Dark Matter Constraints from S-Stars Near Sgr A*

Observations have revealed a cluster (known as the S-Cluster) of star which travel extremely close to Sgr A* during their highly eccentric orbits. In models where dark matter accumulates in stellar interiors and then annihilates, dark matter annihilation could significantly affect the energetics of these stars. We find three observable phenomena: (1) dark matter annihilation can prevent stars from initially forming near Sgr A*, they must instead migrate from large distances, (2) dark matter annihilation can partially replace nuclear fusion, slowing down stellar evolution, (3) large dark matter annihilation rates can potentially disrupt the star, causing it to expand and cool off of the main sequence.

First Detection of TeV Gamma-Ray Emission from the Sun

Unlike the most powerful astrophysical accelerators, the Sun is not expected to have sufficient magnetic fields to accelerate TeV cosmic rays (and thus directly produce TeV gamma rays). Instead, the Sun is expected to act as a ``target", where the collisions of TeV cosmic rays with gas in the upper atmosphere of the Sun are expected to produce a dim gamma ray flux. However, this mechanism is theoretically expected to only produce gamma rays at low energy (below ~0.005 TeV). This paper builds on two Fermi-LAT analyses, which showed that the solar gamma-ray flux extended to at least 0.2 TeV, and uses a high-energy instrument (HAWC) to now extend the detection of this emission to at least 2 TeV. This is extremely unexpected, and very hard to model as either an astrophysical or dark matter process.

Using Unusual Supernovae to Probe Dark Matter Interactions

Observations have uncovered a strange sub-category of supernovae, Ca-Rich Gap Transients, which are dim Type Ia supernovae that have spectral lines indicating an unusual abundence of Ca (indicative of an origin as low-mass white dwarfs), and a morphological distribution that makes them overabundent in the outskirts of galaxies. The fact that these systems are consistent with the explosion of low-mass (~0.5 solar mass) white dwarfs makes it difficult to utilize standard stellar or binary synthesis models to explain their evolution. The fact that they are found primarily in the outskirts of galaxies also suggests physics that does not correlate with the total star-formation rate. We propose a model where asteroid-mass primordial black holes collide with, and precipitate the explosions of these white dwarfs. We show that the morphology of these events is dominated by dwarf spheroidal galaxies orbiting around a host galaxy, and naturally explains the morphology of these systems.

Full Publication List:

20. 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

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

18. Dark Branches of Immortal Stars at the Galactic Center
Isabelle John, Rebecca Leane, Tim Linden
To Be Submitted

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

16. Dark Matter Scattering Constraints from Observations of Stars Surrounding Sgr A*
Isabelle John, Rebecca Leane, Tim Linden
Physical Review D 109 12 123041 (2024)

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

14. 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)

13. Extraterrestrial Axion Search with the Breakthrough Listen Galactic Center Survey
Joshua Foster, Samuel Witte, Matthew Lawson, Tim Linden, Vishal Gajjar, Christoph Weniger, Ben Safdi
Physial Review Letters 129, 251102 (2022)

12. Dark Matter Microhalos in the Solar Neighborhood: Pulsar Timing Signatures of Early Matter Domination
Sten Delos, Tim Linden
Physical Review D 105 123514

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

10. Celestial-Body Focused Dark Matter Annihilation Throughout the Galaxy
Rebecca Leane, Tim Linden, Payel Mukhopadhyay, Natalia Toro
Physical Review D, 103 (2021) 7

9. 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

8. 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

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

6. 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

5. 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

4. Searching for Dark Matter with Neutron Star Mergers and Quiet Kilonovae
Joe Bramante, Tim Linden, Yu-Dai Tsai
Physical Review D 97 055016

3. 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

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

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

Tim Linden

Assistant Professor, Stockholm University

linden@fysik.su.se

Celestial Body Searches