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.

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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.

IGRB Limits on Dark Matter Cusps

Recent work has indicated that prompt ``cusps" containing highly compact dark matter structures can be produced in the early universe and survive until the present day throughout most of the galactic and extragalactic volume. In such scenarios, these cusps can dominate the total dark mattter annihilation rate of the universe due to their compact nature. This changes the morphology of the expected dark matter signal, making it more isotropic. We produce a new model for the isotropic gamma-ray background flux using 14 years of Fermi-LAT observations, and set strong constraints on dark matter annihilation in these scenarios, ruling out standard thermal models annihilation to bb up to masses of 120 GeV.

Dark Matter signals become brighter righter when inverse-Compton is modeled more accurately

Searches for cosmic-ray positrons are one of the most sensitive probes of dark matter annihilation. This is particularly true for leptophilic dark matter models that can be difficult to detect via other channels. The principle impediment to detecting the sharply peaked positron spectrum from leptophilic dark matter is the cooling of the positrons via synchrotron and inverse-Compton scattering, which has normally been treated via a simplified model that assumes these losses are continuous. We show that when the stochasticity of inverse-Compton losses is taken into account, the expected signal becomes twice as bright, making it easier to detect heavy dark matter.

Strongly Coupled Dark Matter Models Can Produce Antihelium 4

AMS-02 has tentatively detected a flux of antihelium nuclei (around 10 events in total). This observation, if confirmed, would be very exciting because antihelium is not supposed to be produced by standard astrophysics -- and would point towards new physics. More confusingly, a handful of these events may be consistent with antihelium 4 -- which is difficult to produce even in dark matter models. We propose the first realistic dark matter model capable of producing an observable antihelium 4 flux. If the WIMP is very heavy (near unitarity scale), and annihilates through a strongly coupled dark sector, then each annihilation will produce a shower of dark pions with very high multiplicity. If these particles quickly decay to standard model quarks through a portal interaction -- the result is a high multiplicity of baryons produced promptly at the same vertex. This significantly enhances the formation of heavy antinuclei, and makes observations of antihelium-4 possible.

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.

Dark Matter Microhalos in the Solar Neighborhood: Pulsar Timing Signatures of Early Matter Domination

The impressive regularity of pulsating neutron stars allow them to be used as extremely acurate "clocks" that operate on Myr or even Gyr timescales. Pulsar timing arrays have taken repeated observations of nearby MSPs over 20 Myr, looking for small deviations which may be due to local gravitational effects affecting either the neutron star environment or local solar neighborhood. We note that these arrays are quickly becoming sensitive to the enhanced dark matter substructure that is expected when the universe goes through a period of Early Matter Domination before the onset of radiation domination during big-bang nucleosynthesis. Current, or near-future, observations (20 years with approximately 70 pulsars), could begin to constrain novel EMDE parameter space -- while future studies including 200 pulsars over 40 years could raise the minimum energy floor for early matter domination as high as 150 MeV.

Full Publication List:

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

18. Dark Matter Scattering Constraints from Observations of Stars Surrounding Sgr A*
Isabelle John, Rebecca Leane, Tim Linden
Accepted by Physical Review D

17. 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)

16. Accurate Inverse-Compton Models Strongly Enhance Leptophilic Dark Matter Signals
Isabelle John, Tim Linden
Physical Review D 108 10 103022

15. Cosmic Ray Antihelium from a Strongly Coupled Dark Sector
Martin Winkler, Pedro De la Torre Luque, Tim Linden
Physical Review D 107 12 123035 (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. Dark Matter Microhalos in the Solar Neighborhood: Pulsar Timing Signatures of Early Matter Domination
Sten Delos, Tim Linden
Physical Review D 105 123514

12. Response to Comment on: "Dark Matter Annihilation Can Produce a Detectable Antihelium Flux through Λb Decays
Martin Winkler, Tim Linden
ArXiv Only

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

10. Dark Matter Annihilation Can Produce a Detectable Antihelium Flux through Λb Decays
Martin Wolfgang Winkler, Tim Linden
Physical Review Letters 126 101101

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

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

6. 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

5. 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

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

3. 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

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

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

Tim Linden

Assistant Professor, Stockholm University