Publications

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.

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.

Extraterrestrial Axion Search with the Breakthrough Listen Galactic Center Survey

The extremely strong magnetic fields of neutron stars potentially provide an optimal environment to initiate the conversion of dark matter axions into photon signals. The galactic center remains an exciting target for these searches, because previous studies indicate that it has both a large dark matter and neutron star density. We utilize detailed sureys the galactic center produced by the Breakthrough Listen Project, which are directly targeted at finding evidence of extraterrestrial life in our universe. The high energy resolution of these surveys is optimal for the detection of an axion signal. We find no evidence for a radio line produced by axion-photon conversion, and set strong limits on the axion-photon conversion cross-section, the exact strength of which depends on uncertainties in the underlying neutron star and dark matter population -- uncertainties which can be significantly reduced by upcoming experiments over the next decade.

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.

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.

Full Publication List:

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

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

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