Publications

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.

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.

AGN-Driven Models for the Neutrino and Gamma-Ray Flux from NGC 1068

Recent observations by IceCube have found an extremely high neutrino flux from the Seyfert II Galaxy NGC 1068. These models are hard to reconcile with the relatively low gamma-ray flux from this galaxy, unless the neutrinos are produced in a region where the gamma-ray emission is strongly attenuated. However, gamma-ray attenuation produces lower-energy gamma-rays that should be obsrved by future telescopes at MeV energies. We study the parameter space which can simultaneously produce the bright neutrino emission and dim GeV gamma-ray emission, finding that this source must have extremely high magnetic and radiation fields near the central black hole. We forecast that future MeV telescopes will detect bright emission from this galaxy.

Where are the Cascades from Blazar Jets? An Emerging Tension in the Gamma-Ray Sky

Blazars are expected to be bright sources of TeV gamma-ray emission. However, their gamma-ray signal is attenuated by extragalactic light before making it to Earth. However, this emission is not invisible, as it cascades down to GeV energies, where it should be observed as a bright isotropic GeV gamma-ray flux. Using up to date blazar models and isotropic gamma-ray background measurements, we set strong constraints on these GeV cascades, finding that they fall far below theoretical predictions. Our study either indicates that blazars have sharp spectral cutoffs and are dim TeV sources, which is in tension with current theory and observation, or that plasma effects are capable of effectively stopping gamma-ray cascades, which in turn indicates that extragalactic magnetic fields are unexpectedly feeble.

Observations of Gamma-Ray Emission from the core of the Sagittarius Dwarf Galaxy

The Sagittarius dwarf spheroidal galaxy is one of the largest satellites of the Milky Way - making in a potentially optimal target in searches for dark matter annihilation. However, it is also unique in that its core holds a population of globular clusters (most notably M54), which are capable of producing bright gamma-ray emission from a population of MSPs at their center. We study 13 years of gamma-ray data, and detect the M54 globular cluster - confirming several previous studies and making this the most distant globular cluster detected by Fermi. We examine the potential for this gamma-ray emission to be produced by either pulsars or potentially by dark matter annihilation from the surrounding Sagittarius dwarf.

CTA will Conclusively Probe Pulsar Models of the Galactic Center Excess

The nature of the Galactic Center excess has been debated for over a decade -- with dark matter and millisecond pulsars as the two leading candidates. We utilize recent evidence which shows that MSPs (like young pulsars) accelerate a significant e+e- population up to TeV energies, in order to argue that any pulsar model capable of explaining the excess would produce a bright diffuse TeV flux that would be detected by the upcoming Cherenkov Telescope Array. If such a feature is not detected, it would strongly constrain pular models of the galactic center excess.

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.

Full Publication List:

61. Strong Constraints on Dark Matter Annihilation in Ursa Major III/UNIONS 1
Milena Crnogorčević, Tim Linden
To Be Submitted

60. 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
To Be Submitted

59. On the Neutrino and Gamma-Ray Emission from NGC 1068
Carlos Blanco, Dan Hooper, Tim Linden, Elena Pinetti
To Be Submitted

58. 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

57. On the gamma-ray emission from the core of the Sagittarius dwarf galaxy
Addy Evans, Louis Strigari, Oskar Svenborn, Andrea Albert, Pat Harding, Dan Hooper, Tim Linden, Andrew Pace
Monthly Notices of the Royal Astronomical Society 524 3 4574

56. The Cherenkov Telescope Array Will Test Whether Pulsars Generate the Galactic Center Gamma-Ray Excess
Celeste Keith, Dan Hooper, Tim Linden
Physical Review D 107 10, 103001

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

54. White Dwarfs in Dwarf Spheroidal Galaxies: A New Class of Compact-Dark-Matter Detectors
Juri Smirnov, Ariel Goobar, Tim Linden, Edvard Mörtsell
Submitted to Physical Review Letters

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

52. Pulsars Do Not Produce Sharp Features in the Cosmic-Ray Electron and Positron Spectra
Isabelle John, Tim Linden
Physical Review D 107 10, 103021 (2023)

51. Anisotropic diffusion cannot explain TeV halo observations
Pedro De la Torre Luque, Ottavio Fornieri, Tim Linden
Physical Review D 106 123033

50. Self-Generated Cosmic-Ray Turbulence Can Explain the Morphology of TeV Halos
Payel Mukhopadhyay, Tim Linden
Physical Review D 105, 123008

49. 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

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

47. Evidence of TeV Halos Around Millisecond Pulsars
Dan Hooper, Tim Linden
Physical Review D 105, 103013

46. 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

45. 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

44. Constraining the Charge-Sign and Rigidity-Dependence of Solar Modulation
Ilias Cholis, Dan Hooper, Tim Linden
Journal of Cosmology and Astroparticle Physics 10 051

43. Millisecond Pulsars Modify the Radio-SFR Correlation in Quiescent Galaxies
Takahiro Sudoh, Tim Linden, John Beacom
Physical Review D 103 083017

42. 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

41. TeV Halos are Everywhere: Prospects for New Discoveries
Takahiro Sudoh, Tim Linden, John Beacom
Physical Review D 100 043016

40. Active Galactic Nuclei and the Origin of IceCube’s Diffuse Neutrino Flux
Dan Hooper, Tim Linden, Abby Vieregg
Journal of Cosmology and Astroparticle Physics 02 012

39. 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

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

37. Self-Generated Cosmic-Ray Confinement in TeV Halos: Implications for TeV γ-ray Emission and the Positron Excess
Carmelo Evoli, Tim Linden, Giovanni Morlino
Physical Review D 98 063017

36. 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

35. 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

34. Millisecond Pulsars, TeV Halos, and Implications for the Galactic Center Gamma-Ray Excess
Dan Hooper, Tim Linden
Physical Review D 98 043005

33. 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

32. Measuring the Local Diffusion Coefficient with HESS Observations of High-Energy Electrons
Dan Hooper, Tim Linden
Physical Review D 98 083009

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

30. TeV Gamma Rays from Galactic Center Pulsars
Dan Hooper, Ilias Cholis, Tim Linden
Physics of the Dark Universe 21 40 2018

29. 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

28. Using HAWC to Detect Invisible Pulsars
Tim Linden, Katie Auchettl, Joseph Bramante, Ilias Cholis, Ke Fang, Dan Hooper, Tanvi Karwal, Shirley Li
Physical Review D 96 103016

27. HAWC Observations Strongly Favor Pulsar Interpretations of the Cosmic-Ray Positron Excess
Dan Hooper, Ilias Cholis, Tim Linden, Ke Fang
Physical Review D 96 103013

26. Evidence for the Stochastic Acceleration of Secondary Antiprotons by Supernova Remnants
Ilias Cholis, Dan Hooper, Tim Linden
Physical Review D 95 123007

25. Low Mass X-Ray Binaries in the Inner Galaxy: Implications for MSPs and the GeV Excess
Daryl Haggard, Craig Heinke, Dan Hooper, Tim Linden
Journal of Cosmology and Astroparticle Physics 1705 05 056

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

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

22. A Predictive Analytic Model for the Solar Modulation of Cosmic Rays
Ilias Cholis, Dan Hooper, Tim Linden
Physical Review D 93 4 043016

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

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

19. Cluster Mergers and the Origin of the ARCADE-2 Excess
Ke Fang, Tim Linden
Journal of Cosmology and Astroparticle Physics 1610 10 004

18. The Galactic Center GeV Excess from a Series of Leptonic Cosmic-Ray Outbursts
Ilias Cholis, Carmelo Evoli, Francesca Calore, Tim Linden, Christoph Weniger, Dan Hooper
Journal of Cosmology and Astroparticle Physics 1512 12 005

17. On the Formation of Ultra-Luminous X-Ray Sources with NS Accretors: The Case of M82-X2
Tassos Fragos, Tim Linden, Vicky Kalogera, Panos Sklias
The Astrophysical Journal Letters, 802 2041

16. 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)

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

14. The Circular Polarization of Pulsar Wind Nebulae and the Cosmic-Ray Positron Excess
Tim Linden
The Astrophysical Journal 799 200 (2015)

13. Is the Ultra-High Energy Cosmic-Ray Excess Correlated with IceCube Neutrinos?
Ke Fang, Toshihiro Fujii, Tim Linden, Angela Olinto
The Astrophysical Journal, 794 126

12. Probing the Pulsar Origin of the Positron Fraction with Atmospheric Cherenkov Telescopes
Tim Linden, Stefano Profumo
The Astrophysical Journal, 772 18

11. Ultraluminous X-Ray Sources in the Most Metal-Poor Galaxies
A. Prestwich, M. Tsantaki, A. Zezas, F. Jackson, T. Roberts, R. Foltz, Tim Linden, V. Kalogera
The Astrophysical Journal 769 2 92

10. Testing the WMAP-Planck Haze with Spiral Galaxies
Eric Carlson, Dan Hooper, Tim Linden, Stefano Profumo
Journal of Cosmology and Astroparticle Physics, 1307 026 (2013)

9. Chandra Observations of the Collisional Ring Galaxy NGC 922
A. Prestwich, J. Galache, Tim Linden, V. Kalogera, A. Zezas, T. Roberts, R. Kilgard, A. Wolter, G. Trinchieri
The Astrophysical Journal, 747 2 150

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

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

6. 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

5. On The Rarity of X-Ray Binaries with Naked Helium Donors
Tim Linden, Francesca Valsecchi, Vicky Kalogera
The Astrophysical Journal, 748 2 114

4. The Effect of Starburst Metallicity on Bright X-Ray Binary Formation Pathways
Tim Linden, Vicky Kalogera, Jeremy Sepinsky, Andrea Prestwich, Andreas Zezas, Jay Gallagher
The Astrophysical Journal, 725 2 1984

3. 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

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

1. Probing Electron-Capture Supernovae: X-Ray Binaries in Starbursts
Tim Linden, Jeremy Sepinsky, Vicky Kalogera, Chris Belczynski
The Astrophysical Journal, 699 2 1573 (2009)

Tim Linden

Assistant Professor, Stockholm University

linden@fysik.su.se

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

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

The Sun at GeV--TeV Energies: A New Laboratory for Astroparticle Physics

Astrophysics

Ursa Major III Strongly Constrains Dark Matter Annihilation

Research Topics

IGRB Limits on Dark Matter Cusps

AGN-Driven Models for the Neutrino and Gamma-Ray Flux from NGC 1068

Where are the Cascades from Blazar Jets? An Emerging Tension in the Gamma-Ray Sky

Observations of Gamma-Ray Emission from the core of the Sagittarius Dwarf Galaxy

CTA will Conclusively Probe Pulsar Models of the Galactic Center Excess

First Detection of TeV Gamma-Ray Emission from the Sun

Full Publication List:

Theses

Dark Matter Annihilation at the Galactic Center

Young X-Ray Binary Populations: Metallicity Effects and a Diagnostic for Electron-Capture Supernovae

Modeling the Spatial and Luminosity Distributions of X-Ray Binaries: An Introduction to the StarChart Code

White Papers

Tim Linden