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.

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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.

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.

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.

Constraining Axion-Like Particles with HAWC Observations of TeV Blazars

Very high-energy gamma-rays from distant sources are typically attenuated (absorbed and re-emitted at lower energies) through interactions with the extragalactic background light during their intergalactic propagation. This means that while distant blazars are bright at GeV energies, their emission at TeV energies should be highly suppressed. Interactions with axion-like particles can provide an alterative life for these particles, as TeV photons could convert into axions within blazar magnetic fields, and then propagate effectively through the IGM before reconverting into TeV gamma rays in the Milky Way magnetic field. We search TeV gamma-ray emission from bright blazars using the 3HWC catalog in concert with lower-energy telescopes. We find no evidence for a signal consistent with ALPs, and place constraints on the axion-photon coupling that is consistent with, or slightly stronger than previous limits.

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.

Full Publication List:

55. 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
To Be Submitted

54. The Cherenkov Telescope Array Will Test Whether Pulsars Generate the Galactic Center Gamma-Ray Excess
Celeste Keith, Dan Hooper, Tim Linden
To Be Submitted

53. Cosmic Ray Antihelium from a Strongly Coupled Dark Sector
Martin Winkler, Pedro De la Torre Luque, Tim Linden
To Be Submitted to PRD

52. White Dwarfs in Dwarf Spheroidal Galaxies: A New Class of Compact-Dark-Matter Detectors
Juri Smirnov, Ariel Goobar, Tim Linden, Edvard Mörtsell
To Be Submitted to PRL

51. 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

50. Constraining Axion-Like Particles with HAWC Observations of TeV Blazars
Sunniva Jacobsen, Tim Linden, Katherine Freese
To Be Submitted

49. 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)

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

47. Cosmic-Ray Positrons Strongly Constrain Leptophilic Dark Matter
Isabelle John, Tim Linden
Journal of Cosmology and Astroparticle Physics 12 2021 007

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

45. First Analysis of Jupiter in Gamma Rays and a New Search for Dark Matter
Rebecca Leane, Tim Linden
Submitted to PRL

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

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

42. Anti-Deuterons and Anti-Helium Nuclei from Annihilating Dark Matter
Ilias Cholis, Tim Linden, Dan Hooper
Physical Review D 102 103019

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

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

39. A Robust Excess in the Cosmic-Ray Antiproton Spectrum: Implications for Annihilating Dark Matter
Ilias Cholis, Tim Linden, Dan Hooper
Physical Review D 99 103026

38. 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

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

36. Comment on "Characterizing the Population of Pulsars in the Galactic Bulge with the Fermi Large Area Telescope" [arXiv: 1705.00009v1]
Richard Bartels, Dan Hooper, Tim Linden, Siddharth Mishra-Sharma, Nick Rodd, Ben Safdi, Tracy Slatyer
Physics of the Dark Universe 20 88 2018

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

34. 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

33. 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

32. 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

31. The Gamma-Ray Pulsar Population of Globular Clusters: Implications for the GeV Excess
Dan Hooper, Tim Linden
Journal of Cosmology and Astroparticle Physics 1608 08 018

30. 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

29. 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

28. The High-Energy Tail of the Galactic Center Gamma-Ray Excess
Tim Linden, Nicholas Rodd, Benjamin Safdi, Tracy Slatyer
Physical Review D 94, 103013

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

26. 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

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

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

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

22. 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

21. 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

20. 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

19. 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

18. The Anisotropy of the Extragalactic Radio Background from Dark Matter
Ke Fang, Tim Linden
Physical Review D 91 083501 (2015)

17. What Does the PAMELA Antiproton Spectrum Tell Us About Dark Matter?
Dan Hooper, Tim Linden, Philipp Mertsch
Journal of Cosmology and Astroparticle Physics 03 021 (2015)

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

15. A Critical Reevaluation of Radio Constraints on Annihilating Dark Matter
Ilias Cholis, Dan Hooper, Tim Linden
Physical Review D, 91 083507 (2015)

14. 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)

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

12. 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

11. The Characterization of the Gamma-Ray Signal from the Central Milky Way
Tansu Daylan, Doug Finkbeiner, Dan Hooper, Tim Linden, Stephen Portillo, Nicholas Rodd, Tracy Slatyer
Physics of the Dark Universe 12 2016 1

10. Antihelium from Dark Matter
Eric Carlson, Adam Coogan, Tim Linden, Stefano Profumo, Alejandro Ibarra, Sebastian Wild
Physical Review D, 89 076005

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

8. Pulsars Cannot Account for the Inner Galaxy's GeV Excess
Dan Hooper, Ilias Cholis, Tim Linden, Jennifer Siegal-Gaskins, Tracy Slatyer
Physical Review D, 88 083009

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

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

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

4. The Isotropic Radio Background and Annihilating Dark Matter
Dan Hooper, Alexander Belikov, Tesla Jeltema, Tim Linden, Stefano Profumo, Tracy Slatyer
Physical Review D, 86 10 103003

3. Origin of Gamma-Rays from the Galactic Center
Dan Hooper, Tim Linden
Physical Review D, 84 12 123005

2. Dark Matter and Synchrotron Emission from Galactic Center Radio Filaments
Tim Linden, Dan Hooper, Farhad Yusef-Zadeh
The Astrophysical Journal, 741 2 95

1. Gamma-Rays from the Galactic Center and the WMAP Haze
Dan Hooper, Tim Linden
Physics Review D, 83 8 083517 (2011)



Tim Linden

Assistant Professor, Stockholm University

linden@fysik.su.se