Publications

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.

Weighing the Local Interstellar Medium using Gamma Rays and Dust

The ratio between the gas and dust densities in galactic environments is among the most important parameters in understanding star formation and galaxy evolution. Typical constraints on this ratio stem from radio observations of dust emission and absorption, as well as radio line emission from standard gas tracers. We develop an entirely new method based on gamma-ray observations, which trace the convolution of the well-measured local cosmic-ray density and the well-measured high-latitude gamma-ray emission intensity. We obtain results that are consistent with many previous studies, but depend on an entirely independent set of systematic uncertainties. Moreover, our results have the precision to untangle the tension in previous world leading measurements, quantitatively improving our models of local gas and dust.

Gamma-Rays from Star Forming Activity Appear to Outshine Misaligned Active Galactic Nuclei

The Isotropic Gamma-Ray Background (IGRB) -- produced from the total gamma-ray sources which are too dim to be individually detected -- provides a key diagnostic constraining the energetics of the high-energy universe. The most important contributions are likely gamma-rays from star-forming activity (e.g., SFGs) and hadronic emission from blazars with jets that are not oriented towards Earth (mAGN). Previous studies have attempted to determine the importance of each channel by correlating their gamma-ray emission against their multiwavelength emission. These studies have obtained disparate results, partially because they have not carefully considered "composite" galaxies where both star-formation and mAGN activity is important. We perform the first joint-likelihood analysis that includes both components simultaneously, finding that SFGs are likely the most important contributor to the IGRB, and their contribution remains sizable despite uncertainties in the mAGN contribution.

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.

Pulsars Power Energetic HAWC Sources

Recently, HAWC has released a catalog of 9 sources with detected emission above 56 TeV. We show that all of these sources are likely powered by leptonic (rather than hadronic) processes. The most likely source of the high-energy emission is the young pulsar found near each source. Three distinct observations prefer our leptonic interpretation: (1) the luminosity of each source is consistent with an approximately 10% conversion of spindown power into e+e- acceleration, similar to values found for pulsars such as Geminga and Monogem, (2) a spectral cutoff is observed in each source near 10 TeV, an effect which is naturally explained by the transition from the uncooled to cooled electron spectrum, the position of which can be directly calculated from the known pulsar age, (3) hadronic emission models generically predict too much GeV emission from each source (compared to constraints from Fermi-LAT data). Our results have significant implications for the sources of the positron excess, and the existence of galactic PeVatrons.

Cosmic-Rays Don't Drive Winds in M82

We construct the most detailed models of cosmic-ray injection, propagation, and energy losses in the starburst galaxy M82, including the first two-dimensional model of cosmic-ray injection and propagation in the dense M82 nuclear core. We find that the high-gas density of of the starburst core quickly cools high-energy protons injected by supernovae in teh starburst region. By comparing these gamma-ray constraints with the morphology of observed radio emission, we place strong constraints on the steady state cosmic-ray density above and below the galactic plane. Because we find the cosmic-ray density (and its gradient) to be small, we can rule out models where cosmic-rays drive the galactic winds in galaxies like M82.

Evidence for Cosmic-Ray Escape from the Small Magellanic Cloud

The Small Magellanic Cloud serves as an excellent test case to understand cosmic-ray propagation in small galaxies. Situated only 100 thousand light years away from the Milky Way, its proximity allows us to utilize gamma-ray observations to measure not only its diffuse gamma-ray flux, but also its gamma-ray morphology. We produce a targeted study of the Small Magellanic Cloud, and find that the gamma-ray spectrum includes a peculiar cutoff at energies above 12 GeV, which corresponds to cosmic-ray energies near 200 GeV. These gamma-rays do not appear to be produced by isolated sources within the galaxy, leading us to conclude that high-energy cosmic-rays are produced in the Small Magellanic cloud, but then efficiently escape at high energies before encounting gas. This has implications for cosmic-ray feedback and the dynamics of small galaxies.

Full Publication List:

21. 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

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

19. 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

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

17. 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

16. 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

15. 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

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

13. 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

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

11. 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

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

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

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

7. 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)

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

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

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

3. 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

2. 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

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

Theses

Dark Matter Annihilation at the Galactic Center

Doctoral Thesis
13 June 2013

White Papers

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

AMEGO Collaboration
17 July 2019

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

HAWC Collaboration
18 March 2019

Tim Linden

Assistant Professor, Stockholm University

linden@fysik.su.se