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.

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Pulsars Do Not Produce Lines in the Electron and Positron Spectra

Pulsars are the leading explanation for the excess in the local positron flux -- but it remains an open question as to whether the high-energy positron flux is dominated by handful of nearby systems or by the total contributions of many individually dim pulsars. Models have long predicted that individual pulsars would produce a spectral line at an energy that corresponds to the pulsar age, and the observation or non-observation of such lines could differentiate the pulsar scenarios. We show that this spectral line does not exist -- it is instead of a result of a common approximation that incorrectly treats inverse-Compton energy losses as a continuous, rather than stochastic, process. Pulsars cannot produce sharp features in the cosmic-ray electron and positron spectra, a result which also reopens the possibility that the observation of any such line could be strong evidence for dark matter annihilation.

Anisotropic Diffusion Cannot explain TeV halo observations

The morphology of TeV halos is confusing. They are larger than the region where local magnetic fields from the pulsar and supernova remnant can control particle diffusion. They are smaller than the region that electrons should propagate through if they were diffusing freely through the ISM. Several models have been proposed to explain these observations -- one of which includes anisotropic diffusion. In this model - electrons move freely in one direction, but that direction happens to be aligned with the direction towards the Earth. Since we can't see the ``depth" of electrons easily -- we instead see a smaller halo produced by diffusion in the two perpendicular directions where diffusion is not efficient. Here, we show that these models do not produce the observable characteristics of TeV halos. In particular, the range of diffusion angles between the source and Earth is extremely small - such that the probability of finding several TeV halos in such a configuration is less than one in a billion.

Self-Generated Cosmic-Ray Turbulence Can Explain the Morphology of TeV Halos

TeV halos are a new class of spatially extended high-energy gamma-ray emission sources powered by young and middle aged pulsars. The radial extent of the observed sources is surprising: they are too large to be expalained as compact regions powered by a central object, but too small to be explained by particle diffusion throughout the galactic interstellar medium. Several models have been posited to explain the extremely small diffusion coefficient observed for particles propagating through these halos, but none are satisfactory. We revisit a previous model where the steep cosmic-ray gradient from the energetic source excites a resonant streaming instability that ``self-confines" the cosmic-ray population near the source. We correct an error in previous models, and find that the energetic pulsar is likely powerful enough to contain its own cosmic-ray population, re-establishing this class of models as a leading explanation for the production of TeV halos.

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.

Evidence of TeV Halos Around Millisecond Pulsars

Observations over the last few years have detected bright, spatially extended gamma-ray emission (TeV halos) around young and middle-aged pulsars throughout the Milky Way. This brings forth the question of whether the ancient population of millisecond pulsars -- much more numerous but individually less luminous -- also produce TeV halo activity. Unfortunately, if we expect that MSPs are equally efficient at producing TeV halos (compared to their spindown power) as young and middle-aged pulsars, none of these systems is expected to be bright enough to be individually detected by HAWC. However - they are close! In order to enhance the sensitivity of HAWC, we produce a "stacked" joint-likelihood analysis of the 37 MSPs with the highest expected spindown-flux (spindown-power divided by distance squared), and fit this population with a Geminga-like TeV halo model. We find that this method improves the quality of fit to the HAWC data by 4.24 sigma. However, the HAWC backgorunds are not Poissonian. If we re-fit this data by comparing our model with studies of "blank-sky" locations where no TeV halo emission is expected, we find the significance decreases, but is still approximately 2.5 sigma. Stay Tuned!

Full Publication List:

53. Weighing the Local Interstellar Medium using Gamma Rays and Dust
Axel Widmark, Michael Korsmeier, Tim Linden
To Be Submitted

52. Pulsars Do Not Produce Sharp Features in the Cosmic-Ray Electron and Positron Spectra
Isabelle John, Tim Linden
To Be Submitted

51. Anisotropic diffusion cannot explain TeV halo observations
Pedro De la Torre Luque, Ottavio Fornieri, Tim Linden
To Be Submitted

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
Submitted to JCAP

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

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
Submitted to PRD

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