Publications

Gamma Rays in a Halo Around the Sun

The Sun is one of the brightest gamma-ray sources in the sky. The gamma-ray emission comes from two processes: First, high-energy cosmic-ray protons hit the solar surface and produce outgoing gamma-rays via hadronic interactions. Second, high-energy electrons and positrons upscatter starlight to gamma-ray energies via inverse-Compton scattering. This forms a bright, diffuse halo of gamma-ray emission around the Sun. We use 15 years of Fermi-LAT data to observe the spectrum, morphology, time-variation, and azimuthal distribution of gamma-rays from the solar halo, detecting bright emission between 31.6 MeV and 100 GeV, and out to 45 degress from the Sun. The morphology and spectrum of the emission is primarily governed by solar modulation, and we set strong constraints on the solar modulation potential in regions near the Sun that are inaccessible to local cosmic-ray probes of solar dynamics.

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.

Solar Disk Gamma-Rays

The Sun is one of the brightest gamma-ray sources in the sky. Here we use new Fermi-LAT observations (covering more than 11.5 years of data) to constrain the intensity, spectrum, morphology, and time-variability of gamma-ray emission from the solar disk. Utilizing a novel and advanced background subtraction technique, we obtain percent level precision in observations spanning the energy range 100 MeV to 100 GeV. We uncover several exciting results, including strong evidence for an 11 year solar variability timescale (corresponding to the solar cycle) and strong constraints on an energy-dependence in the amplitude of this correlation. The latter result indicates that strong forces near teh solar surface, rather than modulation throughout the heliosphere, is primarily responsible for the time-dependence of the gamma-ray emission.

Constraining Solar Modulation

A critical uncertainty in understanding cosmic-rays in our Galaxy is that the cosmic-rays we can observe in our solar system are not representative of the galactic cosmic-ray population. To make it to Earth, cosmic-rays must first fight against the magnetic field and charged wind produce by the Sun -- losing energy in a time-dependent, charge-dependent, and rigidity-dependent way. Fortunately, because the cosmic-ray population in interstellar space is constant in time, and the effects induced by the Sun oscillate wildly (even on daily timescales), we can separate these effects and constrain the impact of solar modulation on the observed cosmic-ray population. We use this to better constrain the cosmic-ray population in the interstellar medium, which is a key observable in searches for dark matter or other new physics.

HAWC Constrains Annihilation to Long-Lived Mediators

Dark Matter Annihilation directly to neutrino final states has long been the worst-case scenario for indirect detection searches. One promising direction involves IceCube searches for dark matter annihilation in the Sun, as the neutrinos could easily escape through the dense solar material. A related search, utilizing gamma-ray telescopes, involves dark matter annihilation to meta-stable final states, which can propagate out of the solar interior before decaying to more common particles (e.g., gamma-rays, bottom quarks, etc.). We utilize HAWC observations of the Sun to examine on this unique channel, setting constraints that lie several orders of magnitude below previous experiments.

Upper Limits on TeV Gamma-Ray Flux from Sun

Recently, we made the unexpected detection of solar gamma-rays up to energies exceeing 100 GeV with the Fermi-LAT telescope. These observations showed that, contrary to previous expectations, the solar gamma-ray spectrum during solar minimum continues to much higher energies than expected. The much larger effective area of HAWC will potentially allow the coninued detection of solar gamma-rays up to TeV energies or beyond. While we don't yet have solar minimum observations from HAWC, we can evaluate the signal during solar maximum, when the Fermi-LAT predictions indicate the emission will be dim. We do not find evidence of TeV solar gamma-ray emission, and set the strongest constraints yet available on its flux. This same search strategy will soon be used for searches during solar minimum, which will be much more exciting!

Fermi Data Reveals Two Components of Solar Gamma-Ray Emission

We utilize Fermi-LAT observations of the Sun to uncover a number of surprising new features regarding its intrinsic emission. Most notably, we find that the Sun powers gamma-rays via two distinct processes which have diferent morphologies, spectra time-dependences. The first, observed continuously throughout the solar cycle, is equally bright across the solar surface, and has a spectral cutoff at gamma-ray energies of around 30-50 GeV. The second emission component, which is present only during solar minimum (for about 2-3 years every 11 year cycle), has a hard gamma-ray spectrum, is produced primarily from the Sun's equatorial regions, and is capable of producing gamma-rays with energies of at least 200 GeV. The reason for these spectral differences is unknown, and has the potentially to illuminate new features in solar physics.

Full Publication List:

7. First Observations of Solar Halo Gamma Rays Over a Full Solar Cycle
Tim Linden, Jung-Tsung Li, Bei Zhou, Isabelle John, Milena Crnogorčević, Annika Peter, John Beacom
To Be Submitted

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

5. 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

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

3. 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

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

1. 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

White Papers

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

Mehr Un Nisa et al.
15 March 2019

Tim Linden

Assistant Professor, Stockholm University

linden@fysik.su.se

Solar Astrophysics