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.

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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.

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!

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.

MSPs and the Radio/FIR Correlation

Observations over several decades have established a strong correlation between the far-infrared and non-thermal radio luminosities of star-forming galaxies. Because far-IR emission is produced primarily by massive stars, and non-thermal radio emission primarily by cosmic-ray electrons, this relationship is interpreted as observational evidence for the production of high-energy cosmic-rays by recent star-formation events. Intriguingly, this relation continues over more than four magnitudes in star-formation rate. Recent LOFAR observations, however, find an intriguing radio excess among galaxies with high stellar masses and low-star formation rates -- a result which apperas at odd with star-formation models of the cosmic-ray population. We propose that millisecond pulsars -- which evolve over Gyr timescales and thus trace historic star-formation, can contribute to, or even dominate, the radio flux from these dead galaxies. Such an observation has important implications for our understanding of stellar evolution, as well as cosmic-ray production within the bulge of the Milky Way and other nearby galaxies.

Prospects for Future TeV Halo Detections

Observations by the HAWC telescope have detected a large population of ``TeV Halos", bright, diffuse, and hard spectrum, TeV sources surrounding young and middle aged pulsars. The cleanest TeV halo systems, observed at the largest scales surrounding older pulsars like Geminga and Monogem, were enabled by the large ROI of HAWC observations. However, young pulsars are expected to provide brighter, if more complex, emission sources. We study the observation of younger TeV halos with the HAWC, HESS, and CTA instruments, predicting that upcoming instruments will detect between 50-240 halos!

Full Publication List:

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

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

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

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

12. 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

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

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

9. 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

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

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

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

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

4. 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

3. 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

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

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

Tim Linden

Assistant Professor, Stockholm University