Cosmic-ray Positron Fraction Research Articles

Possible Counterpart Signal of the Fermi Bubbles at the Cosmic-Ray Positrons

The inner Galaxy has hosted cosmic-ray burst events, including those responsible for the gamma-ray Fermi bubbles and the eROSITA bubbles in X-rays. In this work, we study the Alpha Magnetic Spectrometer positron fraction and find three features around 12, 21, and 48 GeV, of which the lowest energy has a 1.4–4.9σ significance, depending on astrophysical background assumptions. Using background simulations that explain the cosmic-ray positron fraction, positron flux, and electron plus positron flux by primary and secondary cosmic rays and cosmic rays from local pulsars, we test these spectral features as originating from electron/positron burst events from the inner Galaxy. We find the 12 GeV feature to be explained by an event of age τ ≃ 3–10 Myr, in agreement with the proposed age of the Fermi bubbles. Furthermore, the energy in cosmic-ray electrons and positrons propagating along the Galactic disk and not within the Fermi bubbles volume is estimated to be 1051.5–1057.5 erg, or O(10−4) − O(1) the cosmic-ray energy causing the Fermi bubbles. We advocate that these positron fraction features are the counterpart signals of the Fermi bubbles, or of substructures within them, or of the eROSITA bubbles.

Observing signals of spectral features in the cosmic-ray positrons and electrons from Milky Way pulsars

The Alpha Magnetic Spectrometer (AMS-02) has provided unprecedented precision measurements of the electron and positron cosmic-ray fluxes and the positron fraction spectrum. At the higher energies, sources as energetic local pulsars, may contribute to both cosmic-ray species. The discreteness of the source population, can result in features both on the positron fraction measurement and in the respective electron and positron spectra. For the latter, those would coincide in energy and would contrast predictions of smooth spectra as from particle dark matter. In this work, using a library of pulsar population models for the local part of the Milky Way, we perform a power-spectrum analysis on the cosmic-ray positron fraction. We also develop a technique to cross-correlate the electron and positron fluxes. We show that both such analyses, can be used to search statistically for the presence of spectral wiggles in the cosmic-ray data. For a significant fraction of our pulsar simulations, those techniques are already sensitive enough to give a signal for the presence of those features above the regular noise, with forthcoming observations making them even more sensitive. Finally, by cross-correlating the AMS-02 electron and positron spectra, we find an intriguing first hint for a positive correlation between them, of the kind expected by a population of local pulsars.

Revisiting GeV-scale annihilating dark matter with the AMS-02 positron fraction

Antimatter cosmic-rays are used to probe new phenomena in physics, including dark matter annihilation. We use the cosmic-ray positron fraction spectrum by the Alpha Magnetic Spectrometer, to search for such an annihilation signal in the Galaxy. We focus on dark matter with mass between 5 and 120 GeV, producing high-energy electrons and positrons. In these cosmic-ray energies the interplay of multiple astrophysical sources and phenomena, makes this search highly sensitive to the underlying astrophysical background assumptions. We use a vast public library of astrophysical models for the cosmic-ray positron fraction background, to derive robust upper limits on the dark matter's annihilation cross section for a number of annihilation channels. This library accounts for different types of cosmic-ray sources and uncertainties on their distribution in space and time. Also, it accounts for uncertainties on those sources' output, their injected into the interstellar medium cosmic-ray spectra and for uncertainties on cosmic-ray propagation. For any given dark matter particle mass and annihilation channel, upper limits on the annihilation cross section are given by bands that stretch a full order of magnitude in its value. Our work provides weaker limits compared to earlier results, that are however robust to all the relevant astrophysical uncertainties. Between 5 and 15 GeV, we find indications for a possible excess flux of cosmic-ray electrons and positrons. That excess is found for most, but not all of our astrophysical background parameter space, and its significance can vary appreciably. Further scrutiny is necessary to improve the understanding of these lower energy cosmic rays. Finally, we note that even if an excess signal is found in these energies, the current background uncertainties do not allow us to accurately deduce its underlying particle properties.

Constraining the local pulsar population with the cosmic-ray positron fraction

Observations of the TeV halos associated with nearby pulsars indicate that these objects inject significant fluxes of very high-energy electron-positrons pairs into the interstellar medium (ISM), thereby likely providing the dominant contribution to the cosmic-ray positron flux. In this paper, we use the cosmic-ray positron fraction as measured by the AMS-02 Collaboration to constrain the characteristics of the local pulsar population. For reasonable model parameters, we find that we can obtain good agreement with the measured positron fraction up to energies of Ee ∼ 300 GeV. At higher energies, the positron fraction is dominated by a small number of pulsars, making it difficult to reliably predict the shape of the expected positron fraction. The low-energy positron spectrum supports the conclusion that pulsars typically transfer approximately η ∼ 5–20% of their total spindown power into the production of very high-energy electron-positron pairs, producing a spectrum of such particles with a hard spectral index, α ∼ 1.5–1.7. Such pulsars typically spindown on a timescale on the order of τ ∼ 104 years. Our best fits were obtained for models in which the radio and gamma-ray beams from pulsars are detectable to 28% and 62% of surrounding observers, respectively.

Red dwarfs as sources of cosmic rays and detection of TeV gamma-rays from these stars

A modern paradigm on the sources of galactic cosmic rays includes supernovae and supernova remnants as the powerful ones to accelerate particles up to ∼1017 eV. Meanwhile, the recent experimental data obtained by PAMELA, AMS-02, Fermi-LAT, CALET, and DAMPE require the existence of other cosmic-ray sources located nearby the solar system, at the distances less than 1 kpc. Presence of such local sources could explain the unexpected rise of the positron fraction in cosmic rays, the observed electron fluxes with the energies more than one TeV, complex shapes of the proton and helium spectra, and anomalous low-energy cosmic rays. Here we consider active dwarf stars as possible sources of Galactic cosmic rays in the energy range up to 1014 eV. These stars are producing powerful stellar flares sometimes with energy release more than 1036 erg. Meanwhile, the generation of high-energy cosmic rays should be accompanied by the high-energy γ-ray emission, which may be observed. Here we present the results of the SHALON long-term observations of generation of γ-ray emission above 800 GeV from the active red dwarf stars.

Red Dwarfs as Sources of Cosmic Rays and First Detection of TeV Gamma-rays from these stars

Conventional sources of Galactic cosmic rays are believed to be supernovae and supernova remnants that are powerful enough to accelerate particles up to 1017 eV. Meanwhile, an interpretation of recent experimental data obtained by PAMELA, Fermi-LAT, and AMS-02 spectrometers hints at a possibility that some cosmic-ray sources may be located in a close proximity of the solar system, at the distances less than 1 kpc. Presence of such local sources could explain the unexpected raise of the positron fraction in cosmic rays, complex shapes of the proton and helium spectra, and even anomalous low-energy cosmic rays. We consider active dwarf stars as possible sources of Galactic cosmic rays in the energy range up to 1014 eV. These stars are producing powerful stellar flares. Meanwhile, generation of high-energy cosmic rays should be accompanied by the high-energy γ-ray emission, which may be observed. Here we present the results of the SHALON long-term observations aimed to search for γ-ray emission above 800 GeV from the active red dwarf stars.

Gamma-ray observations under bright moonlight with VERITAS

Imaging atmospheric Cherenkov telescopes (IACTs) are equipped with sensitive photomultiplier tube (PMT) cameras. Exposure to high levels of background illumination degrades the efficiency of and potentially destroys these photo-detectors over time, so IACTs cannot be operated in the same configuration in the presence of bright moonlight as under dark skies. Since September 2012, observations have been carried out with the VERITAS IACTs under bright moonlight (defined as about three times the night-sky-background (NSB) of a dark extragalactic field, typically occurring when Moon illumination > 35%) in two observing modes, firstly by reducing the voltage applied to the PMTs and, secondly, with the addition of ultra-violet (UV) bandpass filters to the cameras. This has allowed observations at up to about 30 times previous NSB levels (around 80% Moon illumination), resulting in 30% more observing time between the two modes over the course of a year. These additional observations have already allowed for the detection of a flare from the 1ES 1727 + 502 and for an observing program targeting a measurement of the cosmic-ray positron fraction. We provide details of these new observing modes and their performance relative to the standard VERITAS observations.

AMS-02 measurement of cosmic ray positrons and electrons

Precision measurements by AMS of the primary cosmic-ray positron fraction and the positron flux in the energy range 0.5 to 500 GeV, and the electron flux in the range 0.5 to 700 GeV are presented. The results show that above 200 GeV the positron fraction no longer exhibits an increase with energy. The positron flux and the electron flux are significantly different in their magnitude and energy dependence. They each require a description beyond a single power-law spectrum. Between 20 and 200 GeV the positron spectral index is significantly harder than the electron spectral index. The incoming direction of primary CR positrons and electrons are found to be consistent with isotropy within the current accuracy.

Indirect dark matter signatures in the cosmic dark ages. I. Generalizing the bound ons-wave dark matter annihilation from Planck results

Recent measurements of the cosmic microwave background (CMB) anisotropies by Planck provide a sensitive probe of dark matter annihilation during the cosmic dark ages, and specifically constrain the annihilation parameter $f_\mathrm{eff} \langle \sigma v \rangle/m_\chi$. Using new results (Paper II) for the ionization produced by particles injected at arbitrary energies, we calculate and provide $f_\mathrm{eff}$ values for photons and $e^+e^-$ pairs injected at keV-TeV energies; the $f_\mathrm{eff}$ value for any dark matter model can be obtained straightforwardly by weighting these results by the spectrum of annihilation products. This result allows the sensitive and robust constraints on dark matter annihilation presented by the Planck Collaboration to be applied to arbitrary dark matter models with $s$-wave annihilation. We demonstrate the validity of this approach using principal component analysis. As an example, we integrate over the spectrum of annihilation products for a range of Standard Model final states to determine the CMB bounds on these models as a function of dark matter mass, and demonstrate that the new limits generically exclude models proposed to explain the observed high-energy rise in the cosmic ray positron fraction. We make our results publicly available at http://nebel.rc.fas.harvard.edu/epsilon.

Positron fraction, electron and positron spectra measured by AMS-02

A precise measurement by AMS-02 of the electron spectrum up to 700 GeV and of the positron spectrum and positron fraction in primary cosmic rays up 500 GeV are presented. The combined measurement of the cosmic-ray electron and positron energy spectra and fraction provide a unique tool to improve our understanding of the production, acceleration and propagation mechanism of cosmic rays.

Spectrum and fraction of cosmic ray positrons: results of the anomalous diffusion approach

We present the results of new calculations of the energy spectra of cosmic ray electrons, positrons and also positron fraction under assumption that both electrons and positrons are generated by the same Galactic sources, which accelerate particles with same power-law injection spectral index p. The value of the injection index p ≈ 2.85 retrieved in our works from the analysis of the observed CR spectra has been used. The propagation of particles through the highly non-homogeneous interstellar medium has been described by the anomalous diffusion model.We show that proposed approach allows the self-consistent description of the recent AMS-02 data. In contrast to the conclusion made by AMS-02 collaboration about the different origin of the high-energy positrons and electrons, our results demonstrate that the differing behavior of spectral indices of electrons and positrons with energy can be described under the assumption that both positrons and electrons have common Galactic sources.Our prediction (Lagutin et al. Bull. Russian Academy of Sciences. Physics, 2013, Vol. 77, No. 11, p. 1312) that the e+ to total e− + e+ ratio reaches a constant value for E ≫ 200 GeV is confirmed.

A new look at the cosmic ray positron fraction

Context. The positron fraction in cosmic rays has recently been measured with improved accuracy up to 500 GeV, and it was found to be a steadily increasing function of energy above ~10 GeV. This behaviour contrasts with standard astrophysical mechanisms, in which positrons are secondary particles, produced in the interactions of primary cosmic rays during their propagation in the interstellar medium. The observed anomaly in the positron fraction triggered a lot of excitement, as it could be interpreted as an indirect signature of the presence of dark matter species in the Galaxy, the so-called weakly interacting massive particles (WIMPs). Alternatively, it could be produced by nearby sources, such as pulsars.

What can we learn from a sharply falling positron fraction?

Recent results from the AMS-02 data have confirmed that the cosmic ray positron fraction increases with energy between 10 and 200GeV. This quantity should not exceed 50%, and it is hence expected that it will either converge towards 50% or fall. We study the possibility that future data may show the positron fraction dropping down abruptly to the level expected with only secondary production, and forecast the implications of such a feature in term of possible injection mechanisms that include both Dark Matter and pulsars.

WHAT COULD WE LEARN FROM A SHARPLY FALLING POSITRON FRACTION?

Recent results from the AMS-02 data have confirmed that the cosmic ray positron fraction increases with energy between 10 and 200GeV. This quantity should not exceed 50%, and it is hence expected that it will either converge towards 50% or fall. We study the possibility that future data may show the positron fraction dropping down abruptly to the level expected with only secondary production, and forecast the implications of such a feature in term of possible injection mechanisms that include both dark matter and pulsars. {Were a sharp steepening to be found, rather surprisingly, we conclude that pulsar models would do at least as well as dark matter scenarios in terms of accounting for any spectral cut-off.

High statistics measurement of the positron fraction in primary cosmic rays of 0.5-500 GeV with the alpha magnetic spectrometer on the international space station.

A precision measurement by AMS of the positron fraction in primary cosmic rays in the energy range from 0.5 to 500GeV based on 10.9 million positron and electron events is presented. This measurement extends the energy range of our previous observation and increases its precision. The new results show, for the first time, that above ∼200 GeV the positron fraction no longer exhibits an increase with energy.

AMS-02 data confront acceleration of cosmic ray secondaries in nearby sources

We revisit the model proposed earlier to account for the observed increase in the positron fraction in cosmic rays with increasing energy, in the light of new data from the Alpha Magnetic Spectrometer (AMS-02) experiment. The model accounts for the production and acceleration of secondary electrons and positrons in nearby supernova remnants which results in an additional, harder component that becomes dominant at high energies. By fitting this to AMS-02 data we can calculate the expected concomitant rise of the boron-to-carbon ratio, as well as of the fraction of antiprotons. If these predictions are confirmed by the forthcoming AMS-02 data it would conclusively rule out all other proposed explanations, in particular dark matter annihilations or decays.

High statistics measurement of the positron fraction in primary cosmic rays of 0.5-500 GeV with the alpha magnetic spectrometer on the international space station

High statistics measurement of the positron fraction in primary cosmic rays of 0.5-500 GeV with the alpha magnetic spectrometer on the international space station

THE ORIGIN OF THE SPECTRAL INTENSITIES OF COSMIC-RAY POSITRONS

AMS instrument aboard the International Space Station is a high precision instrument capable of collecting large statistics on cosmic ray intensities and as such has started making highly significant contributions to Astroparticle Physics. In order to fully benefit from these observations, the positron fraction in cosmic rays in their recent publication and other results that will follow, the models to interpret the results have to be equally good. We enlarge on this remark by citing examples from the currently popular model and one of our own fitting the observed spectra of positrons, extending it to higher energies. We also show what one might expect to observe from a mono-energetic source, such as the annihilation of dark matter.

Constraining the origin of the rising cosmic ray positron fraction with the boron-to-carbon ratio

The rapid rise in the cosmic ray positron fraction above 10 GeV, as measured by PAMELA and AMS, suggests the existence of nearby primary sources of high energy positrons, such as pulsars or annihilating/decaying dark matter. In contrast, the spectrum of secondary positrons produced through the collisions of cosmic rays in the interstellar medium is predicted to fall rapidly with energy, and thus is unable to account for the observed rise. It has been proposed, however, that secondary positrons could be produced and then accelerated in nearby supernova remnants, potentially explaining the observed rise, without the need of primary positron sources. Yet, if secondary positrons are accelerated in such shocks, other secondary cosmic ray species (such as boron nuclei, and antiprotons) will also be accelerated, leading to rises in the boron-to-carbon and antiproton-to-proton ratios. The measurements of the boron-to-carbon ratio by the PAMELA and AMS collaborations, however, show no sign of such a rise. With this new data in hand, we revisit the secondary acceleration scenario for the rising positron fraction. Assuming that the same supernova remnants accelerate both light nuclei (protons, helium) and heavier cosmic ray species, we find that no more than ~25% of the observed rise in the positron fraction can result from this mechanism (at the 95% confidence level)

Sommerfeld enhancements with vector, scalar, and pseudoscalar force carriers

The first AMS-02 measurement confirms the existence of an excess in the cosmic-ray positron fraction previously reported by the PAMELA and Fermi-LAT experiments. If interpreted in terms of thermal dark matter (DM) annihilation, the AMS-02 result still suggests that the DM annihilation cross section in the present day should be significantly larger than that at freeze out. The Sommerfeld enhancement of DM annihilation cross section as a possible explanation for the observed excesses is subject to constraint from the DM thermal relic density, mainly due to the annihilation of DM particles into force-carrier particles introduced by the mechanism. We show that the effect of the Sommerfeld enhancement and the relic density constraint depend significantly on the nature of the force-carrier. Three cases where the force-carrier is a vector boson, scalar and pseudoscalar particle are investigated and compared with each other. The results show that for the vector force-carrier, the Sommerfeld enhancement can marginally account for the AMS-02 data for $2\mu$ final states, while for scalar force-carrier the allowed Sommerfeld enhancement can be larger by a factor of two. For the case with a pesudoscalar force-carrier, the Sommerfeld enhancement can be large enough to accommodate the AMS-02 and Fermi-LAT result for a variety of DM annihilation final states.