Cosmic Ray Leptons Research Articles

Do the Geminga, Monogem and PSR J0622+3749 γ -ray halos imply slow diffusion around pulsars?

The HAWC Collaboration has reported the detection of an extended $\gamma$-ray emission around the Geminga and Monogem pulsars of a few degree extension. Very recently, the LHAASO Collaboration released also the data for an extended $\gamma$-ray emission around the pulsar PSR J0622+3749. This flux can be explained with electrons and positrons injected from these sources and their inverse Compton Scattering on the interstellar radiation fields. So far the size of such $\gamma-$ray halos has been interpreted as the result of the diffusion coefficient around the sources being about two orders of magnitude smaller than the average in the Galaxy. However, this conclusion is driven by the assumption that particles propagate diffusively right away after the injection without taking into account the ballistic propagation. The propagation of cosmic-ray leptons in the proximity of the Geminga, Monogem and PSR J0622+3749 pulsars is examined here considering the transition from the quasi-ballistic, valid for the most recently injected particles, to the diffusive transport regime. For typical interstellar values of the diffusion coefficient, the quasi-ballistic regime dominates the lepton distribution up to distances of a few tens of parsec from the pulsar for particle energies above $\sim 10$ TeV. In this regime the resulting $\gamma-$ray source tends to be rather compact, despite particles travel a long distance. Indeed, for larger values of the diffusion coefficient, particles propagate ballistically up to larger distances with the result of a more point-like $\gamma-$ray source. When such transition is taken into account, a good fit to the HAWC and LHAASO $\gamma-$ray data around Geminga, Monogem and PSR J0622+3749 is obtained without the need to invoke a strong suppression of the diffusion coefficient.

Features in cosmic-ray lepton data unveil the properties of nearby cosmic accelerators

We present a comprehensive discussion about the origin of the features in the leptonic component of the cosmic-ray spectrum. Working in the framework of a up-to-date CR transport scenario tuned on the most recent AMS-02 and Voyager data, we show that the prominent features recently found in the positron and in the all-electron spectra by several experiments are compatible with a scenario in which pulsar wind nebulae (PWNe) are the dominant sources of the positron flux, and nearby supernova remnants (SNRs) shape the high-energy peak of the electron spectrum. In particular we argue that the drop-off in positron spectrum found by AMS-02 at ∼ 300 GeV can be explained—under different assumptions—in terms of a prominent PWN that provides the bulk of the observed positrons in the ∼ 100 GeV domain, on top of the contribution from a large number of older objects. Finally, we turn our attention to the spectral softening at ∼ 1 TeV in the all-lepton spectrum, recently reported by several experiments, showing that it requires the presence of a nearby supernova remnant at its final stage.

Origin of cosmic ray electrons and positrons

With experimental results of AMS on the spectra of cosmic ray (CR) $e^{-}$, $e^{+}$, $e^{-}+e^{+}$ and positron fraction, as well as new measurements of CR $e^{-}+e^{+}$ flux by HESS, one can better understand the CR lepton ($e^{-}$ and $e^{+}$) spectra and the puzzling electron-positron excess above $\sim$10 GeV. In this article, spectra of CR $e^{-}$ and $e^{+}$ are fitted with a physically motivated simple model, and their injection spectra are obtained with a one-dimensional propagation model including the diffusion and energy loss processes. Our results show that the electron-positron excess can be attributed to uniformly distributed sources that continuously inject into the galactic disk electron-positron with a power-law spectrum cutting off near 1 TeV and a triple power-law model is needed to fit the primary CR electron spectrum. The lower energy spectral break can be attributed to propagation effects giving rise to a broken power-law injection spectrum of primary CR electrons with a spectral hardening above $\sim$40 GeV.

Detection of Nonthermal Hard X-Ray Emission from the “Fermi Bubble” in an External Galaxy

Abstract We report new Chandra hard X-ray (>2 keV) and Jansky Very Large Array C-band observations of the nuclear superbubble of NGC 3079, an analog of the “Fermi bubble” in our Milky Way. We detect extended hard X-ray emission on the southwest (SW) side of the galactic nucleus with coherent multiwavelength features in radio, Hα, and soft X-ray. The hard X-ray feature has a cone shape with possibly a weak cap, forming a bubble-like structure with a diameter of ∼1.1 kpc. A similar extended feature, however, is not detected on the northeast (NE) side, which is brighter in all other wavelengths such as radio, Hα, and soft X-ray. Scattered photons from the nuclear region or other nearby point-like X-ray bright sources, inverse Compton emission from cosmic-ray (CR) electrons via interaction with the cosmic microwave background, or any individually faint stellar X-ray source populations, cannot explain the extended hard X-ray emission on the SW side and the strongly NE/SW asymmetry. A synchrotron emission model, plus a thermal component accounting for the excess at ∼1 keV, can well characterize the broadband radio/hard X-ray spectra. The broadband synchrotron spectra do not show any significant cutoff, and even possibly slightly flatten at higher energy. This rules out a loss-limited scenario in the acceleration of the CR electrons in or around this superbubble. As the first detection of kiloparsec-scale extended hard X-ray emission associated with a galactic nuclear superbubble, the spatial and spectral properties of the multiwavelength emissions indicate that the CR leptons responsible for the broadband synchrotron emission from the SW bubble must be accelerated in situ, instead of transported from the nuclear region of the galaxy.

Breaking the radio – gamma-ray connection in Arp 220

Recent analyses of the gamma-ray spectrum from the ultra-luminous infrared galaxy Arp 220 have revealed a discrepancy in the cosmic ray energy injection rates derived from the gamma-rays versus the radio emission. While the observed radio emission is consistent with the star formation rate inferred from infrared observations, a significantly higher cosmic ray population is necessary to accurately model the measured gamma-ray flux. To resolve this discrepancy between the radio and gamma-ray observations, we find that we must increase the cosmic ray energy injection rate and account for an infrared optical depth greater than unity. Raising the energy injection rate naturally raises the total gamma-ray flux but also raises the radio flux unless there is also an increase in the energy loss rate for cosmic ray leptons. A optically thick medium results in an increase in energy losses via inverse Compton for cosmic ray leptons and preserves agreement with submillimeter, millimeter, and infrared wavelength observations.

Pulsar Wind Nebulae with Bow Shocks: Non-thermal Radiation and Cosmic Ray Leptons

Pulsars with high spin-down power produce relativistic winds radiating a fraction of the power in the range from radio to gamma-rays in the pulsar wind nebulae (PWNe). The rest of the power is dissipated in the interactions of the PWNe with the interstellar medium (ISM). Some of the PWNe are moving relative to the ISM with supersonic speeds producing bow shocks. In this case, the ultrarelativistic particles accelerated at the termination surface of the pulsar wind may be reaccelerated in the converging flow system formed by the outflow from the wind termination shock and the inflow from the bow shock. An outcome of this reacceleration is the creation of particle distributions with hard spectra, such as required to explain the observed synchrotron spectra with photon indices Gamma <~ 1.5. The presence of this hard component is specific to PWNe with bow shocks (BSPWNe). The accelerated particles may end up containing a substantial fraction of the shock ram pressure. For typical ISM and pulsar parameters, the positrons released by these systems are numerous enough to contribute a substantial fraction of the positrons detected as galactic cosmic ray particles above few tens and up to several hundred GeV. The escape of ultrarelativistic particles from a BSPWN and its appearance in the far-UV and X-ray bands is determined by the directions of the interstellar magnetic field, the velocity of the astrosphere and the pulsar rotation axis. In this respect we review the observed appearance and multiwavelength spectra of three different types of BSPWNe: PSR J0437-4715, the Guitar and Lighthouse nebulae, and Vela-like objects. We argue that high resolution imaging of such objects provides unique information on pulsar winds and on the ISM. We discuss the interpretation of imaging observations and estimate the BSPWN contribution to the positron flux observed at the Earth. (ABRIDGED)

Planckintermediate results

Recent models for the large-scale Galactic magnetic fields in the literature have been largely constrained by synchrotron emission and Faraday rotation measures. We use three different but representative models to compare their predicted polarized synchrotron and dust emission with that measured by the Planck satellite. We first update these models to match the Planck synchrotron products using a common model for the cosmic-ray leptons. We discuss the impact on this analysis of the ongoing problems of component separation in the Planck microwave bands and of the uncertain cosmic-ray spectrum. In particular, the inferred degree of ordering in the magnetic fields is sensitive to these systematic uncertainties, and we further show the importance of considering the expected variations in the observables in addition to their mean morphology. We then compare the resulting simulated emission to the observed dust polarization and find that the dust predictions do not match the morphology in the Planck data but underpredict the dust polarization away from the plane. We modify one of the models to roughly match both observables at high latitudes by increasing the field ordering in the thin disc near the observer. Though this specific analysis is dependent on the component separation issues, we present the improved model as a proof of concept for how these studies can be advanced in future using complementary information from ongoing and planned observational projects.

THE CONNECTION BETWEEN THE POSITRON FRACTION ANOMALY AND THE SPECTRAL FEATURES IN GALACTIC COSMIC-RAY HADRONS

Recent data on Galactic cosmic-ray (CR) leptons and hadrons gave rise to two exciting problems: on the lepton side, the origin of the rise of the CR positron fraction e+/(e- + e+) at ~10 - 300 GeV of energy; on the hadron side, the nature of the spectral hardening observed in CR protons and nuclei at ~TeV energies. The lepton anomaly indicates the existence of a nearby e+/- source. It has been proposed that high-energy positrons can be produced inside nearby supernova remnants (SNRs) via interactions of CR hadrons with the ambient medium. A distinctive prediction of this mechanism is a high-energy rise of the boron-to-carbon ratio, which has not been observed. It also requires old SNRs at work (with ineffective magnetic field amplification and slow shock speed), that cannot account for the CR hadronic spectra observed up to the knee energies (~5 PeV). We propose a new picture where, in addition to such a nearby CR accelerator, the high-energy spectrum of CR hadrons is provided by the large-scale population of SNRs, on average younger, that can efficiently accelerate CRs up to the knee. Under this scenario, the spectral hardening of CR hadrons can be naturally interpreted as the transition between the two components. As we will show, our two-component model breaks the connection between the positron fraction and the boron-to-carbon ratio, which is now predicted to decrease with energy in accordance with the data. Forthcoming data from AMS will be crucial for testing this model.

Implications of the AMS-02 positron fraction in cosmic rays

The AMS-02 collaboration has just released its first result of the cosmic positron fraction e+/(e-+e+) with high precision up to ∼350GeV. The AMS-02 result shows the same trend with the previous PAMELA result, which requires extra electron/positron sources on top of the conventional cosmic ray background, either from astrophysical sources or from dark matter annihilation/decay. In this paper we try to figure out the nature of the extra sources by fitting to the AMS-02 e+/(e-+e+) data, as well as the electron and proton spectra by PAMELA and the (e-+e+) spectrum by Fermi and HESS. We adopt the GALPROP package to calculate the propagation of the Galactic cosmic rays and the Markov Chain Monte Carlo sampler to do the fit. We find that under the conventional assumptions about the background and the extra source of the e-+e+, we cannot fit the AMS-02 and Fermi/HESS data well simultaneously. The AMS-02 data require less electrons/positrons from the extra sources than that required by Fermi/HESS. It may indicate that the model needs to be refined or the data between these experiments have systematic uncertainties. The pulsar scenario generally fits the data better than the DM scenario. Furthermore, the constraints from γ-rays also disfavor the DM scenario to explain the cosmic ray lepton data.

Galactic synchrotron emission with cosmic ray propagation models

Cosmic ray (CR) leptons produce radio synchrotron radiation by gyrating in interstellar magnetic fields (B-fields). Details of B-fields, CR electron distributions and propagation are still uncertain. We present developments in our modelling of Galactic radio emission with the galprop code. It now includes calculations of radio polarization, absorption and free–free emission. Total and polarized synchrotron emission are investigated in the context of physical models of CR propagation. Predictions are compared with radio data from 22 MHz to 2.3 GHz and Wilkinson Microwave Anisotropy Probe data at 23 GHz. Spatial and spectral effects on the synchrotron modelling with different CR distribution, propagation halo size and CR propagation models are presented. We find that all-sky total intensity and polarization maps are reasonably reproduced by including an anisotropic B-field, with comparable intensity to the regular one defined by rotation measures. A halo size of 10 kpc, which is larger than usually assumed, is favoured. This work provides a basis for further studies on foreground emission with the Planck satellite and on interstellar gamma-ray emission with Fermi-Large Area Telescope.

Fermi bubble simulations: black hole feedback in the Milky Way

AbstractThe Fermi γ-ray telescope discovered a pair of bubbles at the Galactic center. These structures are spatially-correlated with the microwave emission detected by the WMAP and Planck satellites. These bubbles were likely inflated by a jet launched from the vicinity of a supermassive black hole in the Galactic center. Using MHD simulations, which self-consistently include interactions between cosmic rays and magnetic fields, we build models of the supersonic jet propagation, cosmic ray transport, and the magnetic field amplification within the Fermi bubbles. Our key findings are that: (1) the synthetic Fermi γ-ray and WMAP microwave spectra based on our simulations are consistent with the observations, suggesting that a single population of cosmic ray leptons may simultaneously explain the emission across a range of photon energies; (2) the model fits the observed centrally-peaked microwave emission if a second, more recent, pair of jets embedded in the Fermi bubbles is included in the model. This is consistent with the observationally-based suggestion made by Su &amp; Finkbeiner (2012); (3) the radio emission from the bubbles is expected to be strongly polarized due to the relatively high level of field ordering caused by elongated turbulent vortices. This effect is caused by the interaction of the shocks driven by the jets with the preexisting interstellar medium turbulence; (4) a layer of enhanced rotation measure in the shock-compressed region could exist in the bubble vicinity but the level of this enhancement depends on the details of the magnetic topology.

Three-Dimensional Model of Cosmic-Ray Lepton Propagation Reproduces Data from the Alpha Magnetic Spectrometer on the International Space Station

We study the compatibility of Alpha Magnetic Spectrometer (AMS-02) data on the cosmic-ray (CR) positron fraction with data on the CR electron and positron spectra provided by PAMELA and Fermi LAT. We do that in terms of a novel propagation model in which sources are distributed in spiral arm patterns in agreement with astrophysical observations. While former interpretations assumed an unrealistically steep injection spectrum for astrophysical background electrons, the enhanced energy losses experienced by CR leptons due to the larger average source distance from Earth allow us to reproduce the data with harder injection spectra as expected in a shock acceleration scenario. Moreover, we show that in this approach, and accounting for AMS-02 results, the contribution of nearby accelerators to the fluxes at very high energy can be significantly reduced, thus avoiding any tension with anisotropy upper limits.

A software package for Stellar and solar Inverse Compton emission: StellarICs

We present our software to compute gamma-ray emission from inverse-Compton scattering by cosmic-ray leptons in the heliosphere, as well as in the photospheres of stars. It includes a formulation of modulation in the heliosphere, but can be used for any user-defined modulation model. Profiles and spectra are output to FITS files in a variety of forms for convenient use. Also included are general-purpose inverse-Compton routines with other features like energy loss rates and emissivity for any user-defined target photon and lepton spectra. The software is publicly available and it is under continuing development.

VERITAS deep observations of the dwarf spheroidal galaxy Segue 1

The VERITAS array of Cherenkov telescopes has carried out a deep observational program on the nearby dwarf spheroidal galaxy Segue 1. We report on the results of nearly 48 hours of good quality selected data, taken between January 2010 and May 2011. No significant $\gamma$-ray emission is detected at the nominal position of Segue 1, and upper limits on the integrated flux are derived. According to recent studies, Segue 1 is the most dark matter-dominated dwarf spheroidal galaxy currently known. We derive stringent bounds on various annihilating and decaying dark matter particle models. The upper limits on the velocity-weighted annihilation cross-section are $\mathrm{<\sigma v >^{95% CL} \lesssim 10^{-23} cm^{3} s^{-1}}$, improving our limits from previous observations of dwarf spheroidal galaxies by at least a factor of two for dark matter particle masses $\mathrm{m_{\chi}\gtrsim 300 GeV}$. The lower limits on the decay lifetime are at the level of $\mathrm{\tau^{95% CL} \gtrsim 10^{24} s}$. Finally, we address the interpretation of the cosmic ray lepton anomalies measured by ATIC and PAMELA in terms of dark matter annihilation, and show that the VERITAS observations of Segue 1 disfavor such a scenario.

Charge asymmetric cosmic rays as a probe of flavor violating asymmetric dark matter

The recently introduced cosmic sum rules combine the data from PAMELA and Fermi-LAT cosmic ray experiments in a way that permits to neatly investigate whether the experimentally observed lepton excesses violate charge symmetry. One can in a simple way determine universal properties of the unknown component of the cosmic rays. Here we attribute a potential charge asymmetry to the dark sector. In particular we provide models of asymmetric dark matter able to produce charge asymmetric cosmic rays. We consider spin zero, spin one and spin one-half decaying dark matter candidates. We show that lepton flavor violation and asymmetric dark matter are both required to have a charge asymmetry in the cosmic ray lepton excesses. Therefore, an experimental evidence of charge asymmetry in the cosmic ray lepton excesses implies that dark matter is asymmetric.

Astrophysical models for the origin of the positron “excess”

Over the last three years, several satellite and balloon observatories have suggested intriguing features in the cosmic ray lepton spectra. Most notably, the PAMELA satellite has suggested an “anomalous” rise with energy of the cosmic ray positron fraction. In this article, we summarize the global picture emerging from the data and recapitulate the main features of different types of explanations proposed. The perspectives in testing different scenarios as well as inferring some astrophysical diagnostics from current/near future experiments are also discussed.

Morphology of the Galactic dark matter synchrotron emission with self-consistent cosmic-ray diffusion models

A generic prediction in the paradigm of weakly interacting dark matter is the production of relativistic particles from dark matter pair annihilation in regions of high dark matter density. Ultrarelativistic electrons and positrons produced in the center of the Galaxy by dark matter annihilation should produce a diffuse synchrotron emission. While the spectral shape of the synchrotron dark matter haze depends on the particle model (and secondarily on the Galactic magnetic fields), the morphology of the haze depends primarily on (1) the dark matter density distribution, (2) the Galactic magnetic field morphology, and (3) the diffusion model for high-energy cosmic-ray leptons. Interestingly, an unidentified excess of microwave radiation with characteristics similar to those predicted by dark matter models has been claimed to exist near the Galactic center region in the data reported by the Wilkinson Microwave Anisotropy Probe (WMAP) satellite, and is dubbed the ``WMAP haze.'' In this study, we carry out a self-consistent treatment of the variables enumerated above, enforcing constraints from the available data on cosmic rays, radio surveys, and diffuse gamma rays. We outline and make predictions for the general morphology and spectral features of a ``dark matter haze'' and we compare them to the WMAP haze data. We also characterize and study the spectrum and spatial distribution of the inverse-Compton emission resulting from the same population of energetic electrons and positrons. We point out that the spectrum and morphology of the radio emission at different frequencies are powerful diagnostics to test whether a Galactic synchrotron haze indeed originates from dark matter annihilation.

Cosmic rays from pulsars and magnetars

Abstract We compare the expected abundance of cosmic ray electrons and positrons from pulsars and magnetars. We assume that the distribution of infant pulsars and magnetars follows that of high-mass stars in the Milky Way and that the production rate of cosmic rays is proportional to the spin-down and magnetic-decay power of pulsars and magnetars, respectively. In combination with primary and secondary cosmic ray leptons from other sources (especially supernova remnants), we find that both magnetars and pulsars can easily account for the observed cosmic ray spectrum, in particular the dip seen by HESS (High-Energy Stereoscopic System) at several TeV and the increase in positron fraction found by PAMELA (Payload for Antimatter Exploration and Light Nuclei Astrophysics).

Can multistate dark matter annihilation explain the high-energy cosmic ray lepton anomalies?

Multistate dark matter (DM) models with small mass splittings and couplings to light hidden sector bosons have been proposed as an explanation for the PAMELA/Fermi/H.E.S.S. high-energy lepton excesses. We investigate this proposal over a wide range of DM density profiles, in the framework of concrete models with doublet or triplet dark matter and a hidden SU(2) gauge sector that mixes with standard model hypercharge. The gauge coupling is bounded from below by the DM relic density, and the Sommerfeld enhancement factor is explicitly computable for given values of the DM and gauge boson masses $M$, $\ensuremath{\mu}$ and the (largest) dark matter mass splitting $\ensuremath{\delta}{M}_{12}$. Sommerfeld enhancement is stronger at the galactic center than near the Sun because of the radial dependence of the DM velocity profile, which strengthens the inverse Compton (IC) gamma ray constraints relative to usual assumptions. We find that the PAMELA/Fermi/H.E.S.S. lepton excesses are marginally compatible with the model predictions, and with CMB and Fermi gamma ray constraints, for $M\ensuremath{\cong}800\text{ }\text{ }\mathrm{GeV}$, $\ensuremath{\mu}\ensuremath{\lesssim}200\text{ }\text{ }\mathrm{MeV}$, and a dark matter profile with noncuspy Einasto parameters $\ensuremath{\alpha}\ensuremath{\gtrsim}0.20$, ${r}_{s}\ensuremath{\sim}30\text{ }\text{ }\mathrm{kpc}$. We also find that the annihilating DM must provide only a subdominant ($\ensuremath{\lesssim}0.4$) component of the total DM mass density, since otherwise the boost factor due to Sommerfeld enhancement is too large.

Astrophysical boost factor and dark matter indirect detection

The PAMELA [1], ATIC[2] and Fermi[3] collaborations have recently reported an excess in the cosmic ray positron and electron fluxes. These lepton anomalies might be related to cold dark matter particles annihilating within a nearby dark matter sub-halo. We outline regions of the parameter space for both the dark matter sub-halo and particle model, where data from the different experiments are reproduced. We then confront this interpretation of the data with the results of the cosmological N-body simulation Via Lactea 2. Having a sizeable sub-halo at a distance of only 1.2 kpc could explain the PAMELA excess, but such a configuration has a probability of only 0.37 percent. Reproducing also the ATIC bump would require a very large, nearby sub-halo, which is extremely unlikely (p ~ 3 × 10−5). It is even less probable for the smaller Fermi bump to be caused by the presence of such an object. In either case, we predict Fermi will detect the gamma-ray emission from the sub-halo. We conclude that under canonical assumptions, the cosmic ray lepton anomalies are unlikely to originate from a nearby cold Dark Matter sub-halo.