The millisecond pulsar contribution to the rising positron fraction

Abstract

Pair cascades from millisecond pulsars (MSPs) may be a primary source of Galactic electrons and positrons that contribute to the increase in positron flux above 10 GeV as observed by PAMELA and AMS−02. The Fermi Large Area Telescope (LAT) has increased the number of detected γ-ray MSPs tremendously. Light curve modelling furthermore favours abundant pair production in MSP magnetospheres, so that models of primary cosmic-ray positrons from pulsars should include the contribution from the larger numbers of MSPs and their potentially higher positron output per source. We model the contribution of Galactic MSPs to the terrestrial cosmic-ray electron / positron flux by using a population synthesis code to predict the source properties of presentday MSPs. We simulate pair spectra assuming an offset-dipole magnetic field which boosts pair creation rates. We also consider positrons and electrons that have additionally been accelerated to very high energies in the strong intrabinary shocks in black widow (BW) and redback (RB) binary systems. We transport these particles to Earth by calculating their diffusion and the radiative energy losses they suffer in the Galaxy using a model. Our model particle flux increases for nonzero offsets of the magnetic polar caps. We find that pair cascades from MSP magnetospheres contribute only modestly around a few tens of GeV to the measured fluxes. BW and RB fluxes may reach a few tens of percent of the observed flux up to a few TeV. Future observations should constrain the source properties in this case.

TeV gamma rays from Galactic Center pulsars

“…The main reason for this is that, to date, no TeV-halos have been observed around any millisecond pulsars (MSPs), and thus we do not know what fraction of the spindown power of these sources goes into the production of VHE electrons and positrons. It is widely believed, however, that MSPs are indeed likely to generate such emission [60][61][62], as the modelling of their light curves favor the abundant production of multi-TeV electronpositron pairs [60].…”

Millisecond pulsars, TeV halos, and implications for the Galactic Center gamma-ray excess

“…In particular, only a handful of MSPs within HAWC's fieldof-view have a spindown flux (which we define throughout this study as the spindown power divided by the distance squared) high enough to produce a TeV halo that would currently be detectable by HAWC. Furthermore, it is generally expected that MSPs do indeed generate such emission [8][9][10], as the modeling of their light curves favor the abundant production of multi-TeV electron-positron pairs [8]. On the other hand, the high-surface brightness of TeV halos indicates that cosmic-ray diffusion is inhibited in the vicinity of these sources, and it is unknown whether MSPs are capable of generating such conditions.…”

Recent Progress in Search for Dark Sector Signatures

“…It turns out that secondary electrons have a very flat equilibrium spectrum, which is responsible for the observed positron excess [122]. The origin of the rising positron fraction at high energy is unknown and has been explained by a variety of mechanisms including pulsars [123][124][125], cosmic rays interacting with giant molecular clouds, and DM "primary" particles; the "secondaries" come from these particles colliding with interstellar gas and producing pions and muons, which decay into electrons and positrons. A third, interesting possibility is that electrons and positrons are created by the annihilation of DM particles in the Milky Way and its halo, see [126,127] for recent reviews.…”