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Register a new userEvidence of TeV Halos Around Millisecond Pulsars
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Using data from the HAWC gamma-ray Telescope, we have studied a sample of 37 millisecond pulsars (MSPs), selected for their spindown power and proximity. From among these MSP, we have identified four which favor the presence of very high-energy gamma-ray emission at a level of $(2Delta ln mathcal{L})^{1/2} ge 2.5$. Adopting a correlation between the spindown power and gamma-ray luminosity of each pulsar, we performed a stacked likelihood analysis of these 37 MSPs, finding that the data supports the conclusion that these sources emit very high-energy gamma-rays at a level of $(2Delta ln mathcal{L})^{1/2} = 4.24$. Among sets of randomly selected sky locations within HAWCs field-of-view, less than 1% of such realizations yielded such high statistical significance. Our analysis suggests that MSPs produce very high-energy gamma-ray emission with a similar efficiency to that observed from the Geminga TeV-halo, $eta_{rm MSP} = (0.39-1.08) times eta_{rm Geminga}$. This conclusion poses a significant challenge for pulsar interpretations of the Galactic Center gamma-ray excess, as it suggests that any population of MSPs potentially capable of producing the GeV excess would also produce TeV-scale emission in excess of that observed by HESS from this region. Future observations by CTA will be able to substantially clarify this situation.
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Milagro observations have found bright, diffuse TeV emission concentrated along the galactic plane of the Milky Way. The intensity and spectrum of this emission is difficult to explain with current models where gamma-ray production is dominated by hadronic mechanisms, and has been named the TeV excess. We show that TeV emission from pulsars naturally explains this excess. In particular, recent observations have detected TeV halos surrounding pulsars that are either nearby or particularly luminous. Here, we show that the full population of Milky Way pulsars will produce diffuse TeV emission concentrated along the Milky Way plane. The total gamma-ray flux from TeV halos is expected to exceed the hadronic gamma-ray flux at energies above ~500 GeV. Moreover, the spectrum and intensity of TeV halo emission naturally matches the TeV excess. If this scenario is common to all galaxies, it will decrease the contribution of star-forming galaxies to the IceCube neutrino flux. Finally, we show that upcoming HAWC observations will resolve a significant fraction of the TeV excess into individual TeV halos, conclusively confirming, or ruling out, this model.
Analyses of Fermi Gamma-Ray Space Telescope data have revealed a source of excess diffuse gamma rays towards the Galactic center that extends up to roughly $pm$20 degrees in latitude. The leading theory postulates that this GeV excess is the aggregate emission from a large number of faint millisecond pulsars (MSPs). The electrons and positrons ($e^pm$) injected by this population could produce detectable inverse-Compton (IC) emissions by up-scattering ambient photons to gamma-ray energies. In this work, we calculate such IC emissions using GALPROP. A triaxial three-dimensional model of the bulge stars obtained from a fit to infrared data is used as a tracer of the putative MSP population. This model is compared against one in which the MSPs are spatially distributed as a Navarro-Frenk-White squared profile. We show that the resulting spectra for both models are indistinguishable, but that their spatial morphologies have salient recognizable features. The IC component above $sim$TeV energies carries information on the spatial morphology of the injected $e^pm$. Such differences could potentially be used by future high-energy gamma-ray detectors such as the Cherenkov Telescope Array to provide a viable multiwavelength handle for the MSP origin of the GeV excess.
Radio timing observations of millisecond pulsars (MSPs) in support of Fermi LAT observations of the gamma-ray sky enhance the sensitivity of high-energy pulsation searches. With contemporaneous ephemerides we have detected gamma-ray pulsations from PSR B1937+21, the first MSP ever discovered, and B1957+20, the first known black-widow system. The two MSPs share a number of properties: they are energetic and distant compared to other gamma-ray MSPs, and both of them exhibit aligned radio and gamma-ray emission peaks, indicating co-located emission regions in the outer magnetosphere of the pulsars. However, radio observations are also crucial for revealing MSPs in Fermi unassociated sources. In a search for radio pulsations at the position of such unassociated sources, the Nanc{c}ay Radio Telescope discovered two MSPs, PSRs J2017+0603 and J2302+4442, increasing the sample of known Galactic disk MSPs. Subsequent radio timing observations led to the detection of gamma-ray pulsations from these two MSPs as well. We describe multiwavelength timing and spectral analysis of these four pulsars, and the modeling of their gamma-ray light curves in the context of theoretical models.
Radiation of X-ray bursts and of accretion shocks in weakly magnetized neutron stars in low-mass X-ray binaries is produced in plane-parallel atmospheres dominated by electron scattering. We first discuss polarization produced by single (non-magnetic) Compton scattering, in particular the depolarizing effect of high electron temperature, and then the polarization due to multiply electron scattering in a slab. We further predict the X-ray pulse profiles and polarization properties of nuclear- and accretion-powered millisecond pulsars. We introduce a relativistic rotation vector model, which includes the effect of rotation of polarization plane due to the rapid motion of the hot spot as well as the light bending. Future observations of the X-ray polarization will provide a valuable tool to test the geometry of the emission region in pulsars and its physical characteristics.
Neutron stars spin down over time due to a number of energy-loss processes. We provide tantalizing population-based evidence that millisecond pulsars (MSPs) have a minimum ellipticity of $epsilonapprox10^{-9}$ around their spin axis and that, consequently, some spin down mostly through gravitational-wave emission. We discuss the implications of such a minimum ellipticity in terms of the internal magnetic field strengths and nuclear matter composition of neutron stars and show it would result in the Advanced LIGO and Virgo gravitational-wave detectors, or their upgrades, detecting gravitational waves from some known MSPs in the near future.
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