Millisecond pulsar interpretation of the Galactic center gamma-ray excess

It was found in the Fermi-LAT data that there is an extended $\gamma$-ray excess in the Galactic center region. The proposed sources to be responsible for the excess include the dark matter annihilation or an astrophysical alternative from a population of millisecond pulsars (MSPs). Whether or not the MSP scenario can explain the data self-consistently has very important implications for the detection of particle dark matter, which is however, subject to debate in the literature. In this work we study the MSP scenario in detail, based on the detected properties of the MSPs by Fermi-LAT. We build a model of the Milky Way MSPs which can reproduce the $\gamma$-ray properties of the Fermi-LAT MSPs, and derive the intrinsic luminosity function of the MSPs. The model is then applied to a bulge population of MSPs. We find that the extended $\gamma$-ray excess can be well explained by the bulge MSPs without violating the detectable flux distribution of MSPs by Fermi-LAT. The spatial distribution of the bulge MSPs as implied by the distribution of low mass X-ray binaries follows a $r^{-2.4}$ profile, which is also consistent with the $\gamma$-ray excess data. We conclude that the MSP model can explain the Galactic center $\gamma$-ray excess self-consistently, satisfying all the current observational constraints.Comment: 20 pages, 8 figures and 1 table; match the published version of Journal of High Energy Astrophysics (JHEAp

Perspective of Galactic dark matter subhalo detection on Fermi from the EGRET observation

The perspective of the detectability of Galactic dark matter subhaloes on the Fermi satellite is investigated in this work. Under the assumptions that dark matter annihilation accounts for the "GeV excess" of the Galactic diffuse $\gamma$-rays discovered by EGRET and the $\gamma$-ray flux is dominated by the contribution from subhaloes of dark matter, we calculate the expected number of dark matter subhaloes that Fermi may detect. We show that Fermi may detect a few tens to several hundred subhaloes in 1-year all sky survey. Since EGRET observation is taken as a normalization, this prediction is independent of the particle physics property of dark matter. The uncertainties of the prediction are discussed in detail. We find that the major uncertainty comes from the mass function of subhaloes, i.e., whether the subhaloes are "point like" (high-mass rich) or "diffuse like" (low-mass rich). Other uncertainties like the background estimation and the observational errors will contribute a factor of $2\sim 3$.Comment: 16 pages, 4 figures and 1 table, accepted for publication in Chinese Physics

Diffuse γ\gamma-rays and pˉ\bar{p} flux from dark matter annihilation -- a model for consistent results with EGRET and cosmic ray data

In this work we develop a new propagation model for the Galactic cosmic rays based on the GALPROP code, including contributions from dark matter annihilation. The model predicts compatible Galactic diffuse $\gamma$ ray spectra with EGRET data in all sky regions. It also gives consistent results of the diffuse $\gamma$ ray longitude and latitude distributions. Further the results for B/C, $^{10}$Be/$^9$Be, proton, electron and antiproton spectra are also consistent with cosmic ray measurements. In the model we have taken a universal proton spectrum throughout the Galaxy without introducing large fluctuation for the proton energy loss is negligible. The dark matter annihilation signals are `boosted' after taking the contribution from subhalos into account. Another interesting feature of the model is that it gives better description of the diffuse $\gamma$ rays when taking the source distribution compatible with supernova remnants data, which is different from previous studies.Comment: 29 pages, 13 figures; the published versio

Fermi Large Area Telescope observations of the supernova remnant HESS J1731-347

Context: HESS J1731-347 has been identified as one of the few TeV-bright shell-type supernova remnants (SNRs). These remnants are dominated by nonthermal emission, and the nature of TeV emission has been continuously debated for nearly a decade. Aims: We carry out the detailed modeling of the radio to gamma-ray spectrum of HESS J1731-347 to constrain the magnetic field and energetic particles sources, which we compare with those of the other TeV-bright shell-type SNRs explored before. Methods: Four years of data from Fermi Large Area Telescope (LAT) observations for regions around this remnant are analyzed, leading to no detection correlated with the source discovered in the TeV band. The Markov Chain Monte Carlo method is used to constrain parameters of one-zone models for the overall emission spectrum. Results: Based on the 99.9% upper limits of fluxes in the GeV range, one-zone hadronic models with an energetic proton spectral slope greater than 1.8 can be ruled out, which favors a leptonic origin for the gamma-ray emission, making this remnant a sibling of the brightest TeV SNR RX J1713.7-3946, the Vela Junior SNR RX J0852.0-4622, and RCW 86. The best-fit leptonic model has an electron spectral slope of 1.8 and a magnetic field of about 30 muG, which is at least a factor of 2 higher than those of RX J1713.7-3946 and RX J0852.0-4622, posing a challenge to the distance estimate and/or the energy equipartition between energetic electrons and the magnetic field of this source. A measurement of the shock speed will address this challenge and has implications on the magnetic field evolution and electron acceleration driven by shocks of SNRs.Comment: 7 pages, 3 fogures, A&A in pres