Strong Support for the Millisecond Pulsar Origin of the Galactic Center GeV Excess

Abstract

Using γ-ray data from the Fermi Large Area Telescope, various groups have identified a clear excess emission in the Inner Galaxy, at energies around a few GeV. This excess resembles remarkably well a signal from dark-matter annihilation. One of the most compelling astrophysical interpretations is that the excess is caused by the combined effect of a previously undetected population of dim γ-ray sources. Because of their spectral similarity, the best candidates are millisecond pulsars. Here, we search for this hypothetical source population, using a novel approach based on wavelet decomposition of the γ-ray sky and the statistics of Gaussian random fields. Using almost seven years of Fermi -LAT data, we detect a clustering of photons as predicted for the hypothetical population of millisecond pulsar, with a statistical significance of 10.0σ. For plausible values of the luminosity function, this population explains 100% of the observed excess emission. We argue that other extragalactic or Galactic sources, a mismodeling of Galactic diffuse emission, or the thick-disk population of pulsars are unlikely to account for this observation.

Known radio pulsars do not contribute to the Galactic Center gamma-ray excess

“…First, the average TS value in this region significantly exceeds expectations from either Poisson fluctuations (T S = 0.5) or from null sky observations in the high latitude sky (T S = 0.98 [34]). This result is compatible with the work of [27,29], showing that point sources placed in the GC can pick up significant γ-ray fluxes. However, we caution that this does not conclusively demonstrate that bright point sources reside in the GC region.…”

“…In an upcoming study, we plan to revisit analyses of the GC excess in models with substantial populations of currently undetected point sources, in order to determine whether any missing point source population is indeed capable of explaining the GC excess [35]. The γ-ray spectrum obtained for 7 of the γ-ray point sources indicated as possible pulsars in the analysis of [27]. The proceedure utilized in this paper provides a more detailed spectral model of γ-ray sources compared to that utilized for the 3FGL catalog.…”

Searching for pulsars associated with the Fermi GeV excess

“…With the proper choice of channel widths and integration times for the interferometer setup, pulsar candidates could be further identified based on their diffractive scintillations as measured in variance images (Dai et al 2016). In the mean time, before a full hybrid survey is carried out, we agree with O' Leary et al (2016) and Calore et al (2016) that a search should be made for bulge pulsar candidates toward the "hotspots" identified by Bartels et al (2016) and Lee et al (2016). The detection of PSR J1751−2737 shows that deep imaging-based continuum with targeted pulsation searches offers a complementary approach to existing blind pulsation surveys.…”

“…There are currently two alternative explanations which have been offered to explain this diffuse excess: (a) it is the long-sought annihilation signature of dark matter particles (Vitale et al 2009;Hooper & Goodenough 2011;Weniger 2012;Daylan et al 2016), or (b) the integrated high energy emission from a population of several thousand young and/or millisecond pulsars (Abazajian 2011;Brandt & Kocsis 2015). Recent analysis of the spatial and spectral properties of the gamma-ray excess strongly favor the pulsar hypothesis (Abazajian 2011;Mirabal 2013;Yuan & Zhang 2014;Calore et al 2015;Ajello et al 2016;Lee et al 2016;Bartels et al 2016). …”

Dark matter interpretation of the Fermi -LAT observation toward the Galactic Center

“…As a result, it remains unclear whether this signal arises from DM annihilation rather than from a currently unknown contribution from astrophysics such as a large population of milli-second pulsars, cosmic-ray (CR) proton or electron outbursts, additional cosmic ray sources, and/or emission from a stellar over-density in the Galactic bulge [11,16,[18][19][20][21][22][23]. An interesting development is the use of statistical tools which indicate that GeV photons from the direction of the inner galaxy region show significantly more clustering than would be expected from Poisson noise from smooth components [24][25][26][27]. However, it remains difficult with the current models to disentangle whether this feature represents a property of the excess itself, or unmodelled variation in the background components [28].…”