Millisecond Pulsars and the Gamma-Ray Excess in Andromeda

Abstract

The Fermi Gamma-Ray Space Telescope has provided evidence for diffuse gamma-ray emission in the central parts of the Milky Way and the Andromeda galaxy. This excess has been interpreted either as dark matter annihilation emission or as emission from thousands of millisecond pulsars (MSPs). We have recently shown that old massive globular clusters may move towards the center of the Galaxy by dynamical friction and carry within them enough MSPs to account for the observed gamma-ray excess. In this paper we revisit the MSP scenario for the Andromeda galaxy, by modeling the formation and disruption of its globular cluster system. We find that our model predicts gamma-ray emission ∼ 2 − 3 times larger than for the Milky Way, but still nearly an order of magnitude smaller than the observed Fermi excess in the Andromeda. Our MSP model can reproduce the observed excess only by assuming ∼ 8 times larger number of old clusters than inferred from galaxy scaling relations. To explain the observations we require either that Andromeda deviates significantly from the scaling relations, or that a large part of its high-energy emission comes from additional sources.

Hunting intermediate-mass black holes with LISA binary radial velocity measurements

“…We evolve the clusters for 10 Gyr and find that about 200 clusters survive, ∼ 10,000 clusters get disrupted, and a few dozens of the disrupted clusters end up within 10 pc of the Galactic center. The mass distribution and Galactic density of surviving clusters is consistent with the Harris catalog [54,66].…”

“…Thus, the mass lost via stellar evolution is simply due to the mass of the stars that evolved out of their main sequence to form compact remnants. We parametrize the typical timescale of the mass loss due to star ejections as [54] and the timescale of tidal stripping as [66]…”

Gravitational wave sources from inspiralling globular clusters in the Galactic Centre and similar environments

“…The main observational properties of the Milky Way NSC can be successfully reproduced through this formation mechanism, as widely shown by numerical simulations Tsatsi et al 2017), although a contribution from in-situ star formation cannot be completely ruled out (Baumgardt et al 2018). Moreover, the dry-merger scenario provides a suitable explanation for the intense flux of γ rays coming from the Galactic Centre, which would be due to millisecond pulsars delivered by the infalling clusters (Brandt & Kocsis 2015;Abbate et al 2018;Fragione et al 2018;Arca-Sedda et al 2017), and the Galactic central X-ray excess, which would be due to cataclysmic variables (Arca- .…”

Exploring a cosmic-ray origin of the multiwavelength emission in M31

“…This suggests that although the spectra can be fit well with a multi-component model, the input power needed for the cosmic-ray sources is consistently more than an order of magnitude above what is expected from supernova as galactic cosmic-ray accelerators. Furthermore, as mentioned in the introduction and discussed more thoroughly in the original detection paper [6] along with earlier and subsequent Fermi M31 studies [21,34,40], the gamma-ray emission does not appear to correlate with star-formation or gas-rich regions. CRp produced at larger radii that then diffuse into the emission region may contribute to the observed signal, although this does not address the lack of gas for the CRp to interact with in the interior regions of the galaxy.…”

“…Ref. [34] studied MSPs originating from globular cluster disruption in the bulge of M31, whereas Ref. [21] considered MSPs formed in situ.…”