Gravitationally Focused Dark Matter Around Compact Stars
📝 Abstract
If dark matter self-annihilates then it may produce an observable signal when its density is high. The details depend on the intrinsic properties of dark matter and how it clusters in space. For example, the density profile of some dark matter candidates may rise steeply enough toward the Galactic Center that self-annihilation produces detectable gamma-ray emission. Here, we discuss the possibility that an annihilation signal may arise near a compact object (e.g., neutron star or black hole) even when the density of dark matter in the neighborhood of the object is uniform. Gravitational focusing produces a local enhancement of density, with a profile that falls off approximately as the inverse square-root of distance from the compact star. While geometric dilution may overwhelm the annihilation signal from this local enhancement, magnetic fields tied to the compact object can increase the signal’s contrast relative to the background.
💡 Analysis
If dark matter self-annihilates then it may produce an observable signal when its density is high. The details depend on the intrinsic properties of dark matter and how it clusters in space. For example, the density profile of some dark matter candidates may rise steeply enough toward the Galactic Center that self-annihilation produces detectable gamma-ray emission. Here, we discuss the possibility that an annihilation signal may arise near a compact object (e.g., neutron star or black hole) even when the density of dark matter in the neighborhood of the object is uniform. Gravitational focusing produces a local enhancement of density, with a profile that falls off approximately as the inverse square-root of distance from the compact star. While geometric dilution may overwhelm the annihilation signal from this local enhancement, magnetic fields tied to the compact object can increase the signal’s contrast relative to the background.
📄 Content
Gravitationally Focused Dark Matter Around Compact Stars Benjamin C. Bromley Department of Physics & Astronomy, University of Utah, 115 S 1400 E, Rm 201, Salt Lake City, UT 84112 bromley@physics.utah.edu ABSTRACT If dark matter self-annihilates then it may produce an observable signal when its density is high. The details depend on the intrinsic properties of dark mat- ter and how it clusters in space. For example, the density profile of some dark matter candidates may rise steeply enough toward the Galactic Center that self- annihilation may produce detectable γ-ray emission. Here, we discuss the possi- bility that an annihilation signal may arise near a compact object (e.g., neutron star or black hole) even when the density of dark matter in the neighborhood of the object is uniform. Gravitational focusing produces a local enhancement of density, with a profile that falls offapproximately as the inverse square-root of distance from the compact star. While geometric dilution may overwhelm the annihilation signal from this local enhancement, magnetic fields tied to the compact object can increase the signal’s contrast relative to the background. 1. Introduction Dark matter accounts for the majority of the mass in the Universe, yet its identity remains elusive. Candidates include weakly interacting massive particles (WIMPs) like the neutralino (χ), the supersymmetric partner of the neutrino (Pagels & Primack 1982), al- though their properties are only loosely constrained by theory and experiment. In some cases, plausible values of the mass and cross section suggest that self-annihilation signatures may be detectable in regions where the density of dark matter is high (Berezinsky et al. 1992; Bergstr¨om & Gondolo 1996; Bertone et al. 2004). For example, Bergstr¨om et al. (1998) cal- culate the gamma ray flux from neutralino self-annihilation in the Galactic center (see also Zaharijas & Hooper 2006), while Tyler (2002) and Bergstr¨om & Hooper (2006) provide es- timates of the annihilation signal from the nearby Draco dwarf galaxy. Intermediate-mass black holes may yield a WIMP annihilation signal (Bertone et al. 2005), as may remnant dark arXiv:1112.2355v1 [astro-ph.HE] 11 Dec 2011 – 2 – matter minihalos distributed throughout the Galaxy (e.g. Berezinsky et al. 2003; Sandick et al. 2011). The observability of an annihilation signal critically depends on the number density of particles, since the local event rate is proportional to the density squared. Estimates of the strength of this signal typically derive from the assumption that dark matter density profiles follow a power law, ρ ∼r−γ in many astrophysical contexts. The power-law index γ is between 1 and 2 in the central regions of galaxies according to cosmological simulations (Navarro et al. 1996; Moore et al. 1999; Power et al. 2003), corresponding to a density “cusp.” If the simulations realistically describe the distribution of dark matter, then self-annihilation of WIMPS may indeed be observable in the centers of galaxies. The presence of massive black holes in the centers of galaxies may further enhance the steep rise of a dark matter density profile. Gondolo & Silk (1999) model the adiabatic growth of a central black hole to show that a density “spike,” with a profile steeper than r−2, forms around the black hole. Dynamical processes, such as scattering by stars and capture the black hole, may erode a spike over time, although it may remain largely intact (Bertone & Merritt 2005). More problematic is the inspiral of smaller massive black holes captured from accreted galaxies that may disrupt a density spike (Merritt et al. 2002). Binary black hole coalescence can destroy density structures in the vicinity of the central black hole. However, even if merger event disrupts a density spike, relaxation processes may regenerate a dark matter “crest,” with a density profile that falls offas r−1.5 (Merritt et al. 2007). The time scale for the growth of a dark matter crest can be long, ∼10 Gyr in the case of the Milky Way, so that its structure could reflect the merger history of the Galaxy. From an observational perspective, strong density cusps or spikes are not obviously common. For instance, Kravtsov et al. (1998) find that dwarf and low surface brightness galaxies have shallow core profiles, with γ < 0.5, consistent with results from van den Bosch & Swaters (2001) in a study of dwarf galaxies. Milosavljevi´c et al. (2002) identify an anticorrelation between profile steepness and mass of the central black hole, lending support for a scenario in which the black hole grows through a sequence of merger events which tend to reduce or destroy any density cusp. Even if no density cusp or spike exists in a galactic nucleus, the presence of a massive black hole, such as Sgr A∗in the center of the Milky Way (Melia & Falcke 2001), can facilitate annihilation radiation. The reason is that gravitational focusing inevitably leads to a dark matter density enhancement near the bla
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