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Register a new userThe Distribution and Annihilation of Dark Matter Around Black Holes
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Added by
Jeremy D. Schnittman
Publication date
2015
fields
Physics
and research's language is
English
Authors
Jeremy D. Schnittman
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We use a Monte Carlo code to calculate the geodesic orbits of test particles around Kerr black holes, generating a distribution function of both bound and unbound populations of dark matter particles. From this distribution function, we calculate annihilation rates and observable gamma-ray spectra for a few simple dark matter models. The features of these spectra are sensitive to the black hole spin, observer inclination, and detailed properties of the dark matter annihilation cross section and density profile. Confirming earlier analytic work, we find that for rapidly spinning black holes, the collisional Penrose process can reach efficiencies exceeding $600%$, leading to a high-energy tail in the annihilation spectrum. The high particle density and large proper volume of the region immediately surrounding the horizon ensures that the observed flux from these extreme events is non-negligible.
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We calculate the probability distribution function (PDF) of the expected annihilation luminosities of dark matter subhalos as a function of subhalo mass and distance from the Galactic center using a semi-analytical model of halo evolution. We find that the PDF of luminosities is relatively broad, exhibiting a spread of as much as an order of magnitude at fixed subhalo mass and halo-centric distance. The luminosity PDF allows for simple construction of mock samples of gamma-ray luminous subhalos and assessment of the variance in among predicted gamma-ray signals from dark matter annihilation. Other applications include quantifying the variance among the expected luminosities of dwarf spheroidal galaxies, assessing the level at which dark matter annihilation can be a contaminant in the expected gamma-ray signal from other astrophysical sources, as well as estimating the level at which nearby subhalos can contribute to the antimatter flux.
Gravitational-wave detectors have opened a new window through which we can observe black holes (BHs) and neutron stars (NSs). Analyzing the 11 detections from LIGO/Virgos first gravitational-wave catalog, GWTC-1, we investigate whether the power-law fit to the BH mass spectrum can also accommodate the binary neutron star (BNS) event GW170817, or whether we require an additional feature, such as a mass gap, in between the NS and BH populations. We find that with respect to the power-law fit to binary black hole (BBH) masses, GW170817 is an outlier at the 0.13% level, suggesting a distinction between NS and BH masses. A single power-law fit across the entire mass range is in mild tension with: (a) the detection of one source in the BNS mass range ($sim 1$--$2.5 ,M_odot$), (b) the absence of detections in the mass-gap range ($sim 2.5$--$5 ,M_odot$), and (c) the detection of 10 sources in the BBH mass range ($gtrsim 5 ,M_odot$). Instead, the data favor models with a feature between NS and BH masses, including a mass gap (Bayes factor of 4.6) and a break in the power law, with a steeper slope at NS masses compared to BH masses (91% credibility). We estimate the merger rates of compact binaries based on our fit to the global mass distribution, finding $mathcal{R}_mathrm{BNS} = 871^{+3015}_{-805} mathrm{Gpc}^{-3} mathrm{yr}^{-1}$ and $mathcal{R}_mathrm{BBH} = 47.5^{+57.9}_{-28.8} mathrm{Gpc}^{-3} mathrm{yr}^{-1}$. We conclude that, even in the absence of any prior knowledge of the difference between NSs and BHs, the gravitational-wave data alone already suggest two distinct populations of compact objects.
Reticulum II (Ret II) is a satellite galaxy of the Milky Way and presents a prime target to investigate the nature of dark matter (DM) because of its high mass-to-light ratio. We evaluate a dedicated INTEGRAL observation campaign data set to obtain $gamma$-ray fluxes from Ret II and compare those with expectations from DM. Ret II is not detected in the $gamma$-ray band 25--8000 keV, and we derive a flux limit of $lesssim 10^{-8},mathrm{erg,cm^{-2},s^{-1}}$. The previously reported 511 keV line is not seen, and we find a flux limit of $lesssim 1.7 times 10^{-4},mathrm{ph,cm^{-2},s^{-1}}$. We construct spectral models for primordial black hole (PBH) evaporation and annihilation/decay of particle DM, and subsequent annihilation of positrons produced in these processes. We exclude that the totality of DM in Ret II is made of a monochromatic distribution of PBHs of masses $lesssim 8 times 10^{15},mathrm{g}$. Our limits on the velocity-averaged DM annihilation cross section into $e^+e^-$ are $langle sigma v rangle lesssim 5 times 10^{-28} left(m_{rm DM} / mathrm{MeV} right)^{2.5},mathrm{cm^3,s^{-1}}$. We conclude that analysing isolated targets in the MeV $gamma$-ray band can set strong bounds on DM properties without multi-year data sets of the entire Milky Way, and encourage follow-up observations of Ret II and other dwarf galaxies.
Since the seminal work of Penrose (1969) and Blandford & Znajek (1977), it has been realized that black hole spin may be an important energy source in astrophysics. The radio-loud/radio-quiet dichotomy in the AGN population is usually attributed to differences in black hole spin, with correlations between black hole spin and host galaxy morphology being hypothesized in order to explain why radio-loud AGN occur in early-type galaxies. X-ray observations are uniquely able to answer: Does black hole spin play a crucial role in powering relativistic jets such as those seen from radio-loud active galactic nuclei (AGN), Galactic microquasars, and Gamma-Ray Bursts? Indeed, the importance of black hole spin goes beyond its role as a possible power source: the spin of a supermassive black hole is a fossil record of its formation and subsequent growth history.
We consider the observational properties of a static black hole space-time immersed in a dark matter envelope. We thus investigate how the modifications to geometry, induced by the presence of dark matter affect the luminosity of the black holes accretion disk. We show that the same disks luminosity produced by a black hole in vacuum may be produced by a smaller black hole if surrounded by dark matter under certain conditions. In particular, we demonstrate that the luminosity of the disk is markedly altered by dark matters presence, suggesting that mass estimation of distant super-massive black holes may be changed if they are immersed in dark matter. We argue that a similar effect holds in more realistic scenarios and we discuss about the refractive index related to dark matter lensing. Hence we show how this may help explain the observed luminosity of super-massive black holes in the early universe.
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