Radio signals of particle dark matter

📝 Abstract

In most of particle dark matter (DM) models, the DM candidate injects sizable fluxes of high-energy electrons and positrons through its annihilations or decays. Emitted in regions with magnetic field, they in turn give raise to a synchrotron radiation, which typically covers radio and infrared bands. We discuss the possibility of detecting signatures of Galactic and extra-galactic DM in the total intensity and small-scale anisotropies of the radio background.

💡 Analysis

In most of particle dark matter (DM) models, the DM candidate injects sizable fluxes of high-energy electrons and positrons through its annihilations or decays. Emitted in regions with magnetic field, they in turn give raise to a synchrotron radiation, which typically covers radio and infrared bands. We discuss the possibility of detecting signatures of Galactic and extra-galactic DM in the total intensity and small-scale anisotropies of the radio background.

📄 Content

arXiv:1112.1881v1 [hep-ph] 8 Dec 2011 Radio signals of particle dark matter Marco REGIS∗University of Turin - INFN E-mail: regis.mrc@gmail.com In most of particle dark matter (DM) models, the DM candidate injects sizable fluxes of high- energy electrons and positrons through its annihilations or decays. Emitted in regions with mag- netic field, they in turn give raise to a synchrotron radiation, which typically covers radio and infrared bands. We discuss the possibility of detecting signatures of Galactic and extra-galactic DM in the total intensity and small-scale anisotropies of the radio background. XXIst International Europhysics Conference on High Energy Physics 21-27 July 2011 Grenoble, Rhones Alpes France ∗Speaker. c ⃝Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/ Radio signals of particle dark matter Marco REGIS

1. Introduction From the title of my talk, one may wonder how it can be possible to infer something com- pletely unknown, as non-gravitational signals of dark matter (DM), by means of something quite well-known, as radio-astronomy. Discovery of new physics normally goes through designing new experiments. Here the idea is very simple, namely, that the technique of radio observations is potentially very promising in the quest for particle DM, but requires improved flux and angular sensitivities with respect to current capabilities. The development of ASKAP, EVLA, MeerKAT, and, in particular, SKA makes the near future promisingly bright in this respect. Those new ra- dio telescopes will certainly discover signals of previously unknown physical mechanisms, and we discuss the possibility that an emission induced by particle DM will be among them. Weakly interacting massive particles (WIMPs) are the most investigated class of DM candi- dates in the literature (for a review, see, e.g., [1]). One of the routes to test the hypothesis of WIMP DM stems from the the bases of the framework themselves. Indeed, given the weak interaction, there is a (“weak” but finite) probability that WIMPs in DM halos annihilate in pairs or decay into detectable species. In particular, and with the exception of WIMP models annihilating/decaying into neutrinos only, a sizable branching ratio of annihilation/decay into electrons and positrons is a general feature of WIMP models (see, e.g., Fig. 4 in [2]). Interactions of high-energy e+/e− with the interstellar magnetic field in astrophysical structures give rise to magnetic bremsstrahlung called synchrotron radiation. In the monochromatic approximation of synchrotron power, the en- ergy of an electron emitting at frequency ν is given by E ≃15 p νGHz/BµG GeV (where νGHz is the frequency in GHz and BµG is the magnetic field in µG). It follows that, assuming a magnetic field of few µG (as typical for galaxies), emissions at radio frequencies are mostly generated by electrons with energy around 1-10 GeV. Therefore, assuming a DM mass ≳10 GeV, a sizable DM-induced radio synchrotron emission is a general prediction of WIMP models (with, of course, spectrum and absolute flux depending on the specific model).
2. Targets We now investigate possible targets for detection of DM radio emission and see that in some cases the benchmark ‘thermal’ annihilation rate (σav) = 3·10−26cm3s−1 can be already probed. Galactic Center: Although the nucleus of our Galaxy is a very rich system, where a clean disentanglement of a DM signal from astrophysical emissions is rather complicated, the Galactic Center (GC) is one of the prime targets in WIMP indirect searches, given the large DM overdensity predicted by N-body numerical simulations. Observations of Sgr A∗are not consistent with a DM interpretation and the GC inner-part allows to constrain DM masses below few tens of GeV for a thermal annihilation cross section [2, 3]. Slightly larger scales (∼1◦) could set stronger constraints [2] but dedicated observations would be in order. On the scale of galactic bulge, a hint for a DM signal has been suggested [4] (the so called “WMAP Haze”), which however needs further investigation, given sizable uncertainties related to relevant astrophysical components. Galactic Halo: Mid-high latitudes can be a cleaner test since they involve propagation of e+/e−far from the GC and galactic disc, and so magnetic field and transport parameters suffer of smaller uncertainties. Searches have to be focused on low radio frequencies in the case of WIMP 2 Radio signals of particle dark matter Marco REGIS models inducing an e+/e−spectrum softer than the galactic cosmic-ray one, while at microwave in the opposite scenario. No evidences, but interesting constraints have been derived for PAMELA DM candidates (i.e., heavy and leptophilic WIMPs) [5]. Recently, it has been also shown that for thermal annihilation rate and cuspy profiles, low-frequency galactic radio emission constrains DM candidates with MDM ≲10 GeV [6]. Extragalactic diffuse emis

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