From data recently reported from the IceCube telescope, we derive new bounds on the monochromatic neutrino signal produced from dark matter particle decays. In the few TeV to tens of TeV energy range, these bounds turn out to be better than previous limits by more than an order of magnitude. As a result, intensity constraints on neutrino lines at energies above a few TeV are now comparable to those on gamma-ray lines. From the same data sample, we also perform a detailed search for a neutrino line, showing that there is no significant hint for such a signal.

In the near future, neutrino telescopes are expected to improve their sensitivity to the flux of monochromatic neutrinos produced by dark matter (DM) in our galaxy. This is illustrated by a new limit on the corresponding cross section that we derive from public IceCube data. In this context, we study which DM models could produce an observable flux of monochromatic neutrinos from DM annihilations. To this end, we proceed in two steps. First, within a set of simple and minimal assumptions concerning the properties of the DM particle, we determine the models that could give rise to a significant annihilation into monochromatic neutrinos at the freeze-out epoch. The list of models turns out to be very limited as a result of various constraints, in particular direct detection and neutrino masses at loop level. Given the fact that, even if largely improved, the sensitivities will be far from reaching the thermal annihilation cross section soon, a signal could only be observed if the annihilation into neutrinos today is boosted with respect to the freeze-out epoch. This is why, in a second step, we analyze the possibility of having such a large enhancement from the Sommerfeld effect. For each scenario, we also compute the cross sections into other annihilation products and confront our results with experimental constraints. We find that, within our simple and minimal assumptions, the expectation to observe monochromatic neutrinos is only possible in very specific scenarios. Some will be confirmed or excluded in the near future because they predict signals slightly below the current experimental sensitivities. We also discuss how these prospects change by relaxing our assumptions as well as by considering other types of sharp spectral features. For the latter, we consider boxed-shaped and bremsstrahlung spectra and provide the corresponding limits from IceCube data.

In the energy range from few TeV to 25 TeV, upper bounds on the dark matter decay rate into high energy monochromatic neutrinos have recently become comparable to those on monochromatic gamma-ray lines. This implies clear possibilities of a future double "smoking-gun" evidence for the dark matter particle, from the observation of both a gamma and a neutrino line at the same energy. In particular, we show that a scenario where both lines are induced from the same dark matter particle decay leads to correlations that can already be tested. We study this "double monochromatic" scenario by considering the complete list of lowest dimensional effective operators that could induce such a decay. Furthermore, we argue that, on top of lines from decays into twobody final states, three-body final states can also be highly relevant. In addition to producing a distinct hard photon spectrum, three-body final states also produce a line-like feature in the neutrino spectrum that can be searched for by neutrino telescopes. * Electronic address: celaisat@ulb.ac.be;

If a γ-ray line is observed in the near future, it will be important to determine what kind of dark matter (DM) particle could be at its origin. We investigate the possibility that the γ-ray line would be induced by a slow DM particle decay associated to the fact that the DM particle would not be absolutely neutral. A "millicharge" for the DM particle can be induced in various ways, in particular from a kinetic mixing interaction or through the Stueckelberg mechanism. We show that such a scenario could lead in specific cases to an observable γ-ray line. This possibility can be considered in a systematic modelindependent way, by writing down the corresponding effective theory. This allows for a multi-channel analysis, giving in particular upper bounds on the intensity of the associated γ-ray line from cosmic rays emission. Our analysis includes the possibility that in the twobody decay the photon is accompanied with a neutrino. We show that, given the stringent constraints which hold on the millicharge of the neutrinos, this is not an option, except if the DM particle mass lies in the very light KeV-MeV range, allowing for a possibility of explanation of the recently claimed, yet to be confirmed, ∼ 3.5 KeV X-ray line.

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