Paper of the month:  An unidentified line in X-ray spectra of the Andromeda Galaxy and Perseus galaxy clusters

April 22, 2014 by admin

Well established experimental evidences, most recently the results from the PLANCK experiment, demonstrate that approximately the 27 % of the energy budged of the Universe consists of a dark matter component. Although the most common paradigms refer to stable particles as dark matter candidates, unstable particles can also make up the dark matter of the Universe, provided that their lifetime significantly exceeds the age of the Universe. At the same time, this does not preclude the possibility that a very tiny amount of dark matter particles can decay at present times with a negligible impact on the total abundance. The products of such decays could potentially be detected in the flux of cosmic particles arriving at Earth. By measuring the energies of these particles it is possible to infer the mass of the decaying dark matter particle.

An intriguing scenario of decaying dark matter is the one in which a fermionic DM particle, lighter than the electron, but heavier than approximately 300 eV, as imposed by the so-called Tremain-Gunn limit, decays into a photon and a neutrino. The photons produced in the decay would manifest themselves as a rather narrow peak in the X-ray flux with an energy corresponding to one half the mass of the dark matter particle. From the characteristics of the signal together with some assumptions on the dark matter distribution it is also possible to infer the lifetime of the dark matter particle.

X-ray telescopes have searched for this type of signal in several astrophysical objects, like galaxies and galaxy clusters. So far, however, there has been no detection of a signal which could be attributed to dark matter decay. On the other hand, X-rays signals from dark matter decays are particular elusive, in particular because of the non-optimal resolution of current detectors, and can be confused with instrumental artefacts or with emissions from known astrophysical processes also producing linelike features in the X-ray spectra.

A strategy to overcome this problem is to combine the observation of different astrophysical sources. Indeed, a line produced by the decay of dark matter would be observed with the same characteristics in all the considered objects while astrophysical backgrounds are generally expected to differ.

The authors of 1402.4119 propose a practical realization of this idea combining the observational data collected by the XMM-Newton experiment for two objects, the Andromeda Galaxy and the Perseus galaxy cluster, and they do find an identified excess of events in both of these objects. The spectrum of the Andromeda Galaxy is shown in Figure 1, where a peak can be seen at about 3.5 keV. Furthermore, since these objects are moving away from earth with different velocities, the resulting redshifts should make the observed lines appear at slightly different wavelengths, which indeed they do. The authors also uses data collected of just a blank portion of the sky, where no line is observed. Since many sources of instrumental effects would lead to an excess independent on the portion of the sky observed, this increases the robustness of the observations.

An interpretation of the result is as well proposed in terms of a popular kind of KeV scale mass dark matter candidates, the so-called sterile neutrino. Its lifetime can be calculated in terms of the mass and one additional parameter, the mixing angle , giving the interaction strength of the sterile neutrino. The properties of the observed line can then be translated into a prediction of the mass and mixing angle, reported in figure 2 together with some already known constraints.

To conclude, the properties of the observed X-ray line are consistent with the interpretation in terms of decaying dark matter. However, it is not possible to reach a firm conclusion since the results still suffer from large uncertainties, allowing nondark matter explanations of the data. This result can be strengthen by observing lines in other astrophysical objects, and indeed this result confirms a similar analysis carried out in 1402.2301. The identified lines are at the same energies, and hence compatible with the decay of dark matter. However, alternative explanations, such as an emission line from the plasma in space, is still possible.

The definitive answer will most probably come from the Astro-H experiment which will have the capability to distinguish the possible dark matter line from a line of different origin at similar energy. In absence of a detection by this experiment, the dark matter interpretation of the observed signal would be ruled out.

Figure 1: Spectrum of the Andromeda Galaxy (number of events as function of energy). On top is the total number of events, and on the bottom is the data with the background subtracted. An small excess, associated to the tentative DM line, is visible at around 3.5 keV.

Figure 2: Values of the mass and mixing angle of the sterile neutrino which are compatible with the observed photon line (blue bar). The gray regions correspond to either a dark matter over- or underabundace compared to observations. The red regions is excluded by X-ray searches.

Authors: Giorgio Arcadi and Johannes Bergström