title: Unidentified line in x-ray spectra of the Andromeda galaxy and Perseus galaxy cluster.
abstract: We report a weak line at 3.52±0.02  keV in x-ray spectra of the Andromeda galaxy and the Perseus galaxy cluster observed by the metal-oxide-silicon (MOS) and p-n (PN) CCD cameras of the XMM-Newton telescope. This line is not known as an atomic line in the spectra of galaxies or clusters. It becomes stronger towards the centers of the objects; is stronger for Perseus than for M31; is absent in the spectrum of a deep "blank sky" data set. Although for each object it is hard to exclude that the feature is due to an instrumental effect or an atomic line, it is consistent with the behavior of a dark matter decay line. Future (non-)detections of this line in multiple objects may help to reveal its nature.
authors.affiliation: Instituut-Lorentz for Theoretical Physics, Universiteit Leiden, Niels Bohrweg 2, 2333 CA Leiden, Netherlands.
authors.forename: A
authors.initials: A
authors.lastname: Boyarsky
authors.affiliation: Ecole Polytechnique Fédérale de Lausanne, FSB/ITP/LPPC, BSP, CH-1015 Lausanne, Switzerland.
authors.forename: O
authors.initials: O
authors.lastname: Ruchayskiy
authors.affiliation: Bogolyubov Institute of Theoretical Physics, Metrologichna Street 14-b, 03680 Kyiv, Ukraine and National University "Kyiv-Mohyla Academy", Skovorody Street 2, 04070 Kyiv, Ukraine.
authors.forename: D
authors.initials: D
authors.lastname: Iakubovskyi
authors.affiliation: Instituut-Lorentz for Theoretical Physics, Universiteit Leiden, Niels Bohrweg 2, 2333 CA Leiden, Netherlands and Leiden Observatory, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, Netherlands.
authors.forename: J
authors.initials: J
authors.lastname: Franse