Searching for dark matter in X-rays: how to check the dark matter origin of a spectral feature

A signal from decaying dark matter (DM) can be unambiguously distinguished from spectral features of astrophysical or instrumental origin by studying its spatial distribution. We demonstrate this approach by examining the recent claim of Loewenstein and Kusenko regarding the possible DM origin of the 2.5 keV line in Chandra observations of the Milky Way satellite known as Willman 1. Our conservative strategy is to adopt, among reasonable mass estimates derived here and in the literature, a relatively large dark mass for Willman 1 and relatively small dark masses for the comparison objects. In light of the large uncertainty in the actual DM content of Willman 1, this strategy provides minimum exclusion limits on the DM origin of the reported signal. We analyse archival observations by XMM-Newton of M31 and Fornax dwarf spheroidal galaxy (dSph) and Chandra observations of Sculptor dSph. By performing a conservative analysis of X-ray spectra, we show the absence of a DM decay line with parameters consistent with those of Loewenstein and Kusenko. For M31, the observations of the regions between 10 and 20 kpc from the centre, where the uncertainties in the DM distribution are minimal, make a strong exclusion at the level above 10 Sigma. The Fornax dSph provides a similar to 3.3 Sigma exclusion instead of a predicted 4 Sigma detection, and the Sculptor dSph provides a 3 Sigma exclusion instead of a predicted 2.5 Sigma detection. The observations of the central region of M31 (1-3 kpc off-centre) are inconsistent with having a DM decay line at more than 20 Sigma if one takes the most conservative among the best physically motivated models. The minimal estimate for the amount of DM in the central 40 kpc of M31 is provided by the model of Corbelli et al., assuming the stellar disc's mass to light ratio similar to 8 and almost constant DM density within a core of 28 kpc. Even in this case one gets an exclusion at 5.7 Sigma from central region of M31, whereas modelling all processed data from M31 and Fornax produces more than 14 Sigma exclusion. Therefore, despite possible systematic uncertainties, we exclude the possibility that the spectral feature at similar to 2.5 keV found in Loewenstein and Kusenko is a DM decay line. We conclude, however, that the search for DM decay line, although demanding prolonged (up to 1 Ms) observations of well-studied dSphs, M31 outskirts and other similar objects, is rather promising, as the nature of a possible signal can be checked. An (expected) non-observation of a DM decay signal in the planned observations of Willman 1 should not discourage further dedicated observations.

Decaying dark matter: the case for a deep X-ray observation of Draco

Alexey Boyarsky, Oleg Ruchayskiy

Recent studies of M31, the Galactic Centre (GC), and galaxy clusters have made tentative detections of an X-ray line at similar to 3.5 keV that could be produced by decaying dark matter. We use high-resolution simulations of the Aquarius project to predict the likely amplitude of the X-ray decay flux observed in the GC relative to that observed in M31, and also of the GC relative to other parts of the Milky Way halo and to dwarf spheroidal galaxies. We show that the reported detections from M31 and the GC are compatible with each other, and with upper limits arising from high galactic latitude observations, and imply a decay time tau similar to 10(28) s. We argue that this interpretation can be tested with deep observations of dwarf spheroidal galaxies: in 95 per cent of our mock observations, a 1.3 Ms pointed observation of Draco with XMM-Newton will enable us to discover or rule out at the 3 sigma level an X-ray feature from dark matter decay at 3.5 keV, for decay times tau < 0.8 x 10(28) s.

Oxford University Press2015

Searching for decaying dark matter in deep XMM-Newton observation of the Draco dwarf spheroidal

Alexey Boyarsky, Andrii Neronov, Oleg Ruchayskiy

We present results of a search for the 3.5 keV emission line in our recent very long (similar to 1.4 Ms) XMM-Newton observation of the Draco dwarf spheroidal galaxy. The astrophysical X-ray emission from such dark matter-dominated galaxies is faint, thus they provide a test for the dark matter origin of the 3.5 keV line previously detected in other massive, but X-ray bright objects, such as galaxies and galaxy clusters. We do not detect a statistically significant emission line from Draco; this constrains the lifetime of a decaying dark matter particle to tau > (7-9) x 10(27) s at 95 per cent CL (combining all three XMM-Newton cameras; the interval corresponds to the uncertainty of the dark matter column density in the direction of Draco). The PN camera, which has the highest sensitivity of the three, does show a positive spectral residual (above the carefully modelled continuum) at E = 3.54 +/- A 0.06 keV with a 2.3 sigma significance. The two MOS cameras show less-significant or no positive deviations, consistently within 1 sigma with PN. Our Draco limit on tau is consistent with previous detections in the stacked galaxy clusters, M31 and the Galactic Centre within their 1 - 2 sigma uncertainties, but is inconsistent with the high signal from the core of the Perseus cluster (which has itself been inconsistent with the rest of the detections). We conclude that this Draco observation does not exclude the dark matter interpretation of the 3.5 keV line in those objects.

Wiley-Blackwell2016

New constraints on the mass of fermionic dark matter from dwarf spheroidal galaxies

Diego Blas Temino, Kyrylo Bondarenko, Alexey Boyarsky

Dwarf spheroidal galaxies are excellent systems to probe the nature of fermionic dark matter due to their high observed dark matter phase-space density. In this work, we review, revise, and improve upon previous phase-space considerations to obtain lower bounds on the mass of fermionic dark matter particles. The refinement in the results compared to previous works is realized particularly due to a significantly improved Jeans analysis of the galaxies. We discuss two methods to obtain phase-space bounds on the dark matter mass, one model-independent bound based on Pauli's principle, and the other derived from an application of Liouville's theorem. As benchmark examples for the latter case, we derive constraints for thermally decoupled particles and (non-)resonantly produced sterile neutrinos. Using the Pauli principle, we report a model-independent lower bound of m >= 0.18 keV at 68 per cent CL and m >= 0.13 keV at 95 per cent CL. For relativistically decoupled thermal relics, this bound is strengthened to m >= 0.59 keV at 68 per cent CL and m >= 0.41 keV at 95 per cent CL, while for non-resonantly produced sterile neutrinos the constraint is m >= 2.80 keV at 68 per cent CL and m >= 1.74 keV at 95 per cent CL. Finally, the phase-space bounds on resonantly produced sterile neutrinos are compared with complementary limits from X-ray, Lyman alpha, and big bang nucleosynthesis observations.

OXFORD UNIV PRESS2021