N. Cappelluti, R. Arendt, A. Kashlinsky, Y. Li, G. Hasinger, K. Helgason, M. Urry, P. Natarajan, A. Finoguenov
We present new measurements of the large scale clustering component of the cross-power spectra of the source-subtracted Spitzer-IRAC Cosmic Infrared Background (CIB) and Chandra-ACIS Cosmic X- ray Background (CXB) surface brightness fluctuations Our investigation uses data from the Chandra Deep Field South (CDFS), Hubble Deep Field North (HDFN), EGS/AEGIS field and UDS/SXDF surveys, comprising 1160 Spitzer hours and ∼ 12 Ms of Chandra data collected over a total area of 0.3 deg2. We report the first (>5σ) detection of a cross-power signal on large angular scales >20′′ between [0.5-2] keV and the 3.6μm and 4.5μm bands, at ∼5σ and 6.3σ significance, respectively. The correlation with harder X-ray bands is marginally significant. Comparing the new observations with existing models for the contribution of the known unmasked source population at z <7, we find an excess of about an order of magnitude at 5σ confidence. We discuss possible interpretations for the origin of this excess in terms of the contribution from accreting early black holes, including both direct collapse black holes and primordial black holes, as well as from scattering in the interstellar medium and intra-halo light.
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In this paper we report a 3σ detection of an emission line at ∼3.5 keV in the spectrum of the Cosmic X-ray Background using a total of ∼10 Ms Chandra observations towards the COSMOS Legacy and CDFS survey fields. The line is detected with an intensity is 8.8±2.9×10−7 ph cm−2s−1. Based on our knowledge of Chandra, and the reported detection of the line by other instruments, we can rule out an instrumental origin for the line. We cannot though rule out a background fluctuation, in that case, with the current data, we place a 3σ upper limit at 10−6 ph cm−2s−1. We discuss the interpretation of this observed line in terms of the iron line background, S XVI charge exchange, as well as arising from sterile neutrino decay. We note that our detection is consistent with previous measurements of this line toward the Galactic center, and can be modeled as the result of sterile neutrino decay from the Milky Way when the dark matter distribution is modeled with an NFW profile. In this event, we estimate a mass ms∼7.02 keV and a mixing angle sin2(2θ)= 0.69-2.29 ×10−10. These derived values of the neutrino mass are in agreement with independent measurements toward galaxy clusters, the Galactic center and M31.