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We present simultaneous XMM-Newton and NuSTAR observations spanning 3-78 keV of the nearest radio galaxy, Centaurus A (Cen A). The accretion geometry around the central engine in Cen A is still debated, and we investigate possible…
We present results of a high-resolution soft X-ray (0.2-2 keV) spectroscopic study of a sample of 69 nearby obscured Active Galactic Nuclei (AGN) observed with the Reflection Grating Spectrometer (RGS) on board XMM-Newton. This is the…
We have identified three possible ways in which future XMM-Newton observations can provide significant constraints on the equation of state of neutron stars. First, using a long observation of the neutron star X-ray transient CenX-4 in…
We advocate for the idea that there is a fundamentally new mechanism for axion production on Earth, as recently suggested in Fischer et al. (2018) and Liang & Zhitnitsky (2018). We specifically focus on production of axions within Earth,…
In the two parts of this contribution we describe two related XMM-Newton programs. The first part summarizes our study of the X-ray spectral properties and variability of z>4 quasars (Shemmer et al. 2005). The second part presents…
Context. X-ray spectral variability analyses of active galactic nuclei (AGN) with moderate luminosities and redshifts typically show a softer when brighter behaviour. Such a trend has rarely been investigated for high-luminosity AGNs ($…
We present a NuSTAR and XMM-Newton monitoring campaign in 2014/2015 of the Compton-thick Seyfert 2 galaxy, NGC 1068. During the August 2014 observation, we detect with NuSTAR a flux excess above 20 keV ($32\pm6 \%$) with respect to the…
We review a testable, the axion quark nugget (AQN) model outside of the standard WIMP paradigm. The model was originally invented to explain the observed similarity between the dark and the visible components, $\Omega_{\rm DM}\approx…
We analyse the spectra of the archival XMM-Newton data of the Seyfert 1 AGN Zw 229.015 in the energy range $0.3 - 10.0$ keV. When fitted with a simple power-law, the spectrum shows signatures of weak soft excess below 1.0 keV. We find that…
We present simultaneous XMM-Newton and NuSTAR observations of the `bare' Seyfert 1 galaxy, Ark 120, a system in which ionized absorption is absent. The NuSTAR hard X-ray spectral coverage allows us to constrain different models for the…
We present a theoretical model for detecting axions from neutron stars in a QCD phase of quark matter. The axions would be produced from a quark-antiquark pair $u\bar{u}$ or $d\bar{d}$, in loop(s) involving gluons. The chiral anomaly of QCD…
We present results from a joint Xmm-Newton/NuSTAR monitoring of the Seyfert 1 NGC 4593, consisting of 5x20 ks simultaneous observations spaced by two days, performed in January 2015. The source is variable, both in flux and spectral shape,…
We have measured the spectrum of the Cosmic X-ray Background (CXB) in the 2-8 keV range with the high throughput EPIC/MOS instrument onboard XMM-Newton. A large sample of high galactic latitude observations was used, covering a total solid…
The soft X-ray flux produced by solar axions in the Earth's magnetic field is evaluated in the context of ESA's XMM-Newton observatory. Recent calculations of the scattering of axion-conversion X-rays suggest that the sunward magnetosphere…
Notwithstanding the big efforts devoted to the investigation of the mechanisms responsible for the high-energy (E>100 MeV) gamma-ray emission in active galactic nuclei (AGN), the definite answer is still missing. The X-ray energy band…
We analyze the impact of future hard X-ray observations on the search for indirect signatures of particle dark matter in large extragalactic systems such as nearby clusters or groups of galaxies. We argue that the hard X-ray energy band…
Quark nuggets are theoretical objects composed of approximately equal numbers of up, down, and strange quarks. They are also called strangelets, nuclearites, AQNs, slets, Macros, and MQNs. Quark nuggets are a candidate for dark matter,…
A model is constructed to predict the emission originating from axion-to-photon conversion in the strongly magnetized ultrarelativistic plasma of neutron stars. The acceleration and multiplicity of the charges are observed to shift the…
We present a series of X-ray variability results from a long XMM-Newton + NuSTAR campaign on the bright, variable AGN NGC 3227. We present an analysis of the lightcurves, showing that the source displays typically softer-when-brighter…
We report on a NuSTAR and XMM-Newton program that has observed a sample of three extremely luminous, heavily obscured WISE-selected AGN at z~2 in a broad X-ray band (0.1 - 79 keV). The parent sample, selected to be faint or undetected in…