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The spacetime around astrophysical black holes is thought to be described by the Kerr solution. However, even within general relativity, there is not yet a proof that the final product of the complete collapse of an uncharged body can only be a Kerr black hole. We can thus speculate on the possibility that the spacetime around astrophysical black holes may be described by other solutions of the Einstein Equations and we can test such a hypothesis with observations. In this work, we consider the $\delta$-Kerr metric, which is an exact solution of the field equations in vacuum and can be obtained from a non-linear superposition of the Kerr metric with a static axially symmetric solution, often referred to as the $\delta$-metric. The parameter $\delta=1+q$ quantifies the departure of the source from the Kerr metric and for $q=0$ we recover the Kerr solution. From the analysis of the reflection features in the X-ray spectrum of the Galactic black hole in EXO 1846-031, we find $-0.1 < q < 0.7$ (90% CL), which is consistent with the hypothesis that the spacetime around the compact object in EXO 1846-031 is a Kerr black hole but does not entirely rule out the $\delta$-Kerr metric., Comment: 17 pages, 7 figures. v2: refereed version

The Dirac nodal line (DNL) is a novel form of massless Dirac fermions that reside along lines in momentum space. Here, we verify genuine DNLs in the molecular material, [Ni(dmdt)$_2$], with the combined NMR experiments and numerical simulations. The NMR spectral shift and spin-lattice relaxation rate divided by temperature, $1/T_1T$, decrease linearly and quadratically with temperature, respectively, and become constant at low temperatures, consistent with slightly dispersive DNLs with small Fermi pockets. Comparison of these results with model simulations of DNLs reveals the suppression of the Fermi velocity and the enhancement of antiferromagnetic fluctuations due to electron correlation as well as the influence of the Landau quantization. The present study offers a demonstration to identify the DNL and evaluate the correlation effect with NMR., Comment: 6 pages, 3 figures

Einstein-Maxwell dilaton-axion gravity is a string-inspired model arising from the low energy effective action of heterotic string theory and an important candidate as alternative to General Relativity. Recently, some authors have explored its astrophysical implications in the spectra of accreting black holes and inferred the constraint $r_2 < 0.1$, where $r_2 \ge 0$ is the black hole dilaton charge and General Relativity is recovered for $r_2 = 0$. In the present paper, we study the impact of a non-vanishing black hole dilaton charge on the reflection spectrum of the disk. From the analysis of a NuSTAR spectrum of the black hole binary EXO 1846-031, we find the constraint $r_2 < 0.011$ (90% CL), which is an order of magnitude more stringent., Comment: 8 pages, 5 figures. v2: refereed version

Motivated by the recent discovery of Dirac nodal line in the single-component molecular conductor [Pt(dmdt)$_{2}$], we propose a three-orbital tight-binding model based on the Wannier fitting of the first-principles calculation, and address the problems of edge states, topological properties and magnetic susceptibility. We find that logarithmic peaks of the local density of states emerge near the Fermi energy, owing to pseudo-one-dimensional edge states that appear between the Dirac nodal lines. Magnetic susceptibility calculated in our model can explain the experimental result at a high temperature. In the presence of a realistic spin-orbit coupling, we show that [Pt(dmdt)$_{2}$] is a topological nodal line semimetal with isolated electron and hole pockets.

We construct three-orbital tight-binding models describing single-component molecular conductors [Pt(dmdt)$_2$] and [Ni(dmdt)$_2$] using first-principles calculations. We show that [Ni(dmdt)$_2$] is a Dirac nodal line system with highly one-dimensional edge states at the (001) edge, similar to [Pt(dmdt)$_2$], as demonstrated in prior studies. To investigate possible edge magnetism, we calculate longitudinal and transverse spin susceptibilities using real-space-dependent random-phase approximation (RPA) in three-orbital Hubbard models in the presence of spin--orbit coupling. We find that the edge spin-density wave (SDW) is induced by the Coulomb repulsion and incommensurate nestings of the Fermi arcs. We also find that the magnetic structure of the edge SDW can be changed via extremely small carrier doping, which is controllable in molecular conductors.

The Novikov-Thorne model is the standard framework for the description of geometrically thin and optically thick accretion disks around black holes and is widely used to study the electromagnetic spectra of accreting black holes. One of the assumptions of the model is that the particles of the gas move on nearly-geodesic circular orbits on the equatorial plane. In this work, we propose to test the Keplerian velocity of the particles in the accretion disk using X-ray reflection spectroscopy. We present a modified version of RELXILL in which we introduce a phenomenological parameter, $\alpha$, to quantify possible deviations from Keplerian motion. We use our model to fit a Suzaku observation of the black hole binary GRS 1915+105. We find that the estimate of $\alpha$ is correlated to that of the inclination angle of the disk, $i$, and that we could test the Keplerian disk hypothesis in the presence of a robust and independent measurement of $i$., Comment: 9 pages, 5 figures. v2: refereed version

Asymptotically safe quantum gravity is a promising candidate scenario to provide a UV extension for the effective quantum field theory of Einstein's gravity. The theory has its foundations on the very successful framework of quantum field theory, which has been extensively tested for electromagnetic and nuclear interactions. However, observational tests of asymptotically safe quantum gravity are more challenging. Recently, a rotating black hole metric inspired by asymptotically safe quantum gravity has been proposed, and this opens the possibility of astrophysical tests of the theory. In the present paper, we show the capabilities of X-ray reflection spectroscopy to constrain the inverse dimensionless fixed-point value $\gamma$ from the analysis of a Suzaku observation of the X-ray binary GRS 1915+105. We compare these constraints with those obtained from black hole imaging., Comment: 11 pages, 6 figures. v2: refereed version

In a previous paper, one of us calculated iron K$\alpha$ line profiles emitted from possible accretion disks around traversable wormholes as a first step to use X-ray reflection spectroscopy to search for astrophysical wormholes in active galactic nuclei. In the present paper, we extend that work and construct an XSPEC model for the whole relativistic reflection spectrum. We apply our model to XMM-Newton and NuSTAR observations of the supermassive object in MCG-6-30-15 and we check whether these observations prefer the hypothesis that the central body is a Kerr black hole or a traversable wormhole. We find that the two models provide equally good fits, so with the available data we cannot distinguish the black hole and wormhole scenarios., Comment: 10 pages, 7 figures. v2: refereed version

We construct a relativistic reflection model in a non-Kerr spacetime in which, depending on the value of the deformation parameter of the metric, there are black hole solutions with spin parameter $|a_*| > 1$. We apply our model to fit Suzaku data of four Seyfert galaxies (Ton S180, Ark 120, 1H0419-577, and Swift J0501.9-3239). These galaxies host at the center supermassive black holes that were previously interpreted as near-extremal Kerr black holes. For Ton S180 and 1H0419-577, our measurements are still consistent with the Kerr hypothesis. For Ark 120 and Swift J0501.9-3239, the Kerr solution is not recovered at 3-$\sigma$. We discuss our results and possible systematic uncertainties in the model., Comment: 12 pages, 8 figures. v2: refereed version