A spectral-timing model for ULXs in the supercritical regime

Ultraluminous X-ray sources (ULXs) with luminosities lying between ∼3 × 1039 and 2 × 1040 erg s−1 represent a contentious sample of objects as their brightness, together with a lack of unambiguous mass estimates for the vast majority of the central objects, leads to a degenerate scenario where the accretor could be a stellar remnant (black hole or neutron star) or intermediate-mass black hole (IMBH). Recent, high-quality observations imply that the presence of IMBHs in the majority of these objects is unlikely unless the accretion flow somehow deviates strongly from expectation based on objects with known masses. On the other hand, physically motivated models for supercritical inflows can re-create the observed X-ray spectra and their evolution, although have been lacking a robust explanation for their variability properties. In this paper, we include the effect of a partially inhomogeneous wind that imprints variability on to the X-ray emission via two distinct methods. The model is heavily dependent on both inclination to the line of sight and mass accretion rate, resulting in a series of qualitative and semiquantitative predictions. We study the time-averaged spectra and variability of a sample of well-observed ULXs, finding that the source behaviours can be explained by our model in both individual cases as well as across the entire sample, specifically in the trend of hardness-variability power. We present the covariance spectra for these sources for the first time, which shed light on the correlated variability and issues associated with modelling broad ULX spectra

Observation of the screening signature in the lateral photovoltage of electrons in the Quantum Hall regime

The lateral photovoltage generated in the plane of a two-dimensional electron system (2DES) by a focused light spot, exhibits a fine-structure in the quantum oscillations in a magnetic field near the Quantum Hall conductivity minima. A double peak structure occurs near the minima of the longitudinal conductivity oscillations. This is the characteristic signature of the interplay between screening and Landau quantization.Comment: 4 pages, 4 figures, to be published in Phys. Rev.

Probing exotic phenomena at the interface of nuclear and particle physics with the electric dipole moments of diamagnetic atoms: A unique window to hadronic and semi-leptonic CP violation

The current status of electric dipole moments of diamagnetic atoms which involves the synergy between atomic experiments and three different theoretical areas -- particle, nuclear and atomic is reviewed. Various models of particle physics that predict CP violation, which is necessary for the existence of such electric dipole moments, are presented. These include the standard model of particle physics and various extensions of it. Effective hadron level combined charge conjugation (C) and parity (P) symmetry violating interactions are derived taking into consideration different ways in which a nucleon interacts with other nucleons as well as with electrons. Nuclear structure calculations of the CP-odd nuclear Schiff moment are discussed using the shell model and other theoretical approaches. Results of the calculations of atomic electric dipole moments due to the interaction of the nuclear Schiff moment with the electrons and the P and time-reversal (T) symmetry violating tensor-pseudotensor electron-nucleus are elucidated using different relativistic many-body theories. The principles of the measurement of the electric dipole moments of diamagnetic atoms are outlined. Upper limits for the nuclear Schiff moment and tensor-pseudotensor coupling constant are obtained combining the results of atomic experiments and relativistic many-body theories. The coefficients for the different sources of CP violation have been estimated at the elementary particle level for all the diamagnetic atoms of current experimental interest and their implications for physics beyond the standard model is discussed. Possible improvements of the current results of the measurements as well as quantum chromodynamics, nuclear and atomic calculations are suggested.Comment: 46 pages, 19 tables and 16 figures. A review article accepted for EPJ

A Systematic Study of Variability in a Sample of Ultraluminous X-ray Sources

We present results from a study of short-term variability in 19 archival observations by XMM–Newton of 16 ultraluminous X-ray sources (ULXs). Eight observations (six sources) showed intrinsic variability with power spectra in the form of either a power-law or broken power-law-like continuum and in some cases quasi-periodic oscillations (QPOs). The remaining observations were used to place upper limits on the strength of possible variability hidden within. Seven observations (seven sources) yielded upper limits comparable to, or higher than, the values measured from those observations with detectable variations. These represented the seven faintest sources, all with fx < 3 × 10−12 erg cm−2 s−1. In contrast, there are four observations (three sources) that gave upper limits significantly lower than both the values measured from the ULX observations with detectable variations, and the values expected by comparison with luminous Galactic black hole X-ray binaries (BHBs) and active galactic nuclei (AGN) in the observed frequency bandpass (10−3–1 Hz). This is the case irrespective of whether one assumes characteristic frequencies appropriate for a stellar mass (10 M⊙) or an intermediate mass (1000 M⊙) black hole, and means that in some ULXs the variability is significantly suppressed compared to bright BHBs and AGN. We discuss ways to account for this unusual suppression in terms of both observational and intrinsic effects and whether these solutions are supported by our results

Broad absorption features in wind-dominated ultraluminous X-ray sources?

The luminosities of ultraluminous X-ray sources (ULXs) require an exotic solution with either supercritical accretion modes on to stellar-mass black holes or subcritical accretion on to intermediate-mass black holes (IMBHs) being invoked. Discriminating between the two is non-trivial due to the present lack of a direct mass measurement. A key expectation of the supercritical mode of accretion is the presence of powerful radiatively driven winds. Here we analyse XMM–Newton data from NGC 5408 X-1 and NGC 6946 X-1 and find that strong soft residuals present in the X-ray spectra can be reconciled with broadened, blueshifted absorption by a partially ionized, optically thin phase of this wind. We derive initial values for the physical parameters of the wind; we also discuss other possible origins for the observed features