We present the results of high-quality XMM-NEWTON observations of a ULX in the galaxy NGC 4190. The detection of spectral cutoff in NGC 4190 ULX1 spectra rules out the interpretation of the ULX to be in a standard low/hard canonical accretion state. We report that the high quality EPIC spectra can be better described by broad thermal component, such as a slim disk. In addition we found long term spectral and flux variability in the source using several XMM-NEWTON and Swift data. A clear anti-correlation between flux and power-law photon index is found which further confirms the unusual spectral state evolution of the ULX. Spectral properties of the ULX suggest that the source is in a broadened disk state with luminosities (≈(3 − 10) × 1039 ergs s−1) falling in the ultraluminous regime. The positive Luminosity-temperature relation further suggests that the multi colour disk model follows the L∝T4 relation which is expected for a blackbody disk emission from a constant area and the slim disk model seems to favour L∝T2 relation consistent with an advection dominated disk emission . From the broadened disk like spectral feature at such luminosity, we estimated the upper limit of the mass of the central compact object from the inner disk radius and found that the ULX hosts a stellar mass black hole.

We report the detection of flaring events in NGC 4395 ULX1, a nearby ultraluminous X-ray source (ULX), for the first time, using recent XMM-Newton observations. The flaring episodes are spectrally harder than the steady-emission intervals, resulting in higher fractional variability in the high-energy regime. A thin Keplerian and a slim accretion disk provide the best-fit continuum for XMM-Newton spectra. All observations show a broad hump-like feature around ∼0.9 keV, which can be associated with a collection of blended emission lines, and suggests the presence of a wind or outflow in this ULX through comparison with other ULXs that show a similar feature. The flaring spectra correspond to higher slim-disk temperatures due to a higher mass accretion rate under an advection-dominated accretion scenario. The luminosity–temperature (L-T) values in different flux states show a positive trend. When characterized with a power-law relation, the L-T profile is broadly consistent with both L ∝ T 2 and L ∝ T 4 relations for the analyzed data. The empirical predictions for a slim accretion disk in the case of super-Eddington accretion onto a stellar-mass compact object is L ∝ T 2, which is a possible scenario in ULX1. The origin of the flaring events is understood as an intrinsic change in accretion rate or presence of variable clumpy wind in the inner region of the accretion disk.

The origin of spectral curvature at energies E ≃ 10 keV in ultraluminous X-ray sources (ULXs) is not well understood. In this paper, we propose a novel mechanism based on synchrotron radiation to explain this cutoff. We show that relativistic plasma can give rise to the observed spectral curvature for neutron star magnetic fields due to the variation in the latitude of synchrotron radiation. We analyze the NuSTAR data of two bright pulsar ULXs, NGC 5907 ULX1 and NGC 7793 P13, and provide estimates of the physical parameters of these sources. We fit the data for synchrotron emission at various latitudes and show that the spectral cutoff in these cases can be explained for a large range of acceptable physical parameters, e.g., a semirelativistic plasma with γ ≃ 20 for high latitudes or a highly relativistic plasma (γ ≃ 105) for emission close to the electron’s orbital plane in a typical magnetic field of B ≃ 1012 G. We also discuss how such an emission mechanism can be distinguished from other proposed models. A corollary to our study is that most ULXs might be neutron stars as they display such a spectral cutoff.

We show the appearance of geometric phase in a Dirac particle traversing in nonrelativistic limit in a time-independent gravitational field. This turns out to be similar to the one originally described as a geometric phase in magnetic fields. We explore the geometric phase in the Kerr and Schwarzschild geometries, which have significant astrophysical implications. Nevertheless, the work can be extended to any spacetime background including that of time-dependent. In the Kerr background, i.e. around a rotating black hole, geometric phase reveals both the Aharonov–Bohm effect and Pancharatnam–Berry phase. However, in a Schwarzschild geometry, i.e. around a nonrotating black hole, only the latter emerges. We expect that our assertions can be validated in both the strong gravity scenarios, like the spacetime around black holes, and weak gravity environment around Earth.

We report X-ray spectral variability in an ultraluminous X-ray source NGC 1042 ULX1, using archival XMM-Newton and recent NuSTAR observations. In long-term evolution, the source has shown a trend of variation in spectral hardness. The variability in different XMM-Newton observations is prominent above ∼1 keV. Cool thermal disk component with a characteristic temperature of ∼0.2 keV manifests that the spectral state of NGC 1042 ULX1 in all epochs is similar to that of the ultraluminous state sources. An apparent anti-correlation between luminosity and powerlaw index demonstrates that the source becomes spectrally harder when it is in a brighter state. That is conceivably related to variation in accretion rate, strength of Comptonization, wind/outflow in the system or a manifestation of varying disk occultation. Typical hard ultraluminous type spectra indicate that NGC 1042 ULX1 is a low inclination system in general. Spectral properties suggest that, like many other ULXs which show spectral curvature around ∼6–10 keV, NGC 1042 ULX1 could be another stellar-mass super-Eddington accretor.