Spectral variability in NGC 1042 ULX1

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 $\sim 1$ keV. Cool thermal disk component with a characteristic temperature of $\sim 0.2$ 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 $\sim 6-10$ keV, NGC 1042 ULX1 could be another stellar-mass super-Eddington accretor.Comment: 9 pages, 5 figures, accepted for publication in MNRA

Constraint on the Accretion of NGC 6946 X-1 Using Broadband X-Ray Data

We analyze broadband X-ray data of NGC 6946 X-1 and probe plausible accretion scenarios in this ULX. NGC 6946 X-1 is a persistent soft source with broadband continuum spectra described by two thermal disk components. The cool accretion disk temperature T _cool ∼ 0.2 keV and the presence of a ∼0.9 keV emission/absorption broad feature suggest evidence of an optically thick wind due to supercritical accretion. The hot geometrically modified accretion disk has an inner temperature of T _hot ∼ 2 keV with a radially dependent profile T ( r ) ∝ r ^−0.5 , expected in a slim-disk scenario. Further, the measurement based on a realistic inclination angle of the disk indicates that the mass of the host compact object is comparable to a ∼6–10 M _⊙ nonrotating black hole or the system hosts a moderately magnetized neutron star with a B ≲ 2 × 10 ^11 G magnetic field. Overall, the detected spectral curvature, high luminosity, flux contribution from two thermal disk components, and estimated accretion rate support the super-Eddington accretion scenario