Studying the nature of Ultraluminous X-ray sources in NGC 1453 with XMM-Newton

Abstract

This paper presents a multi epoch, detailed spectral and temporal analysis of the non-nuclear X-ray point sources of the massive elliptical galaxy NGC 1453, based on two XMM-Newton observational data. In the observation with Obs ID 0901620101, seven sources having net counts $\ge$ 100 were identified for PN data, which along with their corresponding MOS data were considered for the analysis. For the observation with Obs ID 0673770601, only three sources were found for PN and two sources for MOS-1 and MOS-2. The spectra of all the sources were simultaneously fitted using two empirical models: an absorbed power-law model and an absorbed disk blackbody model. Based on the estimated bolometric luminosities of the sources, six sources - X-1, X-2, X-3, X-4, X-6 and X-7 are categorized as HLXs with X-ray luminosity, $L_X>10^{41}erg{s}^{-1}$ while one source, X-5, as an ELX with $L_X>10^{40}erg{s}^{-1}$, within the error limit. Notably, for source X-1, the disk blackbody component is the dominant feature in both the observations. In the 2012 observation (Obs ID 0673770601), the source exhibits a soft, cool accretion disk with an inner disk temperature of $k{T}_{in}\sim$ 0.27 keV. By 2022 (Obs ID 0901620101), X-1 presents an even softer, supersoft spectrum, characterized by a significantly lower inner disk temperature of $k{T}_{in}\sim$ 0.17 keV and photon index $\Gamma >$ 5. This shift over the span of a decade indicates a further softening of the source. In the present study, due to limited data availability, the Luminosity-Temperature (L-T) relation could not be strictly constrained. However, for the purpose of mass estimation, we have assumed that the L$\sim T^4$ relation holds. The observed further softening of the source (X-1), accompanied by a slight increase in luminosity in the later observation, suggests a potential inverse correlation between the inner disk temperature of the soft component and luminosity, which is consistent with the characteristics of beamed disk emission resulting from radiatively driven, strong outflowing winds, as expected in super-Eddington accretion system. The spectra of sources X-4 and X-7 were found to be relatively soft, with $k{T}_{in}\sim$ 0.5–0.7 keV and $\Gamma >$ 2. Sources X-2, X-5, and X-6 exhibit a hard spectral state ($\Gamma \sim$ 1.5–1.8), while the source, X-2 was detected in both observations without any significant spectral variations. For the source, X-3, a slight hardening of the spectrum was observed in the later observation. The hard spectral states of the sources may arise from the inverse Comptonization or accretion flow in the innermost region. Temporal analysis using the Chi-square probability of constancy test reveals no variability for all sources at kilosecond time binnings, at a 99% confidence level, except for source X-1 at 1 ks time binning for the observation (Obs ID 0901620101). In the case of X-1, intrinsic red noise dominates over Poisson noise at 10 ks time scale, indicating the possibility of potential beaming effects or instabilities within the accretion disk. Further, the variability was quantified by calculating the RMS fractional variability ($F_{var}$) using the lcstat tool, at different time bins of 0.1 ks, 0.5 ks, and 1 ks. Moreover, the absence of pulsations in their power density spectra suggests that these Ultra-Luminous X-ray Sources (ULXs)- the HLXs and ELX are unlikely to be powered by pulsating neutron stars.