Transient and Variable Ultra-luminous X-ray Sources in NGC 4552

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Original abstract

We searched for transient and variable X-ray sources in the elliptical galaxy NGC 4552 using the {\it Chandra} and {\it XMM-Newton} observations from 2001-2012. We detected 14 transient and variable X-ray sources within the 4$R_{e}$ region of the galaxy, which exhibited peak state and undetected flux behaviour in the {\it Chandra} observations. Among them, two sources (T1 and T14) exceed the X-ray luminosity $10^{39}\rm~erg~s^{-1}$ in the detected observations and have the peak state and undetected luminosity ratio of $\geq 14.3$ and $\geq 8.7$, respectively, which we classified as transient and variable ultra-luminous X-ray sources (ULXs). For the third relatively bright transient source, the flux varies by a factor of 3 across the detected observations, and the hardness ratio analysis suggests a normal X-ray binary system. The remaining sources have luminosities in the $10^{37}-10^{38} \rm~erg~s^{-1}$ range, and the spectral analysis indicates that the sources may belong to neutron star or black hole X-ray binary systems, either in the hard or thermal-dominated state. The identified transient and variable ULXs exhibited high variability and soft spectral characteristics, which are consistent with ULXs accreting at super-Eddington rates. The near-infrared (NIR) counterpart search using {\it JWST} observations identified potential NIR counterparts for three mildly variable sources, and their magnitudes in different NIR bands suggest a red supergiant companion for each source. Deep X-ray monitoring observations can shed further light on the new transient and variable ULXs.

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