Spectral and Temporal Properties of Ultraluminous X-Ray Source ULX-1 in NGC 4088

Abstract We present the results of a multi-epoch, detailed spectral and temporal analysis of the ultraluminous X-ray source (ULX), ULX-1, in the galaxy NGC 4088 using XMM-Newton, Chandra, and Swift observations. The presence of a hard powerlaw spectral slope supports the interpretation of ULX-1 as a hard ULX. The observed inner disk temperature of kT in > 1.5 keV is inconsistent with the presence of an intermediate mass black hole, but favors the super-Eddington accretion state. Moreover, the physically acceptable value of the parameter controlling the radial temperature profile of the disk( p ) derived from fitting the first XMM-Newton observation with the slim disk model further points towards the possible presence of a broadened disk, indicating the super-Eddington accretion nature. Slight overall long-term flux variability is evident for this ULX, and a hint towards a positive correlation between flux and the powerlaw photon index is also observed when the relatively better data of XMM-Newton and Chandra are considered. The L-T relationship is observed to follow a positive trend, with the L–T profile consistent with either relation ( L ∝ T 4 or L ∝ T 2 ) in both cases. The source exhibits no significant short-term variability at different time binnings of the light curve as indicated by the chi-square probability of constancy and fractional RMS variability values. The power density spectrum created shows no evidence of intrinsic variability of the source above the white noise. Further, no sign of pulsation was detected for this source. Assuming this ULX to be powered by an accreting black hole and using the slim disk geometry, the upper limit of the black hole mass was estimated and found to be less than 100 M ⊙ .