Revealing the nature of the ULX and X-ray population of the spiral galaxy NGC 4088
Abstract
We present the first Chandra and Swift X-ray study of the spiral galaxy NGC\,4088 and its ultraluminous X-ray source (ULX N4088--X1). We also report very long baseline interferometry (VLBI) observations at 1.6 and 5 GHz performed quasi-simultaneously with the Swift and Chandra observations, respectively. Fifteen X-ray sources are detected by Chandra within the D25 ellipse of NGC\,4088, from which we derive the X-ray luminosity function (XLF) of this galaxy. We find the XLF is very similar to those of star-forming galaxies and estimate a star-formation rate of 4.5 M yr-1. The Chandra detection of the ULX yields its most accurate X-ray position, which is spatially coincident with compact radio emission at 1.6 GHz. The ULX Chandra X-ray luminosity, L0.2-10.0 keV = 3.4\ × 1039 erg s-1, indicates that N4088--X1 could be located at the high-luminosity end of the high-mass X-ray binary (HMXB) population of NGC\,4088. The estimates of the black hole mass and ratio of radio to X-ray luminosity of N4088--X1 rule out a supermassive black hole nature. The Swift X-ray spectrum of N4088--X1 is best described by a thermal Comptonization model and presents a statistically significant high-energy cut-off. We conclude that N4088--X1 is most likely a stellar remnant black hole in a HMXB, probably fed by Roche lobe overflow, residing in a super-Eddington ultraluminous state. The 1.6 GHz VLBI source is consistent with radio emission from possible ballistic jet ejections in this state.
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