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.'
author:
- 'M. Mezcua, G. Fabbiano, J.C. Gladstone, S.A. Farrell, and R. Soria'
bibliography:
- 'referencesALL.bib'
title: |
    Revealing the nature of the ULX and X-ray population\
    of the spiral galaxy NGC4088
---

Introduction
============

Early X-ray observations of nearby galaxies with the *Einstein Observatory*, and later with *ROSAT*, *ASCA*, *XMM-Newton*, and *Chandra*, revealed the presence of a population of off-nuclear X-ray point sources with X-ray luminosities above the Eddington limit for a stellar-mass black hole ($L_\mathrm{X} > 10^{39}$ erg s$^{-1}$; ). These high luminosities imply black holes (BHs) of masses $>$ 10 $M_{\odot}$, if the accretion is sub-Eddington and the radiation is isotropic. This suggested that ULXs could be intermediate mass BHs (IMBHs) with BH masses in the range $100 \leq M_\mathrm{BH} \leq 10^{5} M_{\odot}$ (e.g., @1999ApJ...519...89C). The alternative is stellar-remnant BHs ($M_\mathrm{BH} < 100 M_{\odot}$) accreting at around or above the Eddington limit (e.g., @2001ApJ...552L.109K; @2005Sci...307..533F; @2011NewAR..55..166F).

Studies of the X-ray luminosity function (XLF) of the X-ray populations of these nearby galaxies 