tokens: be plausible for an isolated black hole accreting from the interstellar medium (see, e.g., Maccarone ( 2005 ) and references therein). At the same time, in this context there is an indication that the observed X-ray spectrum of source XMM0838+24 does not coincide with predictions for various forms of radiatively inefficient accretion flows that are expected to have a power law spectrum with a photon index 1.4–2.5 (see, e.g., Narayan et al. 1998 ; Ball et al. 2001 ). High-resolution X-ray and deeper optical/NIR imaging and spectroscopic observations are required to shed light on the association of XMM0838+24 and its host galaxy and thus derive its X-ray luminosity. In this study we do not attempt to analyze X-ray spectra of XMM0838+24 and XMM1226+12 in detail because given their faint X-ray fluxes it would not provide any selectivity to discriminate conventional X-ray object types. However, we compare their X-ray colors from the XMM-Newton source catalog to a sample of known AGNs and ESO 243–49 HLX–1. In Figure 6 we overplotted two “corner” states of ESO 243–49 HLX–1 and our HLX candidates on hardness diagrams of the known AGN population from Lin et al. ( 2012 ). Clearly, broadband properties of ESO 243–49 HLX–1 do not allow its unambigous separation from AGNs. At the same time, selected HLX candidates are situated aside from ESO 243–49 HLX–1 and most AGNs due to systematically harder HR2 and HR3 colors. This, however, does not favor any particular interpretation of the nature of the candidates. Figure 6. The X-ray color–color diagrams for known AGN (from Lin et al. ( 2012 ), black dots), selected HLX candidates (this paper, gray stars) and two spectral states of ESO 243–49 HLX–1, high luminosity consistent with steep power law and low luminosity hard power law states (gray triangles). X-ray colors are taken from the 3XMM-DR5 catalog and are defined as HR , where f i is the X-ray flux in i -th XMM-Newton band (band 1 is 0.2–0.5 keV, band 2 is 0.5–1.0 keV, band 3 is 1.0–2.0 keV, band 4 is 2.0–4.5 keV, band 5 is 4.5–12 keV). As claimed by Lin et al. ( 2012 ) systematic uncertainties of these colors can reach 0.1 which is below symbol size for ESO 243–49 HLX–1 and HLX candidates. 5.2. Base Sample Properties We compute the number density of the HLX candidates from our base sample. From a total of 98 HLX candidates we expect to have at least 21 true HLXs. Given the searched volume and solid angle this gives us an HLX number density of 1 object per ≈2 × 10 5 Mpc 3 . This is approximately a factor of several thousand less frequent than ULXs (Swartz et al. 2011 ). We can normalize this density to galactic halos. In total there are 33,879 SDSS galaxies falling in the 3XMM-DR5 footprint (total observed part of the sky) that we included in our off-nuclear cross-match procedure. Hence, the density of our HLX candidates is about 6 × 10 −4 per galaxy. If they are all accreting IMBHs, this is too large a density for a population of “naked” IMBHs that
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