2006ApJ...653..536F

The temporal and spectral properties of ultraluminous X-ray sources (ULXs, L_X>2x10³⁹ ergs/s) and bright X-ray sources (L_X>3x10³⁸ ergs/s) are examined and compared in two extremely different host environments: the old elliptical galaxy NGC 1399 and the young, star-forming Antennae galaxies (NGC 4038/4039). ULXs in NGC 1399 show little variability on either long or short timescales. Only 1 of 8 ULXs and 10 of 63 bright sources in NGC 1399 are variable at a confidence level of 90%. On long timescales, the NGC 1399 sources are steadier than most Galactic black hole X-ray binaries, but are similar to GRS 1915+105. The outburst duration of the NGC 1399 sources is about 20 yr, again, similar to that of GRS 1915+105. The bright X-ray sources in NGC 1399 may be black hole X-ray binaries with giant-star companions similar to GRS 1915+105. The brightest ULX (PSX-1) in NGC 1399 is coincident with a globular cluster, shows a hard spectrum with a photon index around 1.5, and has a nearly constant luminosity around 5x10³⁹ ergs/s. It may be an intermediate-mass black hole (IMBH) in a hard spectral state. In contrast to NGC 1399, the ULXs in the Antennae are all variable, and a large fraction of the bright sources (9 of 15) are also variable. The variability and luminosity of ULXs in the Antennae suggest that they are black hole high-mass X-ray binaries accreting via Roche lobe overflow. A flare with a duration of about 5 ks is found from Antennae X-42. The most luminous ULX, X-16, with a very hard spectrum (Γ=1.0-1.3) and a luminosity that varies by a factor of 10, could be an IMBH candidate.