A Comparison of Ultraluminous X-Ray Sources in NGC 1399 and the Antennae Galaxies (NGC 4038/4039)
Abstract
The temporal and spectral properties of ultraluminous X-ray sources (ULXs, LX>2×1039 ergs s-1) and bright X-ray sources (LX>3×1038 ergs s-1) 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 5×1039 ergs s-1. 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- December 2006
- DOI:
- 10.1086/508141
- arXiv:
- arXiv:astro-ph/0608024
- Bibcode:
- 2006ApJ...653..536F
- Keywords:
-
- Accretion;
- Accretion Disks;
- Black Hole Physics;
- galaxies: individual (NGC 1399 [the Antennae]);
- Galaxies: Individual: NGC Number: NGC 4038/4039;
- X-Rays: Binaries;
- X-Rays: Galaxies;
- Astrophysics
- E-Print:
- 10 pages, 9 figures, accepted for publication in ApJ