A Transition to a Low/Soft State in the Ultraluminous Compact X-Ray Source Holmberg II X-1
Abstract
We present three XMM-Newton observations of the ultraluminous compact X-ray source Holmberg II X-1 in its historical brightest and faintest states. The source was in its brightest state in 2002 April with an isotropic X-ray luminosity of ~2×1040 ergs s-1 but changed to a peculiar low/soft state in 2002 September in which the X-ray flux dropped by a factor of ~4 and the spectrum softened. In all cases, a soft excess component, which can be described by a simple or multicolor disk blackbody (kT~120-170 eV), is statistically required in addition to a power-law continuum (Γ~2.4-2.9). Both spectral components became weaker and softer in the low/soft state; however, the dramatic variability is seen in the power-law component. This spectral transition is opposite to the ``canonical'' high/soft-low/hard transitions seen in many Galactic black hole binaries. There is a possible contribution from an optically thin thermal plasma. When this component is taken into account, the spectral transition appears to be normal-a drop of the power-law flux and a slightly softer blackbody component in the low state.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- June 2004
- DOI:
- 10.1086/422338
- arXiv:
- arXiv:astro-ph/0401223
- Bibcode:
- 2004ApJ...608L..57D
- Keywords:
-
- Accretion;
- Accretion Disks;
- Black Hole Physics;
- Stars: Individual: Alphanumeric: Holmberg II X-1;
- X-Rays: Stars;
- Astrophysics
- E-Print:
- Revised and shortened version, To appear in ApJ Letters