SIMBAD references

2004ApJ...608L..57D - Astrophys. J., 608, L57-L60 (2004/June-2)

A transition to a low/soft state in the ultraluminous compact X-ray source Holmberg II X-1.


Abstract (from CDS):

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 ∼2x1040 ergs/s 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.

Abstract Copyright:

Journal keyword(s): Accretion, Accretion Disks - Black Hole Physics - Stars: Individual: Alphanumeric: Holmberg II X-1 - X-Rays: Stars

CDS comments: Parag. 4 : NGC 5048 X-1 is a misprint for NGC 5408 X-1. Holmberg II X-1 is [KWB2002] 1 in Simbad. X-37 in NGC 4038-4039 (The Antennae) is [ZFR2002] 37 in Simbad.

Simbad objects: 8

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