Detection of Strong Short-term Variability in NGC 6946 X-1
Abstract
Using two archival XMM-Newton observations, we identify strong X-ray flux variations in NGC 6946 X-1 indicating that it is the most variable ultraluminous X-ray source (ULX) on mHz timescales known so far. The 1-10 keV light curve exhibits variability with a fractional rms amplitude of 60% integrated in the frequency range of 1-100 mHz. The power spectral density of the source shows a flat-topped spectrum that breaks at about 3 mHz with possible quasi-periodic oscillations near 8.5 mHz. Black hole binaries usually produce strong fast variability in the hard or intermediate state. The energy spectrum of NGC 6946 X-1 is dominated by two components, a 0.18 keV thermal disk and a power law with a photon index of ≈ 2.2, which is consistent with the intermediate state. The characteristic timescales of the X-ray emission suggest that the ULX may contain a black hole with a mass on the order of 103 solar masses.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- October 2010
- DOI:
- 10.1088/0004-637X/722/1/620
- arXiv:
- arXiv:1008.2065
- Bibcode:
- 2010ApJ...722..620R
- Keywords:
-
- accretion;
- accretion disks;
- black hole physics;
- X-rays: binaries;
- X-rays: individual: NGC 6946 X-1;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 5 pages, 2 figures, 2 tables. ApJ to appear