A systematic study of variability in a sample of ultraluminous X-ray sources

Abstract

We present results from a study of short‐term variability in 19 archival observations by XMM–Newton of 16 ultraluminous X‐ray sources (ULXs). Eight observations (six sources) showed intrinsic variability with power spectra in the form of either a power‐law or broken power‐law‐like continuum and in some cases quasi‐periodic oscillations (QPOs). The remaining observations were used to place upper limits on the strength of possible variability hidden within. Seven observations (seven sources) yielded upper limits comparable to, or higher than, the values measured from those observations with detectable variations. These represented the seven faintest sources, all with fx < 3 × 10−12 erg cm−2 s−1. In contrast, there are four observations (three sources) that gave upper limits significantly lower than both the values measured from the ULX observations with detectable variations, and the values expected by comparison with luminous Galactic black hole X‐ray binaries (BHBs) and active galactic nuclei (AGN) in the observed frequency bandpass (10−3–1 Hz). This is the case irrespective of whether one assumes characteristic frequencies appropriate for a stellar mass (10 M⊙) or an intermediate mass (1000 M⊙) black hole, and means that in some ULXs the variability is significantly suppressed compared to bright BHBs and AGN. We discuss ways to account for this unusual suppression in terms of both observational and intrinsic effects and whether these solutions are supported by our results.

From ultraluminous X-ray sources to ultraluminous supersoft sources: NGC 55 ULX, the missing link

“…The resulting power density spectra are shown in Fig. A4 and confirm that the source has very limited variability in 2010 compared to 2001 (see also Heil et al 2009). We therefore focus on the 2001 observation only in the remainder of this Section.…”

“…The power density spectrum is very flat and the Poisson noise dominates above a mHz (see, e.g., Heil et al 2009). We selected three frequency intervals for which it is possible to measure any lag and covariance.…”

The ultraluminous X-ray source NGC 5643 ULX1: a large stellar mass black hole accreting at super-Eddington rates?

“…Indeed, a number of other ULXs showed similar degeneracies between spectral shape, luminosity and short-term variability when comparing low and highquality data, and it has been proposed that these ULXs may be sMBHs or massive stellar BHs accreting at super-Eddington rates (e.g. Stobbart et al 2006;Roberts 2007;Gladstone et al 2009;Heil et al 2009;Middleton et al 2011;Sutton et al 2013b;Pintore et al 2014;Middleton et al 2015). If the power-law-dominated spectra of NGC 5643 ULX1 are indeed simply due to the poor data quality and the intrinsic spectra are actually similar to the highest quality observation, then the marginally different levels of short-term variability could be explained by the effects of an outflow ejected by the accretion disc at radii where it is locally super-Eddington (Poutanen et al 2007;Ohsuga & Mineshige 2011;Takeuchi et al 2014).…”

“…Shakura & Sunyaev 1973;Ohsuga & Mineshige 2007;Poutanen et al 2007;Ohsuga et al 2009). Partially inhomogeneous winds in ULXs may explain the puzzling short-term variability reported in some ULXs (Heil, Vaughan & Roberts 2009), through a combination of inclination angle and accretion rate (e.g. Middleton, Sutton & Roberts 2011;Takeuchi, Ohsuga & Mineshige 2013;Middleton et al 2015).…”

A search for periodicities from a ULX in the LINER galaxy NGC 4736

“…It has a centroid frequency of 202.5 mHz with an rms of 6% in the (0.2−10) keV energy band. However, Heil et al (2009) reported that while the source Holmberg IX X-1 shows variability, it does not show a significant QPO feature. Strohmayer et al (2007) found a ULX in NGC 5408 (X-1) which shows a pair of QPOs with a 4:3 ratio.…”