Accretion states of atoll sources

Using more than a decade of Rossi X-ray Timing Explorer data, we performed a systematic study of the luminosity, timing and spectral properties of nine atoll sources across different source states.  We show that soft states can have luminosities as low as 1% of the Eddington luminosity, while the luminosity of hard states can be as high as 15% Eddington (Fig. 1). By comparing the spectral and timing properties in each source we find an anticorrelation between the hardness of the X-ray spectrum and the variability frequencies (Fig. 2). Furthermore, we show that for constant variability frequency, luminosity and hardness are positively correlated across different sources (Fig. 3). This luminosity-hardness correlation is clearly at odds with the common perception that harder sources are less luminous, and holds for luminosities between 0.5% and 30% Eddington.

From Linares & van der Klis (2009, in prep.):

 
Fig. 1: ``Color-color-color'' diagrams of the nine atoll sources studied. The color scale shows the 2-200 keV luminosity, in Eddington units and logarithmic scale. The luminosity ranges from 2.5 x 10³⁵ erg s^-1 (0.1% L_Edd) to 1.3 x 10³⁸ erg s^-1 (50% L_Edd). In a given source hard states are on average less luminous, but hard color does not determine luminosity , and hard states in some sources (e.g. 1820, 1705) can be more luminous than soft states in other sources (e.g. 0614, 1608).

Fig. 2: Top right: Break frequency versus spectral hardness for the eight sources where flat-topped noise is present.
Bottom right: upper kHz QPO frequency versus spectral hardness for the seven sources that show upper kHz QPOs. The frequency-hardness anticorrelation is visible in both cases, and ``saturates'' towards soft and hard states.
Top left: Break frequency versus luminosity for the eight sources where flat-topped noise is present.
Bottom left: upper kHz QPO frequency versus luminosity for the seven sources that show upper kHz QPOs (see also Ford et al. 2000). Note that the same range in break frequency (more than two orders of magnitude) is observed at very different luminosities, both in a single source (spanning ~1 order of magnitude in 1608, Aql X-1 and 1705) and across sources (spanning ~2 orders of magnitude).

Fig. 3: Luminosity-hardness correlation across sources for a given timing state. Left: Observations with upper kHz QPO frequency near 1000 Hz and 500 Hz. Right: Observations with break frequency near 30 Hz and 1 Hz.