Our previous work discussed the nature of the possible supernova remnant, CXOU J123029.5 +413927, indicating that the long-term light curve of this source did reveal a decay in the emission of this source. The previous observations showed that this object must have
Figure 4.11: Extended long-term light curve for possible supernova remnant, CXOU J123029.5+413927. Symbols are as per Figure 4.10.
brightened (detonated?) some time after 1991, which would indicate that we may be look- ing at a young SNR, but that this rate of decay was too shallow for such a source. This lead to the suggestion that an additional component could be diluting the emission of the SNR and affecting the observed decay rate. An alternate idea was proposed by Fridriksson et al. (2008), stating that we may after all simply be observing an X-ray binary system.
Upon examination of the extended long term light curve of this source (shown in Figure 4.11) we find a spike in the emission of this source during X2 & X3. Such a spike would be very unlikely if we were observing a pure and undiluted SNR. We find that during this spike the source shows preference towards a MCDBB-like spectrum (for observation X2)
if we use the lowest value of χ2/DoF, with the possibility of a power-law fit discounted
via the null hypothesis criteria. A MCDBB-like spectrum would indicate the presence of an accretion disc and therefore an X-ray binary system. We can therefore rule out the presence of an isolated young SNR. So the question remaining is whether we are observing emission that is purely from an X-ray binary system, or if the shallow decay witnessed in earlier in the long-term light curve of this source was the result of a young SNR in combination with an X-ray binary system.
4.6.6 Summary
Two recent observations made by the XMM-Newton telescope have provided us with the opportunity to revisit the pair of interacting galaxies, NGC 4485 & NGC 4490, and their large sample of ultraluminous X-ray sources, to test our previous conclusions and explore the nature of these systems in more detail. Whilst all sources continue to show no statis- tical signs of variability on short time-scales, the long term spectral and temporal trends observed previously in the majority of our sample appear to continue. We find that their spectra continue to exhibit a PL-like spectra at lower luminosities transiting to spectrum more easily described by a MCDBB at higher luminosities. The are two exceptions to this, the first is CXOU J123030.8+413911, which continues to display PL-like spectral
shape up to a new high of ∼ 3.2 × 1039 erg s−1. This could indicated that we are looking
at a slightly more massive BH than previously considered (but still possible as the end
point of one star i.e. MBH ∼< 100M⊙) . The other is CXOU J123030.6+414142, which
in observation X2 displays a PL-like spectrum at 2.6 × 1039 erg s−1. This result may
challenge our previous ideas or may be the result of hysteresis.
Considering some of the other sources we have observed, we find that the near-nuclear source in NGC 4490, CXOU J123936.3+413837, exhibits a large amount of variability over longer time-scales, indicating that we may be observing a range of spectral states from sub to super-Eddington accretion rates. We also find that the transient source, identified in the previous sections as CXOU J123038.3+413820, has dropped below detection limits once again during X2 & X3, indicating that our previous detection may have been during a short outburst.
Finally we note that the spectrum of the possible young SNR, identified initially in RWWM02, is best modelled by a MCDBB in X2. This spectral shape discounts the possibility that we are observing a pure SNR, although we cannot at present rule out that the emission of such a source is present in conjunction with that of an X-ray binary system.
These new data highlight the need for more observations of high signal-to-noise in order to properly characterise the spectrum of these sources and to gain a greater understanding of their spectral variability.
Chapter 5
The Ultraluminous
State
5.1
Introduction
Multi-wavelength approaches have started to open many doors within ULX research, but the true nature of these systems is still an enigma. Here we revisit the question of what the X-ray spectra of ULXs can tell us about the nature of their accretion flows, and how this constrains the nature of the accreting object. One method is to analyse the X-ray spectra of these sources, applying simple models of accretion systems, e.g. disc plus power- law. Results from such studies have revealed ∼ 0.2 keV disc temperatures with a hard
(Γ < 2) tail, implying black hole masses of ∼ 103 M
⊙ (e.g. Miller et al. 2003), which
supports the IMBH interpretation. However, analysis of high quality data demonstrates the presence for curvature of the tail at the highest energies, with a photon deficit above 5 keV (Roberts et al. 2005; Stobbart, Roberts & Wilms 2006, SRW06 hereafter within this chapter; Miyawaki et al. 2009). Such curvature is never seen in the low/hard state at these low energies. Only the very high state shows similar curvature, but these generally have spectra which are steep. Thus the accretion flows in ULXs may not simply be scaled up versions of those seen in black hole binaries, as would be the case for the IMBH model. Instead, observation of ULXs may allow us to probe a new regime of accretion physics, a new ‘ultraluminous state’ (Roberts 2007; Soria 2007).
In this chapter we choose to use the best data available in the XMM-Newton public archives, as previous studies have shown the limitations of more moderate signal-to-noise data in samples of ULXs (e.g. Berghea et al. 2008). By using only the highest quality data from the widest band pass, highest sensitivity instruments available we can hope to
avoid the ambiguity of previous analyses, and make definitive statements on the accretion processes in ULXs.