Idea rectora

The primary aim of this thesis is to consider the nature of the SFXT class of SgXRBs, the extent to which they can indeed be considered a single class and their place within the HMXB hierarchy. As the detection in large numbers of SFXTs and some of their key characteristics, such as orbital periods, outburst durations and X-ray dynamic range, were provided by INTEGRAL, Chapter 2 provides an overview of the observatory and the data analysis methods utilised for the hard X-ray characterisation of SFXTs using IGR J17544−2619 as an example. Chapters 3, 4 and 5 then undertake in-depth analysis of three individual SFXTs using a variety of observatories. Chapter 3 provides an analysis of IGR J16418−4532 using archival INTEGRAL/IBIS data along with recent co-ordinated, orbital phase constrained INTEGRAL and XMM-Newton observations. Chapter 4 describes a temporal and spectral study of IGR J17544−2619 using RXTE. Chapter 5 then describes predominantly INTEGRAL studies of XTE J1739−302 and IGR J17354−3255 to identify the orbital periods of the systems and discusses their implications on the nature of these binaries. Finally the results of all the studies presented are combined, along with other recent works, to consider the current understanding of SFXTs and conclude on their nature in Chapter 6. The future direction of this field of study is also considered, including the short term, approved observational programmes and longer term scientific goals.

Chapter 2

INTEGRAL data analysis

methods and techniques

The impact of the INTEGRAL observatory on the SFXT field cannot be understated, with the telescope providing not only the first detections of the recurrent, fast transient sources that revealed the SFXT class, but also characterising many aspects of the nature of these extreme X-ray sources. Of greatest importance perhaps is the determination of orbital periods in SFXTs allowing the outbursts detected in the hard X-ray band and the targeted follow-up by focusing soft X-ray telescopes to be placed in a dynamical context. Given the importance of such determinations in developing an understanding of the physical processes acting to generate the X-ray behaviour observed in SFXTs, this Chapter is devoted to detailing the analyses used to extract periodicities and outbursts from long baseline INTEGRAL datasets.

Initially the INTEGRAL mission is outlined with specific reference being made to the ‘Coded Aperture’ nature of the imaging utilised in the hard X-ray band and the challenges this presents for the detection of transient sources. The techniques employed to detect orbital periodicities in long baseline INTEGRAL/IBIS light curves and identify outbursts in the dataset are then discussed along with the statistical significance testing performed in each case. Examples of these techniques and the results they yield are presented using, predominantly, the case of the prototypical SFXT IGR J17544−2619.

2.1

The INTEGRAL observatory

The ’International Gamma-ray Astrophysics Laboratory’ INTEGRAL (Winkler et al., 2003) was the most sensitive hard X-ray/soft γ-ray observatory ever built at the time of its launch, being only recently surpassed by NuStar (Harrison et al., 2010).

Figure 2.1: Schematic diagram showing the scientific instruments aboard the INTEGRAL observatory. The three main science instruments, IBIS, JEM-X and SPI, are the highlighted sections in a diagonal line from the top left to bottom right of the schematic. All three instruments are co-aligned and the coded aperture mask of IBIS can be seen on the top left. Image Credit: ESA.

The telescope was launched on a Proton rocket into a 72 hour elliptical orbit which allows long, uninterrupted observations of celestial high energy sources. The

observatory utilises ‘Coded Aperture’ techniques to perform fine high energy imaging and spectroscopy over a range of 3 keV to 10 MeV across three separate co-aligned telescopes. The ‘Joint European X-ray Monitor’ JEM-X (Lund et al., 2003) is the soft X-ray telescope and provides arc minute imaging in the 3−35 keV band for a field of view (FOV) of diameter 13.2o (7.5o) at zero (half) response. The ‘Imager on Board the INTEGRAL satellite’ IBIS (Ubertini et al., 2003) provides fine hard X-ray imaging (PSF FWHM 12’), timing and coarse spectroscopy between 15 keV and 10 MeV across a FOV of 29.1o by 29.4o (at zero response) allowing the serendipitous detection of large numbers of hard X-ray sources in each observation and facilitating the accumulation of deep survey data. Finally the ‘Spectrometer on INTEGRAL’ SPI (Vedrenne et al., 2003) performs coarse imaging (PSF FWHM 2.5o) and fine γ-ray spectroscopy of point and extended sources in the 18 keV to 8 MeV energy range. Figure 2.1 shows a schematic of the INTEGRAL observatory in which the three main science instruments are highlighted.

Due to the Galactic Plane location, fast transient behaviour and relatively low fluxes of SFXTs the IBIS instrument has been the main source of SFXT discoveries.

The coarse imaging capabilities, small effective area and optimisation for spectral analysis of SPI prevents the unambiguous detection of fainter sources in the Galactic Plane with this instrument. The smaller FOV and lower effective area of JEM-X compared to IBIS (500 and 2600 cm2 respectively, although this is

somewhat offset by the soft X-ray spectra of SFXTs) results in a coarser sampling of the Galactic Plane at lower sensitivity with this instrument which restricts the rate of increase of exposure, and therefore the number of outbursts detected (see Section 2.4.3), compared to IBIS. Hence JEM-X does not contribute greatly to the knowledge of the global properties of SFXTs and from this point only methods for the analysis of IBIS data are discussed. It should be noted, however, that JEM-X has made some specific, significant contributions to the study of SFXTs such as the identification of IGR J18483−0311 as a 21 s pulsar (Sguera et al., 2007).

The coded aperture instruments aboard INTEGRAL facilitated the effective production of hard X-ray/soft γ-ray images at a time when γ-ray focusing optics were not technically feasible. Instead a ‘mask’ is placed above the detector plane with a characteristic pattern of opaque and transparent ‘cells’ such that a γ-ray source illuminating the mask casts a shadow of the pattern on to the detector plane. The mask used in IBIS is a 16 mm thick tungsten mask with ∼50%

transparency (i.e. half of the cells are tungsten and opaque to γ-ray photons up to energies of several MeV and the other half are transparent). Figure 2.2 illustrates the effect of two sources illuminating the IBIS coded mask and generating a superimposed pattern on the detector plane, the exact form of which is dependant upon the relative position and intensities of the two sources. To convert these ‘shadowgrams’ in to sky maps the detector plane image is ‘de-convolved’ by correlating the known mask pattern with the detector plane image at all positions. The resulting ‘correlation map’ outlines the relative positions and intensities of sources in the FOV during the observation. The correlation map can then be ‘back projected’ to form an image of the sky by combining the known position,

orientation and pointing of the satellite with a preliminary source list (nominally the INTEGRAL ‘General reference catalog’, Ebisawa et al. 2003) to model the detector plane image and reconstruct the source content of the IBIS FOV during the observation. The production of sky images is performed by the INTEGRAL ‘Offline Science Analysis (OSA)’ software described by Goldwurm et al. (2003), to which the reader is referred for a full and detailed discussion of this complex process of image reconstruction. Here the implications of this method of X-ray imaging on the subsequent data analysis are briefly outlined and the methods of overcoming them are discussed in detail in the following sections.

Figure 2.2: Illustration of a coded aperture telescope being illuminated by two sources. A superimposed mask pattern is accumulated on the detector plane, the exact form of which depends on the relative positions and intensities of the two sources. Image Credit: ISDC/M. T¨urler.