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3.1. Population studies

Populations of discrete X-ray sources in different galaxies and different galactic environments (bulge, disk, spiral arms, etc.) have different morphologies for their luminosity distributions (Fig. 3). It has been suggested that the slope and the break are indicative of the star-formation history (Wu 2001): an unbroken power-law indicates continuous star formation; a break (ie, a lack of bright sources) may be caused by aging of the X-ray source population, indicating the look-back time to the last major episode of star formation. This in turn provides information on the dynamical history of a galaxy, because star formation is often triggered by close encounters and mergers with other galaxies.

Figure 3a Figure 3b

Figure 3. Left: the cumulative luminosity distribution of the discrete sources in M81 shows a break at L approx 3 × 1037 erg s-1, suggesting that bulge sources belong to a much older population; continuous star formation in the disk results in an unbroken power-law instead (Tennant et al. 2001). Right: in M83, the luminosity function of the sources inside the inner 60" (nuclear starburst region + bar) does not show a break (lower curve). This can be interpreted as due to continuous star formation in the nuclear region. However, the luminosity function of the X-ray sources outside 60" (upper curve) has a break at Lx approx 1038 erg s-1, interpreted either as the upper limit of the NS population or as due to population aging (Soria & Wu 2002).

Color-color plots can separate different classes of X-ray sources, and distiguish XRBs in the soft and hard state (Fig. 4). XRBs are, in turn, a mixture of young (timescale of ~ 107 yr after star formation), wind-accreting, high-mass XRBs (generally seen through higher intrinsic absorption), and old (timescale of ~ 109 yr after star formation), Roche-lobe accreting low-mass XRBs. For Galactic sources, the color and spectral separation between the hard and soft state is generally larger in XRBs with a BH accretor than in those with a NS.

Figure 4a Figure 4b

Figure 4. Left: The model lines in the X-ray color-color diagram constrain the expected locations (for 4 × 1020 leq nH leq 5 × 1022 cm-2) of different classes of X-ray sources in M83: XRBs in a hard state (power-law spectrum with 1.3 ltapprox Gamma ltapprox1.7), XRBs in a soft state (diskbb with 0.5 ltapprox kT ltapprox 1.0 keV), SNRs (Raymond-Smith thermal spectrum with kT approx 0.5 keV), supersoft sources (blackbody spectrum with kT approx 0.06 keV). Here S = 0.3-1.0 keV, M = 1.0-2.0 keV, H = 2.0-8.0 keV. For the brightest sources, we can obtain individual spectral fits. Datapoints of sources whose X-ray spectra are consistent with hard-state XRBs have been plotted in blue; plotted in magenta: soft-state XRBs; in green: SNRs; in red: supersoft sources. Right: Same model lines, for a different choice of X-ray colors. Here Col1 ident (CH + CM) / 21/2, Col2 ident (CH - CM) / 21/2, where CH = (H - S) / (H + S), CM = (M - S) / (M + S).

Studies of luminosity and color variability offer another criterion to separate XRBs (which often show state transitions) from SNRs (which do not). The age of different classes of X-ray sources can be inferred from a study of their spatial correlation with various indicators of recent star formation, eg, the spiral arms defined by the HII regions (Fig. 5).

Figure 5a Figure 5b

Figure 5. Left: the diffuse X-ray emission (0.3-8.0 keV Chandra contours) is associated to the spiral arms and is an indicator of recent star formation, similar to the Halpha emission (greyscale image from the Anglo-Australian Telescope). Right: Most of the discrete X-ray sources (red circles) are associated with star-forming regions or young stellar populations (greyscale VLT B-band image, tracing OBA stars). The radio flux (12 cm contours from a VLA image) is a combination of free-free and synchrotron emission and is also an indicator of star formation. Size of both images: 6' × 6'. North is up, East is left.

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