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Astron. Astrophys. 359, 573-585 (2000)

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5. The Be/X-ray binary population of the SMC

The spatial distribution of the SMC HMXBs including the new candidates from this work is shown in Fig. 1. Nearly all new candidates are located along the main body of the SMC where most of the optically identified Be/X-ray binaries are concentrated. Only one new candidate is found in the eastern wing near the supergiant HMXB SMC X-1, where already two other Be/X-ray pulsars are known. The distribution is not biased due to incomplete coverage, neither in the optical nor in X-rays and makes the strong concentration of Be/X-ray binaries in certain areas of the SMC more pronounced.

[FIGURE] Fig. 1. Distribution of HMXBs in the SMC. X-ray pulsars are marked with circle. The 25 new candidates from this work are indicated by the larger crosses

The X-ray luminosity distribution of Be/X-ray binaries and candidates in the SMC is compared to that of systems in the Galaxy in Fig. 2. To do this we intensively searched the literature on galactic Be/X-ray binary systems. We derived 31 galactic sources with [FORMULA] [FORMULA] erg s-1 which are summarized in Table 3 (which should be mostly complete). The luminosity estimates of galactic systems are often hampered by uncertain distances and different energy bands of the observing instrument. This may cause luminosity uncertainties by a factor of [FORMULA]10 in some cases but should not change the overall distribution drastically.

[FIGURE] Fig. 2. Distribution of observed maximum X-ray luminosity of Be/X-ray binaries in the SMC and the Galaxy. Including the candidates from this work, which form a population of low-luminosity systems, 47 Be/X-ray binaries are found in the SMC


Table 3. Be/X-ray binaries and likely candidates located in the Galaxy

The new candidates in the SMC mainly raise the number of Be/X-ray binaries with luminosities log([FORMULA] @ [FORMULA] 35.5 (21 out of 24 are new candidates). This can easily be explained by the high sensitivity of the ROSAT instruments which allowed to detect Be/X-ray binaries in their low-state while most of the higher luminosity Be/X-ray binaries were discovered during outburst. X-ray luminosities derived from detectors sensitive at higher energies (typically 0.5 - 10 keV) might be up to a factor [FORMULA]2 higher than those derived from ROSAT count rates (see Sect. 3) which would shift the low-luminosity end in Fig. 2 by 0.3 dex to the right. However, such a shift would not change the overall distribution significantly. Recently, in the Galaxy several likely low-luminosity Be/X-ray binaries were discovered by BeppoSAX and ASCA (1SAX J1324.4-6200, Angelini et al. 1998; AX J1820.5-1434, Kinugasa et al. 1998; AX J1749.2-2725, Torii et al. 1998; 1SAX J1452.8-5949, Oosterbroek et al. 1999; AX J1700062-4157, Torii et al. 1999). ROSAT also contributed new low-luminosity systems (RX J0440.9+4431, RX J0812.4-3114, RX J1037.5-5647, RX J0146.9+6121, Motch et al. 1997). However, the high absorption in the galactic plane makes the detection of low-luminosity X-ray sources in the ROSAT X-ray band and their optical identification difficult. This might explain the smaller number of low-luminosity Be/X-ray binaries discovered so far in the Galaxy compared to the SMC and might suggest that the luminosity distribution of Be/X-ray binaries is very similar in the SMC and our Galaxy. In this case many more such systems are expected to be found in our Galaxy which would significantly contribute to the hard X-ray galactic ridge emission (Warwick et al. 1985).

Various authors have suggested the existence of a population of low-luminosity systems which are usually persistent X-ray sources showing moderate outbursts and long pulse periods (e.g. Kinugasa et al. 1998; Mereghetti et al. 2000), somewhat different to the high-luminosity systems with strong outbursts and shorter pulse periods. The SMC results suggest that the low-luminosity sources even dominate the Be/X-ray binaries in number. From the fact that about one third of the already identified Be/X-ray binaries is not listed in current emission-line catalogues even more such systems are expected to be found in the ROSAT X-ray source catalogues of the SMC. On the other hand some of them will be observed with higher maximum luminosity in future outbursts, but if they indeed form a class of low-luminosity Be/X-ray binaries like X Per, the outbursts are expected to be small changing the luminosity distribution immaterial.

There is a large difference in the number of OB supergiant HMXBs between the Galaxy and the SMC. In the SMC at most two such systems are identified (SMC X-1 and maybe EXO 0114.6-7361) resulting in an overall ratio of Be to supergiant X-ray binaries of more than 20. In the Galaxy this proportion is more of order 2 (12 supergiant systems in the Galaxy are listed in the reviews of White et al. 1995 and Bildsten et al. 1997). It is remarkable that the SMC supergiant HMXBs are all located in the eastern wing giving rise to a local Be/supergiant HMXB ratio similar to that in the Galaxy. In contrast no supergiant HMXB is known in the SMC main body making the difference more extreme. One possible explanation is a different star formation history. Be/X-ray binaries evolve from binary star systems with typical total mass of [FORMULA]20 [FORMULA] within about 15 My (van den Heuvel 1983) while the more massive supergiant HMXBs are formed on shorter time scales. The latter therefore would trace more recent epochs of star formation than the Be/X-ray binaries. The comparatively large number of Be/X-ray binaries in the SMC in this view suggests a burst of star formation about 15 My ago while relatively few massive early-type stars were born during the last few million years.

It is remarkable that the Large Magellanic Cloud (LMC) may also have experienced a burst of star formation about 16 My ago as was derived from optical photometric surveys by Harris et al. (1999). LMC and SMC resemble in the relative composition of their X-ray binary populations, both rich in HMXBs but very few old low-mass X-ray binaries (Cowley et al. 1999), suggesting a common star formation history triggered by tidal interaction during close encounters of LMC, SMC and Milky Way. However, according to present day modeling the last encounter occurred [FORMULA]0.2 Gy ago (Gardiner & Noguchi 1996), too early for the formation of the Be/X-ray binaries we see today in X-rays. Therefore, the event which caused the origin of the frequent SMC (and LMC?) Be/X-ray binaries remains still unclear. Also the different numbers of HMXBs detected in LMC and SMC relative to their total mass need to be explained.

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Online publication: July 7, 2000