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As we have seen, evolutionary considerations based on our current understanding of massive binary systems lead to the conclusion that BH+PSR binaries must constitute a rather numerous subclass of all binary PSR (1 per 1000 isolated pulsars; Lipunov et al., 1994b). It seemed a priori that the binary radio pulsar PSR B0042-73 in the SMC (Kaspi et al., 1994) could belong to one of these two classes. A 16th magnitude B1 star detected inside the radio error box of this pulsar was suggested as a possible optical companion to the pulsar. Reliable identification (or absence) of the optical companion would be detecting the orbital variations of the B-star radial velocity. This indeed was found by Bell et al. (1995), so that PSR B0042-73 is the first dual-line pulsar. However, some important questions about this pulsar do remain unanswered, and we think it is worth repeating some of the unusual properties of the optical companion to this pulsar.
In addition, the low stellar wind rates cannot be a general property of all B-stars in MC, because this would contradict the observational fact of high X-ray luminosity of massive wind-fed MC X-ray binary sources. We also note, that stellar wind from B-stars which is transparent for pulsar radioemission in a binary with separation, would imply a high fraction 10 percent of visible radiopulsars with massive normal stars (50 per 700 observed isolated pulsars). This also does not fit current observations.
where and are masses of the pulsar and its companion in solar units, respectively. With and we obtain km s , much higher than peculiar velocity dispersion in the SMC.
Minimum companion mass of , as suggested by the companion's mass function (Kaspi et al., 1994), corresponds to at least the B3 spectral class implying that we would observe it as a star, which is not the case.
The pulsar itself is a very efficient one, i.e. it has the highest ratio of radio luminosity ( erg s) to total rotational energy loss of neutron star ( erg s) (Kaspi et al., 1994). The number of such pulsars in the SMC can be estimated by scaling the galactic pulsar number () by the ratio of the amount of SMC massive X-ray sources to that in our Galaxy, 1/10. Further, this figure must be reduced by a factor 1/100 (fraction of the efficient galactic isolated pulsars; Taylor et al. (1993)) times 1/20 (fraction of galactic pulsars with high radio luminosity; Taylor et al. (1993)). Possible selection effects, such as pulsar beaming, leave only a few pulsars to be detected at the current sensitivity of 1 mJy.
The fact of the binarity of the first pulsar discovered in the SMC may seem to be very strange, because in our Galaxy the fraction of binary pulsars with massive companions is 1/500. Independently of the nature of the secondary companion, this fact can be explained only by a noticeable deficit of single stars and wide binaries at least among massive stars in the SMC. This independently follows from the observed lack of type II supernovae in the MC (Shklovskii, 1983).
The second independent factor that can augment the fraction of binary PSR with massive companions, is a burst-like behavior of star formation in the MC. Indeed, if one assumes a star formation burst to occur some million years ago, one can expect a substantially larger fraction of massive binary pulsars, because the bulk of single pulsars are from stars, which have not yet exploded. These massive binaries with pulsars preferentially will hide BH formed from the most massive stars (> ). To illustrate this idea, we modeled the evolution of different types of radiopulsars (single and entering binary systems) after a star formation burst using the Scenario Machine. The results are presented in Figure 35. The relative numbers of different types of radiopulsars depend very much on time after the burst, especially during the first millions years. So it becomes clear that in a galaxy with a non-stationary star formation one can expect quite different ratios of different types of radiopulsars.
Figure 35: Relative number of visible binary radiopulsars with
different secondary companions among single radiopulsars as a function
of time (in million years) after a star formation burst
(Lipunov et al., 1995c). The evolution of
binary systems was tracked. Initial binary distributions and evolutionary
parameters were the same as in Lipunov et al. (1994b),
but the fraction of mass that collapses to form BH was
Within the framework of different assumptions about parameters of the binary evolution scenario, we have estimated the relative number of binary radiopulsars with BH companions, not subjected to selection effects. For the probable parameters of BH formation ( - , -0.5) we obtained the expected number of BH+PSR binaries to be of order of unity per 700 radiopulsars (see Table 11). This means that such binary pulsars should be discovered in the near future. From the point of view of future observations of relativistic effects, all such systems must be highly eccentric (e>0.5 with 90 percent probability), with orbital periods lying in the range of 10 days to several years. We also estimated the number of BH+PSR binaries, formed through possible AIC of NS during the common envelope stage of binary evolution, to be an order of magnitude less than those formed directly from very massive stars.