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PSR B0042-73 in the Small Magellanic Cloud


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 (tex2html_wrap_inline89451 per 1000 isolated pulsars; Lipunov et al., 1994b[124]). It seemed a priori that the binary radio pulsar PSR B0042-73  in the SMC (Kaspi et al., 1994[81]) 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)[11], 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.

  1. The total absence of surrounding plasma that screens radioemission at the periastron passage (Kaspi et al., 1994[81]). Taking the observed parameters of PSR J0045-7319, the condition of transparency of the fully ionized spherically-symmetric stellar wind  with constant velocity v against free-free absorption of the 430-MHz radioemission (i.e. optical depth tex2html_wrap_inline11732 close to periastron), requires tex2html_wrap_inline11734 , where tex2html_wrap_inline11736 yrtex2html_wrap_inline8853) is the rate of the wind, tex2html_wrap_inline11740 cm stex2html_wrap_inline8853 (see more detailed considerations of pulsar  with optical companions in Lipunov et al., 1994a[123]) A similar restriction on tex2html_wrap_inline9166 follows from the observed tex2html_wrap_inline11746 upper limit of variation in dispersion measure 3.2 pc cmtex2html_wrap_inline11748 (Kaspi et al., 1994[81]). Standard radiation-driven stellar wind for a B1 star with bolometric luminosity tex2html_wrap_inline8945 3 tex2html_wrap_inline8845 tex2html_wrap_inline11754  erg stex2html_wrap_inline8853 is of the order of tex2html_wrap_inline11758 - tex2html_wrap_inline11760 yrtex2html_wrap_inline8853 , which would completely absorb radioemission, unless one assumes an unusually weak stellar wind for the normal B1 star. In principle, weaker stellar mass loss  rates in the Magellanic Clouds (MC)   could be connected to the observed deficit of metals ( tex2html_wrap_inline11764 ), as absorption in their lines leads to radiation-driven stellar winds. However, analysis shows that tex2html_wrap_inline11766 (Kudritzki et al., 1987[89]); this reduces tex2html_wrap_inline9166 maximum two to three times, and the observed lack of absorption of the pulsar's radiation and dispersion measure constancy cannot be fully explained by the stellar wind chemical composition.
  2. 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 tex2html_wrap_inline11770 separation, would imply a high fraction tex2html_wrap_inline894510 percent of visible radiopulsars with massive normal stars (tex2html_wrap_inline894550 per 700 observed isolated pulsars). This also does not fit current observations.

  3. The observed radial velocity of the B-star is surprisingly close to the mean velocity of a field star in the SMC tex2html_wrap_inline9044 165 km stex2html_wrap_inline8853 , whereas high orbital eccentricity tex2html_wrap_inline11780 , that must have been acquired during the last supernova explosion  in this system, implies a high space velocity of the barycenter after the supernova explosion,
  4. displaymath11782

    where tex2html_wrap_inline11784 and tex2html_wrap_inline11786 are masses of the pulsar and its companion in solar units, respectively. With tex2html_wrap_inline11788 and tex2html_wrap_inline11790 we obtain tex2html_wrap_inline11792  km stex2html_wrap_inline8853 , much higher than peculiar velocity dispersion in the SMC.

  5. If we interpret the observed apse line motion tex2html_wrap_inline11796  yrtex2html_wrap_inline8853 as the sum of tidal quadrupole and relativistic periastron advance for a BV-star, even the tex2html_wrap_inline11746 upper limit yields the upper mass limit tex2html_wrap_inline11802 , too small for a reported 16th magnitude candidate (Kaspi et al., 1994[81]). Instead, if we assume pure relativistic periastron advance that high, we get a total mass of the binary PSR J0045-7319 tex2html_wrap_inline11804  yrtex2html_wrap_inline8853) tex2html_wrap_inline11808 . This does not contradict the existence of a massive BH in this system.
  6. Minimum companion mass of tex2html_wrap_inline9044 tex2html_wrap_inline11812 , as suggested by the companion's mass function tex2html_wrap_inline11814 (Kaspi et al., 1994[81]), corresponds to at least the B3 spectral class implying that we would observe it as a tex2html_wrap_inline11816 star, which is not the case.

The pulsar itself is a very efficient one, i.e. it has the highest ratio of radio luminosity (tex2html_wrap_inline8945tex2html_wrap_inline9563  erg stex2html_wrap_inline8853) to total rotational energy loss of neutron star (tex2html_wrap_inline8945 tex2html_wrap_inline11826  erg stex2html_wrap_inline8853) (Kaspi et al., 1994[81]). The number of such pulsars in the SMC can be estimated by scaling the galactic pulsar number (tex2html_wrap_inline8945tex2html_wrap_inline9939) by the ratio of the amount of SMC massive X-ray sources to that in our Galaxy, tex2html_wrap_inline89451/10. Further, this figure must be reduced by a factor tex2html_wrap_inline89451/100 (fraction of the efficient galactic isolated pulsars; Taylor et al. (1993)[190]) times tex2html_wrap_inline89451/20 (fraction of galactic pulsars with high radio luminosity; Taylor et al. (1993)[190]). 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 tex2html_wrap_inline89451/500. 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[181]).

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 tex2html_wrap_inline8945tex2html_wrap_inline9178 stars, which have not yet exploded. These massive binaries  with pulsars preferentially will hide BH formed from the most massive stars (> tex2html_wrap_inline11558 ). 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 tex2html_wrap_inline8905 binary systems was tracked. Initial binary distributions and evolutionary parameters were the same as in Lipunov et al. (1994b[124]), but the fraction of mass that collapses to form BH was tex2html_wrap_inline8907 . 

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 ( tex2html_wrap_inline11862 - tex2html_wrap_inline11864 , tex2html_wrap_inline10334 -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.

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Next: X-ray Luminosity Evolution Up: Black Holes in Binaries Previous: Cyg X-1 as a

Mike E. Prokhorov
Sat Feb 22 18:38:13 MSK 1997