On the apparent lack of Be X-ray binaries with black holes in the galaxy and in the Magellanic Clouds

Abstract

Abstract. In the Galaxy there are 67 Be X-ray binaries known to-date. Out of those, 45 host a neutron star, and for the reminder the nature of a companion is not known. None, so far, is known to host a black hole. This disparity is referred to as a missing Be -black hole X-ray binary problem. The stellar population synthesis calculations following the formation of Be X-ray binaries (Belczyński & Zió lkowski 2009) predict that the ratio of the binaries with neutron stars to the ones with black holes is rather high F N S/ B H ∼ 30 − 50. A comparison of this ratio with the number of confirmed Be -neutron star X-ray binaries (45) indicates that the expected number of Be -black hole X-ray binaries is of the order of only ∼ 0 − 2. This is entirely consistent with the observed Galactic sample. Therefore, there is no problem of the missing Be+BH X-Ray Binaries for the Galaxy In the Magellanic Clouds there are 94 Be X-ray binaries known to-date. Out of those, 60 host a neutron star. Again, none hosts a black hole. The stellar population synthesis calculations carried out specifically for the Magellanic Clouds (Zió lkowski & Belczyński 2010) predict that the ratio of the Be X-ray binaries with neutron stars to the ones with black holes is only F N S/ B H ∼ 10. This value is rather too low, as it implies the expected number of Be+BH X-ray binaries of the order of ∼ 6, while none is observed. We found, that to remove the discrepancy, one has to take into account a different history of the star formation rate in the Magellanic Clouds, with the respect to the Galaxy. New stellar population synthesis calculations are currently being carried out.

A Be-type star with a black-hole companion

“…3). There are currently ~81 BeXBs known in the Galaxy with ~48 pulsating neutron stars 4,20 , and thus our discovery of a black-hole companion to MWC 656 is consistent with these model predictions. However, it should be noted that the X-ray spectra of the remaining BeXBs, whenever they are available, also indicate the presence of a neutron star.…”

“…The large dynamical mass rules out a white dwarf or a neutron star, so the only viable alternative is a black hole. It should be noted that none of the ~170 BeXBs curently known 4 shows any evidence for an accretion disk, providing circumstantial evidence for a difference in the nature of the compact stars. The accretion disk in MWC 656 is expected to also radiate Balmer and He I lines but these are blended with the corresponding (stronger) emission lines from the Be disk and thus are not detected.…”