Discovery of Be/X-ray stars in two supernova remnants in the Small Magellanic Cloud

We present ROSAT high resolution x-ray images of two previously catalogued supernova remnants in the Small Magellanic Cloud (SMC): 0101-72.4 and 0104-72.3. These remnants were known to show optical, x-ray, and radio emission based on earlier observations: the present data show the first evidence for arcsecond-scale x-ray structure. There is no diffuse x-ray emission associated with the optically emitting shell in 0101-72.4; we set a 3σ upper limit of 7 × 10³⁴ erg s^-1 on the 0.2-2 keV luminosity from the region. The x-ray emission comes instead from a weak pointlike object near the limb of the remnant. Optical observations of this source reveal a m_v=14.8 blue star with Hα and Hβ in emission; we identify this as a Be star in the SMC. No evidence for variability down to time scales of about 1 s was found in the ROSAT data; however, a comparison of the ROSAT and Einstein fluxes indicates possible long term variability by a factor of ∼2 over several years. The other SNR, 0104-72.3, also contains a pointlike x-ray source with a blue optical counterpart (m_v=16.7) and Ha emission. We tentatively identify this as a Be star as well. In addition to the point source there is weak diffuse x-ray emission from 0104-72.3 (L_X∼1.4 × 10³⁵ erg s^-1), but the remnant's appearance in the x-ray band is considerably different from that in either the radio or optical band. We argue for a physical association between the SNRs and Be/x-ray stars. A large space velocity (≳ 100 km s^-1) for the Be/x-ray binaries is required if the explosions that produced the remnants also formed the neutron stars in the binaries. Alternatively, the associations could be the result of common membership in OB associations in the SMC.