SALT spectroscopy of the HMXB associated with the LMC supernova remnant MCSNR\,J0513−-6724

We report the results of optical \'echelle spectroscopy with the Southern African Large Telescope (SALT) of the mass donor star BSDL\,923 in the neutron star (NS) high-mass X-ray binary XMMU\,J051342.6$-$672412 associated with the LMC supernova remnant (SNR) MCSNR\,J0513$-$6724. We found that BSDL\,923 is a B0.7\,III star with double peaked emission lines originating in a circumbinary disk-like structure. This classification and the presence of double-peaked emission lines imply that BSDL\,923 is a Be star. Modelling with the stellar atmosphere code {\sc fastwind} was used to derive the effective temperature $T_{\rm eff}=27\pm1$\,kK, surface gravity $\log g=3.22\pm0.10$, projected rotational velocity v\sin i\approx100\pm45 \, \kms, colour excess $E(B-V)=0.53\pm0.05$\,mag, and luminosity \log(L_*/\lsun)=5.46\pm0.10 of BSDL\,923, as well as to show that the surface of this star is polluted with $\alpha$-elements (O, Mg and Si) from the supernova ejecta. We found also that the NS is orbiting BSDL\,923 in an eccentric ($e=0.158\pm0.061$) orbit with the orbital period of $1.280\pm0.006$\,d and the semi-major axis of 17\pm3 \, \rsun, and the radius of BSDL\,923 is 25\pm5 \,\rsun. We speculate that the NS is embedded in the atmosphere of BSDL\,923 either because it was kicked at birth towards this star or because of inflation of BSDL\,923 caused by the energy input from the supernova blast wave. Using long-slit spectroscopy with SALT, we searched for possible signs of the SNR shell in the 2D spectrum, but did not find them. This lack of detection is consistent with the young age ($\approx4^{+2} _{-1}$\,kyr) of MCSNR\,J0513$-$6724, implying that it is still in the adiabatic (non-radiative) phase.Comment: 13 pages, 11 figures and 6 tables. Accepted for publication by MNRA

On the Nature of the Apparent Ring Galaxy SDSS J075234.33+292049.8

(abridged) An object classified as a galaxy in on-line data bases and revealed on sky survey images as a distant ring galaxy is a rare case of polar ring galaxy where the ring is only slightly inclined to the equatorial plane of the central body. SDSS imaging indicates that the diameter of the ring is about 36 kpc. The SDSS data was combined with long-slit spectroscopic observations and with Fabry-Perot Interferometer H-beta mapping obtained at the Russian Academy of Sciences 6-m telescope. We derived the complex morphologies of this presumed ring galaxy from a combination of SDSS images and from the kinematical behaviour of the central body and of the ring, and determined the stellar population compositions of the two components from SDSS colours, spectroscopy, and evolutionary stellar synthesis models. The ring metallicity is slightly under-abundant. The total luminosity and the total mass of the system are not extreme, but the rather high M/L~20 indicates the presence of large amounts of dark matter. Two alternative explanations of this object are proposed (1) a ring formed by two semi-circular and tight spiral arms at the end of a central bar with a warp or precession of the ring material. The object could, therefore, be explained as an extreme SBa(R) galaxy, or (2) a Polar Ring Galaxy where the inner object is an S0 and the ring is significantly more luminous than the central object. The compound object would then be similar to the NGC 4650A galaxy, being a rare object with a polar component only modestly inclined to the equatorial plane of the central body. Arguments for (and against) both explanations are given and discussed, with the second alternative being more acceptable.Comment: 16 pages, 11 figures, MNRAS in pres