Mata Sánchez, D.D.Mata SánchezCharles, P. A.P. A.CharlesArmas Padilla, M.M.Armas PadillaBuckley, D. A. H.D. A. H.BuckleyISRAEL, Gian LucaGian LucaISRAELLinares, M.M.LinaresMuñoz-Darias, T.T.Muñoz-Darias2020-12-092020-12-0920170035-8711http://hdl.handle.net/20.500.12386/28739The new recurrent X-ray transient MAXI J1957+032 has had four X-ray outbursts within 16 months, all very briefly detected (lasting <5 d). During the most recent event (2016 September/October), we obtained with the Southern African Large Telescope the first optical spectrum of the transient counterpart, showing the classic blue continuum of an X-ray irradiated disc in an LMXB and no other features. At high Galactic latitude below the plane (-13°) reddening is low but there is no quiescent counterpart visible on any of the existing sky surveys, nor any other known X-ray source in the region. Swift monitoring of three of the four events is presented, showing rapidly fading X-ray outbursts together with significant UVOT detections in the UV (W1,M2,W2), U and B bands. The optical properties are most like those of the short-period LMXBs, which, combined with the softening witnessed during the decay to quiescence would place the system at d < 13 kpc. The short duration and short recurrence time of the outbursts are reminiscent of the accreting millisecond X-ray pulsars, which exhibit peak luminosities of ∼ 1 per cent L<SUB>Edd</SUB>. Assuming this peak luminosity would place MAXI J1957+032 at a distance of d ∼ 5-6 kpc.STAMPAenArticle10.1093/mnras/stx4792-s2.0-85017226359000398419200041https://academic.oup.com/mnras/article/468/1/564/30516812017MNRAS.468..564MFIS/05 - ASTRONOMIA E ASTROFISICAERC sectors::Physical Sciences and Engineering::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation::PE9_6 Stars and stellar systems