Bibcode
Mata-Sanchez, D.; Charles, P. A.; Armas Padilla, M.; Buckley, D. A. H.; Israel, G. L.; Linares, M.; Muñoz-Darias, T.
Bibliographical reference
Monthly Notices of the Royal Astronomical Society, Volume 468, Issue 1, p.564-569
Advertised on:
6
2017
Citations
9
Refereed citations
7
Description
The 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 LEdd. Assuming this peak luminosity would
place MAXI J1957+032 at a distance of d ∼ 5-6 kpc.
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Montserrat
Armas Padilla