Swift and SALT observations of the multiple outbursts of MAXI J1957+032

Journal
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
Date Issued
2017
Author(s)
Mata Sánchez, D.
•
Charles, P. A.
•
Armas Padilla, M.
•
Buckley, D. A. H.
•
•
Linares, M.
•
Muñoz-Darias, T.
DOI
10.1093/mnras/stx479
Abstract
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.
Volume
468
Issue
1
Start page
564
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