# Swift and SALT observations of the Multiple Outbursts of MAXI J1957+032

## TL;DR

This study reports multi-wavelength observations of MAXI J1957+032, a recurrent X-ray transient with brief outbursts, providing insights into its optical spectrum, outburst behavior, and potential classification as a short-period low-mass X-ray binary.

## Contribution

First optical spectrum of MAXI J1957+032's transient counterpart and detailed multi-wavelength analysis of its outbursts and properties.

## Key findings

- Optical spectrum shows blue continuum typical of X-ray irradiated disc in an LMXB.
- Outbursts are short, recurrent, and resemble those of AMXPs.
- Estimated distance of the system is 5-13 kpc based on luminosity and spectral analysis.

## 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 days). During the most recent event (Sep/Oct 2016), we obtained with SALT 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 deg) 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 3 of the 4 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 AMXPs, which exhibit peak luminosities of 1 % L_Edd. Assuming this peak luminosity would place MAXI J1957+032 at a distance of d= 5-6 kpc.