# The complex evolution of the X-ray binary transient MAXI J1807+132 along the decay of its discovery outburst

## TL;DR

This study monitors the X-ray transient MAXI J1807+132 over 125 days, revealing complex outburst decay, re-flares, and spectral features that suggest it could be a black hole or neutron star system, with evidence leaning towards a black hole.

## Contribution

First detailed optical and X-ray long-term monitoring of MAXI J1807+132, analyzing spectral and variability features to infer its nature and behavior during outburst decay.

## Key findings

- Re-flares recur approximately every 6.5 days.
- Spectral features are consistent with both black hole and neutron star accretors.
- Optical and X-ray data suggest a possible black hole system.

## Abstract

MAXI J1807+132 is an X-ray transient discovered during the decay of an outburst in 2017. We present optical and X-ray monitoring of the source over more than 125 days, from outburst to quiescence. The outburst decay is characterized by the presence of several re-flares with a quasi-periodic recurrence time of $\sim 6.5$ days. We detect broad H and He emission lines during outburst, characteristic of transient low mass X-ray binaries. These emission lines show strong variability from epoch to epoch and, in particular, during the early stages are found embedded into deep and very broad absorption features. The quiescent spectrum shows H$\alpha$ in emission and no obvious signatures of the donor star. XMM-Newton and Swift spectra can be fitted with standard X-ray models for accreting black-holes and neutron stars, although the obtained spectral parameters favour the latter scenario. Conversely, other observables such as the optical/X-ray flux ratio, the likely systemic velocity ($\gamma \sim -150$ km s$^{-1}$) and the re-flares recurrence time suggest a black hole nature. We discuss all the above possibilities with emphasis on the strong similarities of MAXI J1807+132 with short orbital period systems.