# The very faint hard state of the persistent neutron star X-ray binary SLX 1737-282 near the Galactic centre

## TL;DR

This study characterizes the spectral and timing properties of the neutron star binary SLX 1737-282, revealing a faint hard state with unusual low electron temperature and constraining its orbital inclination.

## Contribution

First detailed spectral and timing analysis of SLX 1737-282 in a faint hard state, highlighting its low electron temperature and constraining system inclination.

## Key findings

- X-ray luminosity in the range 3-6 x10^35 erg/s.
- Spectral fit with a cool black body and Comptonized component.
- Electron temperature varies from >7 keV to ~2 keV between observations.

## Abstract

We report on a detailed study of the spectral and temporal properties of the neutron star low mass X-ray binary SLX 1737-282, which is located only ~1degr away from Sgr A. The system is expected to have a short orbital period, even within the ultra-compact regime, given its persistent nature at low X-ray luminosities and the long duration thermonuclear burst that it has displayed. We have analysed a Suzaku (18 ks) observation and an XMM-Newton (39 ks) observation taken 7 years apart. We infer (0.5-10 keV) X-ray luminosities in the range 3-6 x10^35erg s-1, in agreement with previous findings. The spectra are well described by a relatively cool (kTbb = 0.5 keV) black body component plus a Comptonized emission component with {\Gamma} ~1.5-1.7. These values are consistent with the source being in a faint hard state, as confirmed by the ~ 20 per cent fractional root mean square amplitude of the fast variability (0.1 - 7 Hz) inferred from the XMM-Newton data. The electron temperature of the corona is >7 keV for the Suzaku observation, but it is measured to be as low as ~2 keV in the XMM-Newton data at higher flux. The latter is significantly lower than expected for systems in the hard state. We searched for X-ray pulsations and imposed an upper limit to their semi-amplitude of 2 per cent (0.001 - 7 Hz). Finally, we investigated the origin of the low frequency variability emission present in the XMM-Newton data and ruled out an absorption dip origin. This constraint the orbital inclination of the system to 65 degr unless the orbital period is longer than 11 hr (i.e. the length of the XMM-Newton observation).