The very faint hard state of the persistent neutron star X-ray binary SLX 1737–282 near the Galactic Centre. Issue 3 (28th September 2017)

• Record Type: Journal Article
• Authors: Armas Padilla, M.; Ponti, G.; De Marco, B.; Muñoz-Darias, T.; Haberl, F.
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 473:Issue 3(2018)
• Page Start: 3789
• Page End: 3795
• Publication Date: 2017-09-28
• Subjects: accretion, accretion discs -- stars: individual: (SLX 1737–282) -- stars: neutron -- X-rays: binaries; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stx2538
• Languages: English
• ISSNs: 0035-8711

Abstract

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 ∼1° away from Sgr A*. The system is expected to have a short orbital period, even within the ultracompact 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 yr apart. We infer (0.5–10 keV) X-ray luminosities in the range of 3–6 × 10 35 ergs −1, in agreement with previous findings. The spectra are well described by a relatively cool ( kT bb = 0.5 keV) blackbody component plus a Comptonized emission component with Γ ∼ 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° unless the orbital period is longer than 11 h (i.e. the length of the XMM–Newton observation).