NuSTAR and AstroSat observations of GX 9+1: spectral and temporal studies

Abstract

ABSTRACT We have studied the spectro-temporal properties of the neutron star low-mass X-ray binary GX 9+1 using data from NuSTAR/Focal Plane Module and AstroSat/Soft X-ray Telescope and Large Area X-ray Proportional Counter (LAXPC). The hardness–intensity diagram of the source showed it to be in the soft spectral state (banana branch) during both observations. NuSTAR spectral analysis yielded an inclination angle (θ) = 29${^{+3}_{-4}}^{\circ }$ and inner disc radius (Rin) ≤ 19 km. Assuming that the accretion disc was truncated at the Alfvén radius during the observation, the upper limits of the magnetic dipole moment (μ) and the magnetic field strength (B) at the poles of the neutron star in GX 9+1 were calculated to be 1.5 × 10 26 G cm3 and 2.1 × 10 8 G, respectively (for kA = 1). Furthermore, the thickness of the boundary layer was found to be ≃7.5 km, which yielded the radius of the neutron star to be ≤11.5 km. Flux-resolved spectral analysis with AstroSat data showed the source to be disc dominated (Fdisc/Ftotal ∼ 0.9) with a monotonic increase in mass accretion rate ($\dot{m}$) along the banana branch. The analysis also showed the presence of absorption edges at ∼1.9 and ∼2.4 keV, likely due to Si xiii and S xv, respectively. Temporal analysis with LAXPC-20 data in the 0.02–100 Hz range revealed the presence of noise components, which could be characterized by broad Lorentzian components.