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

TL;DR

This study uses NuSTAR and AstroSat data to analyze the spectral and temporal properties of the neutron star binary GX 9+1, revealing its soft spectral state, disk parameters, magnetic field estimates, and variability features.

Contribution

It provides the first combined spectral and temporal analysis of GX 9+1 with detailed disk, magnetic, and variability measurements using multiple observatories.

Findings

GX 9+1 was in a soft spectral state during observations.

Estimated the neutron star's magnetic field strength as approximately 2.08×10^8 G.

Detected broad Lorentzian components indicating noise in the temporal data.

Abstract

We have studied the spectro-temporal properties of the neutron star low mass X-ray binary GX 9$+$1 using data from \textit{NuSTAR/FPM} and \textit{AstroSat/SXT} and \textit{LAXPC}. The hardness-intensity diagram of the source showed it to be in the soft spectral state during both observations. \textit{NuSTAR} spectral analysis yielded an inclination angle ($\theta$) $=$ 29$\substack{+3\\-4}^{\circ}$ and inner disk radius ($R_{in}$) $\leq$ 19.01 km. Assuming that the accretion disk was truncated at the Alfv\'en radius during the observation, the upper limit of the magnetic dipole moment ($\mu$) and the magnetic field strength ($B$) at the poles of the neutron star in GX 9$+$1 were calculated to be 1.45$\times$$10^{26}$ G cm$^3$ and 2.08$\times$$10^8$ G, respectively (for $k_A$ $=$ 1). Flux resolved spectral analysis with \textit{AstroSat} data showed the source to be in the soft spectral state ($F_{disk}$/$F_{total}$ $\sim$0.9) with a monotonic increase in mass accretion rate ($\dot{m}$) along the banana branch. The analysis also showed the presence of absorption edges at $\sim$1.9 and $\sim$2.4 keV, likely due to Si XIII and S XV, respectively. Temporal analysis with \textit{LAXPC-20} data in the 0.02 $-$ 100 Hz range revealed the presence of noise components, which could be characterized with broad Lorentzian components.