Soft X-ray transients are systems that are detected when they go into an outburst, wherein their X-ray luminosity increases by several orders of magnitude. These outbursts are markers of the poorly understood change in the spectral state of these systems from the low/hard state to the high/soft state. We report the spectral properties of one such soft X-ray transient: MAXI J0637−430, with data from the SXT and LAXPC instruments on board the AstroSat mission. The source was observed for a total of ∼60 ks in two observations on 2019 November 8 and 21 soon after its discovery. Flux-resolved spectral analysis of the source indicates the presence of a multicolor blackbody component arising from the accretion disk and a thermal Comptonization component. The stable low temperature (∼0.55 keV) of the blackbody component points to a cool accretion disk with an inner disk radius of the order of a few hundred kilometers. In addition, we report the presence of a relativistically broadened Gaussian line at 6.4 keV. The disk-dominated flux and photon power-law index of ⪆2 and a constant inner disk radius indicate the source to be in the soft state. From the study we conclude that MAXI J0637−430 is a strong candidate for a black hole X-ray binary.
GX 3+1, an atoll type neutron star low-mass X-ray binary, was observed four times by Soft X-ray Telescope and The Large Area X-ray Proportional Counters on-board AstroSat between October 5, 2017 and August 9, 2018. The hardness-intensity-diagram of the source showed it to be in the soft spectral state during all the four observations. The spectra of the source could be adequately fit with a model consisting of blackbody ($\mathtt {bbody}$) and power-law ($\mathtt {powerlaw}$) components. This yielded the blackbody radius and mass accretion rate to be ∼8 km and ∼2 × 10−9 M⊙ y−1, respectively. In one of the observations, a Type I X-ray burst having a rise and e-folding time of 0.6 and 5.6 s, respectively, was detected. Time-resolved spectral analysis of the burst showed that the source underwent a photospheric radius expansion. The radius of the emitting blackbody in GX 3+1 and its distance were estimated to be 9.19 $\substack{+0.97\\-0.82}$ km and 10.17 $\substack{+0.07\\-0.18}$ kpc, respectively. Temporal analysis of the burst yielded upper limits of the fractional RMS amplitude of 7%, 5% and 6% during burst start, burst maximum and right after the radius expansion phase, respectively.
We have studied the spectro-temporal properties of the neutron star low mass X-ray binary GX 9+1 using data from NuSTAR/FPM and AstroSat/SXT and LAXPC. The hardness-intensity diagram of the source showed it to be in the soft spectral state during both observations. NuSTAR spectral analysis yielded an inclination angle (θ) = 29${^{+3}_{-4}}^{\circ }$ and inner disk radius (Rin) ≤ 19.01 km. Assuming that the accretion disk was truncated at the Alfvén radius during the observation, the upper limit 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 1.45×1026 G cm3 and 2.08×108 G, respectively (for kA = 1). Flux resolved spectral analysis with AstroSat data showed the source to be in the soft spectral state (Fdisk/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 with broad Lorentzian components.
GX 3+1, an atoll type neutron star low-mass X-ray binary, was observed four times by Soft X-ray Telescope and The Large Area X-ray Proportional Counters on-board AstroSat between October 5, 2017 and August 9, 2018. The hardness-intensity-diagram of the source showed it to be in the soft spectral state during all the four observations. The spectra of the source could be adequately fit with a model consisting of blackbody (bbody) and power-law (powerlaw) components. This yielded the blackbody radius and mass accretion rate to be ∼8 km and ∼2 × 10 −9 M y −1 , respectively. In one of the observations, a Type I X-ray burst having a rise and e-folding time of 0.6 and 5.6 s, respectively, was detected. Time-resolved spectral analysis of the burst showed that the source underwent a photospheric radius expansion. The radius of the emitting blackbody in GX 3+1 and its distance were estimated to be 9.19 +0.97 −0.82 km and 10.17 +0.07−0.18 kpc, respectively. Temporal analysis of the burst yielded upper limits of the fractional RMS amplitude of 7%, 5% and 6% during burst start, burst maximum and right after the radius expansion phase, respectively.
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