Gx 3+1: The Stability of Spectral Index as a Function of Mass Accretion Rate

Abstract

We present an analysis of the spectral and timing properties observed in X-rays from neutron star binary GX 3+1 (4U 1744-26) during long-term transitions between the faint and bright phases superimposed on short-term transitions between lower banana (LB) and upper banana (UB) branches in terms of its color-color diagram. We analyze all observations of this source obtained with the Rossi X-ray Timing Explorer and BeppoSAX satellites. We find that the X-ray broad-band energy spectra during these spectral transitions can be adequately reproduced by a composition of a low-temperature blackbody component, a Comptonized component (COMPTB) and Gaussian component. We argue that the electron temperature kT e of the Compton cloud monotonically increases from 2.3 keV to 4.5 keV, when GX 3+1 makes a transition from UB to LB. We also detect an evolution of noise components (a very low frequency noise and a high frequency noise) during these LB -UB transitions. Using a disk seed photon normalization of COMPTB, which is proportional to the mass accretion rate, we find that the photon power-law index Γ is almost constant (Γ = 2.00 ± 0.02) when mass accretion rate changes by factor 4. In addition, we find that the emergent spectrum is dominated by the strong Comptonized component. We interpret this quasi-stability of the index Γ and a particular form of the spectrum in the framework of a model in which the energy release in the transition layer located between the accretion disk and neutron star surface dominates that in the disk. Moreover, this index stability effect now established for

Study of the reflection spectrum of the accreting neutron star GX 3+1 using XMM–Newton and INTEGRAL

“…Previous works (e.g. Seifina & Titarchuk 2012) have shown that the high energy component of GX 3+1 is weakly variable.…”

“…This component is dominant at energies between 1.5-30 keV, corresponding to about 70% of the total flux emission, and the stability of this high energy component has been widely investigated (e.g. Seifina & Titarchuk 2012). This makes feasible our choice to stack all the available INTEGRAL observations.…”

“…Mainardi et al 2010;Seifina & Titarchuk 2012;Piraino et al 2012). We find a neutral column density (NH) of ∼ 1.6 × 10 22 cm −2 , using the abundances of Anders & Grevesse (1989).…”

“…van den Berg et al (2014) identified a counterpart of GX 3+1 in the near-infrared (NIR) with a Ks=15.8 +/-0.1 mag suggesting that the donor star is not a late-type giant and that the NIR emission is dominated by the heated outer accretion disc. Spectral analyses of the source showed that its X-ray spectrum can be described by a model comprising of a soft blackbody component, most likely associated with the accretion disc, and a comptonizing component, produced by an optically thick electron population located close to the NS boundary layer (Mainardi et al 2010;Piraino et al 2012;Seifina & Titarchuk 2012). Seifina & Titarchuk (2012), using all the Rossi X-ray Timing Explorer (RXTE) and BeppoSAX data, showed that the comptonizing component is quite stable suggesting that the energy release in the transition layer between the accretion disc and the NS dominates over the energy release in the accretion disc.…”

“…Spectral analyses of the source showed that its X-ray spectrum can be described by a model comprising of a soft blackbody component, most likely associated with the accretion disc, and a comptonizing component, produced by an optically thick electron population located close to the NS boundary layer (Mainardi et al 2010;Piraino et al 2012;Seifina & Titarchuk 2012). Seifina & Titarchuk (2012), using all the Rossi X-ray Timing Explorer (RXTE) and BeppoSAX data, showed that the comptonizing component is quite stable suggesting that the energy release in the transition layer between the accretion disc and the NS dominates over the energy release in the accretion disc. A number of broad emission lines were also reported by Piraino et al (2012) and associated with the Kα transitions of Ar, Ca and Fe.…”