GX3+1: the stability of spectral index as a function of mass accretion rateстатья
Статья опубликована в высокорейтинговом журнале
Информация о цитировании статьи получена из
Web of Science,
Scopus
Статья опубликована в журнале из списка Web of Science и/или Scopus
Дата последнего поиска статьи во внешних источниках: 18 июля 2013 г.
Аннотация:We present an analysis of the spectral and timing properties observed in X-rays from neutron star (NS) binary GX3+1 (4U1744-26) 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 broadband energy spectra during these spectral transitions can be adequately reproduced by a composition of a low-temperature blackbody component, a Comptonized component (COMPTB), and a Gaussian component. We argue that the electron temperature kTe of the Compton cloud monotonically increases from 2.3 keV to 4.5 keV, when GX3+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 a factor
of four. 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 NS surface dominates that in the disk. Moreover, this index stability effect now established for GX3+1 was previously found in the atoll source 4U1728-34 and suggested for a number of other low-mass X-ray NS binaries (see Farinelli & Titarchuk). This intrinsic behavior of NSs, in particular for atoll sources, is fundamentally different from that seen in black hole binary sources where the index monotonically increases during spectral transition from the low state to the high state and then finally saturates at high values of mass accretion rate.