http://arxiv.org/abs/1810.13307
We report on the discovery of a dramatic change in the energy spectrum of the X-ray pulsar GX 304-1 appearing at low luminosity. Particularly, we found that the cutoff power-law spectrum typical for accreting pulsars, including GX 304-1 at higher luminosities of $L_{\rm X}\sim 10^{36} – 10^{37}$ erg s$^{-1}$, transformed at lower luminosity of $L_{\rm X}\sim 10^{34}$ erg s$^{-1}$ to a two-component spectrum peaking around 5 and 40 keV. We suggest that the observed transition corresponds to a change of the dominant mechanism responsible for the deceleration of the accretion flow. We argue that the accretion flow energy at low accretion rates is released in the atmosphere of the neutron star, and the low-energy component in the source spectrum corresponds to the thermal emission of the optically thick, heated atmospheric layers. We suggest that the high-energy component can be associated with thermally broadened cyclotron emission of the upper atmospheric layers overheated to $kT \sim 10 -100$ keV. Alternative scenarios are also discussed.
S. Tsygankov, A. Escorial, V. Suleimanov, et. al.
Thu, 1 Nov 18
69/76
Comments: 5 pages, 2 figures, submitted to MNRAS Letters