Abstract
The origin of radiation from Anomalous X-ray Pulsars (AXPs) has remained mysterious since this group of objects was recognized. Using the BeppoSAX archival data for two Anomalous X-ray Pulsars, RX J170849-400910 and 4U 0142+61, we investigate the thermal flux fractions in the radiation from these two sources. Due to the stellar atmospheric effect and the temperature distribution over the stellar surface, using a single blackbody component to account for the surface thermal emission is too idealized. In this paper we adopt two blackbody components, to better approximate the surface thermal emission. Being careful not to over-sample the instrument spectral resolution, acceptable spectral fits are all found for a single-power-law model (PL), a power-law-plus-blackbody model (PL+BB), and a three-component one (one power law plus two blackbodies, PL+BB+BB). Two major differences appear in the fitting results for the different models: (i) The thermal flux fraction is only 17% (RX J170849-400910) and 19% (4U 0142+61) for the PL+BB model. This increases to 74% and 62%, respectively, for the PL+BB+BB model. (ii) The photon power index of the power law is 3.21±0.09 for RX J170849-400910 and 4.28±0.09 for 4U 0142+61 with the PL model, and 2.94±0.18 (RX J170849-400910) and 3.97±0.21 (4U 0142+61) with the PL+BB model. It becomes significantly harder for RX J170849-400910 (power index 1.76±0.16), but only slightly for 4U 0142+61 (power index 3.47±0.46) for the PL+BB+BB model. The degree of partitioning between the thermal and nonthermal fluxes certainly constrains a successful AXP model, and a harder power law helps in weakening the distinction between Anomalous X-ray Pulsars and Soft Gamma Repeaters (SGRs). We also performed a phase-resolved spectral analysis for RX J170849-400910. Our fitting results indicate that its energy-dependent pulse profiles may not be due only to power index variation, but also to the variation of thermal emission in different phases.