TEMPORAL PROPERTIES OF GX 301−2 OVER A YEAR-LONG OBSERVATION WITH SuperAGILE

Abstract

ABSTRACT We present the long-term monitoring of the high-mass X-ray binary GX 301−2 performed with the SuperAGILE (SA) instrument on-board the Astro-rivelatore Gamma ad Immagini LEggero (AGILE) mission. The source was monitored in the 20–60 keV energy band during the first year of the mission from 2007 July 17 to 2008 August 31, covering about one whole orbital period and three more pre-periastron (PP) passages for a total net observation time of about 3.7 Ms. The SA data set represents one of the most continuous and complete monitoring at hard X-ray energies of the 41.5 days long binary period available to date. The source behavior was characterized at all orbital phases in terms of hard X-ray flux, spectral hardness, spin-period history, pulsed fraction, and pulse shape profile. We also complemented the SA observations with the soft X-ray data of the Rossi X-Ray Timing Explorer/All-Sky Monitor. Our analysis shows a clear orbital modulation of the spectral hardness, with peaks in correspondence with the PP flare and near phase 0.25. The hardness peaks, we found, could be related with the wind-plus-stream accretion model proposed in order to explain the orbital light-curve modulation of GX 301−2. Timing analysis of the pulsar spin period shows that the secular trend of the ∼680 s pulse period is consistent with the previous observations, although there is evidence of a slight decrease in the spin-down rate. The analysis of the hard X-ray-pulsed emission also showed a variable pulse shape profile as a function of the orbital phase, with substructures detected near the passage at the periastron, and a clear modulation of the pulsed fraction, which appears in turn strongly anticorrelated with the source intensity.