A Compton reflection dominated spectrum in a peculiar accreting neutron star

Abstract

We report on a puzzling event that occurred during a long BeppoSAX observation of the slowly rotating binary pulsar GX 1+4. During this event, lasting about 1 d, the source X-ray flux was over a factor 10 lower than normal. The low-energy pulsations disappeared while at higher energies they were shifted in phase by ∼0.25. The continuum spectrum taken outside this low-intensity event was well fitted by an absorbed cut-off power law and exhibited a broad iron line at ∼6.5 keV probably due to the blending of the neutral (6.4 keV) and ionized (6.7 keV) Ka iron lines. The spectrum during the event was Compton reflection dominated and it showed two narrow iron lines at ∼6.4 and ∼7.0 keV, the latter never revealed before in this source. We also present a possible model for this event in which a variation of the accretion rate thickens a torus-like accretion disc that hides for a while the direct neutron star emission from our line of sight. In this scenario, the Compton reflected emission observed during the event is well explained in terms of emission reflected by the side of the torus facing our line of sight. © 2005 RAS.