INTEGRAL and BeppoSAX Observations of the Transient Atoll Source 4U 1608–522: From Quiescent to Hard Spectral State

We report on the spectral evolution of 4U 1608–522 using observations performed as part of the long-term Galactic bulge monitoring program with INTEGRAL. The data set includes the 2005 April outburst. BeppoSAX archival data (two observations, from 1998 and 2000) are also analyzed and compared with the INTEGRAL data. Three different spectral states have been identified from the hardness-intensity diagram derived from INTEGRAL: the canonical hard and soft states, as well as an intermediate state. The hard-state spectrum is well described by a weak blackbody component plus a Comptonized plasma with high electron temperature (kTe ≃ 60 keV) extending up to 200 keV without any additional cutoff. The soft spectra are characterized by a cold Comptonized plasma (kTe = 2-3 keV; 7 keV for the intermediate state) and a strong disk blackbody component. A reflection component, indicating reflection of the X-rays from the accretion disk, is also present in the soft state seen by BeppoSAX in 1998. The 2000 BeppoSAX observation reveals the source to have been in its quiescent state, which can be modeled with a neutron star atmosphere (assuming a neutron star radius of 10 km and mass 1.4 M☉) with an effective temperature kTeff of 0.1 keV, plus a power-law component with Γ ~ 3, detected for the first time in this source. This spectrum can also be modeled with a simple blackbody compatible with emission originating from a small fraction of the neutron star's surface, of radius 0.4 km.