Spectral variation in the supergiant fast X-ray transient SAX J1818.6-1703 observed by XMM-Newton and INTEGRAL

We present the results of a 30 ks XMM-Newton observation of the supergiant fast X-ray transient (SFXT) SAX J1818.6-1703 - the first in-depth soft X-ray study of this source around periastron. INTEGRAL observations shortly before and after the XMM-Newton observation show the source to be in an atypically active state. Over the course of the XMM-Newton observation, the source shows a dynamic range of ∼100 with a luminosity greater than 1 × 10³⁵ erg s^-1 for the majority of the observation. After an ∼6 ks period of low-luminosity (∼10³⁴ erg s^-1) emission, SAX J1818.6-1703 enters a phase of fast flaring activity, with flares ∼250 s long, separated by ∼2 ks. The source then enters a larger flare event of higher luminosity and ∼8 ks duration. Spectral analysis revealed evidence for a significant change in spectral shape during the observation with a photon index varying from Γ ∼ 2.5 during the initial low-luminosity emission phase, to Γ ∼ 1.9 through the fast flaring activity, and a significant change to Γ ∼ 0.3 during the main flare. The intrinsic absorbing column density throughout the observation (n_H ∼ 5 × 10²³ cm^-2) is among the highest measured from an SFXT, and together with the XMM-Newton and INTEGRAL luminosities, consistent with the neutron star encountering an unusually dense wind environment around periastron. Although other mechanisms cannot be ruled out, we note that the onset of the brighter flares occurs at 3 × 10³⁵erg s^-1, a luminosity consistent with the threshold for the switch from a radiative-dominated to Compton cooling regime in the quasi-spherical settling accretion model.