INTEGRAL and XMM-Newton observations of AX J1845.0-0433

Abstract

AX J1845.0-0433 is a transient high-mass X-ray binary discovered by ASCA. The source displays bright and short flares observed recently with INTEGRAL. The transient behaviour and the bright and short flares are studied in order to understand the accretion mechanisms and the nature of the source. Public INTEGRAL data and a pointed XMM-Newton observation are used to study in details the flaring and quiescent phases. AX J1845.0-0433 is a persistent X-ray binary with a O9.5I supergiant companion emitting at a low 0.2-100 keV luminosity of ~1e35 erg/s with seldom flares reaching luminosities of 1e36 erg/s. The most-accurate X-ray position is R.A. (2000) =18h45m01s.4 and Dec. = -04deg33'57''.7 (uncertainty 2''). Variability factors of 50 are observed on time scale as short as hundreds of seconds. The broad-band high-energy spectrum is typical of wind-fed accreting pulsars with an intrinsic absorption of NH=(2.6+/-0.2)e22 cm-2, a hard continuum of Gamma=(0.7-0.9)+/-0.1 and a high-energy cutoff at Ecut=16+/-3 keV. An excess at low energies is also observed fitted with a black body with a temperature of kT=0.18+/-0.05 keV. Optically-thin and highly-ionised iron (Fe XVIII-XIX) located near the supergiant star is detected during the quiescence phase. The spectral shape of the X-ray continuum is constant. The flare characteristics in contrast to the persistent quiescent emission suggest that clumps of mass M~1e22 g are formed within the stellar wind of the supergiant companion.