IGR J17354-3255 as a candidate intermediate supergiant fast X-ray transient possibly associated with the transient MeV AGL J1734-3310

Abstract

We present spectral and temporal results from INTEGRAL long-term monitoring of the unidentified X-ray source IGR J17354-3255. We show that it is a weak persistent hard X-ray source spending a major fraction of the time in an out-of-outburst state with an average of 18-60 keV X-ray flux of ∼1.1 milliCrab, occasionally interspersed with fast X-ray flares (duration from a few hours to a few days) with a dynamic range as high as ∼200. From archival Swift/X-ray telescope observations, we also show that the dynamic range from non-detection to highest level of measured X-ray activity is >300. Our imager on board the lntegral satellite (IBIS) timing analysis strongly confirms the ∼8.4 d orbital period previously detected with Swift/burst alert telescope (BAT); in addition we show that the shape of the orbital profile is rather smooth and appears to be dominated by low-level X-ray emission rather than by bright outbursts; the measured degree of outburst recurrence is ∼25 per cent. The spectral and temporal characteristics of IGR J17354-3255 are highly indicative of a supergiant high-mass X-ray binary (SGXB) nature. However, our inferred dynamic ranges in both soft and hard X-rays are significantly greater than those of classical SGXB systems, but instead are typical of intermediate supergiant fast X-ray transients (SFXTs). Finally, we note for the first time that the observed fast flaring X-ray behaviour of IGR J17354-3255 is very similar to that detected with AGILE from the spatially associated MeV source AGL J1734-3310, suggesting a possible physical link between the two objects.