X-ray, optical, and infrared investigation of the candidate supergiant fast X-ray transient IGR J18462 - 0223

Abstract

We report on a broad-band X-ray study (0.5-60 keV) of the poorly known candidate supergiant fast X-ray transient (SFXT) IGR J18462 - 0223, and on optical and near-infrared (NIR) followup observations of field objects. The out-of-outburst X-ray state has been investigated for the first time with archival INTEGRAL/IBIS, ASCA, Chandra, and Swift/XRT observations. This allowed us to place stringent 3σ upper limits on the soft (0.5-10 keV) and hard (18-60 keV) X-ray emission of 2.9 × 10^-13 erg cm^-2 s^-1 and 8 × 10^-12 erg cm^-2 s^-1, respectively. The source was also detected during an intermediate soft X-ray state with flux equal to 1.6 × 10^-11 erg cm^-2 s^-1 (0.5-10 keV). In addition, we report on the INTEGRAL/IBIS discovery of three fast hard X-ray flares (18-60 keV) having a duration in the range 1-12 h and the flaring behavior was investigated in soft X-rays (3-10 keV) with archival INTEGRAL/JEM-X observations. The duty cycle (1.2%) and the dynamic ranges ( > 1380 and > 190 in the energy bands 0.5-10 keV and 18-60 keV, respectively) were measured for the first time. Archival UKIDSS JHK NIR data, together with our deep R-band imaging of the field, unveiled a single, very red object inside the intersection of the Swift/XRT and XMM-Newton error circles. This source has optical/NIR photometric properties compatible with a very heavily absorbed blue supergiant located at ~11 kpc, so is a strong candidate for a counterpart to IGR J18462 - 0223. NIR spectroscopy is advised to confirm the association. Finally, a hint of a possible orbital period was found at ~2.13 days. If confirmed by further studies, this would make IGR J18462 - 0223 the SFXT with the shortest orbital period among the currently known systems.