X-ray and near-infrared observations of the obscured accreting pulsar IGR J18179-1621

IGR J18179-1621 is an obscured accreting X-ray pulsar discovered by INTEGRAL on 2012 February 29. We report on our 20ks Chandra-High Energy Transmission Gratings Spectrometer observation of the source performed on 2012 March 17, on two short contemporaneous Swift observations, and on our two near-infrared (K_s , H_n , and J_n ) observations performed on 2012 March 13 and 26. We determine the most accurate X-ray position of IGR J18179-1621, α_J2000 = 18^h17^m5218, δ_J2000 = -16°21′3168 (90% uncertainty of 06). A strong periodic variability at 11.82 s is clearly detected in the Chandra data, confirming the pulsating nature of the source, with the light-curve softening at the pulse peak. The quasi-simultaneous Chandra-Swift spectra of IGR J18179-1621 can be well fit by a heavily absorbed hard power law (N _H = 2.2 ± 0.3 × 10²³ cm^-2 and photon index Γ = 0.4 ± 0.1) with an average absorbed 2-8keV flux of 1.4 × 10 ^-11 erg cm^-2 s^-1. At the Chandra-based position, a source is detected in our near-infrared (NIR) maps with K _s = 13.14 ± 0.04mag, H_n = 16 ± 0.1mag, and no J_n -band counterpart down to 18mag. The NIR source, compatible with 2MASS J18175218-1621316, shows no variability between 2012 March 13 and 26. Searches of the UKIDSS database show similar NIR flux levels at epochs six months prior to and after a 2007 February 11 archival Chandra observation where the source's X-ray flux was at least 87times fainter. In many ways IGR J18179-1621 is unusual: its combination of a several week long outburst (without evidence of repeated outbursts in the historical record), high absorption column (a large fraction of which is likely local to the system), and 11.82 s period does not fit neatly into existing X-ray binary categories.