X-RAY AND NEAR-INFRARED OBSERVATIONS OF THE OBSCURED ACCRETING PULSAR IGR J18179–1621

Abstract

IGR J18179–1621 is an obscured accreting X-ray pulsar discovered by INTEGRAL on 2012 February 29. We report on our 20 ks 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[subscript s] , H[subscript n] , and J[subscript n] ) observations performed on 2012 March 13 and 26. We determine the most accurate X-ray position of IGR J18179–1621, α[subscript J2000] = 18[superscript h]17[subscript m]52[s over .]18, δ[subscript J2000] = –16°21'31['' over .]68 (90% uncertainty of 0['' over .]6). 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 [subscript H] = 2.2 ± 0.3 × 10[superscript 23] cm[superscript –2] and photon index Γ = 0.4 ± 0.1) with an average absorbed 2-8 keV flux of 1.4 × 10[superscript –11] erg cm[superscript –2] s[superscript –1]. At the Chandra-based position, a source is detected in our near-infrared (NIR) maps with K[subscript s] = 13.14 ± 0.04 mag, H[subscript n] = 16 ± 0.1 mag, and no J[subscript n] -band counterpart down to ~18 mag. 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 87 times 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.