X-ray and Near Infrared observations of the obscured accreting pulsar IGR J18179-1621

TL;DR

This study provides detailed X-ray and near-infrared observations of the obscured pulsar IGR J18179-1621, revealing its pulsation, high absorption, and unique outburst characteristics, contributing to understanding its nature and classification.

Contribution

First detailed multi-wavelength analysis of IGR J18179-1621, identifying its pulsation, precise position, and unusual outburst behavior, challenging existing X-ray binary classifications.

Findings

Detected 11.82 s pulsation confirming pulsar nature

Measured high local absorption with N_H ~ 2.2 x 10^{23} cm^{-2}

Observed no significant NIR variability over months

Abstract

IGR J18179-1621 is an obscured accreting X-ray pulsar discovered by INTEGRAL on 2012 February 29. We report on our 20 ksec 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 March 26. We determine the most accurate X-ray position of IGR J18179-1621, alpha_{J2000}=18 h 17m 52.18 s, delta_{J2000}=-16^\circ 21', 31.68" (90% uncertainty of 0.6"). A strong periodic variability at 11.82 s is clearly detected in the Chandra data, confirming the pulsating nature of the source, with the lightcurve 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 \times 10^{23} cm^{-2}, and photon index \Gamma = 0.4+/-0.1) with an average absorbed 2-8 keV flux of 1.4\times 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+/-0.04 mag, H_n = 16+/-0.1 mag, and no J_n band counterpart down to ~18 mag. The NIR source, compatible with 2MASS J18175218-1621316, shows no variability between 2012 March 13 and March 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.