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IGRJ16181-5407 is likely an AGN

ATel #11890; F. Ursini, L. Bassani, A. Malizia, V. Sguera (INAF-OAS Bologna), A. Bazzano, P. Ubertini, M. T. Fiocchi (INAF-IAPS Rome), A. J. Bird (University of Southampton)
on 26 Jul 2018; 08:35 UT
Credential Certification: Francesco Ursini (francesco.ursini@inaf.it)

Subjects: X-ray, AGN

We use archival Chandra and NuSTAR observations to discuss the X-ray counterpart of IGR J16181-5407, a persistent but still unidentified source seen by INTEGRAL/IBIS (Bird et al. 2016, ApJS, 223, 15). Within the IBIS positional uncertainty one bright and hard X-ray source was detected (Landi et al. 2012, ATel #4233) but no unambiguous optical counterpart was found.

Chandra pointed at the region of this source on 2017-05-20, with an exposure time of 5 ks (Obs. Id. 18975) and was able to locate the source (RA(J2000)=16:18:07.73, Dec(J2000)=-54:06:12.14) with an uncertainty of only 2 arcsec, allowing us to pinpoint a single optical/IR object: USNO-B1.0 0358-0591923/2MASSJ 16180771-5406122, also listed in the ALL-WISE catalogue with colours W1-W2=1.176 and W2-W3=2.566, i.e. typical of AGNs (Secrest et al. 2015, ApJS, 221, 12). The source spectrum is absorbed (N_H~10²³ cm^-2) and bright (0.3-10 keV flux=3x10^-12 ergs cm^-2 s^-1).

This source was also observed by NuSTAR on 2017-01-29 (Obs. Id. 30161006002), with a net exposure time of 59 ks, as part of the Galactic Legacy Survey. The NuSTAR spectrum is well fitted by an absorbed power law (Chi^2/dof=192/171), with positive residuals around 6 keV suggesting the presence of a Fe K alpha emission line. Adding a narrow Gaussian line improves the fit (Delta Chi^2 of -15 for 2 dof less; the probability of chance improvement is 0.001 from an F-test). The equivalent width of the line is 100+/-50 eV, and its energy (observer's frame) is 5.85+\-0.15 keV. If interpreted as the neutral Fe K alpha line at 6.4 keV, it would indicate a redshift of 0.085+/-0.015. Finally, we measure a photon index Gamma=1.69+/-0.08 and an absorbing column density of (8+/-2)x10²² cm^-2 . The extrapolated 0.3-10 keV flux is 4x10^-12 ergs cm^-2 s^-1, while the extrapolated 20-100 keV flux is 1.2x10^-11 ergs cm^-2 s^-1 (a factor of 1.5 larger than the IBIS flux).

We encourage multiwavelength observations of this source to confirm its extragalactic nature.