NuSTAR Observations of the Compton-thick Active Galactic Nucleus and Ultraluminous X-Ray Source Candidate in NGC 5643
Rigby, J. R.; Harrison, F. A.; Stern, D.; Alexander, D. M.; Bauer, F. E.; Brandt, W. N.; Gandhi, P.; Matt, G.; Arévalo, P.; Brightman, M.; Del Moro, A.; Lansbury, G. B.; Ballantyne, D. R.; Boggs, S. E.; Christensen, F. E.; Craig, W. W.; Hailey, C. J.; Hickox, R. C.; Puccetti, S.; Ricci, C.; Baloković, M.; Walton, D. J.; Zappacosta, L.; Zhang, W.; Annuar, A.
United Kingdom, Chile, United States, Denmark, Italy, Japan
Abstract
We present two Nuclear Spectroscopic Telescope Array (NuSTAR) observations of the local Seyfert 2 active galactic nucleus (AGN) and an ultraluminous X-ray source (ULX) candidate in NGC 5643. Together with archival data from Chandra, XMM-Newton, and Swift-BAT, we perform a high-quality broadband spectral analysis of the AGN over two decades in energy (∼0.5-100 keV). Previous X-ray observations suggested that the AGN is obscured by a Compton-thick (CT) column of obscuring gas along our line of sight. However, the lack of high-quality ≳10 keV observations, together with the presence of a nearby X-ray luminous source, NGC 5643 X-1, have left significant uncertainties in the characterization of the nuclear spectrum. NuSTAR now enables the AGN and NGC 5643 X-1 to be separately resolved above 10 keV for the first time and allows a direct measurement of the absorbing column density toward the nucleus. The new data show that the nucleus is indeed obscured by a CT column of NH ≳ 5 × 1024 cm-2. The range of 2-10 keV absorption-corrected luminosity inferred from the best-fitting models is L2-10,int = (0.8-1.7) × 1042 erg s-1, consistent with that predicted from multiwavelength intrinsic luminosity indicators. In addition, we also study the NuSTAR data for NGC 5643 X-1 and show that it exhibits evidence of a spectral cutoff at energy E ∼ 10 keV, similar to that seen in other ULXs observed by NuSTAR. Along with the evidence for significant X-ray luminosity variations in the 3-8 keV band from 2003 to 2014, our results further strengthen the ULX classification of NGC 5643 X-1.