NuSTAROBSERVATIONS OF THE COMPTON-THICK ACTIVE GALACTIC NUCLEUS AND ULTRALUMINOUS X-RAY SOURCE CANDIDATE IN NGC 5643

Abstract

We present two 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, had 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 N H 5 × 10 24 cm −2 . The range of 2-10 keV absorption-corrected luminosity inferred from the best fitting models is L 2−10,int = (0.8-1.7) × 10 42 erg s −1 , consistent with that predicted from multiwavelength intrinsic luminosity indicators. We also study the NuSTAR data for NGC 5643 X-1, and show that it exhibits evidence for a spectral cut-off 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 2003-2014, our results further strengthen the ULX classification of NGC 5643 X-1.

The ultraluminous X-ray source NGC 5643 ULX1: a large stellar mass black hole accreting at super-Eddington rates?

“…Here we instead report that during a long XMM-Newton observation, the ULX had a spectrum which could be well modelled either in terms of a cold, optically thick Comptonizing corona or a very hot (2.5-3 keV), advection-dominated disc. The high-energy roll-over was further confirmed by a NuSTAR observation of the source (Annuar et al 2015), indicating that the spectral shape of NGC 5643 ULX1 was more complex than the single power law as found in low-quality data. Furthermore, it is important to note that we observed a change in luminosity of a factor of 2-3 between the observations, without significant spectral changes.…”

“…The AGN is known to be heavily absorbed and characterized by a large iron emission feature, probably associated with Compton reflection of hard photons (e.g. Guainazzi et al 2004;Bianchi et al 2006;Annuar et al 2015). Other emission features have also been found in the XMM-Newton spectra of the AGN, and these have been associated with photoionized emission regions.…”

“…Other emission features have also been found in the XMM-Newton spectra of the AGN, and these have been associated with photoionized emission regions. Annuar et al (2015) studied a combination of NuSTAR and XMM-Newton data with the aim of well characterizing the spectral properties of the AGN. They found that different reflection/obscuration models can well describe the data, indicating that both Compton thick absorption by a toroidal absorber and reflection from distant, cold material are important in explaining the AGNs spectral properties.…”

“…We make use of the high-quality data from this XMM-Newton observation (∼100 ks) to test whether we can rule out either sub-or super-Eddington accretion in NGC 5643 ULX1. Besides the ULX, NGC 5643 hosts also an AGN (Guainazzi et al 2004;Bianchi, Guainazzi & Chiaberge 2006;Annuar et al 2015). Results of the spectral analysis of the AGN are also briefly reported in and discussed in Appendix A.…”

NuSTAR UNVEILS A HEAVILY OBSCURED LOW-LUMINOSITY ACTIVE GALACTIC NUCLEUS IN THE LUMINOUS INFRARED GALAXY NGC 6286

“…However, a significant fraction of U/LIRGs might be heavily obscured (e.g., Bauer et al 2010;Treister et al 2010b) and X-rays at energies 10 keV are strongly attenuated in Compton-thick (CT, N H 10 24 cm −2 ) AGNs. Observations carried out in the hard X-ray band ( 10 keV) are less affected by absorption and can be used to probe nuclear X-ray emission even in highly obscured systems (e.g., Arévalo et al 2014;Baloković et al 2014;Gandhi et al 2014;Annuar et al 2015;Bauer et al 2015;Koss et al 2015;Lansbury et al 2015;Puccetti et al 2015;Ricci et al 2015). Previous hard X-ray observations of U/LIRGs were carried out with BeppoSAX (e.g., Vignati et al 1999), Suzaku (e.g., Teng et al 2009), and Swift/BAT (Koss et al 2013).…”