AChandraX‐Ray Study of NGC 1068. II. The Luminous X‐Ray Source Population

Abstract

We present an analysis of the compact X-ray source population in the Seyfert 2 galaxy NGC 1068, imaged with a $50 ks Chandra observation. We find a total of 84 compact sources on the S3 chip, of which 66 are located within the 25.0 B-mag arcsec À2 isophote of the galactic disk of NGC 1068. Spectra have been obtained for the 21 sources with at least 50 counts and modeled with both multicolor disk blackbody and power-law models. The power-law model provides the better description of the spectrum for 18 of these sources. For fainter sources, the spectral index has been estimated from the hardness ratio. Five sources have 0.4-8 keV intrinsic luminosities greater than 10 39 ergs s À1 , assuming that their emission is isotropic and that they are associated with NGC 1068. We refer to these sources as intermediate-luminosity X-ray objects (IXOs). If these five sources are X-ray binaries accreting with luminosities that are both sub-Eddington and isotropic, then the implied source masses are e7 M , and so they are inferred to be black holes. Most of the spectrally modeled sources have spectral shapes similar to Galactic black hole candidates. However, the brightest compact source in NGC 1068 has a spectrum that is much harder than that found in Galactic black hole candidates and other IXOs. The brightest source also shows large amplitude variability on both shortterm and long-term timescales, with the count rate possibly decreasing by a factor of 2 in $2 ks during our Chandra observation, and the source flux decreasing by a factor of 5 between our observation and the grating observations taken just over 9 months later. The ratio of the number of sources with luminosities greater than 2:1 Â 10 38 ergs s À1 in the 0.4-8 keV band to the rate of massive (>5 M ) star formation is the same, to within a factor of 2, for NGC 1068, the Antennae, NGC 5194 (the main galaxy in M51), and the Circinus galaxy. This suggests that the rate of production of X-ray binaries per massive star is approximately the same for galaxies with currently active star formation, including '' starbursts.''

XMM-Newton spectral properties of the ultraluminous IRAS galaxy Mrk 273

“…This is often observed in Compton-thick Seyfert 2 galaxies (Sambruna et al 2001;Kinkhabwala et al 2002;Done et al 2003;Bianchi et al 2005), which, in some cases, also show evidence for starburst activity (e.g. Circinus, Ruiz et al 2001;NGC 1068, Smith & Wilson 2003. As was observed in Paper I, two bright clumps of soft X-ray emission coincide with [OIII] emission.…”

A deficit of ultraluminous X-ray sources in luminous infrared galaxies

“…Previous work has shown it has a very hard spectrum: its power-law photon index is ∼0.8-1 (Smith & Wilson 2003;Swartz et al 2004;Berghea et al 2008). However, this is not due to excessive pile-up on the Chandra detectors; its observed count rate of ∼0.03 count s −1 is no more than ∼5 per cent piled-up.…”

“…Lehmer et al (2010). These are: (1) Smith & Wilson (2003); (2) Smith et al (2012). d Galaxies with multiple observations; further details of these can be found in Lehmer et al (2010).…”

X‐Ray Sources in the Star‐Forming Galaxies NGC 4194 and NGC 7541

“…For NGC 4579, we estimate three sources from the results of Eracleous et al (2002). For NGC 1068, we estimate 18 net sources from the 0.5-8 keV band results of Smith & Wilson (2003). For these galaxies, we have adopted the SFRs quoted by GGS03 rescaled to the distances listed in Table 7 except for NGC 1068 and NGC 4945, where we have calculated the SFR from the far-IR (FIR) luminosities quoted in Koribalski (1996) rescaled to the distances quoted in Table 7.…”