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Register a new userNuSTAR reveals that the heavily obscured nucleus of NGC 2785 was the contaminant of IRAS 09104+4109 in the BeppoSAX/PDS hard X-rays
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The search for heavily obscured active galactic nuclei (AGNs) has been revitalized in the last five years by NuSTAR, which has provided a good census and spectral characterization of a population of such objects, mostly at low redshift, thanks to its enhanced sensitivity above 10 keV compared to previous X-ray facilities, and its hard X-ray imaging capabilities. We aim at demonstrating how NGC2785, a local (z=0.009) star-forming galaxy, is responsible, in virtue of its heavily obscured active nucleus, for significant contamination in the non-imaging BeppoSAX/PDS data of the relatively nearby (~17 arcmin) quasar IRAS 09104+4109 (z=0.44), which was originally mis-classified as Compton thick. We analyzed ~71 ks NuSTAR data of NGC2785 using the MYTorus model and provided a physical description of the X-ray properties of the source for the first time. We found that NGC2785 hosts a heavily obscured (NH~3*10^{24} cm^{-2}) nucleus. The intrinsic X-ray luminosity of the source, once corrected for the measured obscuration (L(2-10 keV)~10^{42} erg/s), is consistent within a factor of a few with predictions based on the source mid-infrared flux using widely adopted correlations from the literature. Based on NuSTAR data and previous indications from the Neil Gehrels Swift Observatory (BAT instrument), we confirm that NGC2785, because of its hard X-ray emission and spectral shape, was responsible for at least one third of the 20-100 keV emission observed using the PDS instrument onboard BeppoSAX, originally completely associated with IRAS 09104+4109. Such emission led to the erroneous classification of this source as a Compton-thick quasar, while it is now recognized as Compton thin.
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We present a long (~76 ks) Chandra observation of IRAS 09104+4109, a hyper-luminous galaxy, optically classified as a Type 2 AGN hosted in a cD galaxy in a cluster at z=0.442. We also report on the results obtained by fitting its broad-band spectral energy distribution. The Compton-thick nature of this source (which has been often referred to as an archetype of Compton-thick Type 2 quasars) was formerly claimed on the basis of its marginal detection in the PDS instrument onboard BeppoSAX, being then disputed using XMM-Newton data. Both Chandra analysis and optical/mid-IR spectral fitting are consistent with the presence of heavy (~1-5 10^{23} cm^{-2}) but not extreme (Compton-thick) obscuration. However, using the mid-IR and the [OIII] emission as proxies of the nuclear hard X-ray luminosity suggests the presence of heavier obscuration. The 54-month Swift BAT map shows excess hard X-ray emission likely related to a nearby (z=0.009) Type 2 AGN, close enough to IRAS 09104+4109 to significantly enhance and contaminate its emission in the early BeppoSAX PDS data.
We present NuSTAR hard X-ray (3-79 keV) observations of three Type 2 quasars at z ~ 0.4-0.5, optically selected from the Sloan Digital Sky Survey (SDSS). Although the quasars show evidence for being heavily obscured Compton-thick systems on the basis of the 2-10 keV to [OIII] luminosity ratio and multiwavelength diagnostics, their X-ray absorbing column densities (N_H) are poorly known. In this analysis: (1) we study X-ray emission at >10 keV, where X-rays from the central black hole are relatively unabsorbed, in order to better constrain N_H; (2) we further characterize the physical properties of the sources through broad-band near-UV to mid-IR spectral energy distribution (SED) analyses. One of the quasars is detected with NuSTAR at >8 keV with a no-source probability of <0.1%, and its X-ray band ratio suggests near Compton-thick absorption with N_H gtrsim 5 x 10^23 cm^-2. The other two quasars are undetected, and have low X-ray to mid-IR luminosity ratios in both the low energy (2-10 keV) and high energy (10-40 keV) X-ray regimes that are consistent with extreme, Compton-thick absorption (N_H gtrsim 10^24 cm^-2). We find that for quasars at z ~ 0.5, NuSTAR provides a significant improvement compared to lower energy (<10 keV) Chandra and XMM-Newton observations alone, as higher column densities can now be directly constrained.
NuSTAR observed the bright Compton-thin, narrow line Seyfert 1 galaxy, NGC 5506, for about 56 ks. In agreement with past observations, the spectrum is well fit by a power law with Gamma~1.9, a distant reflection component and narrow ionized iron lines. A relativistically blurred reflection component is not required by the data. When an exponential high energy cutoff is added to the power law, a value of 720(+130,-190) keV (90% confidence level) is found. Even allowing for systematic uncertainties, we find a 3 sigma lower limit to the high-energy cutoff of 350 keV, the highest lower limit to the cutoff energy found so far in an AGN by NuSTAR.
We analyzed the spectroscopic data from the PN and the MOS cameras in the 0.4-10 keV band. We also used an archival BeppoSAX 1-50 keV observation of IRAS 09104+4109 to investigate possible variations of the quasar emission. The X-ray emission in the EPIC band is dominated by the intra-cluster medium thermal emission. We found that the quasar contributes ~35% of the total flux in the 2-10 keV band. Both a transmission- (through a Compton-thin absorber with a Compton optical depth of tau_C~0.3, i.e. Nh~5 x 10^{23} cm^-2) and a reflection-dominated (tau_C>1) model provide an excellent fit to the quasar continuum emission. However, the value measured for the EW of Fe Kalpha emission line is only marginally consistent with the presence of a Compton-thick absorber in a reflection-dominated scenario, which had been suggested by a previous, marginal (i.e. 2.5sigma) detection with the hard X-ray (15-50 keV), non-imaging BeppoSAX/PDS instrument. Moreover, the value of luminosity in the 2-10 keV band measured by the transmission-dominated model is fully consistent with that expected on the basis of the bolometric luminosity of IRAS 09104+4109. From the analysis of the XMM-Newton data we therefore suggest the possibility that the absorber along the line of sight to the nucleus of IRAS 09104+4109 is Compton-thin. Alternatively, the absorber column density could have changed from Compton-thick to -thin in the five years elapsed between the observations. If this is the case, then IRAS 09104+4109 is the first changing-look quasar ever detected.
Using the BeppoSAX observatory, we have observed a nearby LINER/Seyfert 2 galaxy, NGC 3079, which is known as an outflow galaxy and a bright H_2O-maser source. Using the PDS detector, we have revealed that the NGC 3079 nucleus suffers from a Compton-thick absorption, with a hydrogen column density sim 10^{25} cm^{-2}. After corrected the absorption, the 2--10 keV luminosity becomes 10^{42-43} erg s^{-1} at a distance of 16 Mpc. It is 2-3 orders of magnitude higher than that observed in the MECS band (below 10 kev). We also detected a strong Fe-K line at 6.4^{+0.3}_{-0.2} keV with an equivalent width of 2.4^{+2.9}_{-1.5} keV, which is consistent with the heavy absorption.
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