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Register a new userA New Population of Compton-Thick AGN Identified Using the Spectral Curvature Above 10 keV
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We present a new metric that uses the spectral curvature (SC) above 10 keV to identify Compton-thick AGN in low-quality Swift BAT X-ray data. Using NuSTAR, we observe nine high SC-selected AGN. We find that high-sensitivity spectra show the majority are Compton-thick (78% or 7/9) and the remaining two are nearly Compton-thick (NH~5-8x10^23 cm^-2). We find the SC_bat and SC_nustar measurements are consistent, suggesting this technique can be applied to future telescopes. We tested the SC method on well-known Compton-thick AGN and find it is much more effective than broad band ratios (e.g. 100% using SC vs. 20% using 8-24/3-8 keV). Our results suggest that using the >10 keV emission may be the only way to identify this population since only two sources show Compton-thick levels of excess in the OIII to X-ray emission ratio (F_OIII/F_2-10 keV>1) and WISE colors do not identify most of them as AGN. Based on this small sample, we find that a higher fraction of these AGN are in the final merger stage than typical BAT AGN. Additionally, these nine obscured AGN have, on average, four times higher accretion rates than other BAT-detected AGN (Edd ratio=0.068 compared to 0.016). The robustness of SC at identifying Compton-thick AGN implies a higher fraction of nearby AGN may be Compton-thick (~22%) and the sum of black hole growth in Compton-thick AGN (Eddington ratio times population percentage), is nearly as large as mildly obscured and unobscured AGN.
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To fully understand cosmic black hole growth we need to constrain the population of heavily obscured active galactic nuclei (AGN) at the peak of cosmic black hole growth ($zsim$1-3). Sources with obscuring column densities higher than $mathrm{10^{24}}$ atoms $mathrm{cm^{-2}}$, called Compton-thick (CT) AGN, can be identified by excess X-ray emission at $sim$20-30 keV, called the Compton hump. We apply the recently developed Spectral Curvature (SC) method to high-redshift AGN (2<z<5) detected with Chandra. This method parametrizes the characteristic Compton hump feature cosmologically redshifted into the X-ray band at observed energies <10 keV. We find good agreement in CT AGN found using the SC method and bright sources fit using their full spectrum with X-ray spectroscopy. In the Chandra deep field south, we measure a CT fraction of $mathrm{17^{+19}_{-11}%}$ (3/17) for sources with observed luminosity $mathrm{>5times 10^{43}}$ erg $mathrm{s^{-1}}$. In the Cosmological evolution survey (COSMOS), we find an observed CT fraction of $mathrm{15^{+4}_{-3}%}$ (40/272) or $mathrm{32pm11 %}$ when corrected for the survey sensitivity. When comparing to low redshift AGN with similar X-ray luminosities, our results imply the CT AGN fraction is consistent with having no redshift evolution. Finally, we provide SC equations that can be used to find high-redshift CT AGN (z>1) for current (XMM-Newton) and future (eROSITA and ATHENA) X-ray missions.
We present X-ray observations of the active galactic nucleus (AGN) in NGC 4785. The source is a local Seyfert 2 which has not been studied so far in much detail. It was recently detected with high significance in the 15-60 keV band in the 66 month Swift/BAT all sky survey, but there have been no prior pointed X-ray observations of this object. With Suzaku, we clearly detect the source below 10 keV, and find it to have a flat continuum and prominent neutral iron fluorescence line with equivalent width >~1 keV. Fitting the broadband spectra with physical reflection models shows the source to be a bona fide Compton thick AGN with Nh of at least 2x10^{24} cm^{-2} and absorption-corrected 2-10 keV X-ray power L(2-10) ~ few times 10^{42} erg s^{-1}. Realistic uncertainties on L(2-10) computed from the joint confidence interval on the intrinsic power law continuum photon index and normalization are at least a factor of 10. The local bona fide Compton thick AGN population is highly heterogeneous in terms of WISE mid-infrared source colours, and the nucleus of NGC 4785 appears especially sub-dominant in the mid-infrared when comparing to other Compton thick AGN. Such sources would not be easily found using mid-infrared selection alone. The extent of host galaxy extinction to the nucleus is not clear, though NGC 4785 shows a complex core with a double bar and inner disk, adding to the list of known Compton thick AGN in barred host galaxies.
The Circinus galaxy is one of the nearest obscured AGN, making it an ideal target for detailed study. Combining archival Chandra and XMM-Newton data with new NuSTAR observations, we model the 2-79 keV spectrum to constrain the primary AGN continuum and to derive physical parameters for the obscuring material. Chandras high angular resolution allows a separation of nuclear and off-nuclear galactic emission. In the off-nuclear diffuse emission we find signatures of strong cold reflection, including high equivalent-width neutral Fe lines. This Compton-scattered off-nuclear emission amounts to 18% of the nuclear flux in the Fe line region, but becomes comparable to the nuclear emission above 30 keV. The new analysis no longer supports a prominent transmitted AGN component in the observed band. We find that the nuclear spectrum is consistent with Compton-scattering by an optically-thick torus, where the intrinsic spectrum is a powerlaw of photon index $Gamma = 2.2-2.4$, the torus has an equatorial column density of $N_{rm H} = (6-10)times10^{24}$cm$^{-2}$ and the intrinsic AGN $2-10$ keV luminosity is $(2.3-5.1)times 10^{42}$ erg/s. These values place Circinus along the same relations as unobscured AGN in accretion rate-vs-$Gamma$ and $L_X$-vs-$L_{IR}$ phase space. NuSTARs high sensitivity and low background allow us to study the short time-scale variability of Circinus at X-ray energies above 10 keV for the first time. The lack of detected variability favors a Compton-thick absorber, in line with the the spectral fitting results.
Current measurements show that the observed fraction of Compton-thick (CT) AGN is smaller than the expected values needed to explain the cosmic X-ray background. Prior fits to the X-ray spectrum of the nearby Seyfert-2 galaxy NGC 5347 ($z=0.00792,, D =35.5 rm ~Mpc $) have alternately suggested a CT and Compton-thin source. Combining archival data from $Suzaku$, $Chandra$, and - most importantly - new data from $NuSTAR$, and using three distinct families of models, we show that NGC 5347 is an obscured CTAGN ($N_{rm H} > 2.23times 10^{24}~rm cm^{-2}$). Its 2-30~keV spectrum is dominated by reprocessed emission from distant material, characterized by a strong Fe K$alpha$ line and a Compton hump. We found a large equivalent width of the Fe K$alpha$ line ($rm EW = 2.3 pm 0.3$ keV) and a high intrinsic-to-observed flux ratio ($sim 100$). All of these observations are typical for bona fide CTAGN. We estimate a bolometric luminosity of $L_{rm bol} simeq 0.014 pm 0.005~L_{rm Edd.}$. The $Chandra$ image of NGC 5347 reveals the presence of extended emission dominating the soft X-ray spectrum ($E < 2,rm keV$), which coincides with the [O III] emission detected in the $Hubble ~Space~ Telescope$ images. Comparison to other CTAGN suggests that NGC 5347 is broadly consistent with the average properties of this source class. We simulated $XRISM$ and $Athena$/X-IFU spectra of the source, showing the potential of these future missions in identifying CTAGN in the soft X-rays.
The cold disk/torus gas surrounding active galactic nuclei (AGN) emits fluorescent lines when irradiated by hard X-ray photons. The fluorescent lines of elements other than Fe and Ni are rarely detected due to their relative faintness. We report the detection of K$alpha$ lines of neutral Si, S, Ar, Ca, Cr, and Mn, along with the prominent Fe K$alpha$, Fe K$beta$, and Ni K$alpha$ lines, from the deep Chandra observation of the low-luminosity Compton-thick AGN in M51. The Si K$alpha$ line at 1.74 keV is detected at $sim3sigma$, the other fluorescent lines have a significance between 2 and 2.5 $sigma$, while the Cr line has a significance of $sim1.5sigma$. These faint fluorescent lines are made observable due to the heavy obscuration of the intrinsic spectrum of M51, which is revealed by Nustar observation above 10 keV. The hard X-ray continuum of M51 from Chandra and Nustar can be fitted with a power-law spectrum with an index of 1.8, reprocessed by a torus with an equatorial column density of $N_{rm H}sim7times10^{24}$ cm$^{-2}$ and an inclination angle of $74$ degrees. This confirms the Compton-thick nature of the nucleus of M51. The relative element abundances inferred from the fluxes of the fluorescent lines are similar to their solar values, except for Mn, which is about 10 times overabundant. It indicates that Mn is likely enhanced by the nuclear spallation of Fe.
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