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Recent mid-infrared interferometry observations of nearby active galactic nuclei (AGN) revealed that a significant part of the dust emission extends in the polar direction, rather than the equatorial torus/disk direction as expected by the traditional unification model. We study the X-ray signatures of this polar dusty gas with ray-tracing simulations. Different from those from the ionized gas, the scattered emission from the polar dusty gas produces self-absorption and neutral-like fluorescence lines, which are potentially a unique probe of the kinematics of the polar dusty gas. The anomalously small Fe Ka/Si Ka ratios of type II AGN observed previously can be naturally explained by the polar dusty gas, because the polar emission does not suffer from heavy absorption by the dense equatorial gas. The observed Si Ka lines of the Circinus galaxy and NGC 1068 show blue-shifts with respect to the systemic velocities of the host galaxies, consistent with an outflowing scenario of the Si Ka-emitting gas. The 2.5-3 keV image of the Circinus galaxy is elongated along the polar direction, consistent with an origin of the polar gas. These results show that the polar-gas-scattered X-ray emission of type II AGN is an ideal objective for future X-ray missions, such as Athena.
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X-ray observations of active galactic nuclei (AGN) show variability on timescales ranging from a few hours up to a few days. Some of this variability may be associated with occultation events by clouds in the broad line region. In this work, we aim to model the spectral and polarization variability arising from X-ray obscuration events, serving as probes of the relativistic effects that dominate the emission from the innermost regions. We show that asymmetries can be clearly detected in the AGN spectra as the cloud is shading different parts of the accretion disc. We also show that these effects can be detected in the temporal evolution of the polarization degree ($P$) and the polarization position angle ($Psi$). The variations in $P$ and $Psi$ are highly dependent on the inclination of the system, the position of the primary source and its intrinsic polarization. Considering the disc-corona system only, for an inclination $theta = 30^circ$ (60$^circ$), $P$ increases up to $sim 20$% (30)%, in the 4-8 keV band, when the unpolarized primary source is obscured. However, after accounting for the contribution of parsec-scale material scattering the light in our line of sight (narrow-line region and molecular torus), the variability is smoothed out and the polarization degree can be reduced down to $sim 1$% (2%). Our results suggest that the study of eclipses in AGN with the next generation of X-ray spectral and polarimetric missions could provide unique information on the physics and structure of the innermost regions as well as of the parsec-scale material.
We present new X-ray observations of luminous heavily dust-reddened quasars (HRQs) selected from infrared sky surveys. HRQs appear to be a dominant population at high redshifts and the highest luminosities, and may be associated with a transitional blowout phase of black hole and galaxy co-evolution models. Despite this, their high-energy properties have been poorly known. We use the overall sample of $10$ objects with XMM-Newton coverage to study the high-energy properties of HRQs at $left< L_{rm bol} right> = 10^{47.5}$ erg/s and $left< z right>= 2.5$. For the seven sources with strong X-ray detections, we perform spectral analyses. These find a median X-ray luminosity of $left< L_{rm 2-10,keV} right> = 10^{45.1}$ erg/s, comparable to the most powerful X-ray quasars known. The gas column densities are $N_{rm H}=(1$-$8)times 10^{22}$ cm$^{-2}$, in agreement with the amount of dust extinction observed. The dust to gas ratios are sub-Galactic, but are higher than found in local AGN. The intrinsic X-ray luminosities of HRQs are weak compared to the mid-infrared ($L_{rm 6mu m}$) and bolometric luminosities ($L_{rm bol}$), in agreement with findings for other luminous quasar samples. For instance, the X-ray to bolometric corrections range from $kappa_{rm bol}approx 50$-$3000$. The moderate absorption levels and accretion rates close to the Eddington limit ($left< lambda_{rm Edd} right>=1.06$) are in agreement with a quasar blowout phase. Indeed, we find that the HRQs lie in the forbidden region of the $N_{rm H}$-$lambda_{rm Edd}$ plane, and therefore that radiation pressure feedback on the dusty interstellar medium may be driving a phase of blowout that has been ongoing for a few $10^{5}$ years. The wider properties, including [OIII] narrow-line region kinematics, broadly agree with this interpretation.
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.
If the existence of an obscuring circumnuclear region around the innermost regions of active galactic nuclei (AGN) has been observationally proven, its geometry remains highly uncertain. The morphology usually adopted for this region is a toroidal structure, but other alternatives, such as flared disks, can be a good representative of equatorial outflows. Those two geometries usually provide very similar spectroscopic signatures, even when they are modeled under the assumption of fragmentation. In this lecture note, we show that the resulting polarization signatures of the two models, either a torus or a flared disk, are quite different from each other. We use a radiative transfer code that computes the 2000 - 8000 angstrom polarization of the two morphologies in a clumpy environment, and show that varying the sizes of a toroidal region has deep impacts onto the resulting polarization, while the polarization of flared disks is independent of the outer radius. Clumpy flared disks also produce higher polarization degrees (about 10 % at best) together with highly variable polarization position angles.
We present a mid-infrared (IR) sample study of nearby ultraluminous X-ray sources (ULXs) using multi-epoch observations with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. Spitzer/IRAC observations taken after 2014 were obtained as part of the Spitzer Infrared Intensive Transients Survey (SPIRITS). Our sample includes 96 ULXs located within 10 Mpc. Of the 96~ULXs, 12 have candidate counterparts consistent with absolute mid-IR magnitudes of supergiants, and 16 counterparts exceeded the mid-IR brightness of single supergiants and are thus more consistent with star clusters or non-ULX background active galactic nuclei (AGN). The supergiant candidate counterparts exhibit a bi-modal color distribution in a Spitzer/IRAC color-magnitude diagram, where red and blue ULXs fall in IRAC colors $[3.6] - [4.5]sim0.7$ and $[3.6] - [4.5]sim0.0$, respectively. The mid-IR colors and absolute magnitudes of 4 red and 5 blue ULXs are consistent with that of supergiant B[e] (sgB[e]) and red supergiant (RSG) stars, respectively. While blue, RSG-like mid-IR ULX counterparts likely host RSG mass donors, we propose the red counterparts are ULXs exhibiting the B[e] phenomenon rather than hosts of sgB[e] mass donors. We show that the mid-IR excess from the red ULXs is likely due to thermal emission from circumstellar or circumbinary dust. Using dust as a probe for total mass, we estimate mass-loss rates of $dot{M}sim1times10^{-4}$ M$_odot$ yr$^{-1}$ in dust-forming outflows of red ULXs. Based on the transient mid-IR behavior and its relatively flat spectral index, $alpha=-0.19pm0.1$, we suggest that the mid-IR emission from Holmberg IX X-1 originates from a variable jet.
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