A Deep Chandra Acis Study of NGC 4151. III. The Line Emission and Spectral Analysis of the Ionization Cone (original) (raw)
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The Astrophysical Journal, 2011
We have studied the X-ray emission within the inner ∼150 pc radius of NGC 4151 by constructing high spatial resolution emission line images of blended OVII, OVIII, and NeIX. These maps show extended structures that are spatially correlated with the radio outflow and optical [OIII] emission. We find strong evidence for jet-gas cloud interaction, including morphological correspondences with regions of X-ray enhancement, peaks of near-infrared [FeII] emission, and optical clouds. In these regions, moreover, we find evidence of elevated NeIX/OVII ratios; the X-ray emission of these regions also exceeds that expected from nuclear photoionization. Spectral fitting reveals the presence of a collisionally ionized component. The thermal energy of the hot gas suggests that 0.1% of the estimated jet power is deposited into the host interstellar medium through interaction between the radio jet and the dense medium of the circum-nuclear region. We find possible pressure equilibrium between the collisionally ionized hot gas and the photoionized line-emitting cool clouds. We also obtain constraints on the extended iron and silicon fluorescent emission. Both lines are spatially unresolved. The upper limit on the contribution of an extended emission region to the Fe Kα emission is 5% of the total, in disagreement with a previous claim that 65% of the Fe Kα emission originates in the extended narrow line region.
Astrophysical Journal, 2011
We report on the imaging analysis of 200 ks sub-arcsecond resolution Chandra ACIS-S observations of the nearby Seyfert 1 galaxy NGC 4151. Bright, structured soft X-ray emission is observed to extend from 30 pc to 1.3 kpc in the south-west from the nucleus, much farther than seen in earlier X-ray studies. The terminus of the north-eastern X-ray emission is spatially coincident with a CO gas lane, where the outflow likely encounters dense gas in the host galactic disk. X-ray emission is also detected outside the boundaries of the ionization cone, which indicates that the gas there is not completely shielded from the nuclear continuum, as would be the case for a molecular torus collimating the bicone. In the central r<200 pc region, the subpixel processing of the ACIS data recovers the morphological details on scales of <30~pc (<0.5") first discovered in Chandra HRC images. The X-ray emission is more absorbed towards the boundaries of the ionization cone, as well as perpendicular to the bicone along the direction of a putative torus in NGC 4151. The innermost region where X-ray emission shows the highest hardness ratio, is spatially coincident with the near-infrared resolved H_2 emission and dusty spirals we find in an HST V-H color image. The agreement between the observed H_2 line flux and the value predicted from X-ray-irradiated molecular cloud models supports photo-excitation by X-rays from the active nucleus as the origin of the H_2 line, although contribution from UV fluorescence or collisional excitation cannot be fully ruled out with current data. The discrepancy between the mass of cold molecular gas inferred from recent CO and near-infrared H_2 observations may be explained by the anomalous CO abundance in this X-ray dominated region. The total H_2 mass derived from the X-ray observation agrees with measurement in Storchi-Bergmann et al.
Revisiting the Short-Term X-Ray Spectral Variability of NGC 4151 with Chandra
The Astrophysical Journal, 2010
We present new X-ray spectral data for the Seyfert 1 nucleus in NGC 4151 observed with Chandra for ∼200 ks. A significant ACIS pileup is present, resulting in a non-linear count rate variation during the observation. With pileup corrected spectral fitting, we are able to recover the spectral parameters and find consistency with those derived from unpiled events in the ACIS readout streak and outer region from the bright nucleus. The absorption corrected 2-10 keV flux of the nucleus varied between 6 × 10 −11 erg s −1 cm −2 and 10 −10 erg s −1 cm −2 (L 2−10keV ∼ 1.3 − 2.1 × 10 42 erg s −1). Similar to earlier Chandra studies of NGC 4151 at a historical low state, the photon indices derived from the same absorbed power-law model are Γ ∼ 0.7 − 0.9. However, we show that Γ is highly dependent on the adopted spectral models. Fitting the power-law continuum with a Compton reflection component gives Γ ∼ 1.1. By including passage of non-uniform X-ray obscuring clouds, we can reproduce the apparent flat spectral states with Γ ∼ 1.7, typical for Seyfert 1 AGNs. The same model also fits the hard spectra from previous ASCA "long look" observation of NGC 4151 in the lowest flux state. The spectral variability during our observation can be interpreted as variations in intrinsic soft continuum flux relative to a Compton reflection component that is from distant cold material and constant on short time scale, or variations of partially covering absorber in the line of sight towards the nucleus. An ionized absorber model with ionization parameter log ξ ∼ 0.8 − 1.1 can also fit the low-resolution ACIS spectra. If the partial covering model is correct, adopting a black hole mass M BH ∼ 4.6 × 10 7 M ⊙ we constrain the distance of the obscuring cloud from the central black hole to be r 9 light-days, consistent with the size of broad emission line region of NGC 4151 from optical reverberation mapping.
The Ionized Nuclear Environment in NGC 985 as seen by Chandra and BeppoSAX
2004
We investigate the ionized environment of the Seyfert 1 galaxy NGC 985 with a new Chandra-HETGS observation and an archival BeppoSAX observation. Both spectra exhibit strong residuals to a single powerlaw model, indicating the presence of an ionized absorber and a soft excess. A detailed model over the Chandra data shows that the 0.6-8 keV intrinsic continuum can be well represented by a powerlaw (Γ ≈ 1.6) plus a blackbody component (kT=0.1 keV). Two absorption components are clearly required to fit the absorption features observed in the Chandra spectrum. The components have a difference of 29 in ionization parameter and 3 in column density. The presence of the low ionization component is evidenced by an Fe M-shell unresolved transition array (UTA) produced by charge states vii to xiii. The high ionization phase is required by the presence of broad absorption features arising from several blends of Fe L-shell transitions (Fe xvii-xxii). A third highly ionized component might also be present, but the data does not allow to constrain its properties. Though poorly constrained, the outflow velocities of the components (581 ± 206 km s −1 for the high ionization phase and 197 ± 184 km s −1 for the low ionization one) are consistent with each other, and with the outflow velocities of the absorption components observed in the UV. In addition, the low ionization component produces significant amounts of O vi, N v, and C iv which suggests that a single outflow produces the UV and X-ray features. The broad band (0.1-100 keV) continuum in the BeppoSAX data can be parameterized by a powerlaw (Γ ≈ 1.4), a blackbody (kT=0.1 keV), and a high energy cutoff (E c ≈ 70 keV). An X-ray luminosity variation by a factor 2.3 is observed between the BeppoSAX and Chandra observations (separated by almost 3 years). Variability in the opacity of the absorbers is detected in response to the continuum variation, but while the colder component is consistent with a simple picture of photoionization equilibrium, the ionization state of the hotter component seems to increase while the continuum flux drops. The most striking result in our analysis is that during both the Chandra and the BeppoSAX observations, the two absorbing components appear to have the same pressure. Thus, we suggest that the absorption arises from a multi-phase wind. Such scenario can explain the change in opacity of both absorption components during the observations, but requires that a third hotter component is pressure-confining the two phases. Hence, our analysis points to a 3-phase medium similar to the wind found in NGC 3783, and further suggests that such a wind might be a common characteristic in AGN. The pressure-confining scenario requires fragmentation of the confined phases into a large number of clouds.
The Astrophysical Journal, 2009
We report high resolution imaging of the nucleus of the Seyfert 1 galaxy NGC 4151 obtained with a 50 ks Chandra HRC observation. The HRC image resolves the emission on spatial scales of 0.5 ′′ , ∼30 pc, showing an extended X-ray morphology overall consistent with the narrow line region (NLR) seen in optical line emission. Removal of the bright point-like nuclear source and image deconvolution techniques both reveal X-ray enhancements that closely match the substructures seen in the Hubble Space Telescope [OIII] image and prominent knots in the radio jet. We find that most of the NLR clouds in NGC 4151 have [OIII] to soft X-ray ratio ∼10, despite the distance of the clouds from the nucleus. This ratio is consistent with the values observed in NLRs of some Seyfert 2 galaxies, which indicates a uniform ionization parameter even at large radii and a density decreasing as r −2 as expected for a nuclear wind scenario. The [OIII]/X-ray ratios at the location of radio knots show an excess of X-ray emission, suggesting shock heating in addition to photoionization. We examine various mechanisms for the X-ray emission and find that, in contrast to jet-related X-ray emission in more powerful AGN, the observed jet parameters in NGC 4151 are inconsistent with synchrotron emission, synchrotron self-Compton, inverse Compton of CMB photons or galaxy optical light. Instead, our results favor thermal emission from the interaction between radio outflow and NLR gas clouds as the origin for the X-ray emission associated with the jet. This supports previous claims that frequent jet-ISM interaction may explain why jets in Seyfert galaxies appear small, slow, and thermally dominated, distinct from those kpc scale jets in the radio galaxies.
Spatially Resolved Chandra HETG Spectroscopy of the NLR Ionization Cone in NGC 1068
2010
We present initial results from a new 440-ks Chandra HETG GTO observation of the canonical Seyfert 2 galaxy NGC 1068. The proximity of NGC 1068, together with Chandra's superb spatial and spectral resolution, allow an unprecedented view of its nucleus and circumnuclear NLR. We perform the first spatially resolved high-resolution X-ray spectroscopy of the 'ionization cone' in any AGN, and use the sensitive line diagnostics offered by the HETG to measure the ionization state, density, and temperature at discrete points along the ionized NLR. We argue that the NLR takes the form of outflowing photoionized gas, rather than gas that has been collisionally ionized by the small-scale radio jet in NGC 1068. We investigate evidence for any velocity gradients in the outflow, and describe our next steps in modeling the spatially resolved spectra as a function of distance from the nucleus.
The Chandra HRC View of the Subarcsecond Structures in the Nuclear Region of NGC 1068
The Astrophysical Journal, 2012
We have obtained a high spatial resolution X-ray image of the nucleus of NGC 1068 using the High Resolution Camera (HRC-I) on board the Chandra X-ray Observatory, which provides an unprecedented view of the innermost 1 arcsec radius region of this galaxy. The HRC image resolves the narrow line region into X-ray emission clumps matching bright emission-line clouds in the HST [OIII] λ5007 images and allows comparison with sub-arcsec scale radio jet for the first time. Two distinct X-ray knots are revealed at 1.3-1.4 arcsec northeast and southwest of the nucleus. Based on the combined X-ray, [OIII], and radio continuum morphology, we identify the locations of intense radio jet-cloud interaction. The [OIII] to soft X-ray ratios show that some of these clouds are strongly affected by shock heating, whereas in other locations the jet simply thrusts through with no signs of strong interaction. This is further strengthened by the presence of a kT ∼ 1 keV collisionally ionized component in the ACIS spectrum of a shock heated cloud HST-G. We estimate that the kinematic luminosity of the jet-driven shocks is 6 × 10 38 erg s −1 , a negligible fraction (10 −4) of the estimated total jet power.
Chandra LETGS and XMM-Newton observations of NGC�4593
Astronomy and Astrophysics, 2003
In this paper, we analyze spectra of the Seyfert 1 galaxy NGC 4593 obtained with the Chandra Low Energy Transmission Grating Spectrometer (LETGS), the Reflection Grating Spectrometer (RGS) and the European Photon Imaging Camera's (EPIC) onboard of XMM-Newton. The two observations were separated by ∼7 months. In the LETGS spectrum we detect a highly ionized warm absorber corresponding to an ionization state of 400×10 −9 W m, visible as a depression at 10 − 18Å. This depression is formed by multiple weak Fe and Ne lines. A much smaller column density was found for the lowly ionized warm absorber, corresponding to ξ = 3×10 −9 W m. However, an intermediate ionization warm absorber is not detected. For the RGS data the ionization state is hard to constrain. The EPIC results show a narrow Fe Kα line.
The complex X-ray spectrum of NGC 4507
Arxiv preprint astro-ph/ …, 2004
XMM-N ewton and Chandra/HETG spectra of the Compton-thin (NH ∼ 4 × 10 23 cm −2 ) Seyfert 2 galaxy, NGC 4507, are analyzed and discussed. The main results are: a) the soft X-ray emission is rich in emission lines; an (at least) two-zone photoionization region is required to explain the large range of ionization states. b) The 6.4 keV iron line is likely emitted from Compton-thick matter, implying the presence of two circumnuclear cold regions, one Compton-thick (the emitter), one Compton-thin (the cold absorber). c) Evidence of an Fe xxv absorption line is found in the Chandra/HETG spectrum. The column density of the ionized absorber is estimated to be a few×10 22 cm −2 .
Imaging the Circumnuclear Region of NGC 1365 with Chandra
The Astrophysical Journal, 2009
We present the first Chandra/ACIS imaging study of the circumnuclear region of the nearby Seyfert galaxy NGC 1365. The X-ray emission is resolved into point-like sources and complex, extended emission. The X-ray morphology of the extended emission shows a biconical soft X-ray emission region extending ∼5 kpc in projection from the nucleus, coincident with the high excitation outflow cones seen in optical emission lines particularly to the northwest. Harder X-ray emission is detected from a kpc-diameter circumnuclear ring, coincident with the star-forming ring prominent in the Spitzer mid-infrared images; this X-ray emission is partially obscured by the central dust lane of NGC 1365. Spectral fitting of spatially separated components indicates a thermal plasma origin for the soft extended X-ray emission (kT = 0.57 keV). Only a small amount of this emission can be due to photoionization by the nuclear source. Detailed comparison with [OIII]λ5007 observations shows the hot interstellar medium (ISM) is spatially anticorrelated with the [OIII] emitting clouds and has thermal pressures comparable to those of the [OIII] medium, suggesting that the hot ISM acts as a confining medium for the cooler photoionized clouds. The abundance ratios of the hot ISM are fully consistent with the theoretical values for enrichment from Type II supernovae, suggesting that the hot ISM is a wind from the starburst circumnuclear ring. X-ray emission from a ∼ 450 pc long nuclear radio jet is also detected to the southeast.