arXiv subjects

Emanuele Nardini

Publications and source records attributed to Emanuele Nardini.

50 records · Page 3Linked to original sources

arXiv2018

A new relativistic component of the accretion disk wind in PDS 456

Past X-ray observations of the nearby luminous quasar PDS 456 (at $z=0.184$) have revealed a wide angle accretion disk wind (Nardini et al. 2015), with an outflow velocity of $\sim-0.25c$. Here we unveil a new, relativistic component of the wind through hard X-ray observations with NuSTAR and XMM-Newton, obtained in March 2017 when the quasar was in a low flux state. This very fast wind component, with an outflow velocity of $-0.46\pm0.02c$, is detected in the iron K band, in addition to the $-0.25c$ wind zone. The relativistic component may arise from the innermost disk wind, launched from close to the black hole at radius of $\sim10$ gravitational radii. The opacity of the fast wind also increases during a possible obscuration event lasting for 50 ks. We suggest that the very fast wind may only be apparent during the lowest X-ray flux states of PDS 456, becoming overly ionized as the luminosity increases. Overall, the total wind power may even approach the Eddington value.

astro-ph.HE↗

arXiv2018

Does the X-ray outflow quasar PDS 456 have a UV outflow at 0.3c?

The quasar PDS 456 (at redshift ~0.184) has a prototype ultra-fast outflow (UFO) measured in X-rays. This outflow is highly ionized with relativistic speeds, large total column densities log N_H(cm^-2) > 23, and large kinetic energies that could be important for feedback to the host galaxy. A UV spectrum of PDS 456 obtained with the Hubble Space Telescope in 2000 contains one well-measured broad absorption line (BAL) at ~1346A (observed) that might be Ly-alpha at v ~ 0.06c or NV 1240 at v ~ 0.08c. However, we use photoionisation models and comparisons to other outflow quasars to show that these BAL identifications are problematic because other lines that should accompany them are not detected. We argue that the UV BAL is probably CIV 1549 at v ~ 0.30c. This would be the fastest UV outflow ever reported, but its speed is similar to the X-ray outflow and its appearance overall is similar to relativistic UV BALs observed in other quasars. The CIV BAL identification is also supported indirectly by the tentative detection of another broad CIV line at v ~ 0.19c. The high speeds suggest that the UV outflow originates with the X-ray UFO crudely 20 to 30 r_g from the central black hole. We speculate that the CIV BAL might form in dense clumps embedded in the X-ray UFO, requiring density enhancements of only >0.4 dex compared clumpy structures already inferred for the soft X-ray absorber in PDS 456. The CIV BAL might therefore be the first detection of low-ionisation clumps proposed previously to boost the opacities in UFOs for radiative driving.

astro-ph.GA↗

arXiv2017

A deep X-ray view of the bare AGN Ark 120. III. X-ray timing analysis and multiwavelength variability

We present the spectral/timing properties of the bare Seyfert galaxy Ark 120 through a deep ~420ks XMM-Newton campaign plus recent NuSTAR observations and a ~6-month Swift monitoring campaign. We investigate the spectral decomposition through fractional rms, covariance and difference spectra, finding the mid- to long-timescale (~day-year) variability to be dominated by a relatively smooth, steep component, peaking in the soft X-ray band. Additionally, we find evidence for variable FeK emission red-ward of the FeK-alpha core on long timescales, consistent with previous findings. We detect a clearly-defined power spectrum which we model with a power law with a slope of alpha ~ 1.9. By extending the power spectrum to lower frequencies through the inclusion of Swift and RXTE data, we find tentative evidence of a high-frequency break, consistent with existing scaling relations. We also explore frequency-dependent Fourier time lags, detecting a negative ('soft') lag for the first time in this source with the 0.3-1 keV band lagging behind the 1-4 keV band with a time delay of ~900s. Finally, we analyze the variability in the optical and UV bands using the Optical/UV Monitor on-board XMM-Newton and the UVOT on-board Swift and search for time-dependent correlations between the optical/UV/X-ray bands. We find tentative evidence for the U-band emission lagging behind the X-rays with a time delay of 2.4 +/- 1.8 days, which we discuss in the context of disc reprocessing.

astro-ph.HE↗

arXiv2017

Nuclear absorption and emission in the AGN merger NGC 6240: the hard X-ray view

We present the analysis of four NuSTAR observations of the luminous infrared galaxy merger NGC 6240, hosting a close pair of highly obscured active galactic nuclei (AGN). Over a period of about two years, the source exhibits hard X-ray variability of the order of 20 per cent, peaking around 20 keV. When the two AGN are resolved with Chandra, column densities in the range $N_\textrm{H} \sim 1-2 \times 10^{24}$ cm$^{-2}$ are estimated for both of them. The exact values are hard to determine, as they appear to depend on aspects that are sometimes overlooked in Compton-thick objects, such as the covering factor of the absorber, iron abundance, and the contamination in the Fe-K band from foreground hot-gas emission. Nearly spherical covering and slightly subsolar iron abundance are preferred in this case. While the southern nucleus is suggested to be intrinsically more powerful, as also implied by the mid-IR and 2-10 keV brightness ratios, solutions involving a similar X-ray luminosity of the two AGN cannot be ruled out. The observed variability is rather limited compared to the one revealed by the Swift/BAT light curve, and it can be fully explained by changes in the continuum flux from the two AGN, without requiring significant column density variations. NGC 6240 is hereby confirmed to represent a unique opportunity to investigate the X-ray (and broad-band) properties of massive galaxy mergers, which were much more frequent in the early Universe.

astro-ph.HE↗

arXiv2017

Spatially resolved Fe K spectroscopy of NGC 4945

We present the imaging and spectroscopic analysis of the combined Chandra ACIS-S observations of the Compton-thick Seyfert 2 galaxy NGC 4945. We performed a spatially-resolved spectroscopy of the circumnuclear environment of the source, picturing the innermost 200 parsecs around the highly absorbed nucleus. The additional 200 ks ACIS-S data with respect to the previous campaign allowed us to map with even greater detail the central structure of this source and to discover an enhanced iron emission in the innermost nuclear region, with respect to the associated Compton reflection continuum. We revealed that the Equivalent Width of the iron K$α$ line is spatially variable (ranging from 0.5 to 3 keV), on scales of tens of parsecs, likely due to the ionization state and orientation effects of the reprocessing material, with respect to the central X-ray illuminating source. A clump of highly ionized Fe XXV He-$α$ is also detected, 40 parsecs east to the nucleus. When observations taken years apart are considered, the central unresolved reflected emission is found to remain constant.

astro-ph.HE↗

arXiv2017

A high spectral resolution map of the nuclear emitting regions of NGC 7582

We present the results of the spatial and spectral analysis of the deep (~200 ksec) Chandra HETG observation of the changing look AGN NGC7582. During this observation NGC7582 was in a highly obscured state. Therefore, we considered also a short Suzaku observation, which caught NGC7582 in a Compton thick state. This allows us to determine the underlying continuum and the amount of absorption ($N_H \sim~1.2\times10^{24}$ cm$^{-2}$). A wealth of emission lines are detected in the Chandra data, which allow us to map the structure of the circum-nuclear emitters. The high resolution spectrum reveals that the soft X-ray emission originates in a hybrid gas, which is ionized in part by the starforming activity and in part by the central AGN. The high resolution images confirm that the emitting region is inhomogeneous and extends up to a few hundred pc from the nuclear source. The X-ray images are more extended in the lower energy lines (Ne and Mg) than in the higher energy lines (Si, Fe), where the former are dominated by the collisionally ionised gas and the latter by the photoionized AGN emission. This is supported by the analysis of the He-like triplets. We deduce that a low density photoionized gas is responsible for the strong forbidden components, which is likely to originate from extended AGN Narrow Line Region gas at distances of 200-300 pc from the black hole. We also detected an absorption feature at ~ 6.7 keV consistent with the rest frame energy of the resonance absorption line from FeXXV, which traces the presence of a sub-parsec scale ionized absorber. The emerging picture is in agreement with our view of the circumnuclear gas in AGN, where the medium is clumpy and stratified in both density and ionization. These absorbers and emitters are located on different scales: from the sub-pc Broad Line Region gas out to the kpc scale of the galactic absorber.

astro-ph.HE↗

arXiv2017

High Resolution X-ray Spectroscopy of the Seyfert 1, Mrk 1040. Revealing the Failed Nuclear Wind with Chandra

High resolution X-ray spectroscopy of the warm absorber in the nearby X-ray bright Seyfert 1 galaxy, Mrk 1040 is presented. The observations were carried out in the 2013-2014 timeframe using the Chandra High Energy Transmission Grating with a total exposure of 200 ks. A multitude of absorption lines from Ne, Mg and Si are detected from a wide variety of ionization states. In particular, the detection of inner K-shell absorption lines from Ne, Mg and Si, from charge states ranging from F-like to Li-like ions, suggests the presence of a substantial amount of low ionization absorbing gas, illuminated by a steep soft X-ray continuum. The observations reveal at least 3 warm absorbing components ranging in ionization parameter from $\logξ= 0-2$ and with column densities of $N_{\rm H} =1.5-4.0 \times 10^{21}$cm$^{-2}$. The velocity profiles imply that the outflow velocities of the absorbing gas are low and within $\pm100$ km s$^{-1}$ of the systemic velocity of Mrk 1040, which suggests any outflowing gas may have stalled in this AGN on large enough scales. The warm absorber is likely located far from the black hole, within 300 pc of the nucleus and is spatially coincident with emission from an extended Narrow Line Region as seen in the HST images. The iron K band spectrum reveals only narrow emission lines, with Fe K$α$ at 6.4 keV consistent with originating from reflection off Compton thick pc-scale reprocessing gas.

astro-ph.HE↗

arXiv2016

A deep X-ray view of the bare AGN Ark 120. I. Revealing the Soft X-ray Line Emission

The Seyfert 1 galaxy, Ark 120, is a prototype example of the so-called class of bare nucleus AGN, whereby there is no known evidence for the presence of ionized gas along the direct line of sight. Here deep ($>400$ ks exposure), high resolution X-ray spectroscopy of Ark 120 is presented, from XMM-Newton observations which were carried out in March 2014, together with simultaneous Chandra/HETG exposures. The high resolution spectra confirmed the lack of intrinsic absorbing gas associated with Ark 120, with the only X-ray absorption present originating from the ISM of our own Galaxy, with a possible slight enhancement of the Oxygen abundance required with respect to the expected ISM values in the Solar neighbourhood. However, the presence of several soft X-ray emission lines are revealed for the first time in the XMM-Newton RGS spectrum, associated to the AGN and arising from the He and H-like ions of N, O, Ne and Mg. The He-like line profiles of N, O and Ne appear velocity broadened, with typical FWHM widths of $\sim5000$ km s$^{-1}$, whereas the H-like profiles are unresolved. From the clean measurement of the He-like triplets, we deduce that the broad lines arise from gas of density $n_{\rm e}\sim10^{11}$ cm$^{-3}$, while the photoionization calculations infer that the emitting gas covers at least 10 percent of $4π$ steradian. Thus the broad soft X-ray profiles appear coincident with an X-ray component of the optical-UV Broad Line Region on sub-pc scales, whereas the narrow profiles originate on larger pc scales, perhaps coincident with the AGN Narrow Line Region. The observations show that Ark 120 is not intrinsically bare and substantial X-ray emitting gas exists out of our direct line of sight towards this AGN.

astro-ph.HE↗

arXiv2016

Discovery of Broad Soft X-ray Absorption Lines from the Quasar Wind in PDS 456

High resolution soft X-ray spectroscopy of the prototype accretion disk wind quasar, PDS 456, is presented. Here, the XMM-Newton RGS spectra are analyzed from the large 2013-2014 XMM-Newton campaign, consisting of 5 observations of approximately 100 ks in length. During the last observation (hereafter OBS. E), the quasar is at a minimum flux level and broad absorption line profiles are revealed in the soft X-ray band, with typical velocity widths of $σ_{\rm v}\sim 10,000$ km s$^{-1}$. During a period of higher flux in the 3rd and 4th observations (OBS. C and D, respectively), a very broad absorption trough is also present above 1 keV. From fitting the absorption lines with models of photoionized absorption spectra, the inferred outflow velocities lie in the range $\sim 0.1-0.2c$. The absorption lines likely originate from He and H-like neon and L-shell iron at these energies. Comparison with earlier archival data of PDS 456 also reveals similar absorption structure near 1 keV in a 40 ks observation in 2001, and generally the absorption lines appear most apparent when the spectrum is more absorbed overall. The presence of the soft X-ray broad absorption lines is also independently confirmed from an analysis of the XMM-Newton EPIC spectra below 2 keV. We suggest that the soft X-ray absorption profiles could be associated with a lower ionization and possibly clumpy phase of the accretion disk wind, where the latter is known to be present in this quasar from its well studied iron K absorption profile and where the wind velocity reaches a typical value of 0.3$c$.

astro-ph.HE↗

arXiv2016

Short term X-ray spectral variability of the quasar PDS 456 observed in a low flux state

We present an analysis of the 2013 Suzaku campaign on the nearby luminous quasar PDS 456, covering a total duration of ~1 Ms and a net exposure of 455 ks. During these observations, the X-ray flux was suppressed by a factor of >10 in the soft X-ray band when compared to other epochs. We investigated the broadband continuum by constructing a spectral energy distribution, making use of the optical/UV photometry and hard X-ray spectra from the later XMM-Newton/NuSTAR campaign in 2014. The high energy part of this low flux state cannot be accounted for by self-consistent accretion disc and corona models without attenuation by absorbing gas, which partially covers a substantial fraction of the line of sight towards the X-ray source. Two absorption layers are required, of column density $\log (N_{\rm{H,low}}/{\rm cm^{-2}})=22.3\pm0.1$ and $\log (N_{\rm{H,high}}/{\rm cm^{-2}})=23.2\pm0.1$, with average covering factors of ~80% (with typical 5% variations) and 60% ($\pm$10-15%), respectively. In these observations PDS 456 displays significant short term X-ray spectral variability, on timescales of ~100 ks, which can be accounted for by variable covering of the absorbing gas. The partial covering absorber prefers an outflow velocity of $v_{\rm pc} = 0.25^{+0.01}_{-0.05}c$ at the >99.9% confidence level over the case where $v_{\rm pc}=0$. This is consistent with the velocity of the highly ionised outflow responsible for the blueshifted iron K absorption profile. We therefore suggest that the partial covering clouds could be the denser, or clumpy part of an inhomogeneous accretion disc wind. Finally we estimate the size-scale of the X-ray source from its variability. The radial extent of the X-ray emitter is found to be of the order ~15-20 $R_{\rm g}$, although the hard X-ray (>2 keV) emission may originate from a more compact or patchy corona of hot electrons, which is ~6-8 $R_{\rm g}$ in size.

astro-ph.HE↗

arXiv2013

Fast and Furious: Shock Heated Gas as the Origin of Spatially Resolved Hard X-ray Emission in the Central 5 kpc of the Galaxy Merger NGC 6240

We have obtained a deep, sub-arcsecond resolution X-ray image of the nuclear region of the luminous galaxy merger NGC 6240 with Chandra, which resolves the X-ray emission from the pair of active nuclei and the diffuse hot gas in great detail. We detect extended hard X-ray emission from kT~6 keV (~70 million K) hot gas over a spatial scale of 5 kpc, indicating the presence of fast shocks with velocity of ~2200 km/s. For the first time we obtain the spatial distribution of this highly ionized gas emitting FeXXV, which shows a remarkable correspondence to the large scale morphology of H_2(1-0) S(1) line emission and Hαfilaments. Propagation of fast shocks originated in the starburst driven wind into the ambient dense gas can account for this morphological correspondence. With an observed L(0.5-8 keV)=5.3E+41 erg/s, the diffuse hard X-ray emission is 100 times more luminous than that observed in the classic starburst galaxy M82. Assuming a filling factor of 1% for the 70 MK temperature gas, we estimate its total mass (M_{hot}=1.8E+8 Msun) and thermal energy (E_{th}=6.5E+57 ergs). The total iron mass in the highly ionized plasma is M_{Fe}=4.6E+5 Msun. Both the energetics and the iron mass in the hot gas are consistent with the expected injection from the supernovae explosion during the starburst that is commensurate with its high star formation rate. No evidence for fluorescent Fe I emission is found in the CO filament connecting the two nuclei.

astro-ph.CO↗

arXiv2013

The Exceptional Soft X-ray Halo of the Galaxy Merger NGC 6240

We report on a recent ~150-ks long Chandra observation of the ultraluminous infrared galaxy merger NGC 6240, which allows a detailed investigation of the diffuse galactic halo. Extended soft X-ray emission is detected at the 3-sigma confidence level over a diamond-shaped region with projected physical size of ~110x80 kpc, and a single-component thermal model provides a reasonably good fit to the observed X-ray spectrum. The hot gas has a temperature of ~7.5 million K, an estimated density of 2.5x10^{-3} cm^{-3}, and a total mass of ~10^10 M_sun, resulting in an intrinsic 0.4-2.5 keV luminosity of 4x10^41 erg s^{-1}. The average temperature of 0.65 keV is quite high to be obviously related to either the binding energy in the dark-matter gravitational potential of the system or the energy dissipation and shocks following the galactic collision, yet the spatially-resolved spectral analysis reveals limited variations across the halo. The relative abundance of the main alpha-elements with respect to iron is several times the solar value, and nearly constant as well, implying a uniform enrichment by type II supernovae out to the largest scales. Taken as a whole, the observational evidence is not compatible with a superwind originated by a recent, nuclear starburst, but rather hints at widespread, enhanced star formation proceeding at steady rate over the entire dynamical timescale (~200 Myr). The preferred scenario is that of a starburst-processed gas component gently expanding into, and mixing with, a pre-existing halo medium of lower metallicity (Z ~ 0.1 solar) and temperature (kT ~ 0.25 keV). This picture cannot be probed more extensively with the present data, and the ultimate fate of the diffuse, hot gas remains uncertain. Under some favorable conditions, at least a fraction of it might be retained after the merger completion, and evolve into the hot halo of a young elliptical galaxy.

astro-ph.CO↗

arXiv2012

X-ray absorption variability in NGC 4507

We present a complete spectral analysis of an XMM-Newton and Chandra campaign of the obscured AGN in NGC 4507, consisting of six observations spanning a period of six months, ranging from June 2010 to December 2010. We detect strong absorption variability on time scales between 1.5 and 4 months, suggesting that the obscuring material consists of gas clouds at parsec-scale distance. The lack of significant variability on shorter time scales suggests that this event is not due to absorption by broad line region clouds, which was instead found in other studies of similar sources. This shows that a single, universal structure of the absorber (either BLR clouds, or the parsec-scale torus) is not enough to reproduce the observed complexity of the X-ray absorption features of this AGN.

astro-ph.HE↗

arXiv2011

The effects of X-ray absorption variability in NGC 4395

We present a new X-ray analysis of the dwarf Seyfert galaxy NGC 4395, based on two archival XMM-Newton and Suzaku observations. This source is well known for a series of remarkable properties: one of the smallest estimated black hole masses among Active Galactic Nuclei (of the order of ~10^5 M_sun), intense flux variability on very short time-scales (a few tens of seconds), an unusually flat X-ray continuum (Gamma ~ 1.4 over the 2-10 keV energy range). NGC 4395 is also characterized by significant variations of the X-ray spectral shape, and here we show that such behaviour can be explained through the partial occultation by circumnuclear cold absorbers with column densities of ~10^22-10^23 cm^-2. In this scenario, the primary X-ray emission is best reproduced by means of a power law with a standard Gamma ~ 1.8 photon index, consistent with both the spectral slope observed at higher energies and the values typical of local AGN.

astro-ph.CO↗