arXiv subjects

Stefano Bianchi

Publications and source records attributed to Stefano Bianchi.

At least 163 records · Page 9Linked to original sources

arXiv2017

Photoionization instability of the Fe K absorbing plasma in the neutron star transient AX J1745.6-2901

AX J1745.6-2901 is a Low Mass X-ray Binary with an accreting neutron star, showing clear evidence for highly ionized absorption. Strong ionized Fe K$α$ and K$β$ absorption lines are always observed during the soft state, whereas they disappear during the hard states. We computed photoionization stability curves for the hard and the soft state, under different assumptions on the adopted spectral energy distributions and the physical parameters of the plasma. We observe that the ionized absorber lies always on a stable branch of the photoionization stability curve during the soft state, while it becomes unstable during the hard state. This suggests that photoionization instability plays a key role in defining the observable properties of the ionized absorber. The same process might explain the disappearance of the high ionization absorber/wind, during the hard state in other accreting neutron stars and black holes.

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

The NuSTAR view of the true Type 2 Seyfert NGC3147

We present the first NuSTAR observation of a 'true' Type 2 Seyfert galaxy. The 3-40 keV X-ray spectrum of NGC3147 is characterised by a simple power-law, with a standard Γ~1.7 and an iron emission line, with no need for any further component up to ~40 keV. These spectral properties, together with significant variability on time-scales as short as weeks (as shown in a 2014 Swift monitoring campaign), strongly support an unobscured line-of-sight for this source. An alternative scenario in terms of a Compton-thick source is strongly disfavoured, requiring an exceptional geometrical configuration, whereas a large fraction of the solid angle to the source is filled by a highly ionised gas, whose reprocessed emission would dominate the observed luminosity. Moreover, in this scenario the implied intrinsic X-ray luminosity of the source would be much larger than the value predicted by other luminosity proxies, like the [OIII]λ5007 emission line extinction-corrected luminosity. Therefore, we confirm with high confidence that NGC3147 is a true Type 2 Seyfert galaxy, intrinsically characterised by the absence of a BLR.

astro-ph.GA↗

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↗

arXiv2016

Multi-wavelength campaign on NGC7469 I. The rich 640 ks RGS spectrum

Outflows in active galaxies (AGNs) are common, although their launching mechanism, location, and physical impact on the host galaxy remain controversial. We conducted a multiwavelength six-month campaign to observe the nearby Seyfert galaxy NGC 7469 with several observatories in order to better understand and quantify the outflow in this AGN. We report on the time-integrated line-resolved X-ray spectrum of NGC 7469 obtained with the Reflection Grating Spectrometer (RGS) on board XMM-Newton. We use the RGS spectrum to discern the many AGN outflow components. A global fit is applied to obtain their physical parameters. We find that the AGN wind can be well described by three narrow velocity components at -650, -950, and -2050 km/s. The RGS clearly resolves the -20 50 km/s component in C5+ Ly$α$, while the -650 km/s and -950 km/s velocities are blended. Similar velocities are resolved in the UV. The H-equivalent column densities of these components are, respectively, NH = 7x10^20, 2.2x10^21, and 10^20 cm^-2, for a total of 3x10^21 cm^-2. The -650 km/s component shows a broad ionisation distribution. We identify a photo-ionised emission component blue-shifted by -450 km/s which we ascribe to the same outflow that produces the absorption lines. The elemental abundance ratios of C, N, Ne, S, and Fe to O in the outflow tend to be between 1 - 2 times solar. Preliminary estimates of the absorber distance from the AGN center suggest it is at least a few pc away from the center, but more advanced methods need to be applied in order to obtain better constraints. The complex X-ray spectrum of NGC 7469 demonstrates the richness of high energy phenomena taking place in AGN cores. The subtle spectroscopic differences between the various components require deep, high-resolution observations, such as the present RGS spectrum, if one is to resolve them and perform quantitative plasma diagnostics.

astro-ph.HE↗

arXiv2016

Non-linear variability in microquasars in relation with the winds from their accretion disks

The microquasar IGR J17091-3624, which is the recently discovered analogue of the well known source GRS 1915+105, exhibits quasi-periodic outbursts, with a period of 5-70 seconds, and regular amplitudes, referred to as "heartbeat state". We argue that these states are plausibly explained by accretion disk instability, driven by the dominant radiation pressure. Using our GLobal Accretion DIsk Simulation hydrodynamical code, we model these outbursts quantitatively. We also find a correlation between the presence of massive outflows launched from the accretion disk and the stabilization of its oscillations. We verify the theoretical predictions with the available timing and spectral observations. Furthermore, we postulate that the underlying non-linear differential equations that govern the evolution of an accretion disk are responsible for the variability pattern of several other microquasars, including XTE J1550-564, GX 339-4, and GRO J1655-40. This is based on the signatures of deterministic chaos in the observed lightcurves of these sources, which we found using the recurrence analysis method. We discuss these results in the frame of the accretion disk instability model.

astro-ph.HE↗

arXiv2016

Interplay between heartbeat oscillations and wind outflow in microquasar IGR J17091-3624

During the bright outburst in 2011, the black hole candidate IGR J17091-3624 exhibited strong quasi-periodic flare-like events (on timescales of tens of seconds) in some characteristic states, the so-called heartbeat state. From the theoretical point of view, these oscillations may be modeled by the process of accretion disk instability, driven by the dominant radiation pressure and enhanced heating of the plasma. Although the mean accretion rate in this source is probably below the Eddington limit, the oscillations will still have large amplitudes. As the observations show, the source can exhibit strong wind outflow during the soft state. This wind may help to partially or even completely stabilize the heartbeat. Using our hydrodynamical code GLADIS, we modeled the evolution of an accretion disk responsible for X-ray emission of the source. We accounted for a variable wind outflow from the disk surface. We examined the data archive from the Chandra and XMM-Newton satellites to find the observed limitations on the wind physical properties, such as its velocity and ionization state. We also investigated the long-term evolution of this source, which lasted over about 600 days of observations, using the data collected by the Swift and RXTE satellites. During this long period, the oscillations pattern and the observable wind properties changed systematically. We found that this source probably exhibits observable outbursts of appropriate timescales and amplitudes as a result of the disk instability. Our model requires a substantial wind component to explain the proper variability pattern, and even complete suppression of flares in some states. The wind mass-loss rate extracted from the data agrees quantitatively well with our scenario.

astro-ph.HE↗

arXiv2016

NGC5252: a pair of radio-emitting active galactic nuclei?

The X-ray source CXO J133815.6+043255 has counterparts in the UV, optical, and radio bands. Based on the multi-band investigations, it has been recently proposed by Kim et al. (2015) as a rarely-seen off-nucleus ultraluminous X-ray (ULX) source with a black hole mass of >= 10^4 solar mass in the nearby Seyfert galaxy NGC 5252. To explore its radio properties at very high angular resolution, we performed very long-baseline interferometry (VLBI) observations with the European VLBI Network (EVN) at 1.7 GHz. We find that the radio counterpart is remarkably compact among the known ULXs. It does not show a resolved structure with a resolution of a few milliarcsecond (mas), and the total recovered flux density is comparable to that measured in earlier sub-arcsecond-resolution images. The compact radio structure, the relatively flat spectrum, and the high radio luminosity are consistent with a weakly accreting supermassive black hole in a low-luminosity active galactic nucleus. The nucleus of NGC 5252 itself has similar radio properties. We argue that the system represents a relatively rare pair of active galactic nuclei, where both components emit in the radio.

astro-ph.HE↗

arXiv2016

Detection of Faint BLR Components in the Starburst/Seyfert Galaxy NGC 6221 and Measure of the Central BH Mass

In the last decade, using single epoch virial based techniques in the optical band, it has been possible to measure the central black hole mass on large AGN1 samples. However these measurements use the width of the broad line region as a proxy of the virial velocities and are therefore difficult to be carried out on those obscured (type 2) or low luminosity AGN where the nuclear component does not dominate in the optical. Here we present the optical and near infrared spectrum of the starburst/Seyfert galaxy NGC 6221, observed with X-shooter/VLT. Previous observations of NGC 6221 in the X-ray band show an absorbed (N_H=8.5 +/- 0.4 x 10^21 cm^-2) spectrum typical of a type 2 AGN with luminosity log(L_14-195 keV) = 42.05 erg/s, while in the optical band its spectrum is typical of a reddened (A_V=3) starburst. Our deep X-shooter/VLT observations have allowed us to detect faint broad emission in the H_alpha, HeI and Pa_beta lines (FWHM ~1400-2300 km/s) confirming previous studies indicating that NGC 6221 is a reddened starburst galaxy which hosts an AGN. We use the measure of the broad components to provide a first estimate of its central black hole mass (M_BH = 10^(6.6 +/- 0.3) Msol, lambda_Edd=0.01-0.03), obtained using recently calibrated virial relations suitable for moderately obscured (N_H<10^24 cm^-2) AGN.

astro-ph.GA↗

arXiv2015

An Off-Nucleus Nonstellar Black Hole in the Seyfert Galaxy NGC 5252

We report the discovery of a ultraluminous X-ray source (ULX; CXO J133815.6+043255) in NGC 5252. This ULX is an off-nuclear point-source, which is 22$^{\prime\prime}$ away from the center of NGC 5252, and has an X-ray luminosity of 1.5 $\times$ $10^{40}$erg s$^{-1}$. It is one of the rare examples of ULX, which exhibits clear counterparts in radio, optical, UV bands. Follow-up optical spectrum of the ULX shows strong emission lines. The redshift of [O III] emission line coincides with the systematic velocity of NGC 5252, suggesting the ULX is gravitationally bound to NGC 5252. The flux of [O III] appears to be correlated with both X-ray and radio luminosity in the same manner as ordinary AGNs, indicating that the [O III] emission is intrinsically associated with the ULX. Based on the multiwavelength data, we argue that the ULX is unlikely to be a background AGN. A more likely option is an accreting BH with a black hole mass of $\geq 10^4M_\odot$, which might be a stripped remnant of a merging dwarf galaxy.

astro-ph.GA↗

arXiv2015

On the evolution of particle-puffs in turbulence

We study the evolution of turbulent puffs by means of high-resolution numerical simulations. Puffs are bunches of passive particles released from an initially spherical distribution at regular time intervals of the order of the Kolmogorov time. The instantaneous shapes of particle puffs, in particular their asphericity and prolateness, are characterized by measuring the gyration tensor. Analysis has been performed by following, up to one large scale eddy-turn-over time, more than $10^4$ different puffs, each made of 2000 particle tracers, emitted from different places in a homogeneous and isotropic turbulent fluid with Taylor-scale Reynolds number $Re_λ\sim 300$. We also analyze the probability of hitting a given target placed downstream with respect to the local wind at the time of emission, presenting data for three different cases: (i) without any reconstruction of the shape, i.e. considering the bunch of point tracers, and approximating the particle-puff as a (ii) sphere or as an (iii) ellipsoid. The results show a strong dependence on the fluctuations of the instantaneous wind at the moment of the emission and appear to be robust with respect to the approximations (i)-(iii).

physics.flu-dyn↗

arXiv2015

Multiple AGN in the crowded field of the compact group SDSSJ0959+1259

We present a multi-wavelength study of a newly discovered compact group (CG), SDSS J0959+1259, based data from XMM-Newton, SDSS and the Calar Alto optical imager BUSCA. With a maximum velocity offset of 500 km s$^{-1}$, a mean redshift of 0.035, and a mean spatial extension of 480 kpc, this CG is exceptional in having the highest concentration of nuclear activity in the local Universe, established with a sensitivity limit L$_{X}>4\times $10$^{40}$ erg s$^{-1}$ in 2--10 keV band and R-band magnitude $M_R < -19$. The group is composed of two type-2 Seyferts, one type-1 Seyfert, two LINERs and three star forming galaxies. Given the high X-ray luminosity of LINERs which reaches $\sim 10^{41}$ erg s$^{-1}$, it is likely that they are also accretion driven, bringing the number of active nuclei in this group to to 5 out of 8 (AGN fraction of 60\%). The distorted shape of one member of the CG suggests that strong interactions are taking place among its galaxies through tidal forces. Therefore, this system represents a case study for physical mechanisms that trigger nuclear activity and star formation in CGs.

astro-ph.GA↗

arXiv2015

XMM-Newton Observations of Three Interacting Luminous Infrared Galaxies

We investigate the X-ray properties of three interacting luminous infrared galaxy systems. In one of these systems, IRAS 18329+5950, we resolve two separate sources. A second, IRAS 20550+1656, and third, IRAS 19354+4559, have only a single X-ray source detected. We compare the observed emission to PSF profiles and determine that three are extended in emission. One is compact, which is suggestive of an AGN, although all of our profiles have large uncertainties. We then model the spectra to determine soft (0.5--2 keV) and hard (2--10 keV) luminosities for the resolved sources and then compare these to relationships found in the literature between infrared and X-ray luminosities for starburst galaxies. We obtain luminosities of $\log(L_{\textrm{soft}}/\textrm{L}_{\odot}) = 7.32,\:7.06,\:7.68$ and $\log(L_{\textrm{hard}}/\textrm{L}_{\odot}) = 7.33,\: 7.07,\: 7.88$ for IRAS 18329+5950, IRAS 19354+4559, and IRAS 20550+1656, respectively. These are intermediate to two separate predictions in the literature for star-formation-dominated sources. Our highest quality spectrum of IRAS 20550+1656 suggests super-solar abundance of alpha elements at $2σ$ significance, with $\log(\fracα{α_{\odot}}) = [α] = 0.4\pm0.2$. This is suggestive of recent enrichment with Type II supernovae, consistent with a starburst environment. The X-ray properties of the target galaxies are most likely due to starbursts, but we cannot conclusively rule out AGN.

astro-ph.CO↗

arXiv2015

Extending Virial Black Hole Mass Estimates to Low-Luminosity or Obscured AGN: the cases of NGC 4395 and MCG -01-24-012

In the last decade, using single epoch (SE) virial based spectroscopic optical observations, it has been possible to measure the black hole (BH) mass on large type 1 Active Galactic Nuclei (AGN) samples. However this kind of measurements can not be applied on those obscured type 2 and/or low luminosity AGN where the nuclear component does not dominate in the optical. We have derived new SE relationships, based on the FWHM and luminosity of the broad line region component of the Pabeta emission line and/or the hard X-ray luminosity in the 14-195 keV band, which have the prospect of better working with low luminosity or obscured AGN. The SE relationships have been calibrated in the 10^5-10^9 M_sol mass range, using a sample of AGN whose BH masses have been previously measured using reverberation mapping techniques. Our tightest relationship between the reverberation-based BH mass and the SE virial product has an intrinsic spread of 0.20 dex. Thanks to these SE relations, in agreement with previous estimates, we have measured a BH mass of M_BH =1.7^+1.3_-0.7 X 10^5 M_sol for the low luminosity, type 1, AGN NGC 4395 (one of the smallest active galactic BH known). We also measured, for the first time, a BH mass of M_BH = 1.5^+1.1_-0.6 X 10^7 M_sol for the Seyfert 2 galaxy MCG -01-24-012.

astro-ph.GA↗

arXiv2014

Black hole spin and size of the X-ray emitting region(s) in the Seyfert 1.5 galaxy ESO 362-G18

We report results from multi-epoch X-ray observations of the Seyfert 1.5 galaxy ESO 362-G18 performed between November 2005 and June 2010. ESO 362-G18 generally exhibits the typical X-ray spectrum of type 1 Active Galactic Nuclei (AGN). A disc-reflection component accounts for broad residuals in the iron K band and above 10~keV, as well as for a significant soft excess. From our best-fitting reflection model, we measure a black hole spin greater or equal to 0.92 at the 99.99 per cent confidence level. ESO 362-G18 is also (typically) mildly absorbed by a column of neutral gas. The absorber is variable and one observation, performed ~2 months after a typical mildly absorbed one, is heavily absorbed by a cold column density two orders of magnitude higher than that during any other observation. UV variability between the heavily absorbed observation and the others suggests that the absorber can be identified with a dusty, clumpy torus. The absorption variability timescale enables us to locate the X-ray emitting region within the innermost ~50 gravitational radii. Such result holds not only for the X-ray continuum, but also for the soft excess.

astro-ph.HE↗

arXiv2014

Absorption at the dust sublimation radius and the dichotomy between X-ray and optical classification in the Seyfert galaxy H0557-385

In this work, the analysis of multi-epoch (1995-2010) X-ray observations of the Seyfert 1 galaxy H0557-385 is presented. The wealth of data presented in this analysis show that the source exhibits dramatic spectral variability, from a typical unabsorbed Seyfert 1 type spectrum to a Compton-thin absorbed state, on time scales of ~5 years. This extreme change in spectral shape can be attributed to variations in the column density and covering fraction of a neutral absorbing medium attenuating the emission from the central continuum source. Evidence for Compton reflection of the intrinsic nuclear emission is present in each of the spectra, though this feature is most prominent in the low-state spectra, where the associated Fe emission line complex is clearly visible. In addition to the variable absorbing medium, a warm absorber component has been detected in each spectral state. Optical spectroscopy concurrent with the 2010 XMM-Newton observation campaign have detected the presence of broad optical emission lines during an X-ray absorption event.From the analysis of both X-ray and optical spectroscopic data, it has been inferred that the X-ray spectral variability is a result of obscuration of the central emission region by a clumpy absorber covering >80 per cent of the source with an average column density of NH ~7x10^{23} cm^{-2}, and which is located outside the broad line region at a distance from the central source consistent with the dust sublimation radius of the AGN.

astro-ph.HE↗

arXiv2014

A new cosmological distance measure using AGN X-ray variability

We report the discovery of a luminosity distance estimator using Active Galactic Nuclei (AGN). We combine the correlation between the X-ray variability amplitude and the Black Hole (BH) mass with the single epoch spectra BH mass estimates which depend on the AGN luminosity and the line width emitted by the broad line region. We demonstrate that significant correlations do exist which allows one to predict the AGN (optical or X-ray) luminosity as a function of the AGN X-ray variability and either the HBeta or the PaBeta line widths. In the best case, when the PaBeta is used, the relationship has an intrinsic dispersion of ~0.6 dex. Although intrinsically more disperse than Supernovae Ia, this relation constitutes an alternative distance indicator potentially able to probe, in an independent way, the expansion history of the Universe. With this respect, we show that the new mission concept Athena should be able to measure the X-ray variability of hundreds of AGN and then constrain the distance modulus with uncertainties of 0.1 mag up to z~0.6. We also discuss how, using a new dedicated wide field X-ray telescope able to measure the variability of thousands of AGNs, our estimator has the prospect to become a cosmological probe even more sensitive than current Supernovae Ia samples.

astro-ph.CO↗

arXiv2013

XIPE: the X-ray Imaging Polarimetry Explorer

X-ray polarimetry, sometimes alone, and sometimes coupled to spectral and temporal variability measurements and to imaging, allows a wealth of physical phenomena in astrophysics to be studied. X-ray polarimetry investigates the acceleration process, for example, including those typical of magnetic reconnection in solar flares, but also emission in the strong magnetic fields of neutron stars and white dwarfs. It detects scattering in asymmetric structures such as accretion disks and columns, and in the so-called molecular torus and ionization cones. In addition, it allows fundamental physics in regimes of gravity and of magnetic field intensity not accessible to experiments on the Earth to be probed. Finally, models that describe fundamental interactions (e.g. quantum gravity and the extension of the Standard Model) can be tested. We describe in this paper the X-ray Imaging Polarimetry Explorer (XIPE), proposed in June 2012 to the first ESA call for a small mission with a launch in 2017 but not selected. XIPE is composed of two out of the three existing JET-X telescopes with two Gas Pixel Detectors (GPD) filled with a He-DME mixture at their focus and two additional GPDs filled with pressurized Ar-DME facing the sun. The Minimum Detectable Polarization is 14 % at 1 mCrab in 10E5 s (2-10 keV) and 0.6 % for an X10 class flare. The Half Energy Width, measured at PANTER X-ray test facility (MPE, Germany) with JET-X optics is 24 arcsec. XIPE takes advantage of a low-earth equatorial orbit with Malindi as down-link station and of a Mission Operation Center (MOC) at INPE (Brazil).

astro-ph.HE↗