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See detaillnsight on quasar changing-look physics from optical polarimetry
Hutsemekers, Damien ULiege; Agis-Gonzalez, Beatriz ULiege; Sluse, Dominique ULiege et al

Conference (2018, December 11)

A handful of quasars changing from type 1 (strong broad and narrow emission lines) to type 1.9 (strong narrow lines only and dim continuum) on timescales of a few years have been recently uncovered. If ... [more ▼]

A handful of quasars changing from type 1 (strong broad and narrow emission lines) to type 1.9 (strong narrow lines only and dim continuum) on timescales of a few years have been recently uncovered. If the disappearance of the broad emission lines observed in changing-look quasars were caused by the obscuration of the quasar core through moving dust clouds in the torus, high linear polarization typical of type 2 quasars would be expected. We measured the polarization of the changing-look quasar J1011+5442 in which the broad emission lines have disappeared between 2003 and 2015. We found a polarization degree compatible with null polarization. This measurement suggests that the observed change of look is not due to a change of obscuration in a torus hiding the continuum source and the broad line region. Our results thus support the idea that the vanishing of the broad emission lines in J1011+5442 is due to an intrinsic dimming of the ionizing continuum source that is most likely caused by a rapid decrease in the rate of accretion onto the supermassive black hole. New polarization measurements have been secured for a sample of changing-look quasars. They essentially confirm our previous results. [less ▲]

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See detailTRUE2: unveiling the nature of true Seyfert 2 candidates through optical polarimetry
Agis-Gonzalez, Beatriz ULiege; Hutsemekers, Damien ULiege; Acosta Pulido, Jose et al

Poster (2018, December 10)

The so-called true Seyfert 2 candidates [1] are Seyferts galaxies whose optical spectra do not show broad lines. Yet, in the X-ray domain, they exhibit some characteristic behavior of Seyferts 1 such as ... [more ▼]

The so-called true Seyfert 2 candidates [1] are Seyferts galaxies whose optical spectra do not show broad lines. Yet, in the X-ray domain, they exhibit some characteristic behavior of Seyferts 1 such as lack of X-ray obscuration and/or short timescale variability. A true Seyfert 2 candidate will be confirmed as a true Seyfert 2 galaxy if the lack of the broad line region (BLR) emission is not only observational but physical. Since the BLR is hidden behind the circumnuclear, optically-thick, dusty torus, only polar-scattered light can probe the presence or absence of the BLR. Hence, scattering-induced polarization is the only way to probe the existence of hidden-BLRs (HBLR). [less ▲]

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See detailProbing the structure and evolution of active galactic nuclei with the ultraviolet polarimeter POLLUX aboard LUVOIR
Marin, F.; Charlot, S.; Hutsemekers, Damien ULiege et al

in SF2A-2018: Proceedings of the Annual meeting of the French Society of Astronomy and Astrophysics (2018, December 01)

The ultraviolet (UV) polarization spectrum of nearby active galactic nuclei (AGN) is poorly known. The Wisconsin Ultraviolet Photo-Polarimeter Experiment and a handful of instruments on board the Hubble ... [more ▼]

The ultraviolet (UV) polarization spectrum of nearby active galactic nuclei (AGN) is poorly known. The Wisconsin Ultraviolet Photo-Polarimeter Experiment and a handful of instruments on board the Hubble Space Telescope were able to probe the near- and mid-UV polarization of nearby AGN, but the far-UV band (from 1200 Å down to the Lyman limit at 912 Å) remains completely uncharted. In addition, the linewidth resolution of previous observations was at best 1.89 Å. Such a resolution is not sufficient to probe in detail quantum mechanical effects, synchrotron and cyclotron processes, scattering by electrons and dust grains, and dichroic extinction by asymmetric dust grains. Exploring those physical processes would require a new, high-resolution, broadband polarimeter with full ultraviolet-band coverage. In this context, we discuss the AGN science case for POLLUX, a high-resolution UV spectropolarimeter, proposed for the 15-meter primary mirror option of LUVOIR (a multi-wavelength space observatory concept being developed by the Goddard Space Flight Center and proposed for the 2020 Decadal Survey Concept Study). [less ▲]

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See detailTRUE2: Establishing a detectability limit on hidden broad line regions
Agis-Gonzalez, Beatriz ULiege; Hutsemekers, Damien ULiege; Acosta Pulido, Jose et al

Poster (2018, July)

True Seyfert 2 candidates are those Seyferts galaxies whose optical spectral do not show broad lines, nevertheless in the X-ray domain, they exhibit some characteristic behavior of Seyferts 1 such as lack ... [more ▼]

True Seyfert 2 candidates are those Seyferts galaxies whose optical spectral do not show broad lines, nevertheless in the X-ray domain, they exhibit some characteristic behavior of Seyferts 1 such as lack of X-ray obscuration and/or short timescale variability. A true 2 candidate will be confirmed as a true Seyfert 2 if the lack of its broad line region (BLR) is not only observational but physical. These kind of objects are thought to accrete at low Eddington rates, in agreement with theoretical models that predict that the BLR disappears below a certain critical value of accretion rate and/or luminosity. In the last decade, a significant number of true Seyfert 2s with low accretion rates has been claimed in the literature. However, some exceptions as GNS 069 or 2XMM J1231+1106 show high accretion rates, which seem to contradict the generally accepted explanation. A limit on the detection of hidden broad line regions (HBLRs) must be established in order to make sure that BLRs are not present intrinsically. Since true Seyfert 2 candidates are selected by the absence of X-ray obscuration, the most plausible explanation to cause the non-detection of a physically present HBLR would be the absence of an adequate scattering medium. Polarimetry can play a key role to answer this question. The presence of an efficient scattering region would imply a high continuum of polarization. We propose to assess what degrees of polarization are high enough to indicate the presence of a scattering medium able to act as a mirror and thus providing us with the indirect view of the HBLRs. We got new imaging polarimetry data from ISIS@WHT of 10 true 2 candidates which had not been checked in polarized light. If scattering regions are present, undeniable degrees of polarization around 1−3% should be measured. Comparing the measured continuum of polarization with simulations we will be able to estimate a decidability limit on HBLRs. Specifically, we will apply STOKES, a Monte Carlo radiative transfer code which can be used to model, predict, fit and interpret the polarization of AGN [less ▲]

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See detailOptical polarimetry within the changing-look AGN scenario
Agis-Gonzalez, Beatriz ULiege; Hutsemekers, Damien ULiege; Sluse, Dominique ULiege et al

Conference (2018, April 03)

Changing-look AGN are a extraordinary type of AGN which represents a violation of the AGN unified model. Studying this class of objects in polarized light helps us to shade light on the sub-parsec AGN ... [more ▼]

Changing-look AGN are a extraordinary type of AGN which represents a violation of the AGN unified model. Studying this class of objects in polarized light helps us to shade light on the sub-parsec AGN structure and find out which additional ingredients are needed so that the unified model can describe the complex AGN taxonomy. We carried out a detailed analysis of multi-epoch observations of the Seyfert galaxy ESO 362-G18 in X- ray, UV and optical ranges. In total, 45 X-ray observations and 4 optical data sets, 2 of them in polarized light, ranging from January 2003 till March 2016. The four optical analyzed spectra reveal ESO362-G18 as a changing-look Seyfert galaxy, since two spectra are of type 1.5 and the remaining two show ESO362-G18 as a type 1.9 AGN. We have polarized measurements of one of the type 1.5 data sets and also one of the type 1.9, allowing us to compare the polarization properties in both states of a changing-look Seyfert galaxy by first time. From the X-ray data set, we found ESO362-G18 to exhibit relativistic reflection, including a detected soft time lag between continuum and reflection components, whose results support the compact nature of the X-ray emitting regions. We proved a very rapidly spinning Kerr back hole and a very high inclination of ~53o(in two ways, dependent and independent model; see Agís-González et al 2014). Thanks to this derived model, we have also detected two absorption events driven by clouds situated at torus scales. The relatively high inclination we derived is consistent with the idea that our LOS is grazing the obscuring torus (which has a typical half-opening angle of the order of 45o). If the torus is not homogeneous but clumpy, such high inclination may intercept from time to time some of the clumps of the obscuring torus, explaining the detected X-ray absorption events and possibly why ESO362-G18 exhibits changes of look from type 1 to type 2 in its optical spectra. Moreover, the X-ray luminosity keeps roughly constant along the available observation, which discard a change in the accretion rate. Therefore, we will discuss the polarized properties of this intriguing changing-look Seyfert galaxy whose change of look is probably due to absorption in our LOS. At the same time, we will also expose optical polarimetry as a diagnostic tool to disentangle the cause of changes of look. [less ▲]

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See detailA Changing-Look AGN to Be Probed by X-ray Polarimetry
Agis-Gonzalez, Beatriz ULiege; Hutsemekers, Damien ULiege; Miniutti, Giovanni

in Galaxies (2018), 6

Active galactic nuclei (AGN) produce the highest intrinsic luminosities in the Universe from within a compact region. The central engine is thought to be powered by accretion onto a supermassive black ... [more ▼]

Active galactic nuclei (AGN) produce the highest intrinsic luminosities in the Universe from within a compact region. The central engine is thought to be powered by accretion onto a supermassive black hole. A fraction of this huge release of energy influences the evolution of the host galaxy, and in particular, star formation. Thus, AGN are key astronomical sources not only because they play an important role in the evolution of the Universe, but also because they constitute a laboratory for extreme physics. However, these objects are under the resolution limit of current telescopes. Polarimetry is a unique technique capable of providing us with information on physical AGN structures. The incoming new era of X-ray polarimetry will give us the opportunity to explore the geometry and physical processes taking place in the innermost regions of the accretion disc. Here we exploit this future powerful tool in the particular case of changing-look AGN, which are key for understanding the complexity of AGN physics. [less ▲]

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See detailPolarization of the changing-look quasar J1011+5442
Hutsemekers, Damien ULiege; Agis-Gonzalez, Beatriz ULiege; Sluse, Dominique ULiege et al

in Astronomy and Astrophysics (2017), 604

If the disappearance of the broad emission lines observed in changing-look quasars were caused by the obscuration of the quasar core through moving dust clouds in the torus, high linear polarization ... [more ▼]

If the disappearance of the broad emission lines observed in changing-look quasars were caused by the obscuration of the quasar core through moving dust clouds in the torus, high linear polarization typical of type 2 quasars would be expected. We measured the polarization of the changing-look quasar J1011+5442 in which the broad emission lines have disappeared between 2003 and 2015. We found a polarization degree compatible with null polarization. This measurement suggests that the observed change of look is not due to a change of obscuration hiding the continuum source and the broad line region, and that the quasar is seen close to the system axis. Our results thus support the idea that the vanishing of the broad emission lines in J1011+5442 is due to an intrinsic dimming of the ionizing continuum source that is most likely caused by a rapid decrease in the rate of accretion onto the supermassive black hole. Based on observations made with the William Herschel telescope operated on the island of La Palma by the Isaac Newton Group of Telescopes in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. [less ▲]

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See detailPolarimetric view of the changing type Seyfert galaxy ESO 362-G018.
Agis-Gonzalez, Beatriz ULiege; Bagnulo, S.; Hutsemekers, Damien ULiege et al

in Highlights on Spanish Astrophysics IX (2017, March 01)

ESO362-G018 is an active galactic nucleus (AGN) which is classified as a Seyfert 1.5 galaxy e.g. by Bennert et al. (2006), (black data set on figure 1). However, Parisi et. al (2009) found an optical ... [more ▼]

ESO362-G018 is an active galactic nucleus (AGN) which is classified as a Seyfert 1.5 galaxy e.g. by Bennert et al. (2006), (black data set on figure 1). However, Parisi et. al (2009) found an optical spectrum of this source which was taken during the 6dF Galaxy Survey, but it does not show the broad Balmer lines required to classify it as Seyfert 1 galaxy (red data set on figure 1). On the other hand, the results obtained by Agis-Gonzalez et al. (2014❩ in a X-ray analysis of this same source reveal that the inclination of ESO362- G018 i = 53° ± 5° is consistent with the picture of an AGN looked through the upper layers of a clumpy, dusty torus. Thus, according to the Unification Models of AGN and the clumpy nature of the torus, our interpretation of the different spectra is the following one. On 30th of January of 2003 (when the spectrum belonging to the 6dF survey was obtained), our line of sight intercepted a (or several aligned) torus clump(s) with much greater column density than its environment. Accordingly, the nucleus and the broad line region (❨BLR)❩ would be obscured. This allowed only the narrow emission lines to emerge from the narrow line region (NRL). Otherwise, on 18th of September of 2004 (when the spectrum by Bennert et al. 2006 was obtained) there is no clump to intercept and the BLR is not obscured so that the broad Balmer emission lines could be detected. [less ▲]

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See detailEclipsing the innermost accretion disc regions in AGN
Sanfrutos, M.; Miniutti, G.; Dovčiak, M. et al

in Astronomische Nachrichten (2016), 337

Variable X-ray absorption has been observed in active galactic nuclei (AGN) on several time scales. Observations allow us to identify the absorber with clouds associated either with the clumpy torus ... [more ▼]

Variable X-ray absorption has been observed in active galactic nuclei (AGN) on several time scales. Observations allow us to identify the absorber with clouds associated either with the clumpy torus (parsec scales, long timescales) or with the broad line region (BLR) (short timescales). In the latter, the cloud size has been estimated to be of the order of few gravitational radii from the observed absorption variability. Such small cloud sizes are comparable to the X-ray emitting regions so that a detailed modeling of occultation events in AGN has the potential of enabling us to infer accurately the geometry of the system. We have developed a relativistic X-ray spectral model for occultation events and we present here theoretical predictions on the different observables that can be inferred by studying X-ray eclipses in simulated XMM-Newton data. These include the size of the X-ray emitting regions as well as more fundamental parameters such as the black hole spin and the system inclination. We find that absorption varies as a function of the energy range and that its maximum takes place when the approaching part of the accretion disc is covered. Therefore we study the hard-to-soft (H / S) ratio light curves produced during an eclipse and use them to characterise the properties of the inner accretion disc in a new model-independent way. [less ▲]

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See detailThe ionized X-ray outflowing torus in ESO 323-G77: low-ionization clumps confined by homogeneous warm absorbers
Sanfrutos, M.; Miniutti, G.; Krongold, Y. et al

in Monthly Notices of the Royal Astronomical Society (2016), 457

We report on the long- and short-term X-ray spectral analysis of the polar-scattered Seyfert 1.2 galaxy ESO 323-G77, observed in three epochs between 2006 and 2013 with Chandra and XMM-Newton. Four high ... [more ▼]

We report on the long- and short-term X-ray spectral analysis of the polar-scattered Seyfert 1.2 galaxy ESO 323-G77, observed in three epochs between 2006 and 2013 with Chandra and XMM-Newton. Four high-resolution Chandra observations give us a unique opportunity to study the properties of the absorbers in detail, as well as their short time-scale (days) variability. From the rich set of absorption features seen in the Chandra data, we identify two warm absorbers with column densities and ionizations that are consistent with being constant on both short and long time-scales, suggesting that those are the signatures of a rather homogeneous and extended outflow. A third absorber, ionized to a lesser degree, is also present and it replaces the strictly neutral absorber that is ubiquitously inferred from the X-ray analysis of obscured Compton-thin sources. This colder absorber appears to vary in column density on long time-scales, suggesting a non-homogeneous absorber. Moreover, its ionization responds to the nuclear luminosity variations on time-scales as short as a few days, indicating that the absorber is in photoionization equilibrium with the nuclear source on these time-scales. All components are consistent with being co-spatial and located between the inner and outer edges of the so-called dusty, clumpy torus. Assuming co-spatiality, the three phases also share the same pressure, suggesting that the warm / hot phases confine the colder, most likely clumpy, medium. We discuss further the properties of the outflow in comparison with the lower resolution XMM-Newton data. [less ▲]

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See detailShowing variability in AGN by principal component analysis (PCA)
Agis-Gonzalez, Beatriz ULiege; Risaliti, G.; Miniutti, G.

Poster (2015, July 01)

Principal component analysis (PCA) is a powerful tool for studying spectral variability. The technique consists of splitting relatively long exposures into a series of shorter-exposure spectra, and ... [more ▼]

Principal component analysis (PCA) is a powerful tool for studying spectral variability. The technique consists of splitting relatively long exposures into a series of shorter-exposure spectra, and returns a minimal set of independent spectral shapes representing the variable components. If the initial spectra are made up of a linear sum of variable, uncorrelated and spectrally distinct physical components, the PCA will return detailed spectra of each variable component in a model independent way. This is a big advantage to analyze and study the origin of the observed variability without being limited by available spectral models (and by the systematic uncertainties that are inherent to any spectral analysis). We are applying the PCA analysis to several XMM-Newton observations from the brightest and most variable AGN with sufficiently long exposures. We shall present the most interesting results obtained so far. [less ▲]

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See detailShowing spectral variability in AGN by principal component analysis (PCA)
Agis-Gonzalez, Beatriz ULiege

Conference (2015, June)

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See detailBlack hole spin and size of the X-ray-emitting region(s) in the Seyfert 1.5 galaxy ESO 362-G18
Agis-Gonzalez, Beatriz ULiege; Miniutti, G.; Kara, E. et al

in Monthly Notices of the Royal Astronomical Society (2014), 443

We report results from multi-epoch X-ray observations of the Seyfert 1.5 galaxy ESO 362-G18 performed between 2005 November and 2010 June. ESO 362-G18 generally exhibits the typical X-ray spectrum of type ... [more ▼]

We report results from multi-epoch X-ray observations of the Seyfert 1.5 galaxy ESO 362-G18 performed between 2005 November and 2010 June. ESO 362-G18 generally exhibits the typical X-ray spectrum of type 1 active galactic nuclei. 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 a ≥ 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 of ˜ 3-4 × 10[SUP]23[/SUP] cm[SUP]-2[/SUP], nearly 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 time-scale 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. [less ▲]

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See detailESO 362-G18: black hole spin and the size of the X-ray emitting region
Agis-Gonzalez, Beatriz ULiege; Miniutti, Giovanni; Kara, Erin et al

Conference (2014, June 16)

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See detailThe properties of the clumpy torus and BLR in the polar-scattered Seyfert 1 galaxy ESO 323-G77 through X-ray absorption variability
Miniutti, G.; Sanfrutos, M.; Beuchert, T. et al

in Monthly Notices of the Royal Astronomical Society (2014), 437

We report results from multi-epoch (2006-2013) X-ray observations of the polar-scattered Seyfert 1 galaxy ESO 323-G77. The source exhibits remarkable spectral variability from months to years timescales ... [more ▼]

We report results from multi-epoch (2006-2013) X-ray observations of the polar-scattered Seyfert 1 galaxy ESO 323-G77. The source exhibits remarkable spectral variability from months to years timescales. The observed spectral variability is entirely due to variations of the column density of a neutral absorber towards the intrinsic nuclear continuum. The column density is generally Compton-thin ranging from a few times 10[SUP]22[/SUP] cm[SUP]-2[/SUP] to a few times 10[SUP]23[/SUP] cm[SUP]-2[/SUP]. However, one observation reveals a Compton-thick state with column density of the order of 1.5 × 10[SUP]24[/SUP] cm[SUP]-2[/SUP]. The observed variability offers a rare opportunity to study the properties of the X-ray absorber(s) in an active galaxy. We identify variable X-ray absorption from two different components, namely (i) a clumpy torus whose individual clumps have a density of ≤1.7 × 10[SUP]8[/SUP] cm[SUP]-3[/SUP] and an average column density of ˜4 × 10[SUP]22[/SUP] cm[SUP]-2[/SUP], and (ii) the broad-line region (BLR), comprising individual clouds with density of 0.1-8 × 10[SUP]9[/SUP] cm[SUP]-3[/SUP] and column density of 10[SUP]23[/SUP]-10[SUP]24[/SUP] cm[SUP]-2[/SUP]. The derived properties of the clumpy torus can also be used to estimate the torus half-opening angle, which is of the order of 47°. We also confirm the previously reported detection of two highly ionized warm absorbers with outflow velocities of 1000-4000 km s[SUP]-1[/SUP]. The observed outflow velocities are consistent with the Keplerian/escape velocity at the BLR. Hence, the warm absorbers may be tentatively identified with the warm/hot intercloud medium which ensures that the BLR clouds are in pressure equilibrium with their surroundings. The BLR line-emitting clouds may well be the cold, dense clumps of this outflow, whose warm/hot phase is likely more homogeneous, as suggested by the lack of strong variability of the warm absorber(s) properties during our monitoring. [less ▲]

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See detailThe size of the X-ray emitting region in SWIFT J2127.4+5654 via a broad line region cloud X-ray eclipse
Sanfrutos, M.; Miniutti, G.; Agis-Gonzalez, Beatriz ULiege et al

in Monthly Notices of the Royal Astronomical Society (2013), 436

We present results obtained from the time-resolved X-ray spectral analysis of the Narrow-Line-Seyfert 1 galaxy SWIFT J2127.4+5654 during a ˜130 ks XMM-Newton observation. We reveal large spectral ... [more ▼]

We present results obtained from the time-resolved X-ray spectral analysis of the Narrow-Line-Seyfert 1 galaxy SWIFT J2127.4+5654 during a ˜130 ks XMM-Newton observation. We reveal large spectral variations, especially during the first ˜90 ks of the XMM-Newton exposure. The spectral variability can be attributed to a partial eclipse of the X-ray source by an intervening low-ionization/cold absorbing structure (cloud) with column density N[SUB]H[/SUB] = 2.0^{+0.2}_{-0.3}× 10^{22} cm[SUP]-2[/SUP] which gradually covers and then uncovers the X-ray emitting region with covering fraction ranging from zero to ˜43 per cent. Our analysis enables us to constrain the size, number density and location of the absorbing cloud with good accuracy. We infer a cloud size (diameter) of D[SUB]c[/SUB] ≤ 1.5 × 10[SUP]13[/SUP] cm, corresponding to a density of n[SUB]c[/SUB] ≥ 1.5 × 10[SUP]9[/SUP] cm[SUP]-3[/SUP] at a distance of R[SUB]c[/SUB] ≥ 4.3 × 10[SUP]16[/SUP] cm from the central black hole. All of the inferred quantities concur to identify the absorbing structure with one single cloud associated with the broad line region of SWIFT J2127.4+5654. We are also able to constrain the X-ray emitting region size (diameter) to be D[SUB]s[/SUB] ≤ 2.3 × 10[SUP]13[/SUP] cm which, assuming the black hole mass estimated from single-epoch optical spectroscopy (1.5 × 10[SUP]7[/SUP] M[SUB]⊙[/SUB]), translates into D[SUB]s[/SUB] ≤ 10.5 gravitational radii (r[SUB]g[/SUB]) with larger sizes (in r[SUB]g[/SUB]) being associated with smaller black hole masses, and vice versa. We also confirm the presence of a relativistically distorted reflection component off the inner accretion disc giving rise to a broad relativistic Fe K emission line and small soft excess (small because of the high Galactic column density), supporting the measurement of an intermediate black hole spin in SWIFT J2127.4+5654 that was obtained from a previous Suzaku observation. [less ▲]

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See detailWhat is happening in ESO 362-G018?
Agis-Gonzalez, Beatriz ULiege; Miniutti, Giovanni

Poster (2013, May)

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See detailMonitoring of the X-ray absorption variability of ESO 362-G018
Agis-Gonzalez, Beatriz ULiege; Miniutti, Giovanni

Poster (2012, September)

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See detailThe Dust Environment of Main-Belt Comet P/2010 R2 (La Sagra)
Moreno, F.; Lara, L. M.; Licandro, J. et al

in Astrophysical Journal. Letters (2011), 738

We present a model of the dust environment of Main-Belt Comet P/2010 R2 (La Sagra) from images acquired during the period 2010 October-2011 January. The tails are best simulated by anisotropic ejection ... [more ▼]

We present a model of the dust environment of Main-Belt Comet P/2010 R2 (La Sagra) from images acquired during the period 2010 October-2011 January. The tails are best simulated by anisotropic ejection models, with emission concentrated near the nucleus south pole, the spin axis having an obliquity near 90°, indicative of a possible seasonally driven behavior. The dust mass loss rate increases rapidly shortly before perihelion, reaching a maximum value of ~4 kg s[SUP]-1[/SUP], and maintaining a sustained, cometary-like, activity of about 3-4 kg s[SUP]-1[/SUP] up to at least 200 days after perihelion, the date of the latest observation. The size distribution function is characterized by particles in the 5 × 10[SUP]-4[/SUP] cm to 1 cm radius range, assuming a time-constant power-law distribution with an index of -3.5. The ejection velocities are compatible with water-ice sublimation activity at the heliocentric distance of 2.7 AU, with values of 10-20 cm s[SUP]-1[/SUP] for particle radius of 1 cm, and inverse square root dependence on particle size, typical of hydrodynamical gas drag. [less ▲]

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