Visibility of Black Hole Shadows in Low-luminosity AGN

Accreting black holes tend to display a characteristic dark central region called the black-hole shadow, which depends only on spacetime/observer geometry and which conveys information about the black hole’s mass and spin. Conversely, the observed central brightness depression, or image shadow, additionally depends on the morphology of the emission region. In this paper, we investigate the astrophysical requirements for observing a meaningful black-hole shadow in GRMHD-based models of accreting black holes. In particular, we identify two processes by which the image shadow can differ from the black-hole shadow: evacuation of the innermost region of the accretion flow, which can render the image shadow larger than the black-hole shadow, and obscuration of the black-hole shadow by optically thick regions of the accretion flow, which can render the image shadow smaller than the black-hole shadow, or eliminate it altogether. We investigate in which models the image shadows of our models match their corresponding black-hole shadows, and in which models the two deviate from each other. We find that, given a compact and optically thin emission region, our models allow for measurement of the black-hole shadow size to an accuracy of 5%. We show that these conditions are generally met for all MAD simulations we considered, as well as some of the SANE simulations.

Selected aspects of the analysis of molecular line observations of edge-on circumbinary disks

Context. Inner cavities, accretion arms, and density waves are characteristic structures in the density distribution of circumbinary disks. They are the result of the tidal interaction of the non-axisymmetric gravitational forces of the central binary with the surrounding disk and are most prominent in the inner region, where the asymmetry is most pronounced. Aims. The goal of this study is to test the feasibility of reconstructing the gas density distribution and quantifying properties of structures in the inner regions of edge-on circumbinary disks using multiple molecular line observations. Methods. The density distribution in circumbinary disks is calculated with 2D hydrodynamic simulations. Subsequently, molecular line emission maps are generated with 3D radiative transfer simulations. Based on these, we investigate the observability of characteristic circumbinary structures located in the innermost region for spatially resolved and unresolved disks. Results. We find that it is possible to reconstruct the inner cavity, accretion arms, and density waves from spatially resolved multi-wavelength molecular line observations of circumbinary disks seen edge-on. For the spatially unresolved observations only, an estimate can be derived for the density gradient in the transition area between the cavity and the disk’s inner rim.

Physical characterization of 2020 AV2, the first known asteroid orbiting inside Venus orbit

ABSTRACT The first known asteroid with the orbit inside that of Venus is 2020 AV2. This may be the largest member of a new population of small bodies with the aphelion smaller than 0.718 au, called Vatiras. The surface of 2020 AV2 is being constantly modified by the high temperature, by the strong solar wind irradiation that characterizes the innermost region of the Solar system, and by high-energy micrometeorite impacts. The study of its physical properties represents an extreme test-case for the science of near-Earth asteroids. Here, we report spectroscopic observations of 2020 AV2 in the 0.5–1.5-μm wavelength interval. These were performed with the Nordic Optical Telescope and the William Herschel Telescope. Based on the obtained spectra, we classify 2020 AV2 as a Sa-type asteroid. We estimate the diameter of this Vatira to be $1.50_{-0.65}^{+1.10}$ km by considering the average albedo of A-type and S-complex asteroids ($p_V=0.23_{-0.08}^{+0.11}$), and the absolute magnitude (H = 16.40 ± 0.78 mag). The wide spectral band around 1 μm shows the signature of an olivine-rich composition. The estimated band centre BIC = 1.08 ± 0.02 μm corresponds to a ferroan olivine mineralogy similar to that of brachinite meteorites.

Escapees from the bar resonances

Understanding radial migration is a crucial point for building relevant chemical and dynamical evolution models of the Milky Way disk. In this paper we analyze a high-resolution N-body simulation of a Milky Way-type galaxy to study the role that the slowing down of a stellar bar has in generating migration from the inner to the outer disk. Stellar particles are trapped by the main resonances (corotation and outer Lindblad resonance, OLR) which then propagate outward across the disk due to the bar slowing down. Once the bar strength reaches its maximum amplitude, some of the stars delivered to the outer disk escape the resonances and some of them settle on nearly circular orbits. The number of escaped stars gradually increases, also due to the decrease in the bar strength when the boxy/peanut bulge forms. We show that this mechanism is not limited to stars on nearly circular orbits; stars initially on more eccentric orbits can also be transferred outward (out to the OLR location) and can end up on nearly circular orbits. Therefore, the propagation of the bar resonances outward can induce the circularization of the orbits of some of the migrating stars. The mechanism investigated in this paper can explain the presence of metal-rich stars at the solar vicinity and more generally in the outer Galactic disk. Our dynamical model predicts that up to 3% of stars between corotation and the OLR can be formed in the innermost region of the Milky Way. The epoch of the Milky Way bar formation can be potentially constrained by analyzing the age distribution of the most metal-rich stars at the solar vicinity.

Reflection component in the Bright Atoll Source GX 9+9

Context. GX 9+9 (4U 1728−16) is a low mass X-ray binary source harboring a neutron star. Although it belongs to the subclass of the bright Atoll sources together with GX 9+1, GX 3+1, and GX 13+1, its broadband spectrum is poorly studied and apparently does not show reflection features in the spectrum. Aims. To constrain the continuum well and verify whether a relativistic smeared reflection component is present, we analyze the broadband spectrum of GX 9+9 using BeppoSAX and XMM-Newton spectra covering the 0.3−40 keV energy band. Methods. We fit the spectrum adopting a model composed of a disk-blackbody plus a Comptonized component whose seed photons have a blackbody spectrum (Eastern Model). A statistically equivalent model is composed of a Comptonized component whose seed photons have a disk-blackbody distribution plus a blackbody that mimics a saturated Comptonization likely associated with a boundary layer (Western model). Other trials did not return a good fit. Results. The spectrum of GX 9+9 was observed in a soft state and its luminosity is 2.3 × 1037 erg s−1 assuming a distance to the source of 5 kpc. In the Eastern Model scenario, we find the seed-photon temperature and electron temperature of the Comptonized component to be 1.14−0.07+0.10 keV and 2.80−0.04+0.09 keV, respectively, while the optical depth of the Comptonizing corona is 8.9 ± 0.4. The color temperature of the inner accretion disk is 0.86−0.02+0.08 keV and 0.82 ± 0.02 keV for the BeppoSAX and XMM-Newton spectrum, respectively. In the Western Model scenario, instead, we find that the seed-photon temperature is 0.87 ± 0.07 keV and 1.01 ± 0.08 keV for the BeppoSAX and XMM-Newton spectrum, respectively. The electron temperature of the Comptonized component is 2.9 ± 0.2 keV, while the optical depth is 9.4−1.1+1.5. The blackbody temperature is 1.79−0.18+0.09 keV and 1.85−0.15+0.07 keV for the BeppoSAX and XMM-Newton spectrum, respectively. The addition of a relativistic smeared reflection component improved the fit in both the scenarios, giving compatible values of the parameters, even though a significant broad emission line in the Fe-K region is not observed. Conclusions. From the reflection component we estimated an inclination angle of about 43−4+6 deg and 51−2+9 deg for the Eastern and Western Model, respectively. The value of the reflection fraction Ω/2π is 0.18 ± 0.04 and 0.21 ± 0.03 for the Eastern and Western Model, respectively, suggesting that the Comptonized corona should be compact and close to the innermost region of the system.

The star formation history of the Sculptor dwarf spheroidal galaxy

ABSTRACT We present the star formation history (SFH) of the Sculptor dwarf spheroidal galaxy based on deep g, r photometry taken with Dark Energy Camera at the Blanco telescope, focusing our analysis on the central region of the galaxy extended up to ∼3 core radii. We have investigated how the SFH changes radially, subdividing the sampled area into four regions, and have detected a clear trend of star formation. All the SFHs show a single episode of star formation, with the innermost region presenting a longer period of star formation of ∼1.5 Gyr and for the outermost region the main period of star formation is confined to ∼0.5 Gyr. We observe a gradient in the mean age which is found to increase going towards the outer regions. These results suggest that Sculptor continued forming stars after the reionization epoch in its central part, while in the peripheral region, the majority of stars probably formed during the reionization epoch and soon after its end. From our analysis, Sculptor cannot be considered strictly as a fossil of the reionization epoch.

Spectral analysis of the dipping LMXB system XB 1916-053

Context. XB 1916-053 is a low mass X-ray binary system (LMXB) hosting a neutron star (NS) and showing periodic dips. The spectrum of the persistent emission was modeled with a blackbody component having a temperature between 1.31 and 1.67 keV and with a Comptonization component with an electron temperature of 9.4 keV and a photon index Γ between 2.5 and 2.9. The presence of absorption features associated with highly ionized elements suggested the presence of partially ionized plasma in the system. Aims. In this work we performed a study of the spectrum of XB 1916-053, which aims to shed light on the nature of the seed photons that contribute to the Comptonization component. Methods. We analyzed three Suzaku observations of XB 1916-053: the first was performed in November 2006 and the others were carried out in October 2014. We extracted the persistent spectra from each observation and combined the spectra of the most recent observations, obtaining a single spectrum with a higher statistic. We also extracted and combined the spectra of the dips observed during the same observations. Results. On the basis of the available data statistics, we infer that the scenario in which the corona Comptonizes photons emitted both by the innermost region of the accretion disk and the NS surface is not statistically relevant with respect to the case in which only photons emitted by the NS surface are Comptonized. We find that the source is in a soft spectral state in all the analyzed observations. We detect the Kα absorption lines of Fe XXV and Fe XXVI, which have already been reported in literature, and for the first time the Kβ absorption lines of the same ions. We also detect an edge at 0.876 keV, which is consistent with a O VIII K absorption edge. The dip spectrum is well described by a model that considers material in different ionization states covering the persistent spectrum and absorbing part of the rear radiation. From this model we rescale the distance of the absorber to a distance that is lower than 1 × 1010 cm.

The potential of combining MATISSE and ALMA observations: constraining the structure of the innermost region in protoplanetary discs

Context. In order to study the initial conditions of planet formation, it is crucial to obtain spatially resolved multi-wavelength observations of the innermost region of protoplanetary discs. Aims. We evaluate the advantage of combining observations with MATISSE/VLTI and ALMA to constrain the radial and vertical structure of the dust in the innermost region of circumstellar discs in nearby star-forming regions. Methods. Based on a disc model with a parameterized dust density distribution, we apply 3D radiative-transfer simulations to obtain ideal intensity maps. These are used to derive the corresponding wavelength-dependent visibilities we would obtain with MATISSE as well as ALMA maps simulated with CASA. Results. Within the considered parameter space, we find that constraining the dust density structure in the innermost 5 au around the central star is challenging with MATISSE alone, whereas ALMA observations with reasonable integration times allow us to derive significant constraints on the disc surface density. However, we find that the estimation of the different disc parameters can be considerably improved by combining MATISSE and ALMA observations. For example, combining a 30-min ALMA observation (at 310 GHz with an angular resolution of 0.03′′) for MATISSE observations in the L and M bands (with visibility accuracies of about 3%) allows the radial density slope and the dust surface density profile to be constrained to within Δα = 0.3 and Δ(α − β) = 0.15, respectively. For an accuracy of ~1% even the disc flaring can be constrained to within Δβ = 0.1. To constrain the scale height to within 5 au, M band accuracies of 0.8% are required. While ALMA is sensitive to the number of large dust grains settled to the disc midplane we find that the impact of the surface density distribution of the large grains on the observed quantities is small.

A broadband spectral analysis of 4U 1702-429 using XMM-Newton and BeppoSAX data

Context. Most of the X-ray binary systems containing neutron stars classified as Atoll sources show two different spectral states, referred to as soft and hard. Moreover, a large number of these systems show a reflection component relativistically smeared in their spectra, which provides information on the innermost region of the system. Aims. Our aim is to investigate the poorly studied broadband spectrum of the low-mass X-ray binary system 4U 1702-429, which was recently analysed combining XMM-Newton and INTEGRAL data. The peculiar value of the reflection fraction brought us to analyse further broadband spectra of 4U 1702-429. Methods. We re-analysed the spectrum of the XMM-Newton/INTEGRAL observation of 4U 1702-429 in the 0.3–60 keV energy range and we extracted three 0.1–100 keV spectra of the source analysing three observations collected with the BeppoSAX satellite. Results. We find that the XMM-Newton/INTEGRAL spectrum is well fitted using a model composed of a disc blackbody plus a Comptonised component and a smeared reflection component. We used the same spectral model for the BeppoSAX spectra, finding that the addition of a smeared reflection component is statistically significant. The best-fit values of the parameters are compatible to each other for the BeppoSAX spectra. We find that the reflection fraction is 0.05−0.01+0.3 for the XMM-Newton/INTEGRAL spectrum and between 0.15 and 0.4 for the BeppoSAX ones. Conclusions. The relative reflection fraction and the ionisation parameter are incompatible between the XMM-Newton/INTEGRAL and the BeppoSAX observations and the characteristics of the Comptonising corona suggest that the source was in a soft state in the former observation and in a hard state in the latter.