Average Column Density Research Articles

Temporary atmospheres produced by human activities on the Moon

Outgassing from materials, whether through the ascent/descent stages of lunar vehicles, airlock depressurizing, rover or astronaut suit outgassing, may cause an effect of unwanted accumulation of volatiles at the surface and exosphere. This is especially important at (or proximal) to permanently shadowed regions (PSRs) at the lunar poles. Herein, we provide estimates of expected outgassing from various human-landed objects on the Moon, including backpacks, airlocks, rovers, landers, trash and mining operations. Astronaut suits produce some level of oxygen outgassing (Helou et al., 2022), which may transport and condense in these PSRs, even in micro- cold traps (Glavin et al., 2010).We estimate the outgassing from drill mining and trash-to-gas conversion assuming a specific technology is operating. These outgassing systems can create local, temporary atmospheres in the vicinity (∼100 km radius) of the sources. The atmosphere may be particularly high within meters of the source. To obtain column densities for these temporary atmospheres, we first bracket ranges for the gas number loss as a function of time. We then derive the maximum distance traveled and the time the released molecules remain in the exosphere for a single ballistic hop, assuming the molecules are ejected from the surface of the object at its surface temperature. In some cases, such as the astronaut backpack and the rover, the temperature is that of the source. Given this information, an average and peak local exospheric density and column density can be estimated. We find that backpacks, airlock releases, and the Starship lander can create relatively high-density local atmospheres, with local near-lander outgassing water densities exceeding 107/cm3. This local water exosphere is over 106 times greater than the LADEE-derived lower limit of the natural water exosphere at ∼3/cm3. Thus, the anthropogenic temporary water exosphere will likely dominate the environment near the lander, making an assessment of the natural exospheric water environment difficult.

The thermalization of γ-rays in radioactive expanding ejecta: a simple model and its application for Kilonovae and Ia SNe

ABSTRACT A semi-analytic approximation is derived for the time-dependent fraction $f_\gamma (t)$ of the energy deposited by radioactive decay $\gamma$-rays in a homologously expanding plasma of general structure. An analytic approximation is given for spherically symmetric plasma distributions. Applied to Kilonovae (KNe) associated with neutron stars mergers and Type Ia supernovae, our semi-analytic and analytic approximations reproduce, with a few per cent and 10 per cent accuracy, respectively, the energy deposition rates, $\dot{Q}_{\rm dep}$, obtained in numeric Monte Carlo calculations. The time $t_\gamma$ beyond which $\gamma$-ray deposition is inefficient is determined by an effective frequency-independent $\gamma$-ray opacity $\kappa _{\gamma ,\text{eff}}$, $t_\gamma = \sqrt{\kappa _{\gamma ,\text{eff}}\langle \Sigma \rangle t^2}$, where $\langle \Sigma \rangle \propto t^{-2}$ is the average plasma column density. For $\beta$-decay dominated energy release, $\kappa _{\gamma ,\text{eff}}$ is typically close to the effective Compton scattering opacity, $\kappa _{\gamma ,\text{eff}} \approx 0.025$ cm$^{2}$ g$^{-1}$ with a weak dependence on composition. For KNe, $\kappa _{\gamma ,\text{eff}}$ depends mainly on the initial electron fraction $Y_e$, $\kappa _{\gamma ,\text{eff}} \approx 0.03(0.05)$ cm$^{2}$ g$^{-1}$ for $Y_e \gtrsim (\lesssim) 0.25$ (in contrast with earlier work that found $\kappa _{\gamma ,\text{eff}}$ larger by 1–2 orders of magnitude for low $Y_e$), and is insensitive to the (large) nuclear physics uncertainties. Determining $t_\gamma$ from observations will therefore measure the ejecta $\langle \Sigma \rangle t^2$, providing a stringent test of models. For $\langle \Sigma \rangle t^2=2\times 10^{11}~{\rm g\, {cm}^{-2}\, s^2}$, a typical value expected for KNe, $t_\gamma \approx 1$ d.

The X-ray rise and fall of the symbiotic recurrent nova system T CrB

ABSTRACT We present the analysis of publicly available NuSTAR, Suzaku, and XMM–Newton observations of the symbiotic recurrent nova T CrB covering the 2006.77–2022.66 yr period. The X-ray spectra are analysed by adopting a model that includes a reflection component produced by the presence of a disc that mimics the accretion disc and the immediate surrounding medium. Our best-fitting model requires this disc to have a radius of 1 au, effective thickness of 0.1 au, averaged column density 10$^{25}$ cm$^{-2}$ and orientation of 50$^{\circ }$ with respect to the line of sight. This disc is about a factor of two larger than recent estimations for the accretion disc and its presence contributes significantly via reflection to the total X-ray flux detected from T CrB, which naturally produces the emission of the 6.4 keV Fe line. Our analysis suggests that the temperature of the boundary layer evolved from 14.8 keV in the steady-state phase (before 2016), to 2.8 keV in the 2017.24 epoch, to finally stabilize to about $\sim$8 keV in the subsequent epochs. These variations in the plasma temperature of the boundary layer are attributed to the evolution of the mass accretion rate ($\dot{M}_\mathrm{acc}$), which is estimated to have an averaged value of $\dot{M}_\mathrm{acc}$ = 2.6$\times \,10^{-8}$ M$_\odot$ yr$^{-1}$ for the current active phase. The presence of emission lines in the XMM–Newton Reflection Grating Spectrometer spectrum of 2017.24 prevents from adopting a blackbody emission model to fit the soft X-ray range. Instead, we use plasma emission models that suggest the presence of adiabatically shocked gas produced by gas velocities of 110–200 km s$^{-1}$, very likely tracing jet-like ejections similar to what is found in other symbiotic systems. The analysis of X-ray and optical data together show that T CrB has a similar evolution as black hole binaries, accreting neutron stars and active galactic nuclei in the hardness–intensity diagram.

CO emission survey of asymptotic giant branch stars with ultraviolet excesses

Context. The transition from the spherically symmetric envelopes around asymptotic giant branch (AGB) stars to the asymmetric morphologies observed in planetary nebulae is still not well understood, and the shaping mechanisms are a subject of debate. Even though binarity is widely accepted as a promising option, it is limited by the complication of identifying binary AGB stars observationally. Recently, the presence of ultraviolet excesses in AGB stars has been suggested as a potential indicator of binarity. Aims. Our main goals are to characterise the properties of the circumstellar envelopes (CSEs) around candidate AGB binary stars, specifically those selected based on their UV excess emission, and to compare these properties with those derived from previous CO-based studies of AGB stars. Methods. We observed the 12CO (J=1–0) and 12CO (J=2–1) millimetre-wavelength emission in a sample of 29 AGB binary candidates with the IRAM-30 m antenna. We measured the systemic velocities and the terminal expansion velocities from their line profiles. Population diagrams were used to interpret the results, enabling the estimation of excitation temperatures (Tex), mass-loss rates (Ṁ), and the characteristic sizes of the envelope layers where the CO millimetre emission originates (Rs). We explored different trends between the envelope parameters deduced, multiwavelength flux measurements, and other properties of our sample, and compared them with those previously derived from larger samples of AGB stars found in the literature. Results. We detected 12CO emission in 15 sources, of which 5 are first detections. We found relatively low expansion velocities (3 km s−1 ≲ Vexp ≲ 20 km s−1) in our sample. We derived the average excitation temperature and column density of the CO-emitting layers, which we used to estimate self-consistently the average mass-loss rate (10−8 M⊙ yr−1 ≲ Ṁ ≲ 10−5 M⊙ yr−1) and the CO pho-todissociation radius (5 × 1015 cm ≲ RCO ≲ 2 × 1017 cm) of our targets. We find a correlation between CO intensity and IRAS 60 µm fluxes, revealing a CO-to-IRAS 60 µm ratio lower than for AGB stars and closer to that found for pre-planetary nebulae (pPNe). An anti-correlation is observed between 12CO (and IRAS 60 µm) and the near-ultraviolet (NUV), but no such correlation is observed with the far-ultraviolet (FUV). It is also worth noting that there is no correlation between bolometric luminosity and NUV or FUV. Conclusions. For the first time we have studied the mass-loss properties of UV-excess AGB binary candidates and estimated their main CSE parameters. Our sample of uvAGB stars shows similarities with the broader category of AGB stars, except for a distinct CO-to-IRAS 60 µm trend suggesting enhanced CO photodissociation. Our findings, based on single-dish low-J CO line emission observations, support the dust-driven wind scenario and indicate that alternative mass-loss mechanisms are not necessary (in principle) to explain the ~200–2000 yr old mass-loss ejecta in uvAGBs. The different relationships between 12CO and IRAS 60 µm, with NUV and FUV are consistent with an intrinsic origin of NUV emission, but potential dominance of an extrinsic process (e.g. presence of a binary companion) in FUV emission.

FAUST

Context. The origin of the chemical diversity observed around low-mass protostars probably resides in the earliest history of these systems. Aims. We aim to investigate the impact of protostellar feedback on the chemistry and grain growth in the circumstellar medium of multiple stellar systems. Methods. In the context of the ALMA Large Program FAUST, we present high-resolution (50 au) observations of CH3OH, H2CO, and SiO and continuum emission at 1.3 mm and 3 mm towards the Corona Australis star cluster. Results. Methanol emission reveals an arc-like structure at ∼1800 au from the protostellar system IRS7B along the direction perpendicular to the major axis of the disc. The arc is located at the edge of two elongated continuum structures that define a cone emerging from IRS7B. The region inside the cone is probed by H2CO, while the eastern wall of the arc shows bright emission in SiO, a typical shock tracer. Taking into account the association with a previously detected radio jet imaged with JVLA at 6 cm, the molecular arc reveals for the first time a bow shock driven by IRS7B and a two-sided dust cavity opened by the mass-loss process. For each cavity wall, we derive an average H2 column density of ∼7 × 1021 cm−2, a mass of ∼9 × 10−3 M⊙, and a lower limit on the dust spectral index of 1.4. Conclusions. These observations provide the first evidence of a shock and a conical dust cavity opened by the jet driven by IRS7B, with important implications for the chemical enrichment and grain growth in the envelope of Solar System analogues.

Lidar Observations of the Fe Layer in the Mesopause and Lower Thermosphere over Beijing (40.5° N, 116.0° E) and Mohe (53.5° N, 122.4° E)

Lidar observations of metal layers play a significant role in research on the chemistry and dynamics of the mesosphere and lower thermosphere. This work reports on Fe lidar observations conducted in Beijing and Mohe. Utilizing the same laser emission system, a 1064 nm seed laser was injected into an Nd: YAG laser to generate a single longitudinal-mode pulse 532 nm laser, which pumped a dye laser to produce a 572 nm laser. The 572 nm laser and the remaining 1064 nm fundamental frequency laser passed through a sum–frequency module to generate a 372 nm laser to detect the Fe layer. According to a total of 52.6 h of observations for 10 nights in Beijing, the Fe layer has an average column density of 1.24 × 1010 cm−2, an RMS width of 4.4 km and a centroid altitude of 89.4 km. In Mohe, observed for 16 nights and a total of 91.5 h, the Fe layer has an average column density of 1.08 × 1010 cm−2, an RMS width of 4.6 km and a centroid altitude of 89.5 km. The probability of the occurrence of sporadic Fe layers was 42.4% in Beijing and 29.4% in Mohe. Compared to simultaneously observed Na layers, the occurrence probabilities of sporadic Fe layers were higher than those of sporadic Na layers in both stations. Based on the two cases observed in Beijing, it is conjectured that the formation mechanism of sporadic metal layers above approximately 100 km has a more significant influence on sporadic Fe layers than on sporadic Na layers. The lower thermospheric Fe layers with densities significantly larger than those of the main layer were observed during two nights in Mohe. This work contributes to the refinement of the global distribution of Fe layers and provides abundant observational data for the modeling and study of the metal layers.

Dust around Massive Stars Is Agnostic to Galactic Environment: New Insights from PHAT/BEAST

Resolving the environments of massive stars is crucial for understanding their formation mechanisms and their impact on galaxy evolution. An important open question is whether massive stars found in diffuse regions outside spiral arms formed in situ or migrated there after forming in denser environments. To address this question, we use multiresolution measurements of extinction in the Andromeda galaxy (M31) to probe the interstellar medium surrounding massive stars across galactic environments. We construct a catalog of 42,107 main-sequence massive star candidates (M ≥ 8 M ⊙) using resolved stellar photometry from the Panchromatic Hubble Andromeda Treasury (PHAT) program, plus stellar and dust model fits from the Bayesian Extinction and Stellar Tool (BEAST). We quantify galactic environments by computing surrounding stellar densities of massive stars using kernel density estimation. We then compare high-resolution line-of-sight extinction estimates from the BEAST with 25 pc resolution dust maps from PHAT, measuring the total column density distribution of extinction. Our key finding is that, although the average total column density of dust increases with the density of massive stars, the average line-of-sight extinction toward massive stars remains constant across all environments. This suggests that massive stars have a uniform amount of dust in their immediate environment, regardless of their location in the galaxy. One possible explanation for these findings is that small molecular clouds are still capable of forming massive stars, even if they are not resolvable at 25 pc. These results indicate that massive stars are forming in the sparse regions of M31, as opposed to migrating there.

Atmospheric HDO Abundance Measurements in the Tibetan Plateau Based on Laser Heterodyne Radiometer

The Tibet Plateau is known as the “third pole” of the world, and its environmental change profoundly impacts East Asia and even the global climate. HDO is the stable isotope of water vapor, which acts as an ideal tracer for studying the water cycle, and which is commonly used for atmospheric circulation and climatic studies. To monitor the water vapor isotopic abundance in the Tibetan Plateau, a portable laser heterodyne radiometer was operated in Golmud in August 2019. The radiometer utilizes a narrow-linewidth 3.66 μm distributed feedback interband cascade laser as the local oscillator, the heterodyne module is been optimized and the radiometer performs with high resolution and stability in obtaining spectral data. Furthermore, the absorption spectra of atmospheric HDO and H2O are obtained, and the retrieval method for water vapor isotopic abundance is discussed. The optimal estimation method is adopted to retrieve the density of HDO and H2O. The average column density of H2O was 1.22 g/cm2, and the HDO/H2O ratio in Golmud was 178 ± 15 × 10−6 during the observation. For a better understanding of the retrieval, the retrieval errors are analyzed and compared. The results indicate that the smoothing error is significantly higher than the measurement error in this work. The backward trajectory analysis of atmospheric transport is used to investigate the relationship between water vapor density and atmospheric motion. The results indicate that the variation of H2O column density and HDO/H2O ratio have a relationship with atmospheric movements.

Hydrogen column density variability in a sample of local Compton-thin AGN

We present the analysis of multiepoch observations of a set of 12 variable, Compton-thin, local (z < 0.1) active galactic nuclei (AGN) selected from the 100-month BAT catalog. We analyzed all available X-ray data from Chandra, XMM-Newton, and NuSTAR, adding up to a total of 53 individual observations. This corresponds to between three and seven observations per source, probing variability timescales between a few days and ∼20 yr. All sources have at least one NuSTAR observation, ensuring high-energy coverage, which allowed us to disentangle the line-of-sight and reflection components in the X-ray spectra. For each source, we modeled all available spectra simultaneously, using the physical torus models MYTorus, borus02, and UXCLUMPY. The simultaneous fitting, along with the high-energy coverage, allowed us to place tight constraints on torus parameters such as the torus covering factor, inclination angle, and torus average column density. We also estimated the line-of-sight column density (NH) for each individual observation. Within the 12 sources, we detected clear line-of-sight NH variability in five of them, non-variability in five of them, and for two of them it was not possible to fully disentangle intrinsic luminosity and NH variability. We observed large differences between the average values of line-of-sight NH (or NH of the obscurer) and the average NH of the torus (or NH of the reflector), for each source, by a factor between ∼2 to > 100. This behavior, which suggests a physical disconnect between the absorber and the reflector, is more extreme in sources that present NH variability. We note that NH-variable AGN also tend to present larger obscuration and broader cloud distributions than their non-variable counterparts. These trends however require a larger number of sources to confirm (or disprove) this. We observed that large changes in obscuration only occur at long timescales, and used this to place tentative lower limits on torus cloud sizes. Furthermore, we observed a median variation in NH between any two observations of the same source of ∼36%.

Compton-thick AGN in the NuSTAR Era X: Analysing seven local CT-AGN candidates

Context. We present the broad-band X-ray spectral analysis (0.6–50 keV) of seven Compton-thick active galactic nuclei (CT-AGN; line-of-sight (LOS) column density > 1024 cm−2) candidates selected from the Swift-BAT 100 month catalogue using archival NuSTAR data. Aims. We aim to obtain a complete census of the heavily obscured AGN in the local Universe (z < 0.05). Methods. This work is in continuation of the ongoing research of the Clemson-INAF group to classify CT-AGN candidates at redshift z < 0.05 using physically motivated torus models. Results. Our results confirm that three out of seven targets are bona fide CT-AGN. Adding our results to the previously analysed sources using NuSTAR data, we increase the population of bona fide CT-AGN by ∼9%, bringing the total number to 35 out of 414 AGN. We also performed a comparative study using MYTorus and borus02 on the spectra in our sample, finding that both physical models are strongly consistent in the parameter space of LOS column density and photon index. Furthermore, we also investigate the clumpiness of the torus clouds by separately computing the LOS and average torus column densities in each of the seven sources. Adding our results to all the previous 48 CT-AGN candidates analysed by the Clemson-INAF research team for which NuSTAR observations are available, we find that 78% of the sources are likely to have a clumpy distribution of the obscuring material surrounding the accreting supermassive black hole.

What Can We Learn about Compton-thin AGN Tori from Their X-Ray Spectra?<sup>1</sup>

We have developed a Monte Carlo code for simulation of X-ray spectra of active galactic nuclei (AGN) based on a model of a clumpy obscuring torus. Using this code, we investigate the diagnostic power of X-ray spectroscopy of obscured AGN with respect to the physical properties and orientation of the torus, namely: the average column density, \(\langle N_{\textrm{H}}\rangle\), the line-of-sight column density, \(N_{\textrm{H}}\), the abundance of iron, \(A_{\textrm{Fe}}\), the clumpiness (i.e. the average number of gas clouds along the line of sight), \(\langle N\rangle\), and the viewing angle, \(\alpha\). In this first paper of a series, we consider the Compton-thin case, where both \(\langle N_{\textrm{H}}\rangle\) and \(N_{\textrm{H}}\) do not exceed \(10^{24}\) cm\({}^{-2}\). To enable quantitative comparison of the simulated spectra, we introduce five measurable spectral characteristics: the low-energy hardness ratio (ratio of the continuum fluxes in the 7–11 keV and 2–7 keV energy bands), the high-energy hardness ratio (ratio of the continuum fluxes in the 10–100 keV and 2–10 keV energy bands), the depth of the iron K absorption edge, the equivalent width of the Fe K\(\alpha\) line, and the fraction of the Fe K\(\alpha\) flux contained in the Compton shoulder. We demonstrate that by means of X-ray spectroscopy it is possible to tightly constrain \(\langle N_{\textrm{H}}\rangle\), \(N_{\textrm{H}}\) and \(A_{\textrm{Fe}}\) in the Compton-thin regime, while there is degeneracy between clumpiness and viewing direction.

The Complex X-Ray Obscuration Environment in the Radio-loud Type 2 Quasar 3C 223

3C 223 is a radio-loud, Type 2 quasar at z = 0.1365 with an intriguing X-ray Multi-mirror Mission (XMM)-Newton spectrum that implicated it as a rare, Compton-thick (N H ≳ 1.25 × 1024 cm−2) active galactic nucleus (AGN). We obtained contemporaneous XMM-Newton and Nuclear Spectroscopic Telescope Array (NuSTAR) spectra to fit the broadband X-ray spectrum with the physically motivated MYTorus and borus02 models. We confirm earlier results that the obscuring gas is patchy with both high (though not Compton-thick) levels of obscuration (N H > 1023 cm−2) and gas clouds with column densities up to an order of magnitude lower. The spectral fitting results indicate additional physical processes beyond those modeled in the spectral grids of MYTorus and borus02 impact the emergent spectrum: the Compton-scattering region may be extended beyond the putative torus; a ring of heavy Compton-thick material blocks most X-ray emission along the line of sight; or the radio jet is beamed, boosting the production of Fe Kα line photons in the global medium compared with what is observed along the line of sight. We revisit a recent claim that no radio-loud Compton-thick AGN have yet been conclusively shown to exist, finding three reported cases of radio-loud AGN with global average (but not line-of-sight) column densities that are Compton thick. Now that it is possible to separately determine line-of-sight and global column densities, inhomogeneity in the obscuring medium has consequences for how we interpet the spectrum and classify an AGN as “Compton thick.”

The opaque heart of the galaxy IC 860: Analogous protostellar, kinematics, morphology, and chemistry

Compact Obscured Nuclei (CONs) account for a significant fraction of the population of luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs). These galaxy nuclei are compact, with radii of 10–100 pc, with large optical depths at submm and far-infrared wavelengths, and characterized by vibrationally excited HCN emission. It is not known what powers the large luminosities of the CON host galaxies because of the extreme optical depths towards their nuclei. CONs represent an extreme phase of nuclear growth, hiding either a rapidly accreting supermassive black hole or an abnormal mode of star formation. Regardless of their power source, the CONs allow us to investigate the processes of nuclear growth in galaxies. Here we apply principal component analysis (PCA) tomography to high-resolution (0″​​.06) ALMA observations at frequencies 245 to 265 GHz of the nearby CON (59 Mpc) IC 860. PCA is a technique to unveil correlation in the data parameter space, and we apply it to explore the morphological and chemical properties of species in our dataset. The leading principal components reveal morphological features in molecular emission that suggest a rotating, infalling disk or envelope, and an outflow analogous to those seen in Galactic protostars. One particular molecule of astrochemical interest is methanimine (CH2NH), a precursor to glycine, three transitions of which have been detected towards IC 860. We estimate the average CH2NH column density towards the nucleus of IC 860 to be ∼1017cm−2, with an abundance exceeding 10−8 relative to molecular hydrogen, using the rotation diagram method and non-LTE radiative transfer models. This CH2NH abundance is consistent with those found in hot cores of molecular clouds in the Milky Way. Our analysis suggests that CONs are an important stage of chemical evolution in galaxies, that are chemically and morphologically similar to Milky Way hot cores.

The Optical Aurorae of Europa, Ganymede, and Callisto

The tenuous atmospheres of the Galilean satellites are sourced from their surfaces and produced by a combination of plasma-surface interactions and thermal processes. Even though they are thin, these atmospheres can be studied via their auroral emissions, and most work to date has focused on their aurorae at UV wavelengths. Here we present the first detections of the optical aurorae of Ganymede and Callisto, as well as detections of new optical auroral lines at Europa, based on observations of the targets over 10 Jupiter eclipses from 1998 to 2021 with Keck/HIRES. We present measurements of O i emission at 6300/6364, 5577, 7774, and 8446 Å and place upper limits on hydrogen at 6563 Å. These constitute the first detections of emissions at 7774 and 8446 Å at a planetary body other than Earth. The simultaneous measurement of multiple emission lines provides robust constraints on atmospheric composition. We find that the eclipse atmospheres of Europa and Ganymede are composed predominantly of O2, with average column densities of (4.1 ± 0.1) × 1014 cm−2 and (4.7 ± 0.1) × 1014 cm−2, respectively. We find weak evidence for H2O in Europa's bulk atmosphere at an H2O/O2 ratio of ∼0.25, and place only an upper limit on H2O in Ganymede's bulk atmosphere, corresponding to H2O/O2 < 0.6. The column density of O2 derived for Callisto is (4.0 ± 0.9) × 1015 cm−2 for an assumed electron density of 0.15 cm−3, but electron properties at Callisto's orbit are very poorly constrained.

Compton-thick AGN in the NuSTAR Era. IX. A Joint NuSTAR and XMM-Newton Analysis of Four Local AGN

We present the results of the broadband X-ray spectral analysis of simultaneous NuSTAR and XMM-Newton observations of four nearby Compton-thick active galactic nuclei (AGN) candidates selected from the Swift Burst Alert Telescope 150 month catalog. This work is part of a larger effort to identify and characterize all Compton-thick (N H ≥ 1024 cm−2) AGN in the local universe (z ≤ 0.05). We used three physically motivated models—MYTorus, borus02, and UXClumpy—to fit and characterize these sources. Of the four candidates analyzed, 2MASX J02051994-0233055 was found to be an unobscured (N H < 1022 cm−2) AGN, 2MASX J04075215-6116126 and IC 2227 to be Compton-thin (1022 cm−2 < N H < 1024 cm−2) AGN, and one, ESO 362−8, was confirmed to be a Compton-thick AGN. Additionally, every source was found to have a statistically significant difference between their line-of-sight and average torus hydrogen column density, further supporting the idea that the obscuring material in AGN is inhomogeneous. Furthermore, half of the sources in our sample (2MASX J02051994-0233055 and 2MASX J04075215-6116126) exhibited significant luminosity variation in the last decade, suggesting that this might be a common feature of AGN.

Evidence for Large-scale Excesses Associated with Low H i Column Densities in the Sky. I. Dust Excess

Where dust and gas are uniformly mixed, atomic hydrogen can be traced through the detection of far-infrared (FIR) or UV emission of dust. We considered, for the origin of discrepancies observed between various direct and indirect tracers of gas outside the Galactic plane, possible corrections to the zero levels of the Planck High Frequency Instrument (HFI) detectors. We set the zero levels of the Planck-HFI skymaps as well as the 100 μm map from COBE/DIRBE and IRAS from the correlation between FIR emission and atomic hydrogen column density excluding regions of lowest gas column density. A modified blackbody model fit to those new zero-subtracted maps led to significantly different maps of the opacity spectral index β and temperature T and an overall increase in the optical depth at 353 GHz τ 353 of 7.1 × 10−7 compared to the data release 2 Planck map. When comparing τ 353 and the H i column density, we observed a uniform spatial distribution of the opacity outside regions with dark neutral gas and CO except in various large-scale regions of low N H i that represent 25% of the sky. In those regions, we observed an average dust column density 45% higher than predictions based on N H i with a maximum of 250% toward the Lockman Hole region. From the average opacity σ e353 = (8.9 ± 0.1) × 10−27 cm2, we deduced a dust-to-gas mass ratio of 0.53 × 10−2. We did not see evidence of dust associated with a Reynolds layer of ionized hydrogen. We measured a far-ultraviolet isotropic intensity of 137 ± 15 photons s−1 cm−2 sr−1 Å−1 in agreement with extragalactic flux predictions and a near-ultraviolet isotropic intensity of 378 ± 45 photons s−1 cm−2 sr−1 Å−1 corresponding to twice the predicted flux.

SWIFT J0503.7-2819: a nearly synchronous intermediate polar below the period gap?

ABSTRACT Based on the X-ray observations from XMM–Newton and Swift, and optical observations from Transiting Exoplanet Survey Satellite (TESS) and AAVSO, we present temporal and spectral properties of probable intermediate polar SWIFT J0503.7-2819. The X-ray light curve shows two distinctive features, where possibly the second pole seems to be active during the middle of the XMM–Newton observations. Present analysis confirms and also refines the previously reported orbital period of SWIFT J0503.7-2819 as 81.65 ± 0.04 min. The X-ray and optical variations of this target have been found to occur at the period of ∼65 min, which we propose as the spin period of the white dwarf. The energy-dependent modulation at this period, which are due to the photoelectric absorption in the accretion flow, also assures this conjecture. Two-temperature thermal plasma model well explains the X-ray spectra with temperatures of ∼150 eV and ∼18.5 keV, which is absorbed by a dense material with an average equivalent hydrogen column density of 3.8 × 1022 cm−2 that partially covers ∼27 per cent of the X-ray source. An attempt is made to understand the accretion flow in this system using the present data of SWIFT J0503.7-2819. If the proposed spin period is indeed the actual period, then SWIFT J0503.7-2819 could be the first nearly synchronous intermediate polar below the period gap.

A Multiepoch X-Ray Study of the Nearby Seyfert 2 Galaxy NGC 7479: Linking Column Density Variability to the Torus Geometry

Active galactic nuclei are powered by accreting supermassive black holes, surrounded by a torus of obscuring material. Recent studies have shown how the torus structure, formerly thought to be homogeneous, appears to be “patchy”: the detection of variability in the line-of-sight hydrogen column density, in fact, matches the description of an obscurer with a complex structure made of clouds with different column density. In this work, we perform a multiepoch analysis of the X-ray spectra of the Seyfert 2 galaxy NGC 7479 in order to estimate its torus properties, such as the average column density and the covering factor. The measurement of the line-of-sight hydrogen column density variability of the torus allows us to obtain an upper limit on the cloud distance from the central engine. In addition, using the X-ray luminosity of the source, we estimate the Eddington ratio to be in a range of λ Edd = 0.04–0.05 over all epochs.

Gas Column Density Distribution of Molecular Clouds in the Third Quadrant of the Milky Way

We have obtained column density maps for an unbiased sample of 120 molecular clouds in the third quadrant of the Milky Way midplane (b ≤ ∣5∣°) within the Galactic longitude range from 195° to 225°, using the high-sensitivity 12CO and 13CO (J = 1 − 0) data from the Milky Way Imaging Scroll Painting (MWISP) project. The probability density functions of the molecular hydrogen column density of the clouds, N-pdfs, are fitted with both a lognormal (LN) function and a lognormal plus power-law (LN+PL) function. The molecular clouds are classified into three categories according to their shapes of N-pdfs, i.e., LN, LN+PL, and UN (unclear). About 72% of the molecular clouds fall into the LN category, while 18% and 10% fall into the LN+PL and UN categories, respectively. A PL scaling relation, , exists between the width of the N-pdf, σ s , and the average column density, , of the molecular clouds. However, σ s shows no correlation with the mass of the clouds. A correlation is found between the dispersion of normalized column density, σ N/〈N〉, and the sonic Mach number, , of molecular clouds. Overall, as predicted by numerical simulations, the N-pdfs of the molecular clouds with active star formation activity tend to have N-pdfs with PL high-density tails.

Compton-thick AGN in the NuSTAR Era. VIII. A joint NuSTAR–XMM-Newton Monitoring of the Changing-look Compton-thick AGN NGC 1358

We present the multi-epoch monitoring with NuSTAR and XMM-Newton of NGC 1358, a nearby Seyfert 2 galaxy whose properties made it a promising candidate X-ray changing-look active galactic nucleus (AGN), i.e., a source whose column density could transition from its 2017 Compton-thick (having LOS hydrogen column density N H,LOS > 1024 cm−2) state to a Compton-thin (N H,LOS < 1024 cm−2) one. The multi-epoch X-ray monitoring confirmed the presence of significant N H,LOS variability over timescales of weeks to years, and allowed us to confirm the changing-look nature of NGC 1358, which has most recently been observed in a Compton-thin status. Multi-epoch monitoring with NuSTAR and XMM-Newton is demonstrated to be highly effective in simultaneously constraining three otherwise highly degenerate parameters: the torus average column density and covering factor, and the inclination angle between the torus axis and the observer. We find a tentative anticorrelation between column density and luminosity, which can be understood under the framework of chaotic cold accretion clouds driving recursive AGN feedback. The monitoring campaign of NGC 1358 has proven the efficiency of our newly developed method to select candidate N H,LOS-variable, heavily obscured AGN, which we plan to soon extend to a larger sample to better characterize the properties of the obscuring material surrounding accreting supermassive black holes, as well as to constrain AGN feeding models.