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<jats:title>Abstract</jats:title> <jats:p>We present new spectrophotometric and spectropolarimetric observations of Mrk 1239, one of the 8 prototypes that defines type-1 narrow-line Seyfert galaxies (NLS1s). Unlike the other typical NLS1s though, a high degree of polarization (<jats:italic>P</jats:italic> ∼ 5.6%) and red optical–IR (<jats:italic>g–W</jats:italic> <jats:sub>4</jats:sub> = 12.35) colors suggest that Mrk 1239 is more similar to type-2 active galactic nuclei like NGC 1068. Detailed analysis of spectral energy distribution in the UV–optical–IR yields two components from the nucleus: a direct and transmitted component that is heavily obscured (<jats:italic>E</jats:italic> <jats:sub> <jats:italic>B–V</jats:italic> </jats:sub> ≈ 1.6), and another indirect and scattered one with mild extinction (<jats:italic>E</jats:italic> <jats:sub> <jats:italic>B–V</jats:italic> </jats:sub> ∼ 0.5). Such a two-light-paths scenario is also found in previous reports based on the X-ray data. Comparison of emission lines and the detection of He <jats:sc>i</jats:sc>*<jats:italic>λ</jats:italic>10830 BAL at [−3000, −1000] km s<jats:sup>−1</jats:sup> indicates that the obscuring clouds are at physical scale between the sublimation radius and that of the narrow emission line regions. The potential existence of powerful outflows is found as both the obscurer and scatterer are outflowing. Similar to many other type-2s, jet-like structure in the radio band is found in Mrk 1239, perpendicular to the polarization angle, suggesting polar scattering. We argue that Mrk 1239 is very probably a type-2 counterpart of NLS1s. The identification of 1 out of 8 prototype NLS1s as a type-2 counterpart implies that there can be a substantial amount of analogs of Mrk 1239 misidentified as type-1s in the optical band. Properties of these misidentified objects are going to be explored in our future works.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report the detection of an Al <jats:sc>ii</jats:sc> line at 2669.155 Å in 11 metal-poor stars, using ultraviolet spectra obtained with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. We derive Al abundances from this line using a standard abundance analysis, assuming local thermodynamic equilibrium (LTE). The mean [Al/Fe] ratio is −0.06 ± 0.04 (<jats:italic>σ</jats:italic> = 0.22) for these 11 stars spanning − 3.9 &lt; [Fe/H] &lt; −1.3, or [Al/Fe] = −0.10 ± 0.04 (<jats:italic>σ</jats:italic> = 0.18) for 9 stars spanning −3.0 &lt; [Fe/H] &lt; −1.3 if two carbon-enhanced stars are excluded. We use these abundances to perform an empirical test of non-LTE (NLTE) abundance corrections predicted for resonance lines of Al <jats:sc>i</jats:sc>, including the commonly used optical Al <jats:sc>i</jats:sc> line at 3961 Å. The Al <jats:sc>ii</jats:sc> line is formed in LTE, and the abundance derived from this line matches that derived from high-excitation Al <jats:sc>i</jats:sc> lines predicted to have minimal NLTE corrections. The differences between the abundance derived from the Al <jats:sc>ii</jats:sc> line and the LTE abundance derived from Al <jats:sc>i</jats:sc> resonance lines are +0.4 to +0.9 dex, which match the predicted NLTE corrections for the Al <jats:sc>i</jats:sc> resonance lines. We conclude that the NLTE abundance calculations are approximately correct and should be applied to LTE abundances derived from Al <jats:sc>i</jats:sc> lines.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report on a coherent timing analysis of the 163 Hz accreting millisecond X-ray pulsar IGR J17062–6143. Using data collected with the Neutron Star Interior Composition Explorer and XMM-Newton, we investigated the pulsar evolution over a time span of four years. We obtained a unique phase-coherent timing solution for the stellar spin, finding the source to be spinning up at a rate of (3.77 ± 0.09) × 10<jats:sup>−15</jats:sup> Hz s<jats:sup>−1</jats:sup>. We further find that the 0.4–6 keV pulse fraction varies gradually between 0.5% and 2.5% following a sinusoidal oscillation with a 1210 ± 40 day period. Finally, we supplemented this analysis with an archival Rossi X-ray Timing Explorer observation and obtained a phase-coherent model for the binary orbit spanning 12 yr, yielding an orbital period-derivative measurement of (8.4 ± 2.0) × 10<jats:sup>−12</jats:sup> s s<jats:sup>−1</jats:sup>. This large orbital period derivative is inconsistent with a binary evolution that is dominated by gravitational wave emission and is suggestive of highly nonconservative mass transfer in the binary system.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Spectral lines allow us to probe the thermodynamics of the solar atmosphere, but the shape of a single spectral line may be similar for different thermodynamic solutions. Multiline analyses are therefore crucial, but computationally cumbersome. We investigate correlations between several chromospheric and transition region lines to restrain the thermodynamic solutions of the solar atmosphere during flares. We used machine-learning methods to capture the statistical dependencies between six spectral lines sourced from 21 large solar flares observed by NASA’s Interface Region Imaging Spectrograph. The techniques are based on an information-theoretic quantity called <jats:italic>mutual information</jats:italic> (MI), which captures both linear and nonlinear correlations between spectral lines. The MI is estimated using both a categorical and numeric method, and performed separately for a collection of quiet Sun and flaring observations. Both approaches return consistent results, indicating weak correlations between spectral lines under quiet Sun conditions, and substantially enhanced correlations under flaring conditions, with some line-pairs such as Mg <jats:sc>ii</jats:sc> and C <jats:sc>ii</jats:sc> having a normalized MI score as high as 0.5. We find that certain spectral lines couple more readily than others, indicating a coherence in the solar atmosphere over many scale heights during flares, and that all line-pairs are correlated to the GOES derivative, indicating a positive relationship between correlation strength and energy input. Our methods provide a highly stable and flexible framework for quantifying dependencies between the physical quantities of the solar atmosphere, allowing us to obtain a three-dimensional picture of its state.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>A series of measurements of the solar diameter taken in the meridian line of the Basilica of San Petronio (Bologna, Italy) between 1655 and 1736 has been analyzed. This series is of interest because the measurement period includes the Maunder Minimum (1645–1715; hereafter MM) when solar activity was abnormally low. Some authors have suggested an increase of the solar diameter during the MM. Trying to detect these changes, statistical analyses comparing measurements taken in San Petronio during the MM (1655–1715) and other ones taken in a subsequent period (1716–1736) have been performed. Mann–Whitney U tests and Student’s t-tests indicate that there is no statistically significant difference in the medians and averages of the solar diameter in both periods. In fact, we have found differences around 0.″6 in the medians and the averages, which are below the mean accuracy of the instrument. Therefore, we conclude that there is no difference between the solar diameter value measured during the MM (1655–1715) and that for the subsequent period 1716–1736. This implies that there has not been an increase in the solar diameter of several arcseconds during the MM as has been speculated by some authors.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Tidal forces are important for understanding how close binary stars and compact exoplanetary systems form and evolve. However, tides are difficult to model, and significant uncertainties exist about the strength of tides. Here, we investigate tidal circularization in close binaries using a large sample of well-characterized eclipsing systems. We searched TESS photometry from the southern hemisphere for eclipsing binaries. We derive best-fit orbital and stellar parameters by jointly modeling light curves and spectral energy distributions. To determine the eccentricity distribution of eclipsing binaries over a wide range of stellar temperatures (3000–50,000 K) and orbital separations <jats:italic>a</jats:italic>/<jats:italic>R</jats:italic> <jats:sub>1</jats:sub> (2–300), we combine our newly obtained TESS sample with eclipsing binaries observed from the ground and by the Kepler mission. We find a clear dependency of stellar temperature and orbital separation in the eccentricities of close binaries. We compare our observations with predictions of the equilibrium and dynamical tides. We find that while cool binaries agree with the predictions of the equilibrium tide, a large fraction of binaries with temperatures between 6250 K and 10,000 K and orbital separations between <jats:italic>a</jats:italic>/<jats:italic>R</jats:italic> <jats:sub>1</jats:sub> ∼ 4 and 10 are found on circular orbits, contrary to the predictions of the dynamical tide. This suggests that some binaries with radiative envelopes may be tidally circularized significantly more efficiently than usually assumed. Our findings on orbital circularization have important implications also in the context of hot Jupiters, where tides have been invoked to explain the observed difference in the spin–orbit alignment between hot and cool host stars.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present an analysis of 745.8 ks of archival Chandra X-Ray Observatory Advanced CCD Imaging Spectrometer data accumulated between 2000 and 2016 of the millisecond pulsar (MSP) population in the rich Galactic globular cluster Terzan 5. Eight of the 38 MSPs with precise positions are found to have plausible X-ray source matches. Despite the deep exposure, the remaining MSPs are either marginally detected or have no obvious X-ray counterparts, which can be attributed to the typically soft thermal spectra of rotation-powered MSPs, which are strongly attenuated by the high intervening absorbing column (∼10<jats:sup>22</jats:sup> cm<jats:sup>−2</jats:sup>) toward the cluster, and in some instances to severe source crowding/blending. For the “redback” MSP binaries PSR J1748−2446P and PSR J1748−2446ad and the “black widow” binary system PSR J1748−2446O, we find clear evidence for large-amplitude X-ray variability at the orbital period consistent with an intrabinary shock origin. The third redback MSP in the cluster, PSR J1748−2446A, shows order-of-magnitude variations in flux on timescales of years, possibly due to state transitions or intense flaring episodes from a magnetically active secondary star.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present a novel method to detect variable astrophysical objects and transient phenomena using anomalous excess scatter in repeated measurements from public catalogs of Gaia DR2 and Zwicky Transient Facility (ZTF) DR3 photometry. We first provide a generalized, all-sky proxy for variability using only Gaia DR2 photometry, calibrated to white dwarf stars. To ensure more robust candidate detection, we further employ a method combining Gaia with ZTF photometry and alerts. To demonstrate its efficacy, we apply this latter technique to a sample of roughly 12,100 white dwarfs within 200 pc centered on the ZZ Ceti instability strip, where hydrogen-atmosphere white dwarfs are known to pulsate. By inspecting the top 1% of the samples ranked by these methods, we demonstrate that both the Gaia-only and ZTF-informed techniques are highly effective at identifying known and new variable white dwarfs, which we verify using follow-up, high-speed photometry. We confirm variability in all 33 out of 33 (100%) observed white dwarfs within our top 1% highest-ranked candidates, both inside and outside the ZZ Ceti instability strip. In addition to dozens of new pulsating white dwarfs, we also identify five white dwarfs highly likely to show transiting planetary debris; if confirmed, these systems would more than triple the number of white dwarfs known to host transiting debris.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We probe the dusty environment of the archetypical Type 1 active galactic nucleus (AGN) in NGC 4151 with comprehensive IR reverberation mapping over several decades, in the <jats:italic>J</jats:italic> (∼1.22 <jats:italic>μ</jats:italic>m), <jats:italic>H</jats:italic> (∼1.63 <jats:italic>μ</jats:italic>m), <jats:italic>K</jats:italic> (∼2.19 <jats:italic>μ</jats:italic>m), <jats:italic>L</jats:italic> (∼3.45 <jats:italic>μ</jats:italic>m), and <jats:italic>N </jats:italic>bands (∼10.6 <jats:italic>μ</jats:italic>m), plus multiple measurements at 20–40 <jats:italic>μ</jats:italic>m. At 1–4 <jats:italic>μ</jats:italic>m, the hot dust reverberation signals come from two distinct dust populations at separate radii (∼0.033 pc and ∼0.076 pc), with temperatures of ∼1500–2500 K and ∼900–1000 K, consistent with the expected properties of sublimating graphite and silicate dust grains. The domination of the torus infrared output by carbon and silicate grains near their sublimation temperatures and radii may account for the general similarity of AGN near-IR spectral energy distributions. The torus inner edge defined by the hottest dust remains at roughly the same radius independent of the AGN optical luminosity over ∼25 yr. The emission by hot dust warmed directly by the optical/UV AGN output has increased gradually by ∼4% yr<jats:sup>−1</jats:sup>, indicating a possibly growing torus. A third dust component at ∼700 K does not seem to participate directly in the IR reverberation behavior, and its emission may originate deep in the circumnuclear torus. We find a reverberation signal at ∼10 <jats:italic>μ</jats:italic>m with an inferred radius for the warm dust of ∼2.2–3.1 pc. The lack of variability at 20–40 <jats:italic>μ</jats:italic>m indicates that the far-IR emission comes from even more extended regions. The torus properties revealed by dust reverberation analysis are inconsistent with predictions from pure clumpy torus models. Instead, the longer-wavelength emission possibly originates in a flared torus or the polar wind.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Understanding how the magnetic activity of low-mass stars depends on their fundamental parameters is an important goal of stellar astrophysics. Previous studies have shown that activity levels are largely determined by the stellar Rossby number, defined as the rotation period divided by the convective turnover time. However, we currently have little information on the role played by chemical composition. In this work, we investigate how metallicity affects magnetic activity, using photometric variability as an activity proxy. Similarly to other proxies, we demonstrate that the amplitude of photometric variability is well parameterized by the Rossby number, although in a more complex way. We also show that variability amplitude and metallicity are generally positively correlated. This trend can be understood in terms of the effect of metallicity on stellar structure, and hence the convective turnover time (or, equivalently, the Rossby number). Lastly, we demonstrate that the metallicity dependence of photometric variability results in a rotation-period detection bias, whereby the periods of metal-rich stars are more easily recovered for stars of a given mass.</jats:p>