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<jats:title>Abstract</jats:title> <jats:p>We present [C <jats:sc>ii</jats:sc>] synthetic observations of smoothed particle hydrodynamics (SPH) simulations of a dwarf galaxy merger. The merging process varies the star formation rate (SFR) by more than three orders of magnitude. Several star clusters are formed, the feedback of which disperses and unbinds the dense gas through expanding H <jats:sc>ii</jats:sc> regions and supernova (SN) explosions. For galaxies with properties similar to the modeled ones, we find that the [C <jats:sc>ii</jats:sc>] emission remains optically thin throughout the merging process. We identify the warm neutral medium (<jats:inline-formula> <jats:tex-math> <?CDATA $3\lt \mathrm{log}{T}_{\mathrm{gas}}\lt 4$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>3</mml:mn> <mml:mo>&lt;</mml:mo> <mml:mi>log</mml:mi> <mml:msub> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>gas</mml:mi> </mml:mrow> </mml:msub> <mml:mo>&lt;</mml:mo> <mml:mn>4</mml:mn> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac7960ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> with <jats:italic>χ</jats:italic> <jats:sub>HI</jats:sub> &gt; 2<jats:italic>χ</jats:italic> <jats:sub>H2</jats:sub>) to be the primary source of [C <jats:sc>ii</jats:sc>] emission (∼58% contribution), although at stages when the H <jats:sc>ii</jats:sc> regions are young and dense (during star cluster formation or SNe in the form of ionized bubbles), they can contribute ≳50% to the total [C <jats:sc>ii</jats:sc>] emission. We find that the [C <jats:sc>ii</jats:sc>]/far-IR (FIR) ratio decreases owing to thermal saturation of the [C <jats:sc>ii</jats:sc>] emission caused by strong far-UV radiation fields emitted by the massive star clusters, leading to a [C <jats:sc>ii</jats:sc>] deficit medium. We investigate the [C <jats:sc>ii</jats:sc>]−SFR relation and find an approximately linear correlation that agrees well with observations, particularly those from the Dwarf Galaxy Survey. Our simulation reproduces the observed trends of [C <jats:sc>ii</jats:sc>]/FIR versus Σ<jats:sub>SFR</jats:sub> and Σ<jats:sub>FIR</jats:sub>, and it agrees well with the Kennicutt relation of SFR−FIR luminosity. We propose that local peaks of [C <jats:sc>ii</jats:sc>] in resolved observations may provide evidence for ongoing massive cluster formation.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>RX J1301.9+2747 is a unique active galaxy with a supersoft X-ray spectrum that lacks significant emission at energies above 2 keV. In addition, it is one of few galaxies displaying quasiperiodic X-ray eruptions that recur on a timescale of 13–20 ks. We present multiepoch radio observations of RX J1301.9+2747 using GMRT, Very Large Array (VLA), and Very Long Baseline Array (VLBA). The VLBA imaging at 1.6 GHz reveals a compact radio emission unresolved at a scale of &lt;0.7 pc, with a brightness temperature of <jats:italic>T</jats:italic> <jats:sub>b</jats:sub> &gt; 5 × 10<jats:sup>7</jats:sup> K. The radio emission is variable by more than a factor of 2.5 over a few days, based on the data taken from VLA monitoring campaigns. The short-term radio variability suggests that the radio emitting region has a size as small as 8 × 10<jats:sup>−4</jats:sup> pc, resulting in an even higher brightness temperature of <jats:italic>T</jats:italic> <jats:sub>b</jats:sub> ∼ 10<jats:sup>12</jats:sup> K. A similar limit on the source size can be obtained if the observed flux variability is not intrinsic and caused by the interstellar scintillation effect. The overall radio spectrum is steep with a time-averaged spectral index <jats:italic>α</jats:italic> = −0.78 ± 0.03 between 0.89 and 14 GHz. These observational properties rule out a thermal or star formation origin of the radio emission, and appear to be consistent with the scenario of episodic jet ejections driven by a magnetohydrodynamic process. Simultaneous radio and X-ray monitoring observations down to a cadence of hours are required to test whether the compact and variable radio emission is correlated with the quasiperiodic X-ray eruptions.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>On icy worlds, the ice shell and subsurface ocean form a coupled system—heat and salinity flux from the ice shell induced by the ice-thickness gradient drives circulation in the ocean, and in turn, the heat transport by ocean circulation shapes the ice shell. Therefore, understanding the dependence of the efficiency of ocean heat transport (OHT) on orbital parameters may allow us to predict the ice-shell geometry before direct observation is possible, providing useful information for mission design. Inspired by previous works on baroclinic eddies, I first derive scaling laws for the OHT on icy moons, driven by ice topography, and then verify them against high-resolution 3D numerical simulations. Using the scaling laws, I am then able to make predictions for the equilibrium ice-thickness variation knowing that the ice shell should be close to heat balance. The ice shell on small icy moons (e.g., Enceladus) may develop strong thickness variations between the equator and pole driven by the polar-amplified tidal dissipation in the ice; in contrast, the ice shell on large icy moons (e.g., Europa, Ganymede, Callisto, etc.) tends to be flat due to the smoothing effects of the efficient OHT. These predictions are manifested by the different ice-evolution pathways simulated for Enceladus and Europa, considering the ice freezing/melting induced by ice dissipation, conductive heat loss, and OHT as well as the mass redistribution by ice flow.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present measurements of the polarization of X-rays in the 2–8 keV band from the nucleus of the radio galaxy Centaurus A (Cen A), using a 100 ks observation from the Imaging X-ray Polarimetry Explorer (IXPE). Nearly simultaneous observations of Cen A were also taken with the Swift, NuSTAR, and INTEGRAL observatories. No statistically significant degree of polarization is detected with IXPE. These observations have a minimum detectable polarization at 99% confidence (MDP<jats:sub>99</jats:sub>) of 6.5% using a weighted, spectral model-independent calculation in the 2–8 keV band. The polarization angle <jats:italic>ψ</jats:italic> is consequently unconstrained. Spectral fitting across three orders of magnitude in X-ray energy (0.3–400 keV) demonstrates that the SED of Cen A is well described by a simple power law with moderate intrinsic absorption (<jats:italic>N</jats:italic> <jats:sub>H</jats:sub> ∼ 10<jats:sup>23</jats:sup> cm<jats:sup>−2</jats:sup>) and a Fe K<jats:italic>α</jats:italic> emission line, although a second unabsorbed power law is required to account for the observed spectrum at energies below 2 keV. This spectrum suggests that the reprocessing material responsible for this emission line is optically thin and distant from the central black hole. Our upper limits on the X-ray polarization are consistent with the predictions of Compton scattering, although the specific seed photon population responsible for the production of the X-rays cannot be identified. The low polarization degree, variability in the core emission, and the relative lack of variability in the Fe K<jats:italic>α</jats:italic> emission line support a picture where electrons are accelerated in a region of highly disordered magnetic fields surrounding the innermost jet.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We investigate the effect of the presence of lanthanides (<jats:italic>Z</jats:italic> = 57–71) on the kilonova at <jats:italic>t</jats:italic> ∼ 1 hr after the neutron star merger for the first time. For this purpose, we calculate the atomic structures and the opacities for selected lanthanides: Nd (<jats:italic>Z</jats:italic> = 60), Sm (<jats:italic>Z</jats:italic> = 62), and Eu (<jats:italic>Z</jats:italic> = 63). We consider the ionization degree up to 10th (XI), applicable for the ejecta at <jats:italic>t</jats:italic> ∼ a few hours after the merger, when the temperature is <jats:italic>T</jats:italic> ∼ 10<jats:sup>5</jats:sup> K. We find that the opacities for the highly ionized lanthanides are exceptionally high, reaching <jats:inline-formula> <jats:tex-math> <?CDATA ${\kappa }_{\exp }\sim 1000\,{\mathrm{cm}}^{2}\,{{\rm{g}}}^{-1}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>κ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>exp</mml:mi> </mml:mrow> </mml:msub> <mml:mo>∼</mml:mo> <mml:mn>1000</mml:mn> <mml:mspace width="0.25em" /> <mml:msup> <mml:mrow> <mml:mi>cm</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> <mml:mspace width="0.25em" /> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">g</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac7565ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> for Eu, due to the highly dense energy levels. Using the new opacity, we perform radiative transfer simulations to show that the early light curves become fainter by a (maximum) factor of four, in comparison to lanthanide-free ejecta at <jats:italic>t</jats:italic> ∼ 0.1 days. However, the period at which the light curves are affected is relatively brief owing to the rapid time evolution of the opacity in the outermost layer of the ejecta. We predict that for a source at a distance of ∼100 Mpc, UV brightness for lanthanide-rich ejecta shows a drop to ∼21–22 mag at <jats:italic>t</jats:italic> ∼ 0.1 days and the UV peaks around <jats:italic>t</jats:italic> ∼ 0.2 days with a magnitude of ∼19 mag. Future detection of such a kilonova by an existing UV satellite like Swift or the upcoming UV satellite ULTRASAT will provide useful constraints on the abundance in the outer ejecta and the corresponding nucleosynthesis conditions in the neutron star mergers.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In pulsar signal processing, two primary difficulties are (1) radio-frequency interference (RFI) mitigation and (2) information loss due to preprocessing and mitigation itself. Linear mitigation methods have a difficulty in RFI modeling, and accommodate a limited range of RFI morphologies. Thresholding methods suffer from manual factors and adaptability. There is also a distinct lack of methods dedicated to information loss. In this paper, a novel method “CS-Pulsar” is proposed. It carries out compressed sensing (CS) on time-frequency signals to accomplish RFI mitigation and signal restoration simultaneously. Curvelets allow an optimal sparse representation for multichannel pulsar signals containing the time-of-arrival dispersion relationship. CS-Pulsar mitigation is implemented using a regularized least-squares framework that does not require the statistics of RFI to be known beforehand. CS-Pulsar implements channel restoration, and useful signal contents are retrieved from the measurement error by a morphological component analysis aided by the root-mean-square envelope. These two steps allow CS-Pulsar to provide key signal details for special astrophysical purposes. Experiments of signal restoration for pulsar data from the Nanshan 26 m radio telescope reveal the advantage of CS-Pulsar. The method successfully removes false peaks due to on-pulse RFI in multipeaked pulsar profiles. CS-Pulsar also increases the timing accuracy and signal-to-noise ratio proving its feasibilities and prospects in astrophysical measurements.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Recent observations by the Large High Altitude Air Shower Observatory (LHAASO) have paved the way for the observational detection of PeVatrons in the Milky Way, thus revolutionizing the field of <jats:italic>γ</jats:italic>-ray astrophysics. In this paper, we study one such detected source, LHAASO J1908+0621, and explore the origin of multi-TeV <jats:italic>γ</jats:italic>-ray emission from this source. A middle-aged radio supernova remnant SNR G40.5–0.5 and a GeV pulsar PSR J1907+0602 are cospatial with LHAASO J1908+0621. Dense molecular clouds are also found to be associated with SNR G40.5–0.5. We explain the multi-TeV <jats:italic>γ</jats:italic>-ray emission observed from the direction of LHAASO J1908+0621, by the hadronic interaction between accelerated protons that escaped from the SNR shock front and cold protons present inside the dense molecular clouds, and the leptonic emission from the pulsar wind nebula (PWN) associated with the pulsar J1907+0602. Moreover, we explain lower energy <jats:italic>γ</jats:italic>-ray emission by considering the radiative cooling of the electrons that escaped from SNR G40.5–0.5. Finally, the combined lepto-hadronic scenario was used to explain the multiwavelength spectral energy distribution of LHAASO J1908+0621. Although not yet significant, an IceCube hotspot of neutrino emission is spatially associated with LHAASO J1908+0621, indicating a possible hadronic contribution. In this paper, we show that if a hadronic component is present in LHAASO J1908+0621, then the second-generation IceCube observatory will detect neutrinos from this source.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The degree to which the thin accretion disks of black hole X-ray binaries are truncated during hard spectral states remains a contentious open question in black hole astrophysics. During its singular observed outburst in 2009–2010, the black hole X-ray binary XTE J1752−223 spent ∼1 month in a long-stable hard spectral state at a luminosity of ∼0.02–0.1 <jats:italic>L</jats:italic> <jats:sub>Edd</jats:sub>. It was observed with 56 RXTE pointings during this period, with simultaneous Swift-XRT daily coverage during the first 10 days of the RXTE observations. While reflection modeling has been extensively explored in the analysis of these data, there is disagreement surrounding the geometry of the accretion disk and corona implied by the reflection features. We reexamine the combined, high signal-to-noise, simultaneous Swift and RXTE observations, and perform extensive reflection modeling with the latest <jats:monospace>relxill</jats:monospace> suite of reflection models, including newer high disk density models. We show that reflection modeling requires that the disk be within ∼5 <jats:italic>R</jats:italic> <jats:sub>ISCO</jats:sub> during the hard spectral state, while weaker constraints from the thermal disk emission imply higher truncation (<jats:italic>R</jats:italic> <jats:sub>in</jats:sub> = 6–80 <jats:italic>R</jats:italic> <jats:sub>ISCO</jats:sub>). We also explore more complex coronal continuum models, allowing for two Comptonization components instead of one, and show that the reflection features still require only a mildly truncated disk. Finally we present a full comparison of our results to previous constraints found from analyses of the same data set.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present a highly complete sample of broad-line (Type 1) QSOs out to <jats:italic>z</jats:italic> ∼ 3 selected by their mid-infrared colors, a method that is minimally affected by dust reddening. We remove host-galaxy emission from the spectra and fit for excess reddening in the residual QSOs, resulting in a Gaussian distribution of colors for unreddened (blue) QSOs, with a tail extending toward heavily reddened (red) QSOs, defined as having <jats:italic>E</jats:italic>(<jats:italic>B</jats:italic> − <jats:italic>V</jats:italic>) &gt; 0.25. This radio-independent selection method enables us to compare red and blue QSO radio properties in both the FIRST (1.4 GHz) and VLASS (2–4 GHz) surveys. Consistent with recent results from optically selected QSOs from SDSS, we find that red QSOs have a significantly higher detection fraction and a higher fraction of compact radio morphologies at both frequencies. We employ radio stacking to investigate the median radio properties of the QSOs including those that are undetected in FIRST and VLASS, finding that red QSOs have significantly brighter radio emission and steeper radio spectral slopes compared with blue QSOs. Finally, we find that the incidence of red QSOs is strongly luminosity dependent, where red QSOs make up &gt;40% of all QSOs at the highest luminosities. Overall, red QSOs comprise ∼40% of higher luminosity QSOs, dropping to only a few percent at lower luminosities. Furthermore, red QSOs make up a larger percentage of the radio-detected QSO population. We argue that dusty AGN-driven winds are responsible for both the obscuration as well as excess radio emission seen in red QSOs.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present photoionization modeling of galaxy populations at <jats:italic>z</jats:italic> ∼ 0, 2, and &gt;6 to bridge optical and far-infrared (FIR) emission-line diagrams. We collect galaxies with measurements of optical and/or FIR ([O<jats:sc> iii</jats:sc>] 88 <jats:italic>μ</jats:italic>m and [C <jats:sc>ii</jats:sc>] 158 <jats:italic>μ</jats:italic>m) emission-line fluxes and plot them on the [O <jats:sc> <jats:sc>iii</jats:sc> </jats:sc>]<jats:italic>λ</jats:italic>5007/H<jats:italic>β</jats:italic>–[N <jats:sc> <jats:sc>ii</jats:sc> </jats:sc>]<jats:italic>λ</jats:italic>6585/H<jats:italic>α</jats:italic> (BPT) and <jats:italic>L</jats:italic>([O <jats:sc>iii</jats:sc>]<jats:sub>88</jats:sub>)/SFR–<jats:italic>L</jats:italic>([C <jats:sc> <jats:sc>ii</jats:sc> </jats:sc>]<jats:sub>158</jats:sub>)/SFR diagrams, where SFR is the star formation rate and <jats:italic>L</jats:italic>([O <jats:sc> <jats:sc>iii</jats:sc> </jats:sc>]<jats:sub>88</jats:sub>) and <jats:italic>L</jats:italic>([C <jats:sc> <jats:sc>ii</jats:sc> </jats:sc>]<jats:sub>158</jats:sub>) are the FIR line luminosities. We aim to explain the galaxy distributions on the two diagrams with photoionization models that employ three nebular parameters: the ionization parameter <jats:italic>U</jats:italic>, hydrogen density <jats:italic>n</jats:italic> <jats:sub>H</jats:sub>, and gaseous metallicity <jats:italic>Z</jats:italic> <jats:sub>gas</jats:sub>. Our models successfully reproduce the nebular parameters of local galaxies, and then predict the distributions of the <jats:italic>z</jats:italic> ∼ 0, 2, and &gt;6 galaxies in the diagrams. The predicted distributions illustrate the redshift evolution on all the diagrams; e.g., [O <jats:sc> <jats:sc>iii</jats:sc> </jats:sc>]/H<jats:italic>β</jats:italic> and [O <jats:sc> <jats:sc>iii</jats:sc> </jats:sc>]<jats:sub>88</jats:sub>/[C <jats:sc>ii</jats:sc>]<jats:sub>158</jats:sub> ratios continuously decrease from <jats:italic>z</jats:italic> &gt; 6 to 0. Specifically, the <jats:italic>z</jats:italic> &gt; 6 galaxies exhibit ∼0.5 dex higher <jats:italic>U</jats:italic> than low-redshift galaxies at a given <jats:italic>Z</jats:italic> <jats:sub>gas</jats:sub> and show predicted flat distributions on the BPT diagram at <jats:inline-formula> <jats:tex-math> <?CDATA ${\rm{log}}[{\rm{O}}\,{\rm\small{III}}]/{\rm{H}}\beta \,=\,$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi mathvariant="normal">log</mml:mi> </mml:mrow> <mml:mo stretchy="false">[</mml:mo> <mml:mrow> <mml:mi mathvariant="normal">O</mml:mi> </mml:mrow> <mml:mspace width="0.25em" /> <mml:mrow> <mml:mi mathsize="small" mathvariant="normal">III</mml:mi> </mml:mrow> <mml:mo stretchy="false">]</mml:mo> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mi>β</mml:mi> <mml:mspace width="0.25em" /> <mml:mo>=</mml:mo> <mml:mspace width="0.25em" /> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac7fedieqn1.gif" xlink:type="simple" /> </jats:inline-formula> 0.5–0.8. We find that some of the <jats:italic>z</jats:italic> &gt; 6 galaxies exhibit high <jats:italic>L</jats:italic>([O <jats:sc>iii</jats:sc>]<jats:sub>88</jats:sub>)/SFR ratios. To explain these high ratios, our photoionization models require a low stellar-to-gaseous-metallicity ratio or bursty/increasing star formation history at <jats:italic>z</jats:italic> &gt; 6. JWST will test the predictions and scenarios for the <jats:italic>z</jats:italic> &gt; 6 galaxies proposed by our photoionization modeling.</jats:p>