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<jats:title>Abstract</jats:title> <jats:p>With the conclusion of the third observing run for Advanced LIGO/Virgo (O3), we present a detailed analysis of both triggered and serendipitous observations of 17 gravitational-wave (GW) events (7 triggered and 10 purely serendipitous) from the Searches After Gravitational-waves Using ARizona Observatories (SAGUARO) program. We searched a total of 4935 deg<jats:sup>2</jats:sup> down to a median 5<jats:italic>σ</jats:italic> transient detection depth of 21.1 AB mag using the Mt. Lemmon 1.5 m telescope, the discovery engine for SAGUARO. In addition to triggered events within 24 hr, our transient search encompassed a time interval following GW events of &lt;120 hr, providing observations on ∼1/2 of the events accessible to the Mt. Lemmon 1.5 m telescope. We covered 2.1%–86% of the LVC total probability (<jats:italic>P</jats:italic> <jats:sub>total</jats:sub>) for individual events, with a median <jats:italic>P</jats:italic> <jats:sub>total</jats:sub> ≈ 8% within &lt;120 hr. Following improvements to our pipeline and the addition of serendipitous observations, we find a total of seven new optical candidates across five GW events, which we are unable to rule out after searching for additional information and comparing to kilonova models. Using both publicly available and our own late-time data, we investigated a total of 252 optical candidates for these 17 events, finding that only 65% were followed up in some capacity by the community. Of the total 252 candidates, we are able to rule out an additional 12 previously reported counterpart candidates. In light of these results, we discuss lessons learned from the SAGUARO GW counterpart search. We discuss how community coordination of observations and candidate follow-up, as well as the role of archival data, are crucial to improving the efficiency of follow-up efforts and preventing unnecessary duplication of effort with limited electromagnetic resources.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Relativistic magnetic reconnection is a potential particle acceleration mechanism for high-frequency BL Lac objects (HBLs). The Imaging X-ray Polarimetry Explorer (IXPE) scheduled to launch in 2021 has the capability to probe the evolution of magnetic field in HBLs, examining the magnetic reconnection scenario for the HBL flares. In this paper, we make the first attempt to self-consistently predict HBL X-ray polarization signatures arising from relativistic magnetic reconnection via combined particle-in-cell and polarized radiation transfer simulations. We find that although the intrinsic optical and X-ray polarization degrees are similar on average, the X-ray polarization is much more variable in both the polarization degree and angle (PD and PA). Given the sensitivity of the IXPE, it may obtain one to a few polarization data points for one flaring event of nearby bright HBLs Mrk 421 and 501. However, it may not fully resolve the highly variable X-ray polarization. Due to temporal depolarization, where the integration of photons with variable polarization states over a finite period of time can lower the detected PD, the measured X-ray PD can be considerably lower than the optical counterpart or even undetectable. The lower X-ray PD than the optical thus can be a characteristic signature of relativistic magnetic reconnection. For very bright flares where the X-ray polarization is well resolved, relativistic magnetic reconnection predicts smooth X-ray PA swings, which originate from large plasmoid mergers in the reconnection region.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Recent studies of warp kinematics using Gaia DR2 data have produced detections of warp precession for the first time, which greatly exceeds theoretical predictions of models. However, this detection assumes a warp model derived for a young population (few tens of megayears) to fit velocities of an average older stellar population of the thin disk (several gigayears) in Gaia-DR2 observations, which may lead to unaccounted systematic errors. Here, we recalculate the warp precession with the same approach and Gaia DR2 kinematic data, but using different warp parameters based on the fit of star counts of the Gaia DR2 sample, which has a much lower warp amplitude than the young population. When we take into account this variation of the warp amplitude with the age of the population, we find that there is no need for precession. We find the value of warp precession <jats:inline-formula> <jats:tex-math> <?CDATA $\beta ={4}_{-4}^{+6}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>β</mml:mi> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>4</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>6</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabf356ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> km s<jats:sup>−1</jats:sup> kpc<jats:sup>−1</jats:sup>, which does not exclude nonprecessing warp.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present X-ray analysis of the ejecta of supernova remnant (SNR) G350.1–0.3 observed with Chandra and Suzaku, clarify the ejecta’s kinematics over a decade, and obtain a new observational clue to understanding the origin of the asymmetric explosion. Two images from Chandra X-ray Observatory taken in 2009 and 2018 are analyzed with several methods and enable us to measure the velocities in the plane of the sky. A maximum velocity is 4640 ± 290 km s<jats:sup>−1</jats:sup> (0.218 ± 0.014 arcsec yr<jats:sup>−1</jats:sup>) in the eastern region in the remnant. These findings trigger us to scrutinize the Doppler effects in the spectra of the thermal emission, and the velocities in the line-of-sight direction are estimated to be 1000 km s<jats:sup>−1</jats:sup>. The results are confirmed by analyzing the spectra of Suzaku. Combining the proper motions and line-of-sight velocities, the ejecta’s 3D velocities are ∼3000–5000 km s<jats:sup>−1</jats:sup>. The center of the explosion is more stringently constrained by finding the optimal time to reproduce the observed spatial expansion. Our findings that the age of the SNR is estimated at most to be 655 yr and the CCO is observed as a point source object against the SNR strengthen the “hydrodynamical kick” hypothesis on the origin of the remnant.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We investigate the accretion-induced spin-up of the black hole via numerical simulations. Our method is based on general relativistic magnetohydrodynamics of the slowly rotating flows in the Kerr metric, where possibly transonic shock fronts may form. We account for the changing black hole mass and spin during accretion that enforces dynamical evolution of the spacetime metric. We first study nonmagnetized flows with shocks, and we also include magnetic field endowed in the gas. The aim of this study is to verify whether the high-mass black holes may be produced with large spins, even though at birth the collapsars might have contained slowly or moderately spinning cores. In this way, we put constraints on the content of angular momentum in the collapsing massive stars. Our studies are also showing that shock fronts and magnetic fields may halt accretion and limit the black hole spin-up in the exploding supernovae.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report the discovery of diffuse X-ray emission around the supernova remnant (SNR) G106.3+2.7, which is associated with VER J2227+608 and HAWC J2227+610 and is known as a candidate for a PeV cosmic-ray accelerator (PeVatron). We analyze observational data of Suzaku around the SNR and the adjacent pulsar PSR J2229+6114. We find diffuse X-ray emission that is represented by either thermal or nonthermal processes. However, the metal abundance for the thermal emission is &lt;0.13 <jats:italic>Z</jats:italic> <jats:sub>⊙</jats:sub>, which may be too small in the Milky Way and suggests that the emission is nonthermal. The intensity of the diffuse emission increases toward PSR J2229+6114 in the same way as radio emission, and it is in contrast with gamma-ray emission concentrated on a molecular cloud. The X-ray photon index does not change with the distance from the pulsar and it indicates that radiative cooling is ineffective and particle diffusion is not extremely slow. The X-ray and radio emissions seem to be of leptonic origin and the parent electrons may originate from the pulsar. The gamma-ray emission appears to be of hadronic origin because of its spatial distribution. The parent protons may be tightly confined in the cloud separately from the diffusing electrons.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Fast radio bursts (FRBs) are bright radio transient events with durations on the order of milliseconds. The majority of FRB sources discovered so far have a single peak, with the exception of a few showing multiple-peaked profiles, the origin of which is unknown. In this work, we show that the strong lensing effect of a point mass or a point mass + external shear on a single-peak FRB can produce double peaks (i.e., lensed images). In particular, the leading peak will always be more magnified and hence brighter than the trailing peak for a point-mass lens model, while the point-mass + external shear lens model can produce a less magnified leading peak. We find that, for a point-mass lens model, the combination of lens mass <jats:italic>M</jats:italic> and redshift <jats:italic>z</jats:italic> <jats:sub> <jats:italic>l</jats:italic> </jats:sub> in the form of <jats:italic>M</jats:italic>(1 + <jats:italic>z</jats:italic> <jats:sub> <jats:italic>l</jats:italic> </jats:sub>) can be directly computed from two observables—the delayed time Δ<jats:italic>t</jats:italic> and the flux ratio of the leading peak to the trailing peak <jats:italic>R</jats:italic>. For a point-mass + external shear lens model, upper and lower limits in <jats:italic>M</jats:italic>(1 +<jats:italic> z</jats:italic> <jats:sub> <jats:italic>l</jats:italic> </jats:sub>) can also be obtained from Δ<jats:italic>t</jats:italic> and <jats:italic>R</jats:italic> for a given external shear strength. In particular, tighter lens mass constraints can be achieved when the observed <jats:italic>R</jats:italic> is larger. Lastly, we show the process of constraining lens mass using the observed values of Δ<jats:italic>t</jats:italic> and <jats:italic>R</jats:italic> of two double-peaked FRB sources, i.e., FRB 121002 and FRB 130729, as references, although the double-peaked profiles are not necessarily caused by strong lensing.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The ratio of total losses of H and O from the atmosphere is crucial for determining the Martian atmospheric redox state. The H and O escapes are shown to be regulated in a stoichiometric 2:1 ratio in a converged model of present-day Mars over a timescale of ∼10<jats:sup>5</jats:sup> yr, which is called self-regulation. Self-regulation timescales under different atmospheric conditions on early Mars are not well understood. Here we use a 1D photochemical model to calculate the timescales of self-regulation for denser CO<jats:sub>2</jats:sub> atmospheres with various surface temperatures as benchmark cases for early Mars. Self-regulation is driven by variations in the amount of O<jats:sub>2</jats:sub> or CO in the atmosphere, depending on the atmospheric redox state. Self-regulation timescales are likely to be controlled by the net redox balance. A 1 bar CO<jats:sub>2</jats:sub> atmosphere with a surface temperature of 240 K has a self-regulation timescale of a few million years. Denser atmospheres of early Mars have a longer regulation timescale and are less redox-stable than the atmosphere of present-day Mars. Obliquity variations cause atmospheric CO<jats:sub>2</jats:sub> fluctuations, producing a difference in the self-regulation timescale between high and low obliquity. Because an increase in CO<jats:sub>2</jats:sub> suppresses H escape, the net effect of the obliquity cycle could have driven the atmospheric redox states to be more reducing. Our results also suggest the possibility of a CO-dominated atmosphere of 10–100 mbars at 3 Ga. The redox state of ancient Mars might have fluctuated more easily than that of the present.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Protoplanetary disk evolution is strongly impacted by ionization from the central star and local environment, which collectively have been shown to drive chemical complexity and are expected to impact the transport of disk material. Nonetheless, ionization remains a poorly constrained input to many detailed modeling efforts. We use new and archival ALMA observations of N<jats:sub>2</jats:sub>H<jats:sup>+</jats:sup> 3–2 and H<jats:sup>13</jats:sup>CO<jats:sup>+</jats:sup> 3–2 to derive the first observationally motivated ionization model for the IM Lup protoplanetary disk. Incorporating ionization from multiple internal and external sources, we model N<jats:sub>2</jats:sub>H<jats:sup>+</jats:sup> and H<jats:sup>13</jats:sup>CO<jats:sup>+</jats:sup> abundances under varying ionization environments and compare these directly to the imaged ALMA observations by performing non-LTE radiative transfer, visibility sampling, and imaging. We find that the observations are best reproduced using a radially increasing cosmic-ray (CR) gradient, with low CR ionization in the inner disk, high CR ionization in the outer disk, and a transition at ∼80–100 au. This location is approximately coincident with the edge of spiral structure identified in millimeter emission. We also find that IM Lup shows evidence for enhanced UV-driven formation of HCO<jats:sup>+</jats:sup>, which we attribute to the disk’s high flaring angle. In summary, IM Lup represents the first protoplanetary disk with observational evidence for a CR gradient, which may have important implications for IM Lup’s ongoing evolution, especially given the disk’s young age and large size.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Young stellar associations represent a key site for the study of star formation, but to accurately compare observations to models of stellar evolution, the age of an association must be determined. The Upper Scorpius region is the youngest section of the Scorpius–Centaurus OB association, which is the largest collection of nearby, young, low-mass stars. The true age of Upper Scorpius is not clear, and an observed mass-dependent age gradient in Upper Scorpius, as well as in other star-forming regions, complicates age measurements. The age gradient may indicate a genuine astrophysical feature or may be an artifact of unrecognized systematic effects in stellar age measurements. We have conducted a synthetic red-optical low-resolution spectroscopic survey of a simulated analog to the Upper Scorpius star-forming region to investigate the effects of unresolved binary stars (which have mass-dependent demographics) on age measurements of a stellar population. We found that the observed mass-dependent age gradient in Upper Scorpius can be explained by a population of undetected binary stars. For a simulated population with an age of 10 (rms = 2) Myr, we measured an age of 10.5 (rms = 3.5) Myr for F stars and of 7.5 (rms = 5.8) Myr for M stars. This discrepancy is caused by the mass-dependent mass ratio distribution and the variable steepness of the mass–luminosity relation. Our results support the previously suggested 10 Myr age for Upper Scorpius, with a small intrinsic age spread.</jats:p>