The Astrophysical Journal Letters

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The Megamaser Cosmology Project. XIII. Combined Hubble Constant Constraints
We present a measurement of the Hubble constant made using geometric distance measurements to megamaser-hosting galaxies. We have applied an improved approach for fitting maser data and obtained better distance estimates for four galaxies previously published by the Megamaser Cosmology Project: UGC 3789, NGC 6264, NGC 6323, and NGC 5765b. Combining these updated distance measurements with those for the maser galaxies CGCG 074-064 and NGC 4258, and assuming a fixed velocity uncertainty of 250 km s −1 associated with peculiar motions, we constrain the Hubble constant to be H 0 = 73.9 ± 3.0 km s −1 Mpc −1 independent of distance ladders and the cosmic microwave background. This best value relies solely on maser-based distance and velocity measurements, and it does not use any peculiar velocity corrections. Different approaches for correcting peculiar velocities do not modify H 0 by more than ±1 σ , with the full...
In-situ Switchback Formation in the Expanding Solar Wind
Recent near-Sun solar-wind observations from Parker Solar Probe have found a highly dynamic magnetic environment, permeated by abrupt radial-field reversals, or “switchbacks.” We show that many features of the observed turbulence are reproduced by a spectrum of Alfvénic fluctuations advected by a radially expanding flow. Starting from simple superpositions of low-amplitude outward-propagating waves, our expanding-box compressible magnetohydrodynamic simulations naturally develop switchbacks because (i) the normalized amplitude of waves grows due to expansion and (ii) fluctuations evolve toward spherical polarization (i.e., nearly constant field strength). These results suggest that switchbacks form in situ in the expanding solar wind and are not indicative of impulsive processes in the chromosphere or corona.
Detecting Interstellar Objects through Stellar Occultations
Stellar occultations have been used to search for Kuiper Belt and Oort Cloud objects. We propose a search for interstellar objects based on the characteristic durations (∼0.1 s) of their stellar occultation signals and high inclination relative to the ecliptic plane. An all-sky monitoring program of all ∼7 × 10 6 stars with R ≲ 12.5 using 1-m telescopes with 0.1 s cadences is predicted to discover ∼1 interstellar object per year.
De-noising SDO /HMI Solar Magnetograms by Image Translation Method Based on Deep Learning
In astronomy, long-exposure observations are one of the important ways to improve signal-to-noise ratios (S/Ns). In this Letter, we apply a deep-learning model to de-noise solar magnetograms. This model is based on a deep convolutional generative adversarial network with a conditional loss for image-to-image translation from a single magnetogram (input) to a stacked magnetogram (target). For the input magnetogram, we use Solar Dynamics Observatory ( SDO )/Helioseismic and Magnetic Imager (HMI) line-of-sight magnetograms at the center of the solar disk. For the target magnetogram, we make 21-frame-stacked magnetograms, taking into account solar rotation at the same position. We train a model using 7004 pairs of the input and target magnetograms from 2013 January to 2013 October. We then validate the model using 707 pairs from 2013 November and test the model using 736 pairs from 2013 December. Our results from this study are as follows. First, our model successfully d...
Is GW190425 Consistent with Being a Neutron Star–Black Hole Merger?
GW190425 is the second neutron star merger event detected by the Advanced LIGO/Virgo detectors. If interpreted as a double neutron star merger, the total gravitational mass is substantially larger than that of the binary systems identified in the Galaxy. In this work we analyze the gravitational-wave data within the neutron star–black hole merger scenario. For the black hole, we yield a mass of {${2.40}_{-0.32}^{+0.36}{M}_{\odot }$} and an aligned spin of {${0.141}_{-0.064}^{+0.067}$} . As for the neutron star we find a mass of {${1.15}_{-0.13}^{+0.15}{M}_{\odot }$} and the dimensionless tidal deformability of {${1.4}_{-1.2}^{+3.8}\times {10}^{3}$}
Nine New Repeating Fast Radio Burst Sources from CHIME/FRB
We report on the discovery and analysis of bursts from nine new repeating fast radio burst (FRB) sources found using the Canadian Hydrogen Intensity Mapping Experiment (CHIME) telescope. These sources span a dispersion measure (DM) range of 195–1380 pc cm −3 . We detect two bursts from three of the new sources, three bursts from four of the new sources, four bursts from one new source, and five bursts from one new source. We determine sky coordinates of all sources with uncertainties of ∼10′. We detect Faraday rotation measures (RMs) for two sources, with values −20(1) and −499.8(7) rad m −2 , that are substantially lower than the RM derived from bursts emitted by FRB 121102. We find that the DM distribution of our events, combined with the nine other repeaters discovered by CHIME/FRB, is indistinguishable from that of thus far non-repeating CHIME/FRB events. However, as previously reported, the burst widths appear statistically significantly larger than the th...
Spontaneous Onset of Collisionless Magnetic Reconnection on an Electron Scale
Using particle-in-cell simulations, we investigate the onset of magnetic reconnection from a quiescent Harris current sheet in collisionless plasmas. After the current sheet is destabilized by the collisionless tearing mode instability, it proceeds to onset of reconnection, which manifests spontaneous thinning of current sheet and pileup of upstream magnetic flux. Once the current sheet thins to a critical thickness, about two electron inertial lengths, reconnection begins to grow explosively in this electron current sheet. This study shows that the spontaneous onset of collisionless magnetic reconnection is controlled by electron kinetics.
Upper Limit on the Dissipation of Gravitational Waves in Gravitationally Bound Systems
It is shown that a gravitationally bound system with a one-dimensional velocity dispersion σ can at most dissipate a fraction {$\sim 36{\left(\sigma /c\right)}^{3}$} of the gravitational wave (GW) energy propagating through it, even if their dynamical time is shorter than the wave period. The limit is saturated for low-frequency waves propagating through a system of particles with a mean-free-path equal to the size of the system, such as hot protons in galaxy clusters, strongly interacting dark matter particles in halos, or massive black holes in clusters. For such systems with random motions and no resonances, the dissipated fraction, {$\lesssim {10}^{-6}$} , does not degrade the use of GWs as cosmological probes. At high-wave frequencies, the dissipated fraction is additionally su...
Fast Luminous Blue Transients in the Reionization Era and Beyond
To determine the epoch of reionization precisely and to reveal the property of inhomogeneous reionization are some of the most important topics of modern cosmology. Existing methods to investigate reionization that use cosmic microwave background, Ly α emitters, quasars, or gamma-ray bursts have difficulties in terms of accuracy or event rate. We propose that recently discovered fast luminous blue transients (FLBTs) have potential as a novel probe of reionization. We study the detectability of FLBTs at the epoch of reionization with upcoming WFIRST Wide-Field Instrument, using a star formation rate (SFR) derived from galaxy observations and an event rate of FLBTs proportional to the SFR. We find that if FLBTs occur at a rate of 1% of the core-collapse supernova rate, 2 (0.3) FLBTs per year per deg 2 at z > 6 ( z > 8) can be detected by a survey with a limiting magnitude of 26.5 mag in the near-infrared band and a cadence of 10 days. We c...
Probing Cosmic-Ray Transport with Radio Synchrotron Harps in the Galactic Center
Recent observations with the MeerKAT radio telescope reveal a unique population of faint nonthermal filaments pervading the central molecular zone, a region rich in molecular gas near the Galactic center. Some of those filaments are organized into groups of almost parallel filaments, seemingly sorted by their length, so that their morphology resembles a harp with radio-emitting “strings.” We argue that the synchrotron-emitting GeV electrons of these radio harps have been consecutively injected by the same source (a massive star or pulsar) into spatially intermittent magnetic fiber bundles within a magnetic flux tube or via time-dependent injection events. After escaping from this source, the propagation of cosmic-ray (CR) electrons inside a flux tube is governed by the theory of CR transport. We propose to use observations of radio harp filaments to gain insight into the specifics of CR propagation along magnetic fields of which there are two principle modes: CRs could either st...
No Massive Companion to the Coherent Radio-emitting M Dwarf GJ 1151
The recent detection of circularly polarized, long-duration (>8 hr), low-frequency (∼150 MHz) radio emission from the M4.5 dwarf GJ 1151 has been interpreted as arising from a star–planet interaction via the electron cyclotron maser instability. The existence or parameters of the proposed planets have not been determined. Using 20 new HARPS-N observations, we put 99th-percentile upper limits on the mass of any close companion to GJ 1151 at {$M\sin i\lt 5.6\,{M}_{\oplus }$} . With no stellar, brown dwarf, or giant planet companion likely in a close orbit, our data are consistent with detected radio emission emerging from a magnetic interaction between a short-period terrestrial-mass planet and GJ 1151 ( [] ).
GW170817A as a Hierarchical Black Hole Merger
Despite the rapidly growing number of stellar-mass binary black hole mergers discovered through gravitational waves, the origin of these binaries is still not known. In galactic centers, black holes can be brought to each others’ proximity by dynamical processes, resulting in mergers. It is also possible that black holes formed in previous mergers encounter new black holes, resulting in so-called hierarchical mergers. Hierarchical events carry signatures such as higher-than-usual black hole mass and spin. Here we show that the recently reported gravitational-wave candidate, GW170817A, could be the result of such a hierarchical merger. In particular, its chirp mass ∼40 M ⊙ and effective spin of χ eff ∼ 0.5 are the typically expected values from hierarchical mergers within the disks of active galactic nuclei. We find that the reconstructed parameters of GW170817A strongly favor a hierarchical merger origin over having been produced by an isolate...
Excitation Sources of Oscillations in Solar Coronal Loops: A Multi-wavelength Analysis
An investigation into the excitation sources of oscillations detected in a coronal loop structure is carried out using the images obtained with Interface Region Imaging Spectrometer ( IRIS ) and the Atmospheric Imaging Assembly (AIA) instrument on board the Solar Dynamics Observatory ( SDO ). A loop structure in the active region AR 11967 on 2014 January 28, oscillating in the vicinity of a strong eruption and an M3.6 class flare site, is clearly noticeable in SDO /AIA 171 Å images. We study in detail, the oscillations with detected periods between 4 and 13 minutes and their connection in IRIS SJI 1330 Å and SDO /AIA 1700 Å images; both of these wavelengths sample the lower parts of the solar atmosphere. The simultaneous presence of many oscillations in the region of interest in all three wavelength passbands suggest that these oscillations were excited in the lower-chromosphere–photosphere plasma connected to the loop structure and the...
13 CH 3 OH Masers Associated With a Transient Phenomenon in a High-mass Young Stellar Object
We report the first detection of isotopic methanol ( 13 CH 3 OH) maser emission in interstellar space. The emission was detected toward the high-mass young stellar object G358.93-0.03 during monitoring of a flare in the 6.7 GHz methanol (CH 3 OH) maser emission in this source. We find that the spectral and spatial distribution of the 13 CH 3 OH masers differs from the CH 3 OH masers imaged at the same epoch, contrary to expectations from similarity of their pumping. This conclusively demonstrates that isotopic methanol masers are bright under different physical conditions and suggests that they can provide additional, complementary information to the CH 3 OH masers from the same source. We detect a rapid decay of the 13 CH 3 OH maser lines suggesting that they are transient phenomena (masing for only a few months), likely associated with rapid changes in radiation field due to an accretion burst i...
Space Weathering Simulation with Low-energy Laser Irradiation of Murchison CM Chondrite for Reproducing Micrometeoroid Bombardments on C-type Asteroids
Micrometeoroid bombardments are one of the causes of space weathering on airless bodies. We have simulated micrometeoroid bombardments on the surfaces of C-type asteroids by pulse-laser irradiation experiments on Murchison CM2 chondrite samples. In this Letter, we focus in particular on the effect of lower-energy irradiation compared to our previous study, where the laser energy range was set to 5–15 mJ, causing spectral flattening and water absorption band suppression. Murchison powder samples were irradiated with pulse lasers of various laser intensities (0.7, 1, 2, and 5 mJ). The irradiation energies are equivalent to micrometeoroid bombardments on the main-belt asteroids for ∼5.7 × 10 7 yr for 5 mJ and ∼7.9 × 10 6 yr for 0.7 mJ, respectively. We measured reflectance spectra and analyzed chemical compositions and microstructures of the surface of the laser-irradiated Murchison samples. Laser-irradiated Murchison spectra show flattening and darkening in the...
Fast Radio Bursts from Interacting Binary Neutron Star Systems
Recent observations of repeating fast radio bursts (FRBs) suggest that some FRBs reside in an environment consistent with that of binary neutron star (BNS) mergers. The bursting rate for repeaters could be very high and the emission site is likely from a magnetosphere. We discuss a hypothesis of producing abundant repeating FRBs in BNS systems. Decades to centuries before a BNS system coalesces, the magnetospheres of the two neutron stars start to interact relentlessly. Abrupt magnetic reconnection accelerates particles, which emit coherent radio waves in bunches via curvature radiation. FRBs are detected as these bright radiation beams point toward Earth. This model predicts quasi-periodicity of the bursts at the rotation periods of the two merging neutron stars (tens of milliseconds and seconds, respectively) as well as the period of orbital motion (of the order of 100 s). The bursting activities are expected to elevate with time as the two neutron stars get closer. The repeat...
Stellar Kinematics and Environment at z ∼ 0.8 in the LEGA-C Survey: Massive Slow Rotators Are Built First in Overdense Environments
In this Letter, we investigate the impact of environment on integrated and spatially resolved stellar kinematics of a sample of massive, quiescent galaxies at intermediate redshift(0.6 < z < 1.0). For this analysis, we combine photometric and spectroscopic parameters from the UltraVISTA and Large Early Galaxy Astrophysics Census surveys in the COSMOS field and environmental measurements. We analyze the trends with overdensity (1+ δ ) on the rotational support of quiescent galaxies and find no universal trends at either fixed mass or fixed stellar velocity dispersion. This is consistent with previous studies of the local universe; rotational support of massive galaxies depends primarily on stellar mass. We highlight two populations of massive galaxies ( {$\mathrm{log}\,{M}_{\star }/{M}_{\odot }\geqslant 11$} ) that deviate from the average mass relation. First, the mo...
Rapid Transients Originating from Thermonuclear Explosions in Helium White Dwarf Tidal Disruption Events
We study the emission properties of thermonuclear explosions in a helium white dwarf (WD) tidal disruption event (TDE). We consider a TDE where a0.2 M ⊙ helium WD is disrupted by a 10 2.5 M ⊙ intermediate-mass black hole (IMBH). The helium WD is not only tidally disrupted but is also detonated by the tidal compression and by succeeding shocks. We focus on the emission powered by radioactive nuclei in the unbound TDE ejecta. We perform hydrodynamic simulations coupled with nuclear reactions, post-process detailed nucleosynthesis calculations, and radiative transfer simulations. We thus derive multi-band light curves and spectra. The helium WD TDE shows rapid (Δ t 1mag ≃ 5–10 days) and relatively faint ( {${L}_{\mathrm{peak}}\simeq {10}^{42}\,\mathrm{erg}\,{{\rm{s}}}^{-1}$} ) light curves, because the ejecta mass and 5...
Evidence for Spin–Orbit Alignment in the TRAPPIST-1 System
In an effort to measure the Rossiter–McLaughlin effect for the TRAPPIST-1 system, we performed high-resolution spectroscopy during transits of planets e, f, and b. The spectra were obtained with the InfraRed Doppler spectrograph on the Subaru 8.2 m telescope, and were supplemented with simultaneous photometry obtained with a 1 m telescope of the Las Cumbres Observatory Global Telescope. By analyzing the anomalous radial velocities, we found the projected stellar obliquity to be λ = 1 ± 28° under the assumption that the three planets have coplanar orbits, although we caution that the radial-velocity data show correlated noise of unknown origin. We also sought evidence for the expected deformations of the stellar absorption lines, and thereby detected the “Doppler shadow” of planet b with a false-alarm probability of 1.7%. The joint analysis of the observed residual cross-correlation map including the three transits gave ##IMG## {$\lambda ={19}_{-15}^{+13}$}
Spatially Resolved UV Diagnostics of AGN Feedback: Radiation Pressure Dominates in a Prototypical Quasar-driven Superwind
Galactic-scale winds driven by active galactic nuclei (AGN) are often invoked to suppress star formation in galaxy evolution models, but the mechanisms driving these outflows are hotly debated. Two key AGN feedback models are (1) radiation pressure accelerating cool gas and (2) a hot outflowing wind entraining the interstellar medium (ISM). Highly ionized emission-line diagnostics represent a powerful means of differentiating these scenarios because of their sensitivity to the expected compression of the ISM clouds by the hot wind. Here, we report the first spatially resolved UV emission spectroscopy of a prototypical (radio-quiet) quasar-driven superwind around the obscured quasar SDSS J1356+1026 at z = 0.123. We observe ratios of O vi /C iv , N v /C iv , and C iv /He ii that are remarkably similar for outflowing gas clouds ≲100...
X-Ray Spectral Shape Variation in Changing-look Seyfert Galaxy SDSS J155258+273728
We analyze the X-ray, optical, and mid-infrared data of a “changing-look” Seyfert galaxy SDSS J155258+273728 at z ≃ 0.086. Over a period of one decade (2009–2018), its broad H α line intensity increased by a factor of ∼4. Meanwhile, the X-ray emission in 2014 as observed by Chandra was about five times brighter than that in 2010 by Suzaku , and the corresponding emissions in the V -band, mid-infrared W 1 band brighten by ∼0.18, 0.32 mag, respectively. Moreover, the absorption in X-rays is moderate and stable, i.e., {${N}_{{\rm{H}}}\sim {10}^{21}\ {\mathrm{cm}}^{-2}$} , but the X-ray spectrum becomes harder in the 2014 Chandra bright state (i.e., photon index {${\rm{\Gamma }}={1.52}_{-0.06}^{+0.06}$} ) than that of the 2010 Suzaku low state...
Simple Yet Powerful: Hot Galactic Outflows Driven by Supernovae
Supernovae (SNe) drive multiphase galactic outflows, impacting galaxy formation; however, cosmological simulations mostly use ad hoc feedback models for outflows, making outflow-related predictions from first principles problematic. Recent small-box simulations resolve individual SNe remnants in the interstellar medium (ISM), naturally driving outflows and permitting a determination of the wind loading factors of energy η E , mass {${\eta }_{m}$} , and metals {${\eta }_{Z}$} . In this Letter, we compile small-box results, and find consensus that the hot outflows are much more powerful than the cool outflows: (i) hot outflows generally dominate the energy flux, and (ii) their specific energy e s,h is 10–1000 times higher than cool outflows...
Mutual Inclination Excitation by Stellar Oblateness
Ultra-short-period planets (USPs) provide important clues to planetary formation and migration. It was recently found that the mutual inclinations of the planetary systems are larger if the inner orbits are closer (≲5 R * ) and if the planetary period ratios are larger ( P 2 / P 1 ≳ 5). This suggests that the USPs experienced both inclination excitation and orbital shrinkage. Here we investigate the increase in the mutual inclination due to stellar oblateness. We find that the stellar oblateness (within ∼1 Gyr) is sufficient to enhance the mutual inclination to explain the observed signatures. This suggests that the USPs can migrate closer to the host star in a near coplanar configuration with their planetary companions (e.g., disk migration+tides or in situ+tides), before mutual inclination gets excited due to stellar oblateness.
AGILE Observations of Two Repeating Fast Radio Bursts with Low Intrinsic Dispersion Measures
We focus on two repeating fast radio bursts (FRBs) recently detected by the CHIME/FRB experiment in 2018–2019 (Source 1: 180916.J0158+65, and Source 2: 181030.J1054+73). These sources have low excess dispersion measures ( {$\lt 100\,\mathrm{pc}\,{\mathrm{cm}}^{-3}$} and {$\lt 20\,\mathrm{pc}\,{\mathrm{cm}}^{-3}$} , respectively), implying relatively small maximal distances. They were repeatedly observed by AGILE in the MeV–GeV energy range. We do not detect prompt emission simultaneously with these repeating events. This search is particularly significant for the submillisecond and millisecond integrations obtainable by AGILE . The sources are constrained to emit a MeV-fluence in the millisecond range below ##IMG## {$F{{\prime} }_{\mathrm{MeV}}={10}^{-8}\,\mathr...}
An Extremely Massive Quiescent Galaxy at z = 3.493: Evidence of Insufficiently Rapid Quenching Mechanisms in Theoretical Models
We present spectra of the most massive quiescent galaxy yet spectroscopically confirmed at z > 3, verified via the detection of Balmer absorption features in the H - and K -bands of Keck/MOSFIRE. The spectra confirm a galaxy with no significant ongoing star formation, consistent with the lack of rest-frame UV flux and overall photometric spectral energy distribution. With a stellar mass of {${3.1}_{-0.2}^{+0.1}\times {10}^{11}\,{M}_{\odot }$} at z = 3.493, this galaxy is nearly three times more massive than the highest redshift spectroscopically confirmed absorption-line-identified galaxy known. The star formation history of this quiescent galaxy implies that it formed >1000 M ⊙ yr −1 for almost 0.5 Gyr beginning at z ∼ 7.2, strongly suggestive that it is the descendant of massive dusty star-forming galaxies at 5 < ...
Onset of Turbulent Fast Magnetic Reconnection Observed in the Solar Atmosphere
Fast magnetic reconnection powers explosive events throughout the universe, from gamma-ray bursts to solar flares. Despite its importance, the onset of astrophysical fast reconnection is the subject of intense debate and remains an open question in plasma physics. Here we report high-cadence observations of two reconnection-driven solar microflares obtained by the Interface Region Imaging Spectrograph that show persistent turbulent flows preceding flaring. The speeds of these flows are comparable to the local sound speed initially, suggesting the onset of fast reconnection in a highly turbulent plasma environment. Our results are in close quantitative agreement with the theory of turbulence-driven reconnection as well as with numerical simulations in which fast magnetic reconnection is induced by turbulence.
SDSS-IV MaNGA: Variations in the N/O–O/H Relation Bias Metallicity Gradient Measurements
In this Letter we use strong line calibrations of the N/O and O/H in Mapping Galaxies at Apache Point Observatory (MaNGA) galaxy survey spaxel data to explore the systematics introduced by variations in N/O on various strong-line metallicity diagnostics. We find radial variations in N/O at fixed O/H that correlate with total galaxy stellar mass and that can induce ∼40% systematic uncertainties in oxygen abundance gradients when nitrogen-dependent abundance calibrations are used. Empirically, we find that departures from the expected N/O are associated with variation in the local star formation efficiency, as predicted by recent chemical evolution models for galaxies, but we cannot rule out other processes such as radial migration also playing a role.
On the Possibility of GW190425 Being a Black Hole–Neutron Star Binary Merger
We argue that the kilonova/macronova associated with the gravitational-wave event GW190425 could have been bright enough to be detected if it was caused by the merger of a low-mass black hole (BH) and a neutron star (NS). Although tidal disruption occurs for such a low-mass BH is generally expected, the masses of the dynamical ejecta are limited to ≲10 −3 M ⊙ , which is consistent with previous work in the literature. The remnant disk could be as massive as 0.05–0.1 M ⊙ , and the disk outflow of ∼0.01–0.03 M ⊙ is likely to be driven by viscous or magnetohydrodynamic effects. The disk outflow may not be neutron-rich enough to synthesize an abundance of lanthanide elements, even in the absence of strong neutrino emitter, if the ejection is driven on the viscous timescale of ≳0.3 s. If this is the case, the opacity of the disk outflow is kept moderate, and a kilonova/macronova at the distance of GW190425 reaches a detect...
Forecast of Major Solar X-Ray Flare Flux Profiles Using Novel Deep Learning Models
In this Letter, we present the application of a couple of novel deep learning models to the forecast of major solar X-ray flare flux profiles. These models are based on a sequence-to-sequence framework using long short-term memory cell and an attention mechanism. For this, we use Geostationary Operational Environmental Satellite 10 X-ray flux data from 1998 August to 2006 April. Seven hundred sixty events are used for training and 85 for testing. The models forecast 30 minutes of X-ray flux profiles during the rise phase of the solar flare with a minute time cadence. We evaluate the models using the 10-fold cross-validation and rms error (RMSE) based on flux profiles and RMSE based on its peak flux. For comparison we consider two simple deep learning models and four conventional regression models. Major results of this study are as follows. First, we successfully apply our deep learning models to the forecast of solar flare X-ray flux profiles, without any preprocessing t...
Does Gravity Fall Down? Evidence for Gravitational-wave Deflection along the Line of Sight to GW170817
We present a novel test of general relativity (GR): measuring the geometric component of the time delay due to gravitational lensing. GR predicts that photons and gravitational waves follow the same geodesic paths and thus experience the same geometric time delay. We show that for typical systems, the time delays are tens of seconds, and thus can dominate over astrophysical delays in the timing of photon emission. For the case of GW170817, we use a multi-plane lensing code to evaluate the time delay due to four massive halos along the line of sight. From literature mass and distance measurements of these halos, we establish at high confidence (significantly greater than {$5\sigma $} ) that the gravitational waves of GW170817 underwent gravitational deflection to arrive within 1.7 s of the photons.
GJ 1252 b: A 1.2 R ⊕ Planet Transiting an M3 Dwarf at 20.4 pc
We report the discovery of GJ 1252 b, a planet with a radius of1.193 ± 0.074 {${R}_{\oplus }$} and an orbital period of 0.52 days around an M3-type star (0.381 ± 0.019 {${M}_{\odot }$} , 0.391 ± 0.020 {${R}_{\odot }$} ) located 20.385 ± 0.019 pc away. We use Transiting Exoplanet Survey Satellite ( TESS ) data, ground-based photometry and spectroscopy, Gaia astrometry, and high angular resolution imaging to show that the transit signal seen in the TESS data must originate from a transiting planet. We do so by ruling out all false-positive scenarios that attempt to explain the transit signal as originating from an eclipsing stellar binary. Precise Doppler monitoring also le...
Discovery of a Low-mass Companion Embedded in the Disk of the Young Massive Star MWC 297 with VLT/SPHERE
We report the discovery of a low-mass stellar companion around the young Herbig Be star MWC 297. We performed multi-epoch high-contrast imaging in the near-infrared with the Very Large Telescope (VLT)/Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument. The companion is found at a projected separation of 244.7 ± 13.2 au and a position angle of 176.4 ± 0.1 deg. The large separation supports formation via gravitational instability. From the spectrum, we estimate a mass of0.1–0.5 M ⊙ , the range conveying uncertainties in the extinction of the companion and in evolutionary models at young ages. The orbit coincides with a gap in the dust disk inferred from the spectral energy distribution. The young age (≲1 Myr) and mass ratio with the central star (∼0.01) makes the companion comparable to PDS 70 b, suggesting a relation between formation scenarios and disk dynamics.
Nine Localized Deviations from Keplerian Rotation in the DSHARP Circumstellar Disks: Kinematic Evidence for Protoplanets Carving the Gaps
We present evidence for localized deviations from Keplerian rotation, i.e., velocity “kinks,” in 8 of the 18 circumstellar disks observed by the DSHARP program: DoAr 25, Elias 2–27, GW Lup, HD 143006, HD 163296, IM Lup, Sz 129, and WaOph 6. Most of the kinks are detected over a small range in both radial extent and velocity, suggesting a planetary origin, but for some of them foreground contamination prevents us from measuring their spatial and velocity extent. Because of the DSHARP limited spectral resolution and signal to noise in the 12 CO J = 2−1 line, as well as cloud contamination, the kinks are usually detected in only one spectral channel, and will require confirmation. The strongest circumstantial evidence for protoplanets in the absence of higher spectral resolution data and additional tracers is that, upon deprojection, we find that all of the candidate planets lie within a gap and/or at the end of a spiral detected in dust continuum emission. ...
Systematic Variations of CO J = 2−1/1–0 Ratio and Their Implications in The Nearby Barred Spiral Galaxy M83
We present spatial variations of the CO J = 2−1/1–0 line ratio ( {${R}_{21/10}$} ) in the barred spiral galaxy M83 using Total Power Array (single-dish telescopes) data from the Atacama Large Millimeter/submillimeter Array. While the intensities of these two lines correlate tightly, {${R}_{21/10}$} varies over the disk, with a disk average ratio of 0.69, and shows the galactic center and a two-arm spiral pattern. It is high (≳0.7) in regions of high molecular gas surface density (Σ mol ), but ranges from low to high ratios in regions of low Σ mol . The ratio correlates well with the spatial distributions and intensities of far-ultraviolet (FUV) and infrared (IR) emissions, with FUV being the best correlated. It also correlates better with the ratio of specific inten...
Hyperbolic Meteoroids Impacting the Moon
Since the discovery of the Moon’s asymmetric ejecta cloud, the origin of its sunward-canted density enhancement has not been well understood. We propose impact ejecta from meteoroids on hyperbolic trajectories ( β -meteoroids) that hit the Moon’s sunward side could explain this unresolved asymmetry. β -meteoroids are submicron in size, comparable to or smaller than the regolith particles they hit, and can impact the Moon at very high speeds ∼100 km s −1 . Therefore, their impact regime may differ from the significantly larger and slower sporadic meteoroids responsible for generating the bulk of the lunar impact ejecta cloud. We compare lunar impact ejecta production to β -meteoroid fluxes observed by multiple spacecraft. If β -meteoroids are able to liberate similar sized submicron particles, orbital dust detector measurements from the Lunar Dust Experiment on board the Lunar Atmosphere and Dust Environment Explorer spacecraft only need to detect...
Accretion-driven Sources in Spatially Resolved Ly α Emitters
Ly α emission is a standard tracer of starburst galaxies at high redshift. However, a number of local Ly α emitters (LAEs) are X-ray sources, suggesting a possible origin of Ly α photons other than young, hot stars, and which may be active at much later ages relative to the parent starburst. Resolved, nearby LAEs offer the opportunity to discriminate between diffuse X-ray emission arising from supernova-heated gas, high-mass X-ray binaries (HMXBs), or low-luminosity active galactic nuclei (LLAGN). We examine archival X-ray imaging from Chandra and XMM-Newton for 11 galaxies with spatially resolved Ly α imaging to determine the luminosity, morphology, and spectral hardness of the X-ray sources. The data are consistent with 9 of the 12, bright Ly α sources being driven by luminous, > {${10}^{40}\ \mathrm{erg}\ {{\rm{s}}}^{-1}$} X-ray source...
A New Mechanism for Maunder-like Solar Minima: Phase Synchronization Dynamics in a Simple Nonlinear Oscillator of Magnetohydrodynamic Rossby Waves
The long-term solar cycle variability and Grand solar minima remain open questions from a theoretical point of view. Recently, a growing basis of evidence points out to the role of the magnetic Rossby waves in the solar cycle. Here we present a simple deterministic model, based on a low-order spectral representation of the barotropic quasi-geostrophic-magnetohydrodynamic equations for the Solar tachocline. This model supports the idea of the long-term behavior of the solar activity as a result of nonlinear interaction of magnetic Rossby modes. Solutions show that Rossby waves undergo irregular switches between periods of high activity and periods of suppressed activity, resembling the Maunder minimum. Low-energy states in the model are associated with the synchronization of the dynamical phases of the waves. These irregular transitions in the amplitudes of the waves are reminiscent of the observed time series of the solar activity. This suggests that Maunder-like states arise fr...
Shaping the Envelope of the Asymptotic Giant Branch Star W43A with a Collimated Fast Jet
One of the major puzzles in the study of stellar evolution is the formation process of bipolar and multipolar planetary nebulae. There is growing consensus that collimated jets create cavities with dense walls in the slowly expanding (10–20 km s −1 ) envelope ejected in previous evolutionary phases, leading to the observed morphologies. However, the launching of the jet and the way it interacts with the circumstellar material to create such asymmetric morphologies have remained poorly known. Here we present for the first time CO emission from the asymptotic giant branch star W43A that traces the whole stream of a jet, from the vicinity of its driving stellar system out to the regions where it shapes the circumstellar envelope. We found that the jet has a launch velocity of 175 km s −1 and decelerates to a velocity of 130 km s −1 as it interacts with circumstellar material. The continuum emission reveals a bipolar shell with a compact bright dot in th...
Simultaneous Magnetic Polar Cap Heating during a Flaring Episode from the Magnetar 1RXS J170849.0–400910
During a pointed 2018 NuSTAR observation, we detected a flare with a 2.2 hr duration from the magnetar 1RXS J170849.0−400910. The flare, which rose in ∼25 s to a maximum flux 6 times larger than the persistent emission, is highly pulsed with an rms pulsed fraction of 53%. The pulse profile shape consists of two peaks separated by half a rotational cycle, with a peak flux ratio of ∼2. The flare spectrum is thermal with an average temperature of 2.1 keV. Phase-resolved spectroscopy shows that the two peaks possess the same temperature, but differ in size. These observational results, along with simple light curve modeling, indicate that two identical antipodal spots, likely the magnetic poles, are heated simultaneously at the onset of the flare and for its full duration. Hence, the origin of the flare has to be connected to the global dipolar structure of the magnetar. This might best be achieved externally, via twists to closed magnetospheric dipolar field lines seeding bo...
Abrupt Shrinking of Solar Corona in the Late 1990s
We derive the longest uniform record of rotational intensities solar coronal magnetic field since 1968 and compare it with the heliospheric magnetic field (HMF) observed at the Earth. We scale the Mount Wilson Observatory and Wilcox Solar Observatory observations of the photospheric magnetic field to the level of the Synoptic Optical Long-term Investigations of the Sun/Vector Spectro Magnetograph and apply the potential field source surface model to calculate the coronal magnetic field. We find that the evolution of the coronal magnetic field during the last 50 yr agrees with the HMF observed at the Earth only if the effective coronal size, the distance of the coronal source surface of the HMF, is allowed to change in time. We calculate the optimum source surface distance for each rotation and find that it experienced an abrupt decrease in the late 1990s. The effective volume of the solar corona shrunk to less than one half during a short period of only a few years. We note that...
Observational Nonstationarity of AGN Variability: The Only Way to Go Is Down!
To gain insights into long-term active galactic nuclei (AGN) variability, we analyze an AGN sample from the Sloan Digital Sky Survey (SDSS) and compare their photometry with observations from the Hyper Suprime-Cam survey (HSC) observed {$\langle 14.85\rangle $} yr after SDSS. On average, the AGN are fainter in HSC than SDSS. We demonstrate that the difference is not due to subtle differences in the SDSS versus HSC filters or photometry. The decrease in mean brightness is redshift dependent, consistent with expectations for a change that is a function of the rest-frame time separation between observations. At a given redshift, the mean decrease in brightness is stronger for more luminous AGN and for objects with longer time separation between measurements. We demonstrate that the dependence on redshift and luminosity of measured mean brightness decrease is consistent with simple model...
Detection of Diatomic Carbon in 2I/Borisov
2I/Borisov is the first-ever observed interstellar comet (and the second detected interstellar object (ISO)). It was discovered on 2019 August 30 and has a heliocentric orbital eccentricity of ∼3.35, corresponding to a hyperbolic orbit that is unbound to the Sun. Given that it is an ISO, it is of interest to compare its properties—such as composition and activity—with the comets in our solar system. This study reports low-resolution optical spectra of 2I/Borisov. The spectra were obtained by the MDM Observatory Hiltner 2.4 m telescope/Ohio State Multi-Object Spectrograph (on 2019 October 31.5 and November 4.5, UT). The wavelength coverage spanned from 3700 to 9200 Å. The dust continuum reflectance spectra of 2I/Borisov show that the spectral slope is steeper in the blue end of the spectrum (compared to the red). The spectra of 2I/Borisov clearly show CN emission at 3880 Å, as well as C 2 emission at both 4750 and 5150 Å. Using a Haser model to covert the observed flu...
High-resolution Spectral Line Indices Useful for the Analysis of Stellar Populations
The well-known age–metallicity-attenuation degeneracy does not permit unique and good estimates of basic parameters of stars and stellar populations. The effects of dust can be avoided using spectral line indices, but current methods have not been able to break the age–metallicity degeneracy. Here we show that using at least two new spectral line indices defined and measured on high-resolution ( R = 6000) spectra of a signal-to-noise ratio (S/N) ≥ 10, one gets unambiguous estimates of the age and metallicity of intermediate to old stellar populations. Spectroscopic data retrieved with new astronomical facilities, e.g., X-shooter, MEGARA, and MOSAIC, can be employed to infer the physical parameters of the emitting source by means of spectral line index and index–index diagram analysis.
Magnetic Rayleigh–Taylor Instability in an Experiment Simulating a Solar Loop
A hoop force driven magnetic Rayleigh–Taylor instability (MRTI) is observed in a laboratory experiment that simulates a solar coronal loop. Increase of the axial wavelength λ is observed when the axial magnetic field increases. This scaling is consistent with the theoretical MRTI growth rate {${\gamma }^{2}={gk}-2{\left({\boldsymbol{k}}\cdot {{\boldsymbol{B}}}_{0}\right)}^{2}/{\mu }_{0}\rho $} , which implies that if {${\boldsymbol{k}}$} is parallel to {${{\boldsymbol{B}}}_{0}$} (i.e., undular mode), the fastest-growing mode has {$\lambda =2\pi /k=8\pi {B}_{0}^{2}/{\mu }_{0}\rho g$} .
A Fully Kinetic Perspective of Electron Acceleration around a Weakly Outgassing Comet
The cometary mission Rosetta has shown the presence of higher-than-expected suprathermal electron fluxes. In this study, using 3D fully kinetic electromagnetic simulations of the interaction of the solar wind with a comet, we constrain the kinetic mechanism that is responsible for the bulk electron energization that creates the suprathermal distribution from the warm background of solar wind electrons. We identify and characterize the magnetic field-aligned ambipolar electric field that ensures quasi-neutrality and traps warm electrons. Solar wind electrons are accelerated to energies as high as 50–70 eV close to the comet nucleus without the need for wave–particle or turbulent heating mechanisms. We find that the accelerating potential controls the parallel electron temperature, total density, and (to a lesser degree) the perpendicular electron temperature and the magnetic field magnitude. Our self-consistent approach enables us to better understand the underlying plasma...
Detection and Characterization of Oscillating Red Giants: First Results from the TESS Satellite
Since the onset of the “space revolution” of high-precision high-cadence photometry, asteroseismology has been demonstrated as a powerful tool for informing Galactic archeology investigations. The launch of the NASA Transiting Exoplanet Survey Satellite ( TESS ) mission has enabled seismic-based inferences to go full sky—providing a clear advantage for large ensemble studies of the different Milky Way components. Here we demonstrate its potential for investigating the Galaxy by carrying out the first asteroseismic ensemble study of red giant stars observed by TESS . We use a sample of 25 stars for which we measure their global asteroseimic observables and estimate their fundamental stellar properties, such as radius, mass, and age. Significant improvements are seen in the uncertainties of our estimates when combining seismic observables from TESS with astrometric measurements from the Gaia mission compared to when the seismology and astrometry are...
Global Stellar Budget for LIGO Black Holes
The binary black hole mergers observed by Laser Interferometer Gravitational-Wave Observatory (LIGO)–Virgo gravitational-wave detectors pose two major challenges: (i) how to produce these massive black holes from stellar processes; and (ii) how to bring them close enough to merge within the age of the universe? We derive a fundamental constraint relating the binary separation and the available stellar budget in the universe to produce the observed black hole mergers. We find that ≲14% of the entire budget contributes to the observed merger rate of(30+30) M ⊙ black holes, if the separation is around the diameter of their progenitor stars. Furthermore, the upgraded LIGO detector and third-generation gravitational-wave detectors are not expected to find stellar-mass black hole mergers at high redshifts. From LIGO’s strong constraints on the mergers of black holes in the pair-instability mass gap (60–120 M ⊙ ), we find that ≲0.8% of all massive st...
Identification of QPO Frequency of GRS 1915+105 as the Relativistic Dynamic Frequency of a Truncated Accretion Disk
We have analyzed AstroSat observations of the galactic microquasar system GRS 1915+105, when the system exhibited C-type quasi-periodic oscillations (QPOs) in the frequency range of 3.4–5.4 Hz. The broadband spectra (1–50 keV) obtained simultaneously from the Large Area X-ray Proportional Counter and Soft X-ray Telescope can be well described by a dominant relativistic truncated accretion disk along with thermal Comptonization and reflection. We find that while the QPO frequency depends on the inner radii with a large scatter, a much tighter correlation is obtained when both the inner radii and accretion rate of the disk are taken into account. In fact, the frequency varies just as the dynamic frequency (i.e., the inverse of the sound crossing time) does as predicted decades ago by the relativistic standard accretion disk theory for a black hole with a spin parameter of ∼0.9. We show that this identification has been possible due to the simultaneous broadband spectral cov...
An Extreme X-Ray Variability Event of a Weak-line Quasar
We report the discovery of an extreme X-ray flux rise (by a factor of ≳20) of the weak-line quasar Sloan Digital Sky Survey (SDSS) J153913.47+395423.4 (hereafter SDSS J1539+3954) at z = 1.935. SDSS J1539+3954 is the most-luminous object among radio-quiet type 1 active galactic nuclei (AGNs) where such dramatic X-ray variability has been observed. Before the X-ray flux rise, SDSS J1539+3954 appeared X-ray weak compared with the expectation from its ultraviolet (UV) flux; after the rise, the ratio of its X-ray flux and UV flux is consistent with the majority of the AGN population. We also present a contemporaneous HET spectrum of SDSS J1539+3954, which demonstrates that its UV continuum level remains generally unchanged despite the dramatic increase in the X-ray flux, and its C iv emission line remains weak. The dramatic change only observed in the X-ray flux is consistent with a shielding model, where a thick inner accretion disk can block our line...
Carbon Chain Depletion of 2I/Borisov
The composition of comets in the solar system comes in multiple groups thought to encode information about their formation in different regions of the outer protosolar disk. The recent discovery of the second interstellar object, 2I/Borisov, allows for spectroscopic investigations into its gas content and a preliminary classification of it within the solar system comet taxonomies to test the applicability of planetesimal formation models to other stellar systems. We present spectroscopic and imaging observations from 2019 September 20 through October 26 from the Bok, MMT telescope (formerly the Multiple Mirror Telescope, Mount Hopkins, Arizona), and Large Binocular Telescopes. We identify CN in the comet’s spectrum and set precise upper limits on the abundance of C 2 on all dates in October. We use a Haser model to convert our integrated fluxes to production rates and find Q (CN) = (1.1–1.9) ∗ 10 24 mols s −1 increasing over 2019 October 1 to ...
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