Table of contents

Pulsar timing has enabled some of the strongest tests of fundamental physics. Central to the technique is the assumption that the detected radio pulses can be used to accurately measure the rotation of the pulsar. Here, we report on a broadband variation in the pulse profile of the millisecond pulsar J1643−1224. A new component of emission suddenly appears in the pulse profile, decays over four months, and results in a permanently modified pulse shape. Profile variations such as these may be the origin of timing noise observed in other millisecond pulsars. The sensitivity of pulsar-timing observations to gravitational radiation can be increased by accounting for this variability.

We develop a simple model to predict the radial distribution of planetesimal formation. The model is based on the observed growth of dust to millimeter-sized particles, which drift radially, pile-up, and form planetesimals where the stopping time and dust-to-gas ratio intersect the allowed region for streaming instability-induced gravitational collapse. Using an approximate analytic treatment, we first show that drifting particles define a track in metallicity–stopping time space whose only substantial dependence is on the disk's angular momentum transport efficiency. Prompt planetesimal formation is feasible for high particle accretion rates (relative to the gas, ${\dot{M}}_{p}/\dot{M}\gtrsim 3\times {10}^{-2}$ for $\alpha ={10}^{-2}$), which could only be sustained for a limited period of time. If it is possible, it would lead to the deposition of a broad and massive belt of planetesimals with a sharp outer edge. Numerically including turbulent diffusion and vapor condensation processes, we find that a modest enhancement of solids near the snow line occurs for centimeter-sized particles, but that this is largely immaterial for planetesimal formation. We note that radial drift couples planetesimal formation across radii in the disk, and suggest that considerations of planetesimal formation favor a model in which the initial deposition of material for giant planet cores occurs well beyond the snow line.

The phenomenon of solar "torsional oscillations" (TO) represents migratory zonal flows associated with the solar cycle. These flows are observed on the solar surface and, according to helioseismology, extend through the convection zone. We study the origin of the TO using results from a global MHD simulation of the solar interior that reproduces several of the observed characteristics of the mean-flows and magnetic fields. Our results indicate that the magnetic tension (MT) in the tachocline region is a key factor for the periodic changes in the angular momentum transport that causes the TO. The torque induced by the MT at the base of the convection zone is positive at the poles and negative at the equator. A rising MT torque at higher latitudes causes the poles to speed up, whereas a declining negative MT torque at the lower latitudes causes the equator to slow-down. These changes in the zonal flows propagate through the convection zone up to the surface. Additionally, our results suggest that it is the magnetic field at the tachocline that modulates the amplitude of the surface meridional flow rather than the opposite as assumed by flux-transport dynamo models of the solar cycle.

Recently, the Advanced Laser Interferometer Gravitational-wave Observatory detected gravitational-wave (GW) transients from mergers of binary black holes (BHs). The system may also produce a wide-angle, relativistic outflow if the claimed short gamma-ray burst detected by GBM is in real association with GW150914. It was suggested that mergers of double neutron stars (or neutron star-black hole binaries), another promising source of GW transients, also produce fast, wide-angle outflows. In this paper, we calculate the high-energy gamma-ray emission arising from the blast waves driven by these wide-angle outflows. We find that TeV emission arising from the inverse-Compton process in the relativistic outflow, originating from mergers of binary BHs that are similar to those in GW150914, could be detectable by ground-based Imaging Atmospheric Cherenkov Telescopes such as the Cherenkov Telescope Array (CTA) if the sources occur in a dense medium with a density of $n\gtrsim 0.3\,{\mathrm{cm}}^{-3}$. For neutron star–neutron star (NS–NS) and NS–BH mergers, TeV emission from the wide-angle, mildly relativistic outflow could be detected as well, if it occurs in a dense medium with $n\gtrsim 10\mbox{--}100\,{\mathrm{cm}}^{-3}$. Thus, TeV afterglow emission could be a useful probe of the environment of the GW transients, which could shed light on the evolution channels of the progenitors of GW transients.

We report the discovery of the faintest known dwarf galaxy satellite of a Large Magellanic Cloud (LMC) stellar-mass host beyond the Local Group (LG), based on deep imaging with Subaru/Hyper Suprime-Cam. Magellanic Analog Dwarf Companions And Stellar Halos (MADCASH) J074238+652501-dw lies ∼35 kpc in projection from NGC 2403, a dwarf spiral galaxy at D ≈ 3.2 Mpc. This new dwarf has ${M}_{g}=-7.4\pm 0.4$ and a half-light radius of 168 ± 70 pc, at the calculated distance of 3.39 ± 0.41 Mpc. The color–magnitude diagram reveals no evidence of young stellar populations, suggesting that MADCASH J074238+652501-dw is an old, metal-poor dwarf similar to low-luminosity dwarfs in the LG. The lack of either detected HI gas (${M}_{\mathrm{HI}}/{L}_{V}\lt 0.69\,{M}_{\odot }/{L}_{\odot }$, based on Green Bank Telescope observations) or GALEX NUV/FUV flux enhancement is consistent with a lack of young stars. This is the first result from the MADCASH survey, which is conducting a census of the stellar substructure and faint satellites in the halos of Local Volume LMC analogs via resolved stellar populations. Models predict a total of ∼4–10 satellites at least as massive as MADCASH J074238+652501-dw around a host with the mass of NGC 2403, with 2–3 within our field of view, slightly more than the one such satellite observed in our footprint.

Recently a population of large, very low surface brightness, spheroidal galaxies was identified in the Coma cluster. The apparent survival of these ultra-diffuse galaxies (UDGs) in a rich cluster suggests that they have very high masses. Here, we present the stellar kinematics of Dragonfly 44, one of the largest Coma UDGs, using a 33.5 hr integration with DEIMOS on the Keck II telescope. We find a velocity dispersion of $\sigma ={47}_{-6}^{+8}$$\mathrm{km}\,{{\rm{s}}}^{-1}$, which implies a dynamical mass of ${M}_{\mathrm{dyn}}(\lt {r}_{1/2})={0.7}_{-0.2}^{+0.3}\times {10}^{10}\,{M}_{\odot }$ within its deprojected half-light radius of ${r}_{1/2}=4.6\pm 0.2\,\mathrm{kpc}$. The mass-to-light ratio is $M/{L}_{I}(\lt {r}_{1/2})={48}_{-14}^{+21}\,{M}_{\odot }/{L}_{\odot }$, and the dark matter fraction is 98% within ${r}_{1/2}$. The high mass of Dragonfly 44 is accompanied by a large globular cluster population. From deep Gemini imaging taken in $0\buildrel{\prime\prime}\over{.} 4$ seeing we infer that Dragonfly 44 has ${94}_{-20}^{+25}$ globular clusters, similar to the counts for other galaxies in this mass range. Our results add to other recent evidence that many UDGs are "failed" galaxies, with the sizes, dark matter content, and globular cluster systems of much more luminous objects. We estimate the total dark halo mass of Dragonfly 44 by comparing the amount of dark matter within $r=4.6\,\mathrm{kpc}$ to enclosed mass profiles of NFW halos. The enclosed mass suggests a total mass of $\sim {10}^{12}$${M}_{\odot }$, similar to the mass of the Milky Way. The existence of nearly dark objects with this mass is unexpected, as galaxy formation is thought to be maximally efficient in this regime.

Observation of interplanetary scintillation (IPS) beyond Earth-orbit can be challenging due to the necessity to use low radio frequencies at which scintillation due to the ionosphere could confuse the interplanetary contribution. A recent paper by Kaplan et al. presenting observations using the Murchison Widefield Array (MWA) reports evidence of nightside IPS on two radio sources within their field of view. However, the low time cadence of 2 s used might be expected to average out the IPS signal, resulting in the reasonable assumption that the scintillation is more likely to be ionospheric in origin. To check this assumption, this Letter uses observations of IPS taken at a high time cadence using the Low Frequency Array (LOFAR). Averaging these to the same as the MWA observations, we demonstrate that the MWA result is consistent with IPS, although some contribution from the ionosphere cannot be ruled out. These LOFAR observations represent the first of nightside IPS using LOFAR, with solar wind speeds consistent with a slow solar wind stream in one observation and a coronal mass ejection expected to be observed in another.

Centaurs are minor planets orbiting between Jupiter and Neptune that have or had crossing orbits with one or more giant planets. Recent observations and reinterpretation of previous observations have revealed the existence of ring systems around 10199 Chariklo and 2060 Chiron. However, the origin of the ring systems around such a minor planet is still an open question. Here, we propose that the tidal disruption of a differentiated object that experiences a close encounter with a giant planet could naturally form diverse ring–satellite systems around the Centaurs. During the close encounter, the icy mantle of the passing object is preferentially ripped off by the planet's tidal force and the debris is distributed mostly within the Roche limit of the largest remnant body. Assuming the existence of a 20−50 wt% silicate core below the icy mantle, a disk of particles is formed when the objects pass within 0.4–0.8 of the planet's Roche limit with the relative velocity at infinity 3−6 km s⁻¹ and 8 hr initial spin period of the body. The resultant ring mass is 0.1%–10% of the central object's mass. Such particle disks are expected to spread radially, and materials spreading beyond the Roche limit would accrete into satellites. Our numerical results suggest that ring formation would be a natural outcome of such extreme close encounters, and Centaurs can naturally have such ring systems because they cross the orbits of the giant planets.

Recent investigations indicate that solar coronal jets result from eruptions of small-scale chromospheric filaments, called minifilaments; that is, the jets are produced by scaled-down versions of typical-sized filament eruptions. We consider whether solar spicules might in turn be scaled-down versions of coronal jets, being driven by eruptions of microfilaments. Assuming a microfilament's size is about a spicule's width (∼300 km), the estimated occurrence number plotted against the estimated size of erupting filaments, minifilaments, and microfilaments approximately follows a power-law distribution (based on counts of coronal mass ejections, coronal jets, and spicules), suggesting that many or most spicules could result from microfilament eruptions. Observed spicule-base Ca ii brightenings plausibly result from such microfilament eruptions. By analogy with coronal jets, microfilament eruptions might produce spicules with many of their observed characteristics, including smooth rise profiles, twisting motions, and EUV counterparts. The postulated microfilament eruptions are presumably eruptions of twisted-core micro-magnetic bipoles that are ∼10 wide. These explosive bipoles might be built and destabilized by merging and cancelation of approximately a few to 100 G magnetic-flux elements of size $\lesssim 0\buildrel{\prime\prime}\over{.} 5\mbox{--}1\buildrel{\prime\prime}\over{.} 0$. If, however, spicules are relatively more numerous than indicated by our extrapolated distribution, then only a fraction of spicules might result from this proposed mechanism.

We model the time evolution of gaps in tidal streams that are caused by the impact of a dark matter subhalo, while these orbit a spherical gravitational potential. To this end, we make use of the simple behavior of orbits in action-angle space. A gap effectively results from the divergence of two nearby orbits whose initial phase-space separation is, for very cold thin streams, largely given by the impulse induced by the subhalo. We find that in a spherical potential, the size of a gap increases linearly with time for sufficiently long timescales. We have derived an analytic expression that shows how the growth rate depends on the mass of the perturbing subhalo, its scale, and its relative velocity with respect to the stream. We have verified these scalings using N-body simulations and find excellent agreement. For example, a subhalo of mass ${10}^{8}\,{M}_{\odot }$ directly impacting a very cold thin stream on an inclined orbit can induce a gap that may reach a size of several tens of kiloparsecs after a few gigayears. The gap size fluctuates importantly with phase on the orbit, and it is largest close to pericenter. This indicates that it may not be fully straightforward to invert the spectrum of gaps present in a stream to recover the mass spectrum of the subhalos.

Recent surveys have identified a seemingly ubiquitous population of galaxies with elevated [O iii]/Hβ emission line ratios at z > 1, although the nature of this phenomenon continues to be debated. The [O iii]/Hβ line ratio is of interest because it is a main component of the standard diagnostic tools used to differentiate between active galactic nuclei and star-forming galaxies as well as the gas-phase metallicity indicators O₂₃ and R₂₃. Here, we investigate the primary driver of increased [O iii]/Hβ ratios by median-stacking rest-frame optical spectra for a sample of star-forming galaxies in the 3D-HST survey in the redshift range z ∼ 1.4–2.2. Using N = 4220 star-forming galaxies, we stack the data in bins of mass and specific star formation rates (sSFRs), respectively. After accounting for stellar Balmer absorption, we measure [O iii]λ5007 Å/Hβ down to M ∼ 10^9.2M_⊙ and sSFR ∼ 10^−9.6 yr⁻¹, greater than an order of magnitude lower than previous work at similar redshifts. We find an offset of 0.59 ± 0.05 dex between the median ratios at z ∼ 2 and z ∼ 0 at fixed stellar mass, in agreement with existing studies. However, with respect to sSFR, the z ∼ 2 stacks all lie within 1σ of the median SDSS ratios, with an average offset of only −0.06 ± 0.05. We find that the excitation properties of galaxies are tightly correlated with their sSFR at both z ∼ 2 and z ∼ 0, with a relation that appears to be roughly constant over the last 10 Gyr of cosmic time.

We report on the presence of large amounts of million-degree gas in the Milky Way's interstellar and circum-galactic medium. This gas (1) permeates both the Galactic plane and the halo, (2) extends to distances larger than 60–200 kpc from the center, and (3) its mass is sufficient to close the Galaxy's baryon census. Moreover, we show that a vast, ∼6 kpc radius, spherically symmetric central region of the Milky Way above and below the 0.16 kpc thick plane has either been emptied of hot gas or the density of this gas within the cavity has a peculiar profile, increasing from the center up to a radius of ∼6 kpc, and then decreasing with a typical halo density profile. This, and several other converging pieces of evidence, suggest that the current surface of the cavity, at 6 kpc from the Galaxy's center, traces the distant echo of a period of strong nuclear activity of our supermassive black hole, occurring about 6 Myr ago.

The 6.67 hr periodicity and the variable X-ray flux of the central compact object (CCO) at the center of the supernova remnant RCW 103, named 1E 161348–5055, have been always difficult to interpret within the standard scenarios of an isolated neutron star (NS) or a binary system. On 2016 June 22, the Burst Alert Telescope (BAT) on board Swift detected a magnetar-like short X-ray burst from the direction of 1E 161348–5055, also coincident with a large long-term X-ray outburst. Here, we report on Chandra, Nuclear Spectroscopic Telescope Array, and Swift (BAT and XRT) observations of this peculiar source during its 2016 outburst peak. In particular, we study the properties of this magnetar-like burst, we discover a hard X-ray tail in the CCO spectrum during outburst, and we study its long-term outburst history (from 1999 to 2016 July). We find the emission properties of 1E 161348–5055 consistent with it being a magnetar. However, in this scenario, the 6.67 hr periodicity can only be interpreted as the rotation period of this strongly magnetized NS, which therefore represents the slowest pulsar ever detected, by orders of magnitude. We briefly discuss the viable slow-down scenarios, favoring a picture involving a period of fall-back accretion after the supernova explosion, similarly to what is invoked (although in a different regime) to explain the "anti-magnetar" scenario for other CCOs.

We report the detection of a significant infrared variability of the nearest tidal disruption event (TDE) ASASSN-14li using Wide-field Infrared Survey Explorer and newly released Near-Earth Object WISE Reactivation data. In comparison with the quiescent state, the infrared flux is brightened by 0.12 and 0.16 mag in the W1 (3.4 μm) and W2 (4.6 μm) bands at 36 days after the optical discovery (or ∼110 days after the peak disruption date). The flux excess is still detectable ∼170 days later. Assuming that the flare-like infrared emission is from the dust around the black hole, its blackbody temperature is estimated to be ∼2.1 × 10³ K, slightly higher than the dust sublimation temperature, indicating that the dust is likely located close to the dust sublimation radius. The equilibrium between the heating and radiation of the dust claims a bolometric luminosity of ∼10⁴³–10⁴⁵ erg s⁻¹, comparable with the observed peak luminosity. This result has for the first time confirmed the detection of infrared emission from the dust echoes of TDEs.