Terrestrial worlds with P < 1 day, known as ultra-short period planets (USPs), comprise a physically distinct population whose origins may be attributed to various possible formation channels within multi-planet systems. However, the conventional 1 day boundary adopted for USPs is an arbitrary prescription, and it has yet to be evaluated whether this specific cutoff, or any alternatives, may emerge from the data with minimal assumptions. We accordingly present a statistical evaluation of the USP classification boundary for 376 multi-planet systems across Kepler, K2, and TESS. We find that USPs are smaller in size (p = 0.004) and exhibit larger period ratios with their immediate neighbors (𝒫 = P_2/P_1; p < 10^-4) when compared to non-USP short-period (1 < P/days < 5) worlds, and that these discrepancies rapidly transition towards statistical insignificance (p > 0.05) at respective orbital periods of P_R = 0.97^+0.25_-0.19 days and P_𝒫 = 2.09^+0.16_-0.22 days (see Figure 3). We verify that these results are not driven by imprecise planetary parameters, giant companions, low-mass host stars, or detection biases. Our findings provide qualitative support for pathways in which proto-USPs are detached from companions and delivered to P ≲ 2 days via eccentric migration, while a subset of these objects near P ∼ 1 day experience subsequent orbital decay and refractory mass loss to become USPs. These results lend evidence towards an astrophysical basis for the 1 day USP cutoff and encourage consideration of an additional 2 day boundary within future investigations of USP architectures and evolutionary dynamics.

Terrestrial worlds with P < 1 day, known as ultra-short-period planets (USPs), comprise a physically distinct population whose origins may be attributed to various possible formation channels within multiplanet systems. However, the conventional 1 day boundary adopted for USPs is an arbitrary prescription, and it has yet to be evaluated whether this specific cutoff, or any alternatives, may emerge from the data with minimal assumptions. We accordingly present a statistical evaluation of the USP classification boundary for 376 multiplanet systems across Kepler, K2, and TESS. We find that USPs are smaller in size ( p = 0.004) and exhibit larger period ratios with their immediate neighbors ( P = P 2 / P 1 ; p < 10 −4 ) when compared to non-USP short-period (1 < P /days < 5) worlds, and that these discrepancies rapidly transition toward statistical insignificance ( p > 0.05) at respective orbital periods of P R = 0.9 7 − 0.19 + 0.25 day and P P = 2.0 9 − 0.22 + 0.16 days (see Figure 3). We verify that these results are not driven by imprecise planetary parameters, giant companions, low-mass host stars, or detection biases. Our findings provide qualitative support for pathways in which proto-USPs are detached from companions and delivered to P ≲ 2 days via eccentric migration, while a subset of these objects near P ∼ 1 day experience subsequent orbital decay and refractory mass loss to become USPs. These results lend evidence toward an astrophysical basis for the 1 day USP cutoff and encourage consideration of an additional 2 day boundary within future investigations of USP architectures and evolutionary dynamics.

The ubiquity of “peas-in-a-pod” architectural patterns and the existence of the radius valley each presents a striking population-level trend for planets with R _p ≤ 4 R _⊕ that serves to place powerful constraints on the formation and evolution of these subgiant worlds. As it has yet to be determined whether the strength of this peas-in-a-pod uniformity differs on either side of the radius valley, we separately assess the architectures of systems containing only small ( R _p ≤ 1.6 R _⊕ ), rocky planets from those harboring only intermediate-sized (1.6 R _⊕ < R _p ≤ 4 R _⊕ ), volatile-rich worlds to perform a novel statistical comparison of intra-system planetary uniformity across compositionally distinct regimes. We find that, compared to their volatile-rich counterparts, rocky systems are less uniform in mass (2.6 σ ) but more uniform in size (4.0 σ ) and spacing (3.0 σ ). We provide further statistical validation for these results, demonstrating that they are not substantially influenced by the presence of mean-motion resonances, low-mass host stars, alternative bulk compositional assumptions, sample size effects, or detection biases. We also obtain tentative evidence (>2 σ significance) that the enhanced size uniformity of rocky systems is dominated by the presence of super-Earths (1 R _⊕ ≤ R _p ≤ 1.6 R _⊕ ), while their enhanced mass diversity is driven by the presence of sub-Earth ( R _p < 1 R _⊕ ) worlds.

Super-Earths within the same close-in, compact planetary system tend to exhibit a striking degree of uniformity in their radius, mass, and orbital spacing, and this “peas-in-a-pod” phenomenon itself serves to provide one of the strongest constrains on planet formation at large. While it has been recently demonstrated from independent samples that such planetary uniformity occurs for both configurations near and distant from mean motion resonance, the question thus remains if the strength of this uniformity itself differs between near-resonant and nonresonant configurations such that the two modes may be astrophysically distinct in their evolution. We thus provide in this work a novel comparative size uniformity analysis for 48 near-resonant and 251 nonresonant multiplanet systems from the California Kepler Survey catalog, evaluating uniformity both across systems and between planetary pairs within the same system. We find that while multiplanet configurations exhibit strong peas-in-a-pod size uniformity regardless of their proximity to resonance, near-resonant configurations display enhanced intra-system size uniformity as compared to their analogous nonresonant counterparts at the level of both entire systems and subsystem planetary pairs and chains. These results are broadly consistent with a variety of formation paradigms for multiple-planet systems, such as convergent migration within a turbulent protoplanetary disk or planet–planet interactions incited by postnebular dynamical instabilities. Nevertheless, further investigation is necessary to ascertain whether the nonresonant and near-resonant planetary configurations respectively evolve via a singular process or mechanisms that are dynamically distinct.

It has been demonstrated that planets belonging to the same close-in, compact multiple-planet system tend to exhibit a striking degree of uniformity in their sizes. A similar trend has also been found to hold for the masses of such planets, but considerations of such intra-system mass uniformity have generally been limited to statistical samples wherein a majority of systems have constituent planetary mass measurements obtained via analysis of transit timing variations (TTVs). Since systems with strong TTV signals typically lie in or near mean motion resonance, it remains to be seen whether intra-system mass uniformity is still readily emergent for nonresonant systems with non-TTV mass provenance. We thus present in this work a mass uniformity analysis of 17 non-TTV systems with masses measured via radial velocity measurements. Using the Gini index, a common statistic for economic inequality, as our primary metric for uniformity, we find that our sample of 17 non-TTV systems displays intra-system mass uniformity at a level of ∼2.5 σ confidence. We provide additional discussion of possible statistical and astrophysical underpinnings for this result. We also demonstrate the existence of a correlation ( r = 0.25) between characteristic solid surface density (Σ 0 ) of the minimum-mass extrasolar nebula and system mass Gini index, suggesting that more-massive disks may generally form systems with more-unequal planetary masses.

We present cm-band and mm-band afterglow observations of five long-duration $\gamma$-ray bursts (GRBs; GRB 130131A, 130420B, 130609A, 131229A, 140713A) with dust-obscured optical afterglow emission, known as "dark" GRBs. We detect the radio afterglow of two of the dark GRBs (GRB 130131A and 140713A), along with a tentative detection of a third (GRB 131229A) with the Karl G. Jansky Very Large Array (VLA). Supplemented by three additional VLA-detected dark GRBs from the literature, we present uniform modeling of their broadband afterglows. We derive high line-of-sight dust extinctions of $A_{V, \rm GRB} \gtrsim 2.2 - 10.6~{\rm mag}$. Additionally, we model the host galaxies of the six bursts in our sample, and derive host galaxy dust extinctions of $A_{V, \rm Host} \approx 0.3-4.7~{\rm mag}$. Across all tested $\gamma$-ray (fluence and duration) and afterglow properties (energy scales, geometries and circumburst densities), we find dark GRBs to be representative of more typical unobscured long GRBs, except in fluence, for which observational biases and inconsistent classification may influence the dark GRB distribution. Additionally, we find that $A_{V, \rm GRB}$ is not related to a uniform distribution of dust throughout the host, nor to the extremely local environment of the burst, indicating that a larger scale patchy dust distribution is the cause of the high line-of-sight extinction. Since radio observations are invaluable to revealing heavily dust-obscured GRBs, we make predictions for the detection of radio emission from host star formation with the next generation VLA.

We present the discovery of the radio afterglow and near-infrared (NIR) counterpart of the Swift short gamma-ray burst (GRB) GRB 200522A, located at a small projected offset of 1 kpc from the center of a young, star-forming host galaxy at z = 0.5536. The radio and X-ray luminosities of the afterglow are consistent with those of on-axis cosmological short GRBs. The NIR counterpart, revealed by our Hubble Space Telescope observations at a rest-frame time of 2.3 days, has a luminosity of (1.3-1.7) x 10(42) erg s(-1). This is substantially lower than on-axis short GRB afterglow detections but is a factor of 8-17 more luminous than the kilonova of GW170817 and significantly more luminous than any kilonova candidate for which comparable observations exist. The combination of the counterpart's color (i - y = -0.08 0.21; rest frame) and luminosity cannot be explained by standard radioactive heating alone. We present two scenarios to interpret the broadband behavior of GRB 200522A: a synchrotron forward shock with a luminous kilonova (potentially boosted by magnetar energy deposition), or forward and reverse shocks from a 14 degrees, relativistic (Gamma(0) greater than or similar to 10) jet. Models that include a combination of enhanced radioactive heating rates, low-lanthanide mass fractions, or additional sources of heating from late-time central engine activity may provide viable alternate explanations. If a stable magnetar was indeed produced in GRB 200522A, we predict that late-time radio emission will be detectable starting 0.3-6 yr after the burst for a deposited energy of 10(53) erg. Counterparts of similar luminosity to GRB 200522A associated with gravitational wave events will be detectable with current optical searches to 250 Mpc.

Massive, rapidly spinning magnetar remnants produced as a result of binary neutron-star (BNS) mergers may deposit a fraction of their energy into the surrounding kilonova ejecta, powering a synchrotron radio signal from the interaction of the ejecta with the circumburst medium. Here, we present 6.0 GHz Very Large Array (VLA) observations of nine, low-redshift short gamma-ray bursts (GRBs; z < 0.5) on rest-frame timescales of ≈2.4–13.9 yr following the bursts. We place 3σ limits on radio continuum emission of F ν ≲ 6–20 μJy at the burst positions, or L ν ≲ (0.6–8.3) × 1028 erg s−1 Hz−1. Comparing these limits with new light-curve modeling that properly incorporates relativistic effects, we obtain limits on the energy deposited into the ejecta of E ej ≲ (0.6–6.7) × 1052 erg ( erg) for an ejecta mass of 0.03 M ⊙ (0.1 M ⊙). We present a uniform reanalysis of 27 short GRBs with 5.5–6.0 GHz observations, and find that ≳50% of short GRBs did not form stable magnetar remnants in their mergers. Assuming short GRBs are produced by BNS mergers drawn from the Galactic BNS population plus an additional component of high-mass GW194025-like mergers in a fraction f GW190425 of cases, we place constraints on the maximum mass of a nonrotating neutron star (NS; Tolman–Oppenheimer–Volkoff mass; M TOV), finding for f GW190425 = 0.4; this limit increases for larger values of f GW190425. The detection (or lack thereof) of radio remnants in untargeted surveys such as the VLA Sky Survey (VLASS) could provide more stringent constraints on the fraction of mergers that produce stable remnants. If radio remnants are discovered in VLASS, this suggests that short GRBs are a biased population of BNS mergers in terms of the stability of the remnants they produce.

Located in the Perseus galaxy cluster, NGC 1275 (z = 0.0179), is the brightest radio galaxy detected in both high energy (HE; E > 100 MeV) and very high energy (VHE; E > 100 GeV) regimes. Here we present the results from the observations of NGC 1275 with TACTIC for 37.1 h between December, 2016 and February, 2017. In the absence of a statistically significant VHE gamma-ray detection from the TACTIC observations, we estimate the 3 sigma upper limit on the integral flux above 0.85 TeV to be 2.85 x 10(-11) erg cm(-2) s(-1). We also report the results of near simultaneous multiwavelength observations of the source from the Fermi-LAT, Swift-XRT/UVOT and OVRO. We find the source to be in flaring state on the night of 31 December, 2016 - 1 January, 2017 when the HE gamma-ray flux above 0.1 GeV went up to 2.0 x 10(-6) ph cm(-2) s(-1) which is approximately 4 times the flux level during its low state. We present the broad-band spectral energy distributions (SED) of the source during its flaring and low states. It is evident from the SED that the VHE upper limit obtained from the TACTIC observations is consistent with the low emission state of the source.

Context. The addition of a 28 m Cherenkov telescope (CT5) to the H.E.S.S. array extended the experiment’s sensitivity to lower energies. The lowest energy threshold is obtained using monoscopic analysis of data taken with CT5, providing access to gamma-ray energies below 100 GeV for small zenith angle observations. Such an extension of the instrument’s energy range is particularly beneficial for studies of active galactic nuclei with soft spectra, as expected for those at a redshift ≥ 0.5. The high-frequency peaked BL Lac objects PKS2155 − 304 ( z = 0 . 116) PG1553 + (0 . 43 < z < 0 . 58) are among the brightest objects in the gamma-ray sky, both showing clear signatures of gamma-ray absorption at E > 100 GeV interpreted as being due to interactions with the extragalactic background light (EBL). fitted in both cases by a log-parabola spectral model (with a 5.0 σ statistical preference for non-zero curvature for PKS2155 − 304 and 4.5 σ for PG1553 + 113), were found consistent with spectra derived from contemporaneous Fermi -LAT data, indicating a sharp break in the observed spectra of both sources at E ≈ 100 GeV. When corrected for EBL absorption, the intrinsic H.E.S.S. II mono and Fermi -LAT spectrum of PKS2155 − 304 was found to show significant curvature. For PG1553 + 113, however, no significant detection of curvature in the intrinsic spectrum could be found within statistical and systematic uncertainties.