Keywords

We report the discovery of nonstellar hydrogen Balmer and metastable helium absorption lines accompanying a transient, high-velocity (0.05c) broad absorption line (BAL) system in the optical spectra of the tidal disruption event (TDE) AT2018zr (z = 0.071). In the Hubble Space Telescope UV spectra, absorption of high- and low-ionization lines is also present at this velocity, making AT2018zr resemble a low-ionization BALQSO. We conclude that these transient absorption features are more likely to arise in fast outflows produced by the TDE than absorbed by the unbound debris. In accordance with the outflow picture, we are able to reproduce the flat-topped Hα emission in a spherically expanding medium without invoking the typical prescription of an elliptical disk. We also report the appearance of narrow (∼1000 kms⁻¹) N iii λ4640, He ii λ4686, Hα, and Hβ emission in the late-time optical spectra of AT2018zr, which may be a result of UV continuum hardening at late times, as observed by Swift. Including AT2018zr, we find a high association rate (three out of four) of BALs in the UV spectra of TDEs. This suggests that outflows may be ubiquitous among TDEs and less sensitive to viewing angle effects compared to QSO outflows.

A capillary discharge tube was used to record the Ar spectrum in the region of 330–4400 Å. We analyzed a set of 109 lines of Ar v, with 10 of them being classified for the first time. Part of these newly identified lines (six in total) corresponds to electric dipole transitions in the visible and ultraviolet regions of the spectrum. We calculated weighted transition rates (gA) for all experimentally observed lines and the corresponding lifetimes using a relativistic Hartree–Fock method with configuration interaction. The equations were modified to consider core polarization (CP) effects. A study on the Si isoelectronic sequence found that considering CP effects is essential for the correct description of the Ar v structure.

Cosmic photons can be efficiently collected by broadband intensity mapping but information on their emission redshift and frequency is largely lost. We introduce a technique to statistically recover these otherwise collapsed dimensions by exploiting information in spatial fluctuations and apply it to the Galaxy Evolution Explorer (GALEX) All Sky and Medium Imaging Surveys. By spatially cross-correlating photons in the GALEX far-UV (1500 Å) and near-UV (2300 Å) bands with a million spectroscopic objects in the Sloan Digital Sky Survey as a function of redshift, we robustly detect the redshift-dependent intensity of the UV background (UVB) modulated by its clustering bias up to z ∼ 2. These measurements clearly reveal the imprints of UVB spectral features redshifting through the filters. Using a simple parameterization, we simultaneously fit a UVB emissivity and clustering bias factor to these observations and constrain the main spectral features of the UVB spectrum: (i) the Lyman break, (ii) the non-ionizing UV continuum, which agrees with the Haardt & Madau model but does not rely on any assumption regarding the nature of the sources, and (iii) the Lyα emission, the luminosity density of which is consistent with estimates of the combined galaxy and active galactic nucleus contributions at z ∼ 1. Because the technique probes the total background including low surface brightness emission, we place constraints on the amount of UV light originating from the diffuse intergalactic medium (IGM). Finally, the clustering bias of UV photons is found to be chromatic and evolving. Our frequency- and redshift-dependent UVB measurement delivers a summary statistic of the universe's net radiation output from stars, black holes, and the IGM combined.

CHIANTI contains a large quantity of atomic data for the analysis of astrophysical spectra. Programs are available in IDL and Python to perform calculation of the expected emergent spectrum from these sources. The database includes atomic energy levels, wavelengths, radiative transition probabilities, rate coefficients for collisional excitation, ionization, and recombination, as well as data to calculate free–free, free–bound, and two-photon continuum emission. In Version 9, we improve the modeling of the satellite lines at X-ray wavelengths by explicitly including autoionization and dielectronic recombination processes in the calculation of level populations for select members of the lithium isoelectronic sequence and Fe xviii–xxiii. In addition, existing data sets are updated, new ions are added, and new total recombination rates for several Fe ions are included. All data and IDL programs are freely available at http://www.chiantidatabase.org or through SolarSoft, and the Python code ChiantiPy is also freely available at https://github.com/chianti-atomic/ChiantiPy.

We present Keck spectroscopic observations of three probable high-redshift superluminous supernovae (SLSNe) from the Subaru HIgh-Z sUpernova CAmpaign (SHIZUCA), confirming redshifts of 1.851, 1.965, and 2.399. The host galaxies were selected for transient monitoring from multiband photometric redshifts. The supernovae are detected during their rise, and the classically scheduled spectra are collected near maximum light. The rest-frame far-ultraviolet (∼1000–2500 Å) spectra include a significant host galaxy flux contribution, and we compare our host-galaxy-subtracted spectra to UV-luminous SNe from the literature. While the signal-to-noise ratios of the spectra presented here are sufficient for redshift confirmation, supernova spectroscopic type confirmation remains inconclusive. The success of the first SHIZUCA Keck spectroscopic follow-up program demonstrates that campaigns such as SHIZUCA are capable of identifying high-redshift SLSNe with sufficient accuracy, speed, and depth for rapid, well-cadenced, and informative follow-up.

We compare ultraviolet (UV) and optical colors of a sample of 29 type Ia supernovae (SNe Ia) observed with the Swift satellite's UltraViolet Optical Telescope with theoretical models of an asymmetric explosion viewed from different angles from Kasen & Plewa. This includes mid-UV (1600–2700 Å; uvw2 and uvm2) and near-UV (2700–4000 Å; uvw1 and u) filters. We find the observed colors to be redder than the model predictions, and that these offsets are unlikely to be caused by dust reddening. We confirm that high-velocity SNe Ia have red UV-optical observed colors. After correcting the colors for dust reddening by assuming a constant b − v color, we find no correlation between the uvw1 − v or u − v colors and the ejecta velocities for 25 SNe Ia with published velocities and/or spectra. When assuming an optical color–velocity relation, weak correlations of 2 and 3.6σ are found for uvw1 − v and u − v. However, we find that weak correlations can be reproduced with shuffled velocities and colors that are corrected for reddening. The slope and significance of a correlation between the UV colors and the velocity is thus dependent on the slope of the optical color–velocity relation. Even with a correction, a significant scatter still remains in the uvw1 − v colors including a large spread at low velocities, demonstrating that the NUV-blue/red spread is not caused by the photospheric velocity. The uvm2 − uvw1 colors also show a large dispersion uncorrelated with the velocity.

We present results from a systematic selection of tidal disruption events (TDEs) in a wide-area (4800 deg²), g+R band, Intermediate Palomar Transient Factory experiment. Our selection targets typical optically selected TDEs: bright (>60% flux increase) and blue transients residing in the centers of red galaxies. Using photometric selection criteria to down-select from a total of 493 nuclear transients to a sample of 26 sources, we then use follow-up UV imaging with the Neil Gehrels Swift Telescope, ground-based optical spectroscopy, and light curve fitting to classify them as 14 Type Ia supernovae (SNe Ia), 9 highly variable active galactic nuclei (AGNs), 2 confirmed TDEs, and 1 potential core-collapse supernova. We find it possible to filter AGNs by employing a more stringent transient color cut (g − r < −0.2 mag); further, UV imaging is the best discriminator for filtering SNe, since SNe Ia can appear as blue, optically, as TDEs in their early phases. However, when UV-optical color is unavailable, higher-precision astrometry can also effectively reduce SNe contamination in the optical. Our most stringent optical photometric selection criteria yields a 4.5:1 contamination rate, allowing for a manageable number of TDE candidates for complete spectroscopic follow-up and real-time classification in the Zwicky Transient Facility era. We measure a TDE per galaxy rate of ${1.7}_{-1.3}^{+2.9}\times {10}^{-4}\,{\mathrm{gal}}^{-1}\,{\mathrm{yr}}^{-1}$ (90% CL in Poisson statistics). This does not account for TDEs outside our selection criteria, and thus may not reflect the total TDE population, which is yet to be fully mapped.

Laboratory experiments that simulate the photo- and thermo-chemistry of extraterrestrial ices always lead to the formation of semi-refractory organic residues. These residues can be considered as laboratory analogs for the primitive organic matter incorporated into comets and asteroids. Many specific organic molecules have been detected in them. Here we focus on amino acids because of their possible relevance to further prebiotic chemistry on Earth as well as in other solar system bodies. We compare the amino acid content and distribution measured in organic residues produced in our photochemical experiments to those observed in various CM chondrites presenting an increasing degree of aqueous alteration, a process that is thought to impact amino acid chemistry. We find that the amino acid profile of our residues shows similarities with that of the least aqueously altered CM chondrites. In particular, the β-alanine to glycine ratio is comparable to the one measured in the Paris meteorite, a minimally altered CM chondrite, and matches the trend followed by other CM chondrites with different degrees of aqueous alteration. Additionally, the relative abundances of α-, β-, and γ-amino acids in one of our residues are similar to those of the least altered CM chondrites. These results support the idea of a general formation process for amino acids from photo- and thermo-processing of icy grains as an important source for the inventory of amino acids in the early solar system.

We have used GALEX observations of the north and south Galactic poles to study the diffuse ultraviolet background at locations where the Galactic light is expected to be at a minimum. We find offsets of 230–290 photon units in the far-UV (1531 Å) and 480–580 photon units in the near-UV (2361 Å). Of this, approximately 120 photon units can be ascribed to dust-scattered light and another 110 photon units (190 in the near-UV) to extragalactic radiation. The remaining radiation is, as yet, unidentified and amounts to 120–180 photon units in the far-UV and 300–400 photon units in the near-UV. We find that molecular hydrogen fluorescence contributes to the far-UV when the 100 μm surface brightness is greater than 1.08 MJy sr⁻¹.

Comet C/2011 W3 (Lovejoy) is the first sungrazing comet in many years to survive perihelion passage. We report ultraviolet observations with the Ultraviolet Coronagraph Spectrometer (UVCS) spectrometer aboard the Solar and Heliospheric Observatory satellite at five heights as the comet approached the Sun. The brightest line, Lyα, shows dramatic variations in intensity, velocity centroid, and width during the observation at each height. We derive the outgassing rates and the abundances of N, O, and Si relative to H, and we estimate the effective diameter of the nucleus to be several hundred meters. We consider the effects of the large outgassing rate on the interaction between the cometary gas and the solar corona and find good qualitative agreement with the picture of a bow shock resulting from mass loading by cometary neutrals. We obtain estimates of the solar wind density, temperature, and speed, and compare them with predictions of a global magnetohydrodynamic simulation, finding qualitative agreement within our uncertainties. We also determine the sublimation rate of silicate dust in the comet's tail by comparing the visible brightness from the Large Angle Spectroscopic Coronagraphs with the Si iii intensity from UVCS. The sublimation rates lie between the predicted rates for olivines and pyroxenes, suggesting that the grains are composed of a mixture of those minerals.