Ultraviolet Telescope Research Articles

Faint White Dwarf Flux Standards: Data and Models

Abstract Fainter standard stars are essential for the calibration of larger telescopes. This work adds to the calibration spectra (CALSPEC) database of 19 faint white dwarfs (WDs) with all-sky coverage and V magnitudes between 16.5 and 18.7. Included for these stars is a new ultraviolet (UV) Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph between 1150 and 3000 Å with a resolution of ∼500. Pure hydrogen WD models are fit to these UV spectra and to six-band HST/Wide Field Camera 3 photometry at 0.28–1.6 μm to construct predicted model spectral energy distributions covering wavelengths from 900 Å to the James Webb Space Telescope limit of 30 μm using well-established CALSPEC procedures for producing flux standards with the goal of 1% accuracy.

An Elemental Abundance Diagnostic for Coordinated Solar Orbiter/SPICE and Hinode/EIS Observations

Plasma composition measurements are a vital tool for the success of current and future solar missions, but density- and temperature-insensitive spectroscopic diagnostic ratios are sparse, and their underlying accuracy in determining the magnitude of the first ionization potential (FIP) effect in the solar atmosphere remains an open question. Here we assess the Fe viii 185.213 Å/Ne viii 770.428 Å intensity ratio that can be observed as a multispacecraft combination between Solar Orbiter/SPICE and Hinode/EUV Imaging Spectrometer (EIS). We find that it is fairly insensitive to temperature and density in the range of log (T/K) = 5.65–6.05 and is therefore useful, in principle, for analyzing on-orbit EUV spectra. We also perform an empirical experiment, using Hinode/EIS measurements of coronal fan loop temperature distributions weighted by randomly generated FIP bias values, to show that our diagnostic method can provide accurate results as it recovers the input FIP bias to within 10%–14%. This is encouraging since it is smaller than the magnitude of variations seen throughout the solar corona. We apply the diagnostic to coordinated observations from 2023 March and show that the combination of SPICE and EIS allows measurements of the Fe/Ne FIP bias in the regions where the footpoints of the magnetic field connected to Solar Orbiter are predicted to be located. The results show an increase in FIP bias between the main leading polarity and the trailing decayed polarity that broadly agrees with Fe/O in situ measurements from Solar Orbiter/Solar Wind Analyser. Multispacecraft coordinated observations are complex, but this diagnostic also falls within the planned wavebands for Solar-C/Extreme UltraViolet high-throughput Spectroscopic Telescope.

The ultraviolet luminosity function of star-forming galaxies between redshifts of 0.4 and 0.6

Abstract We combine ultraviolet imaging of the 13H survey field, taken with the XMM-Newton Optical Monitor telescope (XMM-OM) and the Neil Gehrels Swift Observatory Ultraviolet and Optical Telescope (UVOT) in the UVM2 band, to measure rest-frame ultraviolet 1,500 Å luminosity functions of star-forming galaxies with redshifts z between 0.4 and 0.6. In total the UVM2 imaging covers a sky area of 641 arcmin2, and we detect 273 galaxies in the UVM2 image with 0.4 < z < 0.6. The luminosity function is fit by a Schechter function with best-fit values for the faint end slope $\alpha =-1.8^{+0.4}_{-0.3}$ and characteristic absolute magnitude $M^{*} = -19.1^{+0.3}_{-0.4}$. In common with XMM-OM based studies at higher redshifts, our best-fitting value for M* is fainter than previous measurements. We argue that the purging of active galactic nuclei from the sample, facilitated by the co-spatial X-ray survey carried out with XMM-Newton is important for the determination of M*. At the brightest absolute magnitudes (M1500 < −18.5) the average UV colour of our galaxies is consistent with that of minimal-extinction local analogues, but the average UV colour is redder for galaxies at fainter absolute magnitudes, suggesting that higher levels of dust attenuation enter the sample at absolute magnitudes somewhat fainter than M*.

Ultraviolet Flux and Spectral Variability Study of Blazars Observed with UVIT/AstroSat

Blazars, the peculiar class of active galactic nuclei, are known to show flux variations across the accessible electromagnetic spectrum. Though they have been studied extensively for their flux variability characteristics across wavelengths, information on their ultraviolet (UV) flux variations on timescales of hours is very limited. Here, we present the first UV flux variability study on intraday timescales of a sample of ten blazars comprising two flat-spectrum radio quasars (FSRQs) and eight BL Lacertae objects (BL Lacs). These objects, spanning a redshift (z) range of 0.034 ≤ z ≤ 1.003, were observed in the far-UV (1300−1800 Å) and near-UV (2000−3000 Å) wavebands using the ultraviolet imaging telescope on board AstroSat. UV flux variations on timescales of hours were detected in nine sources out of the observed ten blazars. The spectral variability analysis showed a bluer-when-brighter trend with no difference in the UV spectral variability behavior between the studied sample of FSRQs and BL Lacs. The observed UV flux and spectral variability in our sample of both FSRQs and BL Lacs revealed that the observed UV emission in them is dominated by jet synchrotron process.

Colorado Ultraviolet Transit Experiment: a mission development history and future possibilities from the National Aeronautics and Space Administration’s first ultraviolet astronomy CubeSat

Colorado Ultraviolet Transit Experiment: a mission development history and future possibilities from the National Aeronautics and Space Administration’s first ultraviolet astronomy CubeSat

Fabrication and high photoresponse performance of a La-doped HfO2 thin film-based UV photodiode

The ability to detect ultraviolet light is crucial for various applications such as energy harvesting, environmental monitoring, sensing, and ultraviolet astronomy. In this paper, we determined the photoelectric properties of a photodiode based on an interfacial layer of a La-doped HfO2 (HfLaO) thin film. We carried out several tests to analyze the electrical behavior and photoresponse of the Au/HLO/p-Si photodiode under ultraviolet (UV) and white light illumination and in the dark. The chemical composition, surface morphology, cross-sectional thickness, optical properties, and crystalline structure of the HfLaO (HLO) film prepared by the sol-gel method were characterized. We found that the prepared photodiode exhibited excellent repeatability and stability. Moreover, the photocurrent increased with increasing UV light intensity. When the UV light intensity was 23.5 mW/cm2, the photocurrent reached 700 μA and remained unsaturated at a 7 V bias voltage. Moreover, we systematically analyzed the changes in the energy bands and carriers under dark and ultraviolet light conditions to further understand the working mechanism of the photodiode. Overall, the Au/HLO/p-Si photodiode had excellent ultraviolet detector performance and could be a strong candidate for optoelectronic applications.

Modeling the Multiwavelength Spectral Energy Distributions of the Fermi-4LAC Bright Flat-spectrum Radio Quasars

In this paper, we present a long-term multiwavelength investigation focusing on 12 distinct samples of Fermi-4LAC bright flat-spectrum radio quasars (FSRQs). Detailed variability and spectral analyses of γ-ray, X-ray, and ultraviolet/optical data obtained by the Fermi Large Area Telescope, the Swift X-ray Telescope, and the Swift Ultraviolet and Optical Telescope were performed over a period of about 14 yr, spanning from 2008 October to 2022 October. These analyses provide insights into characterizing the variations within different activity states. To efficiently reproduce the multiwavelength simultaneous/quasi-simultaneous spectral energy distributions (SEDs) of the samples, we propose a novel approach for constraining the model parameters. By analyzing the parameters of the energy spectral curvature (β), the peak frequency (ν pk), the peak luminosity (L pk), the Compton dominance parameter (A C), and the variability timescale (t var) in different activity states, we can estimate the values of the jet radiation region parameters for the samples. Subsequently, we utilize the synchrotron-self-Compton and external Compton processes, employing a logarithmic-parabolic spectral shape to approximate the observed spectra of the sample sources, while considering the induced regime for the physical parameters. The model results show that: (1) by effectively reproducing SEDs in various active states of bright FSRQs, the parameters within the emission region were reasonably constrained; (2) compared to other active states, the emission region of the jet exhibits a reduced radius during the high state, while the magnetic field strength increases during the low state; and (3) for bright FSRQs in a high-activity state, there is an enhancement of the Doppler factor, often exhibiting a tendency toward energy equipartition.

Are Coronal Loops Projection Effects?

We report results of an in-depth numerical investigation of three-dimensional projection effects that could influence the observed loop-like structures in an optically thin solar corona. Several archetypal emitting geometries are tested, including collections of luminous structures with circular cross sections of fixed and random size, and light-emitting structures with highly anisotropic cross sections, as well as two-dimensional stochastic current density structures generated by fully developed magnetohydrodynamic turbulence. A comprehensive set of statistical signatures is used to compare the line-of-sight (LOS) integrated emission signals predicted by the constructed numerical models with the loop profiles observed by the extreme ultraviolet telescope on board the flight 2.1 of the High-Resolution Coronal Imager (Hi-C). The results suggest that typical cross-sectional emission envelopes of the Hi-C loops are unlikely to have high eccentricity, and that the observed loops cannot be attributed to randomly oriented quasi-two-dimensional emitting structures, some of which would produce anomalously strong optical signatures due to an accidental LOS alignment, as expected in the ''coronal veil“ scenario proposed recently by Malanushenko et al. The possibility of apparent loop-like projections of very small (close to the resolution limit) or very large (comparable with the size of an active region) light-emitting sheets remains open, but the intermediate range of scales commonly associated with observed loop systems is most likely filled with true quasi-one-dimensional (roughly axisymmetric) structures embedded into the three-dimensional coronal volume.

Prolonged and Extremely Non-radial Solar Wind Flows

We present a study of three highly non-radial solar wind events in which the azimuthal solar wind flow angle exceeds 6° for one day or more. None of the events are associated with coronal mass ejections and co-rotating interaction regions observed at 1 AU. For all events, the solar wind outflows at 1 AU exhibit low velocity and density. Based on the significant increase in the oxygen charge state ratio of O7+/O6+ at 1 AU for all of the events, we have traced them back to the Sun and found that their source regions originated in active region and coronal hole (AR-CH) pairs mainly located at the central meridian. Furthermore, examining the dynamical evolutions in their source regions using both the Extreme ultra-violet Imaging Telescope and the Michelson Doppler Imager, we found that the changes taking place in AR-CH boundaries eventually disturbed the stable CH configurations, resulting in a reduction of the CH area and finally its disappearance, leaving only with the AR. Our study provides a possible explanation for discussing the origin of the prolonged and highly non-radial solar wind flows.

The Polstar UV Spectropolarimetry Mission

The Polstar small explorer concept is for an ultraviolet (UV) spectropolarimetry space telescope mission with a focus on massive star astrophysics. The instrument waveband will be from 115 nm to 286 nm for spectroscopy, and from 122 nm to 286 nm for polarimetry. All four Stokes parameters, I, Q, U, and V , will be measured at a resolving power of R=20,000 (15 km s−1 velocity resolution). The telescope aperture will be 40 cm with an effective area of about 22 cm2 at a reference wavelength of 150 nm. The thrust of the science goals will be to determine the astrophysics of angular momentum exchange and transport, and consequences for massive star properties and evolution. This includes the effects of rapid to critical rotation for individual stars (magnetic and non-magnetic), and the effects of mass transfer for massive binaries, including identification of stripped core stars. If selected by the NASA/SMEX program, Polstar would launch around 2031 and observe ∼300 stars to achieve science goals. The mission will include a Guest Observer program to advance discovery in other areas of astrophysics.

Star Formation Characteristics of Galaxies Hosting AGN

We present an analysis of the ultra-violet (UV) observations of two Seyfert type active galactic nuclei (AGN), namely NGC 4051 and NGC 4151. These observations aimed at studying the star formation in the hosts of these AGN were carried out with the ultra-violet imaging telescope on board AstroSat in far-UV. A total of 193 and 328 star-forming regions (SF) were identified using SExtractor in NGC 4051 and NGC 4151, respectively. Using aperture photometry of the identified SF regions, we estimated the star formation rates (SFRs). We found NGC 4051 to have the lowest SFR with a median value of 3.16 × 10−5 M⊙ yr−1 while for NGC 4151, we found a median SFR of 0.012 M⊙ yr−1.

Early-time Ultraviolet and Optical Hubble Space Telescope Spectroscopy of the Type II Supernova 2022wsp

We report early-time ultraviolet (UV) and optical spectroscopy of the young, nearby Type II supernova (SN) 2022wsp obtained by the Hubble Space Telescope (HST)/STIS at about 10 and 20 days after the explosion. The SN 2022wsp UV spectra are compared to those of other well-observed Type II/IIP SNe, including the recently studied Type IIP SN 2021yja. Both SNe exhibit rapid cooling and similar evolution during early phases, indicating a common behavior among SNe II. Radiative-transfer modeling of the spectra of SN 2022wsp with the TARDIS code indicates a steep radial density profile in the outer layer of the ejecta, a solar metallicity, and a relatively high total extinction of E(B − V) = 0.35 mag. The early-time evolution of the photospheric velocity and temperature derived from the modeling agree with the behavior observed from other previously studied cases. The strong suppression of hydrogen Balmer lines in the spectra suggests interaction with a preexisting circumstellar environment could be occurring at early times. In the SN 2022wsp spectra, the absorption component of the Mg ii P Cygni profile displays a double-trough feature on day +10 that disappears by day +20. The shape is well reproduced by the model without fine-tuning the parameters, suggesting that the secondary blueward dip is a metal transition that originates in the SN ejecta.

Determination of the influence of the shape of the nose fairing on the ballistic characteristics of a meteorological missile

Climate change currently stands as one of the key threats and challenges to humanity. Researching atmospheric parameters not only enables the analysis of current climatic variations but also facilitates forecasting future changes and predicting extreme abrupt climatic shifts, natural phenomena, and catastrophes. Meteorological rockets play a significant role in this research, bridging the gap between balloons and meteorological satellites. The main purpose of meteorological rockets is to study the upper layers of the atmosphere, collect information for weather forecasting, and similar rockets can also be used for X-ray and ultraviolet astronomy. Utilizing modern numerical simulation methods allows for swift and precise simulations of complex tasks. Designing rocket technology poses a complex challenge due to the intricate system of differential equations describing the aircraft's motion. Consequently, numerical modeling found extensive application in ballistic design. Through contemporary numerical modeling methods, the flight of a meteorological rocket was simulated, determining its key flight parameters. An analysis was conducted on the influence of three types of nose cone shapes on the rocket's energy-ballistic trajectory.

EUSO-SPB1 mission and science

The Extreme Universe Space Observatory on a Super Pressure Balloon 1 (EUSO-SPB1) was launched in 2017 April from Wanaka, New Zealand. The plan of this mission of opportunity on a NASA super pressure balloon test flight was to circle the southern hemisphere. The primary scientific goal was to make the first observations of ultra-high-energy cosmic-ray extensive air showers (EASs) by looking down on the atmosphere with an ultraviolet (UV) fluorescence telescope from suborbital altitude (33 km). After 12 days and 4 h aloft, the flight was terminated prematurely in the Pacific Ocean. Before the flight, the instrument was tested extensively in the West Desert of Utah, USA, with UV point sources and lasers. The test results indicated that the instrument had sensitivity to EASs of ⪆3 EeV. Simulations of the telescope system, telescope on time, and realized flight trajectory predicted an observation of about 1 event assuming clear sky conditions. The effects of high clouds were estimated to reduce this value by approximately a factor of 2. A manual search and a machine-learning-based search did not find any EAS signals in these data. Here we review the EUSO-SPB1 instrument and flight and the EAS search.

Homogenizing SOHO/EIT and SDO/AIA 171 Å Images: A Deep-learning Approach

Extreme-ultraviolet (EUV) images of the Sun are becoming an integral part of space weather prediction tasks. However, having different surveys requires the development of instrument-specific prediction algorithms. As an alternative, it is possible to combine multiple surveys to create a homogeneous data set. In this study, we utilize the temporal overlap of Solar and Heliospheric Observatory Extreme ultraviolet Imaging Telescope and Solar Dynamics Observatory Atmospheric Imaging Assembly 171 Å surveys to train an ensemble of deep-learning models for creating a single homogeneous survey of EUV images for two solar cycles. Prior applications of deep learning have focused on validating the homogeneity of the output while overlooking the systematic estimation of uncertainty. We use an approach called “approximate Bayesian ensembling” to generate an ensemble of models whose uncertainty mimics that of a fully Bayesian neural network at a fraction of the cost. We find that ensemble uncertainty goes down as the training set size increases. Additionally, we show that the model ensemble adds immense value to the prediction by showing higher uncertainty in test data that are not well represented in the training data.

Long-term X-ray/UV variability in ULXs

ABSTRACT The focus of NASA’s Swift telescope has been transients and target-of-opportunity observing, resulting in many observations of ultraluminous X-ray sources (ULXs) over the last ∼20 yr. For the vast majority of these observations, simultaneous data have been obtained using both the X-ray telescope (XRT) and the ultraviolet and optical telescope (UVOT), providing a unique opportunity to study coupled variability between these bands. Using a sample of ∼40 ULXs with numerous repeat observations, we extract stacked images to characterize the spatial extent of the UV/optical emission and extract long-term light curves to search for first-order linear correlations between the UV and X-ray emission. We find that a small subset may show weakly correlated joint variability, while other sources appear to display non-linear relationships between the bands. We discuss these observations in the context of several theoretical models: precession, irradiation of the outer accretion disc, and irradiation of the companion star. We conclude that more complicated analysis or higher quality data may be required to accurately constrain the nature of the joint X-ray and UV/optical emission in these sources.

The IAU recommended photometric system for ultraviolet astronomy

In the current era when access to space is becoming easier and at a lower cost thanks to the standardised cubesat technology, numerous missions are expected to be launched to observe, particularly, at ultraviolet wavelengths. Given the reduced dimensions of the telescope that a cubesat can carry, most of these missions will be focused on photometric surveys of a reduced sample of targets of interest, and therefore each mission will define their own photometric bands according to their scientific objectives and orbital constraints. However, in order to provide a coherent view of the ultraviolet sky, the data should be post-processed under a common framework. In 2017, the IAU working group on ultraviolet astronomy identified the need to define such a common framework for the upcoming ultraviolet missions, and coordinated the definition of a standard set of photometric bands that could serve for homogenizing the current and future data. This paper presents the procedure adopted by the working group for the definition of the standard photometric system, that was approved by the IAU during the General Assembly Business Sessions held in August, 2021. The photometric system consists of seven bands, denoted as UV1-UV7, all included in the range 115 - 400 nm. Some of these bands are based on existing filters, while others have been defined as theoretical bands with constant throughput. This system is to be regarded as a set of synthetic bands for post-processing the data of any mission, and an example of its application to the SPARCS cubesat is also included. The photometric bands are publicly available and can be downloaded from https://www.nuva.eu/uv-photometry/.

Statistics of electron-multiplying charge-coupled devices

Electron-multiplying charge-coupled devices are efficient imaging devices for low-surface-brightness ultraviolet astronomy from space. The large amplification allows photon counting (PC), the detection of events versus nonevents. This paper provides the statistics of the observation process, the photon-counting process, the amplification process, and the compression. The expression for the signal-to-noise of PC is written in terms of the polygamma function. The optimal exposure time is a function of the clock-induced charge. The exact distribution of amplification process is a simple-to-compute powered matrix. The optimal cutoff for comparing to the read noise is close to a strong function of the read noise and a weak function of the electron-multiplying gain and photon rate. A formula gives the expected compression rate.

On the role of dust and mass-loss in the extended main sequence turnoff of star clusters: the case of NGC 1783

ABSTRACT The colour–magnitude diagram (CMD) morphology of the ‘extended’ main sequence turnoff (eMSTO) and upper main sequence (MS) of the intermediate age (≲ 2 Gyr) Large Magellanic Cloud Cluster NGC 1783 shows the presence of a small group of UV-dim stars, that, in the ultraviolet Hubble Space Telescope filters, are located at colours on the red side of the typical ‘fan’ shape displayed by the eMSTO. We model the UV-dim stars by assuming that some of the stars which would intrinsically be located on the left side of the eMSTO are obscured by a ring of dust due to grain condensation at the periphery of the excretion disc expelled when they spin at the high rotation rates typical of stars in the Be stage. A reasonably low optical depth at 10μ is necessary to model the UV-dim group. Introduction of dust in the interpretation of the eMSTO may require a substantial re-evaluation of previous conclusions concerning the role of age and/or rotation spreads in the MC clusters: the entire eMSTO can be populated by dusty stars, and the reddest UV-dim stars simply represents the tail of the distribution with both maximum obscuration and the dust ring seen along the line of sight. The model stars having higher rotational projected velocity (vsin i) are predicted to be preferentially redder than the slowly rotating stars. The mass-loss responsible for the dust may also cause the non-monotonic distribution of stars in the upper main sequence, with two peaks and gaps showing up in the UV CMD.

Light-curve modelling for the initial rising phase of rapidly evolving transients powered by continuous outflow

ABSTRACT A wind-driven model is a new framework to model observational properties of transients that are powered by continuous outflow from a central system. While it has been applied to fast blue optical transients (FBOTs), the applicability has been limited to post-peak behaviours due to the steady-state assumptions; non-steady-state physics, e.g. expanding outflow, is important to model the initial rising phase. In this paper, we construct a time-dependent wind-driven model, which can take into account the expanding outflow and the time evolution of the outflow rate. We apply the model to a sample of well-observed FBOTs. FBOTs require high outflow rates (∼30 M⊙ yr−1) and fast velocities (∼0.2–0.3c), with the typical ejecta mass and energy budget of ∼0.2 M⊙ and ∼1052 erg, respectively. The energetic outflow supports the idea that the central engine of FBOTs may be related to a relativistic object, e.g. a black hole. The initial photospheric temperature is 105 –106 K, which suggests that FBOTs will show ultraviolet (UV) or X-ray flash similar to supernova shock breakouts. We discuss future prospects of surveys and follow-up observations of FBOTs in the UV bands. FBOTs are brighter in the UV bands than in the optical bands, and the time-scale is a bit longer than that in optical wavelengths. We suggest that UV telescopes with a wide field of view can play a key role in discovering FBOTs and characterizing their natures.