Ultraviolet Imaging Research Articles

A sky survey of ultraviolet sources observed through AstroSat’s UVIT: A point source catalogue

Abstract The Ultra Violet Imaging Telescope (UVIT) onboard India’s first dedicated multiwavelength satellite AstroSat observed a significant fraction of the sky in the ultraviolet with a spatial resolution of 1.4 $^{\prime\prime}$ . We present a catalogue of the point sources observed by UVIT in the far ultraviolet (FUV; 1 300–1 800 Å) and near ultraviolet (NUV; 2 000–3 000 Å). We carried out astrometry and photometry of 428 field pointings in the FUV and 54 field pointings in the NUV band, observed in 5 filter bands in each channel, respectively, covering an area of about 63 square degrees. The final catalogue contains about 102 773 sources. The limiting magnitude(AB) of the F148W band filter, that has the largest number of detections is $\sim$ $21.3$ . For the NUV channel, we find the limiting magnitude at around $\sim$ 23. We describe the final catalogue and present the results of the statistical analysis.

UVIT/AstroSat Investigation of a Low-luminous Blue Straggler Star in NGC 362: Detection of an Extremely Low-mass White Dwarf as Companion

In the present study, we identified an extremely low-mass white dwarf as a companion to a low luminous blue straggler star within the Galactic globular cluster NGC 362. To conduct the analysis, we utilized data obtained from various sources, including AstroSat’s Ultra Violet Imaging Telescope, UVOT, and the 2.2 m ESO telescope. By examining the spectral energy distribution of the blue straggler star candidate, we successfully identified an extremely low-mass white dwarf as its binary companion. We determined the effective temperature, radius, and luminosity for both, the extremely low-mass white dwarf and the blue straggler star candidate. Specifically, the extremely low-mass white dwarf exhibits an effective temperature of 15000 K, a radius of 0.17 R⊙, a luminosity of 1.40 L⊙, and a mass range of 0.19-0.20 M⊙. Furthermore, the position of the straggler star within the cluster suggests their formation via the Case A/B mass-transfer mechanism in a low-density environment.

High-performance solar-blind imaging photodetectors based on micrometer-thick β-Ga2O3 films grown by thermal oxidation of gallium

A preparation method for micrometer-sized β-Ga2O3 films was developed. The MSM device has a responsivity greater than 1.7 A W−1 and has good solar-blind ultraviolet imaging performance.

Realizing high zero-bias gain in a GaN-based bipolar phototransistor through thin-base configuration for ultraviolet imaging

A GaN-based ultraviolet bipolar phototransistor with high zero-bias gain is obtained based on an asymmetric n–p–i–n homo-structure. A thin-base structure is proposed to suppress excess holes recombination and, thus, to promote optical gain.

Cassini UVIS Observations of the Enceladus Auroral Footprint on Saturn in 2017

Ultraviolet Imaging Spectrograph (UVIS) observations show the Enceladus auroral footprint on Saturn on 2017 September 14, near the end of the Cassini mission. A series of Saturn north polar auroral images were obtained by slowly slewing the Cassini spacecraft at right angles to the UVIS long slit. The images were limb-fit to improve the spacecraft geometry. Enhanced extreme-ultraviolet 88–118 nm channel emissions due to electron impact on atomic and molecular hydrogen were seen in the expected location for the Enceladus auroral footprint on five successive images spanning almost 4 hr. Enhanced emissions were also seen in simultaneously obtained far-ultraviolet 111–165 nm images in at least two of these images, with the spectral signature expected for auroral emissions. While most Cassini UVIS auroral images do not show the Enceladus auroral footprint, these 2017 images support the earlier detection of an Enceladus-linked spot on Saturn in 2008 Cassini UVIS data.

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.

Modeling Cassini UVIS Interplanetary Hydrogen Lyα Observations from 1999 to 2017

The Cassini Orbiter Ultraviolet Imaging Spectrograph (UVIS) obtained interplanetary hydrogen Lyα observations from 1999 to 2017, with mid-2004 to 2017 observations obtained from Saturn orbit. During its Saturn orbital phase, the spacecraft moved from mostly downwind and sidewind in the heliosphere to upwind. We analyze the full set of observations with our existing hot hydrogen density model with a solar illumination model most recently used to study Solar and Heliospheric Observatory Solar Wind Anisotropy Experiment data and selected Cassini UVIS observations from 2003 to 2004. We find general agreement between data and model, but with evidence for a decline in UVIS Lyα sensitivity, with a significant decline in 2002 June during a starburn event and an overall roughly linear decline in sensitivity. While earlier work by Pryor et al. fit the UVIS Lyα data from 2003 to 2004 with a hydrogen density in the outer heliosphere (but after filtration at outer heliospheric boundaries) of 0.085 cm−3 using the UVIS laboratory sensitivity calibration, including the sensitivity decline found here leads to a revised hydrogen density estimate of n H = 0.14 ± 0.03 cm−3. This density estimate is consistent with a recent neutral hydrogen density estimate near the termination shock of 0.127 ± 0.015 cm−3 based on models of observations of pick-up hydrogen ions from the New Horizons spacecraft.

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.

Resolving Some Longstanding Issues in Far Ultraviolet Remote Sensing: A Self‐Consistent Analysis of the GUVI and SEE Observations on the TIMED Mission

AbstractFar Ultraviolet (FUV) remote sensing has been used in aeronomy missions as a powerful means to measure the temperature and compositions in the upper atmospheres of terrestrial planets. However, despite decades of continual development, outstanding challenges remain in this field. Particularly, there is currently a lack of methods for the examination and improvement of the data products in FUV remote sensing. As a result, longstanding issues exist in many of those data products. Here we demonstrate that a self‐consistent analysis of multi‐band airglow observations in the spectral range of ∼120–180 nm can serve as a useful means for examination and improvement of the data products in FUV remote sensing. The Global Ultraviolet Imager (GUVI) and Solar EUV Experiment (SEE) observations on the Thermosphere Ionosphere Mesosphere Energetics and Dynamics mission are used for the demonstration. Through a self‐consistent analysis, we reconcile the thermospheric column O/N2 density ratios that are retrieved respectively from the GUVI limb and disk scans, by numerically correcting a systematic offset between those observations. We also reconcile the models with the observed 121.6 and 130.4 nm emission that have shown large discrepancies in the literature. The self‐consistent analysis indicates that the SEE measurements of the solar spectral irradiance below ∼45 nm need to be scaled down by ∼25%, whereas the SEE measurements of the solar 121.6 and 130.4 nm fluxes are highly accurate, with errors less than ∼3%. Our results achieve unprecedented agreement (better than ∼5%) between models and observations and provide a novel method for data examination and improvement in FUV remote sensing.

Ionospheric Response to Solar EUV Radiation Variations Using GOLD Observations and the CTIPe Model

AbstractTo understand the global response of thermospheric‐ionospheric (TI) parameters to variations in solar irradiance measurements from the Global‐Scale Observations of the Limb and Disk (GOLD) ultraviolet imaging spectrograph, solar radio flux F10.7, predictions from the Coupled Thermosphere Ionosphere Plasmasphere electrodynamics (CTIPe) model, and International Global Navigation Satellite System Service total electron content maps (TEC) have been used. Various parameters such as GOLD O/N2, O2, and the nighttime peak electron density (Nmax) have been compared with the CTIPe model simulations. The GOLD observed Nmax shows a number of significant features including a winter anomaly and an equatorial ionization anomaly. The comparison with solar proxies showed that the GOLD QEUV correlates very well with the EUV observations compared to the F10.7 index. The study also examined the relationship between the solar proxies and Nmax on different time scales and found that Nmax responded significantly to QEUV at both medium‐ and long‐term timescales. Furthermore, a low correlation between Nmax in the equatorial region and solar proxies was found. A delayed ionospheric TEC response against solar flux variations within the 27‐day solar rotation was investigated. This ionospheric delay of TEC with respect to solar flux was observed to be less than 1 day, which was reproduced in model simulations. The current study has shown that the GOLD observations can be used to investigate the delayed ionospheric response and to gain a better understanding of the influence of solar activity on the TI system.

What drives the wheels of evolution in NGC 1512?

Context. Environmental and secular processes play a pivotal role in the evolution of galaxies. These can be external processes such as interactions or internal processes linked to the action of bar, bulge, and spiral structures. Ongoing star formation in spiral galaxies can be affected by these processes. By studying the star formation progression in the galaxy, we can gain insights into the role of different processes that regulate the overall evolution of a galaxy. Aims. The ongoing interaction between the barred-spiral galaxy NGC 1512 and its satellite NGC 1510 offers an opportunity to investigate how galactic interactions and the presence of a galactic bar influence the evolution of NGC 1512. We aim to understand the recent star formation activity in the galaxy pair and thus gain insight into the evolution of NGC 1512. Methods. The UltraViolet Imaging Telescope (UVIT) on board AstroSat enables us to characterise the star-forming regions in the galaxy with a superior spatial resolution of ∼85 pc in the galaxy rest frame. We identified and characterised 175 star-forming regions in the UVIT far-ultraviolet (FUV) image of NGC 1512 and correlated with the neutral hydrogen (H I) distribution. Extinction correction was applied to the estimated photometric magnitude. We traced the star-forming spiral arms of the galaxy and studied the star formation properties across the galaxy in detail. Results. We detect localised regions of star-formation enhancement and distortions in the galactic disc. We find this to be consistent with the distribution of H I in the galaxy. This is evidence of past and ongoing interactions affecting the star formation properties of the galaxy. We studied the properties of the inner ring. We find that the regions of the inner ring show maximum star-formation-rate density (log(SFRDmean[M⊙ yr−1 kpc−2]) ∼ −1.7) near the major axis of the bar, hinting at a possible crowding effect in these regions. The region of the bar in the galaxy is also depleted of UV emission. This absence suggests that the galactic bar may have played an active role in the redistribution of gas and quenching of star formation inside the identified bar region. We therefore suggest that both secular and environmental factors might be influencing the evolution of NGC 1512.

UOCS-XII. A study of open cluster NGC 6940 using UVIT/AstroSat cluster properties and exotic populations

ABSTRACT We study an open cluster NGC 6940 using AstroSat/UVIT (Ultraviolet Imaging Telescope) data and other archival data. This is an intermediate age cluster (∼1 Gyr), located at about 770 pc distance, harbouring several exotic populations apart from normal single and binary stars. We identify members of this cluster using a machine learning algorithm, ML-MOC and identify 492 members, including one blue straggler star (BSS), two yellow straggler stars (YSS), 11 blue lurker (BL) candidates, and two red clump (RC) stars. The cluster shows the effect of mass segregation, with massive stars segregated the most into the cluster, followed by the equal-mass binary members and the single low-mass stars. We report the presence of an extended main-sequence turn-off (eMSTO) feature in this cluster and suggest that the age spread may be a contributing factor behind it. However, the effect of stellar rotation, and the dust absorption needs to be examined more comprehensively with a larger fraction of MSTO stars. All the sixteen sources mentioned above have a counterpart in the UVIT/F169M filter. In order to characterize them, we construct multi-wavelength spectral energy distributions (SEDs) of 14 of these objects having no nearby sources within 3 arcsec. The BSS is successfully fitted with a single-component SED. We find that three BLs, two YSS, and one RC star have UV excess greater than 50 per cent and successfully fit two-component SEDs having hot companions. The parameters derived from the SEDs imply that the hot companions of BLs and the RC star are low-mass and normal-mass white dwarfs, whereas the hot companions of YSS are likely to be subdwarf B stars. We suggest that at least 6 out of 14 stars (∼42 per cent) are formed via mass transfer and/or merger pathways.

STRUCTURAL REINFORCEMENT OF FRAGILE ARCHAEOLOGICAL GLASS BY LOSS COMPENSATION USING ACRYLIC SHEETS.

The current paper discusses a new technique for restoring glass artifacts with gaps and loss of fragility. Acrylic slides of Paraloid B-44 are the best in terms of use because the degree of glass transition (Tg) is about 60, which suits the temperature in Egypt. The slides were made with 40% Paraloid B-44 w/v in acetone and used to fill the lost parts for structural reinforcement. The paper also aims to study the glass vessel that was discovered in the excavations of the American Research Centre in Fustat in 1966 and kept in the stores of the Museum of Islamic Art in Cairo. The glass vessel was examined by optical digital microscopy (ODM) and ultraviolet imaging. After that, it was analysed by the scanning electron microscope (SEM) with an EDX unit and Fourier transformed infrared spectroscopy (FT-IR). Then, the conservation processes were performed. The conservation of the glass vessel included disassembling the previous restoration, cleaning, reassembling, filling and supporting the lost parts, and consolidation. The study concluded important results about the composition of the glass, deterioration factors, and identifying the materials of the previous restoration that were found inappropriate and required removal and conservation. It also clarified the method and importance of using acrylic (paraloid B-44) slides in strengthening the fragility and thinness of glass artifacts.

Under-Seawater Immersion β-Ga2O3 Solar-Blind Ultraviolet Imaging Photodetector with High Photo-to-Dark Current Ratio and Fast Response.

This work displays a photovoltaic solar-blind UV photodetector based on a β-Ga2O3 photoelectrode/simulated seawater (NaCl). The photodetector exhibits extremely high photocurrent (6.70 μA); the responsivity can reach 23.47 mA W-1, and the fastest response rise time is 40 ms under 213 nm illumination at zero bias, the responsivity is 25.10 mA W-1 at 0.8 V, and the photo-to-dark current ratio reaches a maximum of 4663, whose responsivity can be effectively adjusted by changing electrolyte concentration, ensuring a good working stability of this device. In addition, with original seawater as the electrolyte, the detector still achieves a high switching ratio (754) and stable detection under zero bias, demonstrating its capability for practical uses. What's more, we present the capability of the photodetector in seawater imaging. This work provides a method for solar-blind UV detection in seawater, which compensates for the limited detection of most current seawater detectors in the visible band, and can provide certain guidance in the field of seawater detection.

Modeling Alpha Particle-Induced Radioluminescence Using Geant4

"Optical detection of alpha particle emitters in the environment by air radioluminescence is a new technology that enables sensing a radiological threat at safe distances, without putting personnel at risk or contaminating equipment. Radiolumi- nescence detection systems need to be fine-tuned to efficiently capture a substantial number of photons while minimizing the contribution from ambient ultraviolet light. The accurate simulation of radioluminescence, in conjunction with ray tracing, facili- tates the design and optimization of such detection systems. In this work, an application within the Geant4 framework has been developed to simulate radioluminescence pho- tons emitted in the vicinity of accelerated alpha particles and at the surface of alpha ra- dioactive samples. The application relies on existing scintillation physics implemented in Geant4 classes such as G4OpticalPhysics and G4Scintillation, which are used to simulate radioluminescence photons as scintillations produced during the passage of alpha particles through air. The application computes the ultraviolet image of alpha particles accelerated at energies of 5.1 MeV and 8.3 MeV, as well as an extended alpha source, M. Luchkov et al., Nucl. Instrum. Meth. Phys. Res. A 1047, 167895 (2023) [1]. The application enables optimization of experimental setups for various scenarios, such as radiological emergency management, radiological crime scene investigations, or decommissioning of nuclear facilities, thus minimizing the use of costly resources and exposure to radiation."

Inferring the Ionospheric State With the Far Ultraviolet Imager on the Fengyun‐4C Geostationary Satellite: Retrieval Algorithm and Verification

AbstractThe Multiband Ultraviolet Spectrum Imager (MUSI) is an optical remote sensing instrument scheduled to launch on the Fengyun‐4C meteorological satellite in 2024. MUSI is designed to measure the airglow emissions between ∼120 and 160 nm above the East Asia and Pacific region from a geostationary orbit to infer ionospheric parameters. Here we develop a lookup‐table algorithm for retrieving the peak electron density (nmF2) and the Total Electron Content (TEC) from nighttime observations of the OI 135.6 nm emission. The algorithm takes into account the north‐south asymmetry of the equatorial ionization anomaly and the atmospheric variations with solar activity and local time. Analysis of synthetic observations shows that the assumptions made in the algorithm only lead to ∼2%–4% uncertainties in the retrievals under the modeled ionospheric conditions. The algorithm is verified by retrieving nmF2 and TEC from the Global‐Scale Observations of the Limb and Disk (GOLD) mission and comparing with coincided radio measurements. The comparison indicates almost negligible systematic differences (typically less than ∼3%) between the GOLD and the ionosonde nmF2, which are within the error limit of the GOLD retrievals. The accuracy of the GOLD TEC retrievals is shown to be comparable to (or in some sense even better than) that of the measurements from the Global Navigation Satellite System. Several simplified yet robust retrieval methods are also developed, which can be readily implemented for both existing and upcoming missions. Our results promote far ultraviolet (FUV) remote sensing to be a key technology for accurate determination of the ionospheric state.

Probing star formation in five of the most massive spiral galaxies observed through ASTROSAT UltraViolet Imaging Telescope

ABSTRACT We present highly resolved and sensitive imaging of the five nearby massive spiral galaxies (with rotation velocities $\rm \gt 300\, km\, s^{-1}$) observed by the UltraViolet Imaging Telescope onboard India’s multiwavelength astronomy satellite ASTROSAT, along with other archival observations. These massive spirals show a far-ultraviolet star formation rate in the range of ∼ 1.4 – 13.7 ${\rm M}_{\odot } \, \mathrm{yr}^{-1}$ and fall in the ‘Green Valley’ region with a specific star formation rate within ∼ 10−11.5 – 10−10.5 yr−1. Moreover, the mean star formation rate density of the highly resolved star-forming clumps of these objects is in the range 0.011 – 0.098 ${\rm M}_{\odot }\, \mathrm{ yr}^{-1}\, \mathrm{kpc}^{-2}$, signifying localized star formation. From the spectral energy distributions, under the assumption of a delayed star formation model, we show that the star formation of these objects had peaked in the period of ∼ 0.8 – 2.8 Gyr after the ‘Big Bang’ and the object that has experienced the peak sooner after the ‘Big Bang’ show relatively less star-forming activity at z ∼ 0 and falls below the main-sequence relation for a stellar content of $\rm \gtrsim 10^{11} \, {\rm M}_{\odot }$. We also show that these objects accumulated much of their stellar mass in the early period of evolution with ∼ 31 – 42 per cent of the total stellar mass obtained in a time of (1/16) – (1/5)th the age of the Universe. We estimate that these massive objects convert their halo baryons into stars with efficiencies falling between ∼ 7 and 31 per cent.

Ultraviolet photo-memory with image storage functions by controlling carriers’ lifetime

Ultraviolet (UV) photodetectors (PDs) have been widely developed for multifunctional applications such as flame detection, space communication and missile detection etc. However, limited attention has been paid to developing UV PDs with photo-memory capabilities, despite their enormous potential for applications in the field of imitating human learning and memory. In this work, we propose a ZnS/ZnO/ZnS heterojunction for UV optoelectronic synapse with optoelectronic memory functions. Through the incorporation of a ZnS shell layer, we achieved a substantial enhancement in the near-band photoluminescence of ZnO and a marked improvement in the lifetime of photo-generated carriers. The ZnS/ZnO/ZnS-3 h UV PD demonstrate a responsivity of 3.6 A/W (5 V bias 0.28 mW/cm2 365 nm light.), which is 12 times of that of ZnO UV PD (0.3 A/W). Additionally, by controlling the thickness of ZnS shell, we extend the decay time of ZnS/ZnO/ZnS-3 h UV PD from 95 s (ZnO) to 378 s, indicating its potential for UV image storage. Also, it shows versatile synaptic functions, including photonic potentiation, paired-pulse facilitation, short-/long-term memory, as well as a “learning” behavior. These results show that UV PDs with long decay time have new potential applications in UV image storage.

Three new identifications of extended UV emission around AGB stars

ABSTRACT Asymptotic giant branch (AGB) stars experience heavy episodes of mass-loss through a slow stellar wind during the thermal pulse phase that form large, pc-scale structures around them. As the AGB stellar wind interacts with the interstellar medium (ISM), the otherwise isotropic ejecta gets distorted, resulting in asymmetric shapes, bow-shock structures and, in the case of fast motion relative to the surrounding ISM, extended wakes and tails as unexpectedly detected in GALEX ultraviolet (UV) images of o Cet, also known as Mira. Since that discovery, another fourteen AGB stars have been reported to exhibit extended UV emission around them. Here, we present the discovery of extended UV emission around another three AGB stars, namely R For, R Hor, and DM Tuc. The analysis of the overall properties of the regions of extended UV emission indicates that these are preferentially detected in closer AGB stars far away from the Galactic plane, i.e. sources less extincted. Faster AGB stars tend to have more elongated shapes, with bow-shocks and wakes or tails of material, while AGB stars farther away from the Galactic plane tend to be surrounded by larger regions of UV emission.

Radioluminescence mapping of 241Am-doped environmental samples and nuclear materials

In the framework of the project RemoteALPHA, several radioluminescence scanning systems were developed to establish the metrological basis for the optical detection of alpha-emitting radionuclides. This work demonstrates an application of the ultraviolet imaging system for the characterization of low-activity alpha sources: contaminated environmental samples, and nuclear materials featuring depleted uranium. The measurements were performed at a reference distance of 2 m from the source. The weakest samples were measured in a chamber with an N2 + NO gas atmosphere. Using the scanning method, the total surface activity was calculated for analyzed samples, with the lowest reported value of just 51 Bq.