Helium Emission Lines Research Articles

SN 2021foa: The “Flip-flop” Type IIn/Ibn Supernova

We present a comprehensive analysis of the photometric and spectroscopic evolution of SN 2021foa, unique among the class of transitional supernovae for repeatedly changing its spectroscopic appearance from hydrogen-to-helium-to-hydrogen dominated (IIn-to-Ibn-to-IIn) within 50 days past peak brightness. The spectra exhibit multiple narrow (≈300–600 km s−1) absorption lines of hydrogen, helium, calcium, and iron together with broad helium emission lines with a full width at half-maximum (FWHM) of ∼6000 km s−1. For a steady, wind mass-loss regime, light-curve modeling results in an ejecta mass of ∼8 M ⊙ and circumstellar material (CSM) mass below 1 M ⊙, and an ejecta velocity consistent with the FWHM of the broad helium lines. We obtain a mass-loss rate of ≈2 M ⊙ yr−1. This mass-loss rate is 3 orders of magnitude larger than derived for normal Type II supernovae. We estimate that the bulk of the CSM of SN 2021foa must have been expelled within half a year, about 12 yr ago. Our analysis suggests that SN 2021foa had a helium-rich ejecta that swept up a dense shell of hydrogen-rich CSM shortly after explosion. At about 60 days past peak brightness, the photosphere recedes through the dense ejecta-CSM region, occulting much of the redshifted emission of the hydrogen and helium lines, which results in an observed blueshift (∼−3000 km s−1). Strong mass-loss activity prior to explosion, such as those seen in SN 2009ip-like objects and SN 2021foa as precursor emission, are the likely origin of a complex, multiple-shell CSM close to the progenitor star.

ExoplaNeT accRetion mOnitoring sPectroscopic surveY (ENTROPY)

Context. Accretion among planetary mass companions is a poorly understood phenomenon, due to the lack of both observational and theoretical studies. The detection of emission lines from accreting gas giants facilitates detailed investigations into this process. Aims. This work presents a detailed analysis of Balmer lines from one of the few known young, planetary-mass objects with observed emission, the isolated L2γ dwarf 2MASS J11151597+1937266 with a mass between 7 and 21 MJup and an age of 5–45 Myr, located at 45 ± 2 pc. Methods. We obtained the first high-resolution (R ~ 50 000) spectrum of the target with VLT/UVES, an echelle spectrograph operating in the near-ultraviolet to visible wavelengths (3200–6800 Å). Results. We report several resolved hydrogen (H I; H3–H6) and helium (He I; λ5875.6) emission lines in the spectrum. Based on the asymmetric line profiles of Hα and Hβ, the 10% width of Hα (199 ± 1 km s−1), tentative He I λ6678 emission, and indications of a disk from mid-infrared excess, we confirm ongoing accretion at this object. Using the Gaia update of the parallax, we revise its temperature to 1816 ± 63 K and radius to 1.5 ± 0.1 RJup. Analysis of observed H I profiles using a 1D planet-surface shock model implies a pre-shock gas velocity, v0 = 120−40+ 80 km s−1, and a pre-shock density, log(n0/cm−3) = 14−5+ 0. The pre-shock velocity points to a mass, Mp = 6−4+ 8 MJup, for the target. Combining H I line luminosities (Lline) and planetary Lline−Lacc (accretion luminosity) scaling relations, we derived a mass accretion rate, Ṁacc = 1.4−0.9+ 2.8 × 10−8 MJup yr−1. Conclusions. The line-emitting area predicted from the planet-surface shock model is very small (~0.03%), and points to a shock at the base of a magnetospherically induced funnel. The Hα profile exhibits a much stronger flux than predicted by the model that best fits the rest of the H I profiles, indicating that another mechanism than shock emission contributes to the Hα emission. Comparison of line fluxes and Ṁacc from archival moderate-resolution SDSS spectra indicate variable accretion at 2MASS J11151597+1937266.

Discovery of the magnetic cataclysmic variable XMM J152737.4–205305.9 with a deep eclipse-like feature

We report the identification and subsequent examination of a polar-type cataclysmic variable named XMM J152737.4−205305.9 newly discovered with the X-ray Multi-Mirror Mission (XMM-Newton). This discovery was made by matching the XMM-Newton data archive with the cataclysmic variable candidate catalog provided by Gaia Data Release 3 (DR3). The use of X-ray photometry led to the identification of two distinct dips that exhibit a recurring pattern with a precise period of 112.4 (1) min in two XMM-Newton observations made one year apart. The data obtained from photometry provided by the Zwicky Transient Facility (ZTF) and the Asteroid Terrestrial-impact Last Alert System (ATLAS) survey consistently indicate the presence of mass-accretion states that differ by up to 2 mag. Following the optical data, the Spectrum Roentgen Gamma(SRG)/eROSITA All Sky Survey observed the system at two different X-ray levels, which may imply different accretion states. Following these observations, the low-resolution spectrum obtained using SALT spectroscopy exposes the prominent hydrogen Balmer and helium emission lines, strongly supporting the categorization of this system as a polar-type magnetic cataclysmic variable. The XMM-Newton observations conducted at various X-ray levels reveal a consistent pattern of a deep dip-like feature with a width of ≈9.1 min. This feature implies the presence of an eclipse in both observations. According to Gaia data, the object is located at a distance of 1156−339+720 pc, and its X-ray luminosity lies within the LX = (3 − 6)×1031 erg s−1 range.

Application of helium line intensity ratio spectroscopy to xenon plasma in E × B Penning discharge

We propose the application of helium line intensity ratio spectroscopy in a low-pressure (0.3 mTorr) xenon E × B discharge with an electron temperature of ∼2 eV and a density of 1010−1011cm−3 . We successfully identified the helium atom line emissions at 388.9, 447.1, 501.6, 504.8, and 706.5 nm with helium pressures of up to ∼20 mTorr. The measured electron temperature, density, and I−V characteristics of the discharge remained almost constant in all helium pressures in the present experiment, indicating the suitability of the helium gas as a diagnostic gas. The results of helium line intensity ratio spectroscopy using the line emissions at 388.9, 447.1, and 504.8 nm showed fair agreement with the Langmuir probe measurement. Considering the trade-off relationship between the disturbance introduced by the helium gas and the signal-to-noise ratio, we conclude that a helium pressure of approximately 4 mTorr (approximately 13 times the partial pressure of xenon) represents the optimal pressure range for the application of the helium line emission intensity ratio method to this xenon plasma. It is found that the use of the line emissions at 501.6 and 706.5 nm result in a significant disturbance in the helium line intensity ratio method due to the radiation trapping effect.

Revisiting the Helium and Hydrogen Accretion Indicators at TWA 27B: Weak Mass Flow at Near-freefall Velocity

TWA 27B (2M1207b) is the first directly imaged planetary-mass (M p ≈ 5 M J) companion and was observed at 0.9–5.3 μm with ​​​JWST/NIRSpec. To understand the accretion properties of TWA 27B, we search for continuum-subtracted near-infrared helium and hydrogen emission lines and measure their widths and luminosities. We detect the He i triplet at 4.3σ and all Paschen-series lines covered by NIRSpec (Paα, Paβ, Paγ, Paδ) at 4σ–5σ. The three brightest Brackett-series lines (Brα, Brβ, Brγ) as well as Pfγ and Pfδ are tentative detections at 2σ–3σ. We provide upper limits on the other hydrogen lines, including on Hα through Hubble Space Telescope archival data. Three lines can be reliably deconvolved to reveal an intrinsic width Δv intrsc = (67 ± 9) km s−1, which is 60% of the surface freefall velocity. The line luminosities seem significantly too high to be due to chromospheric activity. Converting line luminosities to an accretion rate yields Ṁ≈5×10−9MJyr−1 when using scaling relationships for planetary masses, and Ṁ≈0.1×10−9MJyr−1 with extrapolated stellar scalings. Several of these lines represent the first detections at an accretor of such low mass. The weak accretion rate implies that formation is likely over. This analysis shows that JWST can be used to measure low line-emitting mass accretion rates onto planetary-mass objects, motivates deeper searches for the mass reservoir feeding TWA 27B, and hints that other young directly imaged objects might—hitherto unbeknownst—also be accreting.

Development of Zeeman Split-Assisted Tomography for Helium Line Emission Distribution in LHD

Development of Zeeman Split-Assisted Tomography for Helium Line Emission Distribution in LHD

Evidence for inflows and outflows in the nearby black hole transient Swift J1727.8−162

We present 20 epochs of optical spectroscopy obtained with the GTC-10.4m telescope across the bright discovery outburst of the black hole candidate Swift J1727.8−162. The spectra cover the main accretion states and are characterised by the presence of hydrogen and helium emission lines, commonly observed in these objects. They show complex profiles, including double peaks, but also blue-shifted absorptions (with blue-edge velocities of 1150 km s−1), broad emission wings, and flat-top profiles, which are the usual signatures of accretion disc winds. Moreover, red-shifted absorptions accompanied by blue emission excesses suggest the presence of inflows in at least two epochs, although a disc origin cannot be ruled out. Using pre-outburst imaging from Pan-STARRS, we identify a candidate quiescent optical counterpart with a magnitude of g ∼ 20.8. This implies an outburst optical amplitude of ΔV ∼ 7.7, supporting an estimated orbital period of ∼7.6 h, which favours an early K-type companion star. Employing various empirical methods, we derive a distance to the source of d = 2.7 ± 0.3 kpc, corresponding to a Galactic plane elevation of z = 0.48 ± 0.05 kpc. Based on these findings, we propose that Swift J1727.8−162 is a nearby black hole X-ray transient that exhibited complex signatures of optical inflows and outflows throughout its discovery outburst.

A joint SRG/eROSITA + ZTF search: Discovery of a 97-min period eclipsing cataclysmic variable with evidence of a brown dwarf secondary

ABSTRACT Cataclysmic variables (CVs) that have evolved past the period minimum during their lifetimes are predicted to be systems with a brown dwarf donor. While population synthesis models predict that around 40–70 per cent of the Galactic CVs are post-period minimum systems referred to as ‘period bouncers’, only a few dozen confirmed systems are known. We report the study and characterization of a new eclipsing CV, SRGeJ041130.3+685350 (SRGeJ0411), discovered from a joint SRG/eROSITA and ZTF programme. The optical spectrum of SRGeJ0411 shows prominent hydrogen and helium emission lines, typical for CVs. We obtained optical high-speed photometry to confirm the eclipse of SRGeJ0411 and determine the orbital period to be Porb ≈ 97.530 min. The spectral energy distribution suggests that the donor has an effective temperature of ≲ 1800 K. We constrain the donor mass with the period–density relationship for Roche lobe-filling stars and find that Mdonor ≲ 0.04 M⊙. The binary parameters are consistent with evolutionary models for post-period minimum CVs, suggesting that SRGeJ0411 is a new period bouncer. The optical emission lines of SRGeJ0411 are single-peaked despite the system being eclipsing, which is typically only seen due to stream-fed accretion in polars. X-ray spectroscopy hints that the white dwarf in SRGeJ0411 could be magnetic, but verifying the magnetic nature of SRGeJ0411 requires further investigation. The lack of optical outbursts has made SRGeJ0411 elusive in previous surveys, and joint X-ray and optical surveys highlight the potential for discovering similar systems in the near future.

Monitoring broad emission-line components in spectra of the two low-metallicity dwarf compact star-forming galaxies SBS 1420+540 and J1444+4840

ABSTRACT We report the discovery of broad components with P-Cygni profiles of the hydrogen and helium emission lines in the two low-redshift low-metallicity dwarf compact star-forming galaxies SBS 1420+540 and J1444+4840. We found small stellar masses of 106.24 and 106.59 M⊙, low oxygen abundances 12 + log O/H of 7.75 and 7.45, high velocity dispersions reaching σ ∼ 700 and ∼1200 km s−1, high terminal velocities of the stellar wind of ∼1000 and ∼1000–1700 km s−1, respectively, and large EW(H β) of ∼300 Å for both. For SBS 1420+540, we succeeded in capturing an eruption phase by monitoring the variations of the broad-to-narrow component flux ratio. We observe a sharp increase of that ratio by a factor of 4 in 2017 and a decrease by about an order of magnitude in 2023. The peak luminosity of ∼1040 erg s−1 of the broad component in L(H α) lasted for about 6 yr out of a three-decades monitoring. This leads us to conclude that there is probably a luminous blue variable candidate (LBVc) in this galaxy. As for J1444+4840, its very high L(H α) of about 1041 ergs s−1, close to values observed in active galactic nuclei (AGNs) and Type IIn supernovae (SNe), and the variability of no more than 20 per cent of the broad-to-narrow flux ratio of the hydrogen and helium emission lines over a 8 yr monitoring do not allow us to definitively conclude that it contains an LBVc. On the other hand, the possibility that the line variations are due to a long-lived stellar transient of type AGN/SN IIn cannot be ruled out.

Structure of the accretion flow of IX Velorum as revealed by high-resolution spectroscopy

Context. Several high mass-transfer cataclysmic variables show evidence of outflow from the system, which could play an important role in their evolution. We investigate the system IX Vel, which was proposed to show similar characteristics. Aims. We study the structure of the IX Vel system, particularly the structure of its accretion flow and accretion disc. Methods. We used high-resolution time-resolved spectroscopy to construct radial velocity curves of the components in IX Vel. We computed Doppler maps of the system, which we used to estimate the temperature distribution maps. Results. We have improved the spectroscopic ephemeris of the system and its orbital period Porb = 0.19392793(3) d. We constructed Doppler maps of the system based on hydrogen and helium emission lines and the Bowen blend. The maps show features corresponding to the irradiated face of the secondary star, the outer rim of the accretion disc, and low-velocity components located outside the accretion disc and reaching towards L3. We constructed a temperature distribution map of the system using the Doppler maps of Balmer lines. Apart from the features found in the Doppler maps, the temperature distribution map shows a region of high temperature in the accretion disc connecting the expected position of a bright spot and the inner parts of the disc. Conclusions. We interpret the low-velocity emission found in the Doppler map as emission originating in the accretion disc wind and in an outflow region located in the vicinity of the third Lagrangian point L3. This makes IX Vel a member of the RW Sex class of cataclysmic variables.

Nitrogen enhancements 440 Myr after the big bang: supersolar N/O, a tidal disruption event, or a dense stellar cluster in GN-z11?

ABSTRACT Recent observations of GN-z11 with JWST/NIRSpec revealed numerous oxygen, carbon, nitrogen, and helium emission lines at z = 10.6. Using the measured line fluxes, we derive abundance ratios of individual elements within the interstellar medium (ISM) of this superluminous galaxy. Driven by the unusually-bright N iii] λ1750 and N iv] λ1486 emission lines (and by comparison, faint O iii] λλ1660, 1666 lines), our fiducial model prefers log (N/O) > −0.25, greater than four times solar and in stark contrast to lower-redshift star-forming galaxies. The derived log (C/O) > −0.78, (≈30 per cent solar) is also elevated with respect to galaxies of similar metallicity (12 + log (O/H) ≈ 7.82), although less at odds with lower-redshift measurements. We explore the feasibility of achieving these abundance ratios via several enrichment mechanisms using metal yields available in the literature. Given the long time-scale typically expected to enrich nitrogen with stellar winds, traditional scenarios require a very fine-tuned formation history to reproduce such an elevated N/O. We find no compelling evidence that nitrogen enhancement in GN-z11 can be explained by enrichment from metal-free Population III stars. Interestingly, yields from runaway stellar collisions in a dense stellar cluster or a tidal disruption event provide promising solutions to give rise to these unusual emission lines at z = 10.6, and explain the resemblance between GN-z11 and a nitrogen-loud quasar. These recent observations showcase the new frontier opened by JWST to constrain galactic enrichment and stellar evolution within 440 Myr of the big bang.

Identification of a Helium Donor Star in NGC 247 ULX-1

With Very Large Telescope Multi Unit Spectroscopic Explorer (MUSE) observations, we detected highly variable helium emission lines from the optical counterpart of the supersoft ultraluminous X-ray source (ULX) NGC 247 ULX-1. No Balmer lines can be seen in the source spectrum. This is the first evidence for the presence of a helium donor star in ULXs, consistent with a prediction that helium donor stars may be common in ULXs. The helium lines with an FWHM of about 200 km s−1 are likely produced on the outer accretion disk. Their strong variation implies that the central X-ray source can be significantly obscured to the outer disk. Also, a ring or a double-ring structure is revealed in the MUSE image. It is unknown whether or not it is related to the progenitor of the ULX binary.

Use of machine learning for a helium line intensity ratio method in Magnum-PSI

Optical emission spectroscopy (OES) of helium (He) line intensities has been used to measure the electron density, n e , and temperature, T e , in various plasma devices. In this study, a neural network with five hidden layers is introduced to model the relation between the OES data and n e / T e from laser Thomson scattering in the linear plasma device Magnum-PSI and compared to multiple regression analysis. It is shown that the neural network reduces the residual errors of prediction values ( n e and T e ) less than half those of the multiple regression analysis in the ranges of 2 × 10 18 < n e < 8 × 1 0 20 m −3 and 0 . 1 < T e < 4 eV . We checked two different data splitting methods for training and validation data, i.e., with and without considering the unit of discharge. A comparison of the splitting methods suggests that the residual error will decrease to ∼ 10% even for a new discharge data when accumulating a sufficient data set. • The relation between the helium line emission and n e /T e from laser Thomson scattering in the linear plasma device Magnum-PSI are modeled. • A neural network (NN) with five hidden layers is introduced to model and compared to the multiple regression (MR) analysis. • The NN reduces the residual errors of prediction values less than half those of MR. • The residual error can be decreased to ̃10% by accumulating sufficient data set.

Infrared spectroscopy of the 2019 eruption of the recurrent nova V3890 Sgr: Separation into equatorial and polar winds revealed

ABSTRACT We present infrared spectroscopy of the 2019 eruption of the recurrent nova V3890 Sgr, obtained over the period of 5.1–46.3 d after the eruption. The spectrum of the red giant became more prominent as the flux declined, and by day 46.3 dominated the spectrum. Hydrogen and helium emission lines consisted of a narrow component superposed on a broad pedestal. The full width at half-maximum of the narrow components declined with time t as the eruption progressed, as t−0.74, whereas those of the broad components remained essentially constant. Conversely, the line fluxes of the narrow components of Pa β remained roughly constant, while those of the broad components declined by a factor ∼30 over a period of ≲ 25 d. The behaviour of the broad components is consistent with them arising in unencumbered fast-flowing ejecta perpendicular to the binary plane, in material that was ejected in a short ∼3.3-d burst. The narrow components arise in material that encounters the accumulated circumstellar material. The outburst spectra were rich in coronal lines. There were two coronal line phases, one that originated in gas ionized by supersoft X-ray source, the other in shocked gas. From the relative fluxes of silicon and sulphur coronal lines on day 23.4 – when the emitting gas was shocked – we deduce that the temperature of the coronal gas was 9.3 × 105 K, and that the abundances are approximately solar.

Characterization of helium microplasma generated in a flow focusing microfluidic device

Non-thermal microplasmas produced in a microchannel have several potential applications in analytical chemistry, environmental sensing, and surface modification of microfluidic chips for biomedical and lab-on-chip devices. This paper investigates the properties of an atmospheric pressure helium microplasma excited in a polydimethylsiloxane flow focusing microfluidic chip. The influence of input parameters such as applied voltage and gas flow rate on discharge characteristics is investigated in detail. Electron excitation and molecular rotational temperatures are determined with the Boltzmann plot technique. The rotational temperature from the N2+ emission band was calculated in the range of 348–417 K. Electron density and temperature are determined using the well-known plasma diagnostic technique of Stark broadening. The emission lines of hydrogen Balmer (Hα) and neutral helium (501, 667, and 728 nm) are selected to measure the parameters of Stark broadening. The electron density and electron temperature were found to be in the range 0.7×1016–3.39×1016 cm−3 and 10 800–12 493 K, respectively. The evaluated discharge parameters validate the non-thermal equilibrium state of the microplasma. The electrical diagnostics of plasma were performed by monitoring the signals of high voltage and current of the discharge. Moreover, the plasma modified surface (hydrophobic to hydrophilic) was verified by successfully utilizing the microchannel to form an oil-in-water micro-emulsion.

TUVO-21acq: A new cataclysmic variable discovered through a UV outburst

Outbursts from cataclysmic variables, such as dwarf novae (DNe), are prevalent throughout the galaxy and are known to emit strongly in the ultraviolet (UV). However, the UV emission of DN outbursts has not been studied extensively compared with the optical. Detailed characterisation of the physical processes responsible for outburst behaviour requires further UV data, because the UV probes the inner regions of the accretion disc. Here we report, as part of our recently launched Transient UV Objects (TUVO) project, the discovery of TUVO-21acq, a new transient which we detected in the UV using data from the Ultraviolet Optical Telescope (UVOT) aboard Swift. We detected two separate outbursts and used the UVOT data to constrain source properties, focusing on the amplitudes and timescales of the outbursts. We found that during the first outburst the source increased in brightness by at least 4.1, 2.4, and 3.5 mag and during the second outburst by 4.4, 3.4, and 3.6 mag in the UVW1, UVM2, and UVW2 bands, respectively. The outburst durations were in the range of 6–21 days and 11–46 days, and we determined an upper limit for the recurrence time of 316 days. To further characterise the source, we obtained an optical spectrum during quiescence with the Southern African Large Telescope (SALT). The spectrum exhibited hydrogen Balmer series and helium emission lines, and a flat overall spectral shape. We confirm the nature of the source as an accreting white dwarf which underwent DN outburst based on photometric and spectroscopic properties. This study serves as a proof of concept for the TUVO project strategy, demonstrating that it has the capability of discovering and classifying new, interesting UV transients. We also discuss the implications of our findings for our understanding of the physics underlying DN outbursts, in particular with respect to the UV emission. We examine the need for simultaneous UV and optical observations during the onset of DN outbursts in order to help answer remaining questions as to the characteristics and implications of the UV delay, for example.

A comprehensive chemical abundance analysis of the extremely metal poor Leoncino Dwarf galaxy (AGC 198691)

ABSTRACT We re-examine the extremely metal-poor dwarf galaxy AGC 198691 using a high quality spectrum obtained by the LBT’s MODS instrument. Previous spectral observations obtained from KOSMOS on the Mayall 4-m and the Blue channel spectrograph on the MMT 6.5-m telescope did not allow for the determination of sulfur, argon, or helium abundances. We report an updated and full chemical abundance analysis for AGC 198691, including confirmation of the extremely low “direct” oxygen abundance with a value of 12 + log (O/H) = 7.06 ± 0.03. AGC 198691’s low metallicity potentially makes it a high value target for helping determine the primordial helium abundance (Yp). Though complicated by a Na i night sky line partially overlaying the He i λ5876 emission line, the LBT/MODS spectrum proved adequate for determining AGC 198691’s helium abundance. We employ the recently expanded and improved model of Aver et al., incorporating higher Balmer and Paschen lines, augmented by the observation of the infrared helium emission line He i λ10830 obtained by Hsyu et al. Applying our full model produced a reliable helium abundance determination, consistent with the expectation for its metallicity. Although this is the lowest metallicity object with a detailed helium abundance, unfortunately, due to its faintness [EW(Hβ) &amp;lt; 100 Å] and the compromised He i λ5876, the resultant uncertainty on the helium abundance is too large to allow a significant improvement on the measurement of Yp.

Reconfiguration of an Electrothermal-Arc Plasma Source for In Situ PMI Studies

An electrothermal-arc plasma source (ET-Arc) has been developed to produce transient plasma heat and particle fluxes similar to those produced by edge localized modes onto divertor plasma-facing components in tokamaks. The ET-Arc utilizes a capacitive discharge to send current through a 4-mm-diameter, 9-cm-long capillary source liner. The liner material is ablated to form a high-velocity plasma jet that impacts the target downstream. With the current discharge circuit configuration, pulse lengths are 1 to 2 ms in duration and deliver heat fluxes of 0.25 to 2.1 GW m−2. The plasma was previously characterized with optical emission spectroscopy (OES) on helium emission lines. The He I line ratios were interpreted with collisional radiative analysis to calculate ne and Te. The electron temperature and electron density ranged from Te = 1 to 5 eV and ne = 1022 to 1028 electrons/m3, respectively. Recently, the vacuum configuration and target of the ET-Arc device were modified to allow greater diagnostic access for plasma-material interaction (PMI) studies and diagnostic development. The diagnostic suite included two Tektronix high-voltage probes to measure the capacitor and discharge potentials, a discharge current monitor, Edgertronic SC1 high-speed cameras to image the discharge, and a FLIR SC4000 infrared camera to estimate heat flux on the target. The system used OES for plasma characterization, but a new Thomson scattering (TS) diagnostic has been implemented. This system is an Advanced Research Projects Agency - Energy (ARPA-E)-funded, portable diagnostic package for spectroscopic measurements of ne, Te, ni, Ti,, and vi, which includes both TS and OES. Additionally, a novel digital holography (DH) surface-imaging diagnostic was implemented to measure erosion rates in situ. Results from ex situ DH characterization of stainless steel targets exposed to the ET-Arc source indicated that surface erosion of ~150 nm per shot occurred and an in situ DH characterization of similar targets was planned. The arc-triggering system will be revised and optimized to better synchronize with the laser diagnostics. Details of the reconfigured ET-Arc source are reported here.

A Possible Tidal Disruption Event Candidate in the Black Hole Binary System of OJ 287

Abstract The BL Lacertae OJ 287 is a supermassive black hole binary system with complex physics of its irregular flares. During 2016 October–2017 April, a surprising outburst in the X-ray, UV, and optical bands was detected, while no variability was seen in the γ-ray light curve. During the outburst, the X-ray light curves were dominated by the soft X-rays, whose peak in luminosity was ∼1046 erg s−1—more than 10 times higher than the mean level before the outburst—and a “softer-when-brighter” phenomenon was exhibited. These above phenomena have been reported by some previous works. The hardness ratio showed negligible evolution with flare time and soft X-ray luminosity. Critically, the luminosity of the soft X-rays decayed following a power law of t −5/3, which occurs in most tidal disruption events (TDEs), and a similar trend can be seen in the UV and optical bands during the soft X-ray declining period. Helium and oxygen narrow emission lines are strengthened prominently in the optical spectra of postoutburst epochs, that could be attributed to the surrounding gas caused by TDE. We discuss three possible origins of the event, including the jet’s precession, the aftereffects of the black hole–disk impaction, and the TDE. Our results show that the TDE is the more likely scenario to explain the outburst.

NORMAN ROBERT POGSON AND OBSERVATIONS OF THE TOTAL SOLAR ECLIPSE OF 1868 FROM MASULIPATAM, INDIA

Norman Pogson's observations of the 1868 total solar eclipse were significant in many respects. He not only distinguished the helium emission line from the Fraunhofer sodium D line, but also discovered the FeXIV green line. Until recently, he was never properly credited with these discoveries by historians of astronomy. In this paper, we discuss in detail the observations made by Pogson and his small team at Masulipatam, after describing the background of their preparation for the expedition. We also describe how they were helped by some and sidelined by others, and how various factors led to the discoveries that they being hidden from colleagues. It was only with the publication of Nath's book <italic>The Story of Helium and the Birth of Astrophysics</italic> in 2013 that Pogson's achievements in 1868 became widely known and were able to take their rightful place in the annals of science.