arXiv subjects

S. J. Smartt

Publications and source records attributed to S. J. Smartt.

At least 73 records · Page 4Linked to original sources

arXiv2022

Modelling the spectra of the kilonova AT2017gfo -- I: The photospheric epochs

The kilonova (KN) associated with the binary neutron star (BNS) merger GW170817 is the only known electromagnetic counterpart to a gravitational wave source. Here we produce a sequence of radiative transfer models (using $\textsc{tardis}$) with updated atomic data, and compare them to accurately calibrated spectra. We use element compositions from nuclear network calculations based on a realistic hydrodynamical simulation of a BNS merger. We show that the blue spectrum at +1.4 days after merger requires a nucleosynthetic trajectory with a high electron fraction. Our best-fitting model is composed entirely of first $r$-process peak elements (Sr & Zr) and the strong absorption feature is reproduced well by Sr$\,\textsc{ii}$ absorption. At this epoch, we set an upper limit on the lanthanide mass fraction of $X_{\textsc{ln}} \lesssim 5 \times 10^{-3}$. In contrast, all subsequent spectra from $+2.4 - 6.4$ days require the presence of a modest amount of lanthanide material ($X_{\textsc{ln}} \simeq 0.05^{+0.05}_{-0.02}$), produced by a trajectory with $Y_{\rm e} = 0.29$. This produces lanthanide-induced line blanketing below 6000$\,$Å, and sufficient light $r$-process elements to explain the persistent strong feature at $\sim 0.7 - 1.0 \, \micron$ (Sr$\,\textsc{ii}$). The composition gives good matches to the observed data, indicating that the strong blue flux deficit results in the near-infrared (NIR) excess. The disjoint in composition between the first epoch and all others indicates either ejecta stratification, or the presence of two distinct components of material. This further supports the `two-component' kilonova model, and constrains the element composition from nucleosynthetic trajectories. The major uncertainties lie in availability of atomic data and the ionisation state of the expanding material.

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arXiv2022

Progenitor, environment, and modelling of the interacting transient, AT 2016jbu (Gaia16cfr)

We present the bolometric lightcurve, identification and analysis of the progenitor candidate, and preliminary modelling of AT2016jbu (Gaia16cfr). We find a progenitor consistent with a $\sim$22--25~$M_{\odot}$ yellow hypergiant surrounded by a dusty circumstellar shell, in agreement with what has been previously reported. We see evidence for significant photometric variability in the progenitor, as well as strong H$α$ emission consistent with pre-existing circumstellar material. The age of the environment as well as the resolved stellar population surrounding AT2016jbu, support a progenitor age of $>$10 Myr, consistent with a progenitor mass of $\sim$22~$M_{\odot}$. A joint analysis of the velocity evolution of AT2016jbu, and the photospheric radius inferred from the bolometric lightcurve shows the transient is consistent with two successive outbursts/explosions. The first outburst ejected material with velocity $\sim$650$kms^{-1}$, while the second, more energetic event, ejected material at $\sim$4500$kms^{-1}$. Whether the latter is the core-collapse of the progenitor remains uncertain. We place a limit on the ejected $^{56}$Ni mass of $<$0.016$M_{\odot}$. Using the BPASS code, we explore a wide range of possible progenitor systems, and find that the majority of these are in binaries, some of which are undergoing mass transfer or common envelope evolution immediately prior to explosion. Finally, we use the SNEC code to demonstrate that the low-energy explosion within some of these binary systems, together with sufficient CSM, can reproduce the overall morphology of the lightcurve of AT2016jbu.

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arXiv2022

Photometric and spectroscopic evolution of the interacting transient AT 2016jbu (Gaia16cfr)

We present the results from a high cadence, multi-wavelength observation campaign of AT 2016jbu (aka Gaia16cfr), an interacting transient. This dataset complements the current literature by adding higher cadence as well as extended coverage of the lightcurve evolution and late-time spectroscopic evolution. Photometric coverage reveals that AT 2016jbu underwent significant photometric variability followed by two luminous events, the latter of which reached an absolute magnitude of M$_V\sim$-18.5 mag. This is similar to the transient SN 2009ip whose nature is still debated. Spectra are dominated by narrow emission lines and show a blue continuum during the peak of the second event. AT 2016jbu shows signatures of a complex, non-homogeneous circumstellar material (CSM). We see slowly evolving asymmetric hydrogen line profiles, with velocities of 500km$s^{-1}$ seen in narrow emission features from a slow moving CSM, and up to 10,000km$s^{-1}$ seen in broad absorption from some high velocity material. Late-time spectra ($\sim$+1 year) show a lack of forbidden emission lines expected from a core-collapse supernova and are dominated by strong emission from H, He i and Ca ii. Strong asymmetric emission features, a bumpy lightcurve, and continually evolving spectra suggest an inhibit nebular phase. We compare the evolution of H$α$ among SN 2009ip-like transients and find possible evidence for orientation angle effects. The light-curve evolution of AT 2016jbu suggests similar, but not identical, circumstellar environments to other SN 2009ip-like transients.

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arXiv2022

Target of Opportunity Observations of Gravitational Wave Events with Vera C. Rubin Observatory

The discovery of the electromagnetic counterpart to the binary neutron star merger GW170817 has opened the era of gravitational-wave multi-messenger astronomy. Rapid identification of the optical/infrared kilonova enabled a precise localization of the source, which paved the way to deep multi-wavelength follow-up and its myriad of related science results. Fully exploiting this new territory of exploration requires the acquisition of electromagnetic data from samples of neutron star mergers and other gravitational wave sources. After GW170817, the frontier is now to map the diversity of kilonova properties and provide more stringent constraints on the Hubble constant, and enable new tests of fundamental physics. The Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) can play a key role in this field in the 2020s, when an improved network of gravitational-wave detectors is expected to reach a sensitivity that will enable the discovery of a high rate of merger events involving neutron stars (about tens per year) out to distances of several hundred Mpc. We design comprehensive target-of-opportunity observing strategies for follow-up of gravitational-wave triggers that will make the Rubin Observatory the premier instrument for discovery and early characterization of neutron star and other compact object mergers, and yet unknown classes of gravitational wave events.

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arXiv2022

Next Generation Observatories -- Report from the Dawn VI Workshop; October 5-7 2021

The workshop Dawn VI: Next Generation Observatories took place online over three days, 5-7 October, 2021. More than 200 physicists and astronomers attended to contribute to, and learn from, a discussion of next-generation ground-based gravitational-wave detectors. The program was centered on the next generation of ground-based gravitational-wave observatories and their synergy with the greater landscape of scientific observatories of the 2030s. Cosmic Explorer (CE), a concept developed with US National Science Foundation support, was a particular focus; Einstein Telescope (ET), the European next generation concept, is an important complement and partner in forming a network. The concluding summary of the meeting expressed the sentiment that the observational science accessible to CE and ET, also in combination with data from other non-GW observatories, will stimulate a very broad community of analysts and yield insights which are exciting given the access to GWs from the entire universe. The need, and desire, for closer collaboration between ET and CE was expressed; a three-detector network is optimal for delivering much of the science. The science opportunities afforded by CE and ET are broad and compelling, impacting a wide range of disciplines in physics and high energy astrophysics. There was a consensus that CE is a concept that can deliver the promised science. A strong endorsement of Cosmic Explorer, as described in the CE Horizon Study, is a primary outcome of DAWN VI.

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arXiv2022

SN 2020kyg and the rates of faint Iax Supernovae from ATLAS

We present multi-wavelength follow-up observations of the ATLAS discovered faint Iax supernova SN 2020kyg that peaked at an absolute magnitude of $M_g \approx -14.9 \pm 0.2$, making it another member of the faint Iax supernova population. The bolometric light curve requires only $\approx 7 \times 10^{-3}$ M$_{\odot}$ of radioactive $^{56}$Ni, with an ejected mass of $M_{\rm ej} \sim 0.4$ M$_{\odot}$ and a low kinetic energy of $E \approx 0.05 \pm 0.02 \times 10^{51}$ erg. We construct a homogeneous volume-limited sample of 902 transients observed by ATLAS within 100 Mpc during a 3.5 year span. Using this sample, we constrain the rates of faint Iax ($M_r \gtrsim -16$) events within 60 Mpc at $12^{+14}_{-8}\%$ of the SN Ia rate. The overall Iax rate, at $15^{+17}_{-9}\%$ of the Ia rate, is dominated by the low-luminosity events, with luminous SNe Iax ($M_r \lesssim -17.5$) like 2002cx and 2005hk accounting for only $0.9^{+1.1}_{-0.5}\%$ of the Ia rate (a 2$σ$ upper limit of approximately 3\%). We favour the hybrid CONe WD + He star progenitor channel involving a failed deflagration of a near Chandrasekhar mass white dwarf, expected to leave a bound remnant and a surviving secondary companion, as a candidate explanation for faint Iax explosions. This scenario requires short delay times, consistent with the observed environments of SNe Iax. Furthermore, binary population synthesis calculations have suggested rates of $1-18\%$ of the SN Ia rate for this channel, consistent with our rate estimates.

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arXiv2021

Intermediate-luminosity red transients: Spectrophotometric properties and connection to electron-capture supernova explosions

We present the spectroscopic and photometric study of five intermediate-luminosity red transients (ILRTs), namely AT 2010dn, AT 2012jc, AT 2013la, AT 2013lb, and AT 2018aes. They share common observational properties and belong to a family of objects similar to the prototypical ILRT SN~2008S. These events have a rise time that is less than 15 days and absolute peak magnitudes of between $-11.5$ and $-14.5$ mag. Their pseudo-bolometric light curves peak in the range $0.5$ - $9.0 \times10^{40}~\mathrm{erg~s}^{-1}$ and their total radiated energies are on the order of (0.3 - 3) $\times$~10$^{47}$~erg. After maximum brightness, the light curves show a monotonic decline or a plateau, resembling those of faint supernovae IIL or IIP, respectively. At late phases, the light curves flatten, roughly following the slope of the $^{56}$Co decay. If the late-time power source is indeed radioactive decay, these transients produce $^{56}$Ni masses on the order of $10^{-4}$ to $10^{-3}$~\msun. The spectral energy distribution of our ILRT sample, extending from the optical to the mid-infrared (MIR) domain, reveals a clear IR excess soon after explosion and non-negligible MIR emission at very late phases. The spectra show prominent H lines in emission with a typical velocity of a few hundred km~s$^{-1}$, along with Ca~II features. In particular, the [Ca~II] $λ$7291,7324 doublet is visible at all times, which is a characteristic feature for this family of transients. The identified progenitor of SN~2008S, which is luminous in archival Spitzer MIR images, suggests an intermediate-mass precursor star embedded in a dusty cocoon. We propose the explosion of a super-asymptotic giant branch star forming an electron-capture supernova as a plausible explanation for these events.

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arXiv2021

Gaia Photometric Science Alerts

Since July 2014, the Gaia mission has been engaged in a high-spatial-resolution, time-resolved, precise, accurate astrometric, and photometric survey of the entire sky. Aims: We present the Gaia Science Alerts project, which has been in operation since 1 June 2016. We describe the system which has been developed to enable the discovery and publication of transient photometric events as seen by Gaia. Methods: We outline the data handling, timings, and performances, and we describe the transient detection algorithms and filtering procedures needed to manage the high false alarm rate. We identify two classes of events: (1) sources which are new to Gaia and (2) Gaia sources which have undergone a significant brightening or fading. Validation of the Gaia transit astrometry and photometry was performed, followed by testing of the source environment to minimise contamination from Solar System objects, bright stars, and fainter near-neighbours. Results: We show that the Gaia Science Alerts project suffers from very low contamination, that is there are very few false-positives. We find that the external completeness for supernovae, $C_E=0.46$, is dominated by the Gaia scanning law and the requirement of detections from both fields-of-view. Where we have two or more scans the internal completeness is $C_I=0.79$ at 3 arcsec or larger from the centres of galaxies, but it drops closer in, especially within 1 arcsec. Conclusions: The per-transit photometry for Gaia transients is precise to 1 per cent at $G=13$, and 3 per cent at $G=19$. The per-transit astrometry is accurate to 55 milliarcseconds when compared to Gaia DR2. The Gaia Science Alerts project is one of the most homogeneous and productive transient surveys in operation, and it is the only survey which covers the whole sky at high spatial resolution (subarcsecond), including the Galactic plane and bulge.

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arXiv2021

Hubble spectroscopy of LB-1: comparison with B+black-hole and Be+stripped-star models

LB-1 has variously been proposed as either an X-ray dim B-type star plus black hole (B+BH) binary, or a Be star plus an inflated stripped star (Be+Bstr) binary. The Space Telescope Imaging Spectrograph (STIS) on board HST was used to obtain a flux-calibrated spectrum that is compared with non-LTE spectral energy distributions (SED) and line profiles for the proposed models. The Hubble data, together with the Gaia EDR3 parallax, provide tight constraints on the properties and stellar luminosities of the system. In the case of the Be+Bstr model we adopt the published flux ratio for the Be and Bstr stars, re-determine the T$_{eff}$ of the Bstr using the silicon ionization balance, and infer Teff for the Be star from the fit to the SED. We derive stellar parameters consistent with previous results, but with greater precision enabled by the Hubble SED. While the Be+Bstr model is a better fit to the HeI lines and cores of the Balmer lines in the optical, the B+BH model provides a better fit to the Si iv resonance lines in the UV. The analysis also implies that the Bstr star has roughly twice solar silicon abundance, difficult to reconcile with a stripped star origin. The Be star on the other hand has a rather low luminosity, and a spectroscopic mass inconsistent with its possible dynamical mass. The fit to the UV can be significantly improved by reducing the T$_{eff}$ and radius of the Be star, though at the expense of leading to a different mass ratio. In the B+BH model, the single B-type spectrum is a good match to the UV spectrum. Adopting a mass ratio of 5.1$\pm$0.1 (Liu et al. 2020) implies a BH mass of $\sim$21$^{+9}_{-8}M_{\odot}$.

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arXiv2021

The luminous red nova variety: AT 2020hat and AT 2020kog

We present the results of our monitoring campaigns of the luminous red novae (LRNe) AT 2020hat in NGC 5068 and AT 2020kog in NGC 6106. The two objects were imaged (and detected) before their discovery by routine survey operations. They show a general trend of slow luminosity rise, lasting at least a few months. The subsequent major LRN outbursts were extensively followed in photometry and spectroscopy. The light curves present an initial short-duration peak, followed by a redder plateau phase. AT 2020kog is a moderately luminous event peaking at ~7 x 10^40 erg/s, while AT 2020hat is almost one order of magnitude fainter than AT 2020kog, although it is still more luminous than V838 Mon. In analogy with other LRNe, the spectra of AT 2020kog change significantly with time. They resemble those of type IIn supernovae at early phases, then they become similar to those of K-type stars during the plateau, and to M-type stars at very late phases. In contrast, AT 2020hat already shows a redder continuum at early epochs, and its spectrum shows the late appearance of molecular bands. A moderate-resolution spectrum of AT 2020hat taken at +37 d after maximum shows a forest of narrow P Cygni lines of metals with velocities of 180 km/s, along with an Halpha emission with a full-width at half-maximum velocity of 250 km/s. For AT 2020hat, a robust constraint on its quiescent progenitor is provided by archival images of the Hubble Space Telescope. The progenitor is clearly detected as a mid-K type star, with an absolute magnitude of MF606W = -3.33+-0.09 mag and a colour of F606W-F814W = 1.14+-0.05 mag, which are inconsistent with the expectations from a massive star that could later produce a core-collapse supernova. Although quite peculiar, the two objects nicely match the progenitor versus light curve absolute magnitude correlations discussed in the literature.

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arXiv2021

The Young Supernova Experiment: Survey Goals, Overview, and Operations

Time domain science has undergone a revolution over the past decade, with tens of thousands of new supernovae (SNe) discovered each year. However, several observational domains, including SNe within days or hours of explosion and faint, red transients, are just beginning to be explored. Here, we present the Young Supernova Experiment (YSE), a novel optical time-domain survey on the Pan-STARRS telescopes. Our survey is designed to obtain well-sampled $griz$ light curves for thousands of transient events up to $z \approx 0.2$. This large sample of transients with 4-band light curves will lay the foundation for the Vera C. Rubin Observatory and the Nancy Grace Roman Space Telescope, providing a critical training set in similar filters and a well-calibrated low-redshift anchor of cosmologically useful SNe Ia to benefit dark energy science. As the name suggests, YSE complements and extends other ongoing time-domain surveys by discovering fast-rising SNe within a few hours to days of explosion. YSE is the only current four-band time-domain survey and is able to discover transients as faint $\sim$21.5 mag in $gri$ and $\sim$20.5 mag in $z$, depths that allow us to probe the earliest epochs of stellar explosions. YSE is currently observing approximately 750 square degrees of sky every three days and we plan to increase the area to 1500 square degrees in the near future. When operating at full capacity, survey simulations show that YSE will find $\sim$5000 new SNe per year and at least two SNe within three days of explosion per month. To date, YSE has discovered or observed 8.3% of the transient candidates reported to the International Astronomical Union in 2020. We present an overview of YSE, including science goals, survey characteristics and a summary of our transient discoveries to date.

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arXiv2020

Luminous Red Nova AT 2019zhd, a new merger in M 31

We present the follow-up campaign of the luminous red nova (LRN) AT~2019zhd, the third event of this class observed in M 31. The object was followed by several sky surveys for about five months before the outburst, during which it showed a slow luminosity rise. In this phase, the absolute magnitude ranged from M_r=-2.8+-0.2 mag to M_r=-5.6+-0.1 mag. Then, over a four-five day period, AT 2019zhd experienced a major brightening, reaching at peak M_r=-9.61+-0.08 mag, and an optical luminosity of 1.4x10^39 erg/s. After a fast decline, the light curve settled onto a short-duration plateau in the red bands. Although less pronounced, this feature is reminiscent of the second red maximum observed in other LRNe. This phase was followed by a rapid linear decline in all bands. At maximum, the spectra show a blue continuum with prominent Balmer emission lines. The post-maximum spectra show a much redder continuum, resembling that of an intermediate-type star. In this phase, Halpha becomes very weak, Hbeta is no longer detectable and a forest of narrow absorption metal lines now dominate the spectrum. The latest spectra, obtained during the post-plateau decline, show a very red continuum (T_eff ~ 3000 K) with broad molecular bands of TiO, similar to those of M-type stars. The long-lasting, slow photometric rise observed before the peak resembles that of LRN V1309 Sco, which was interpreted as the signature of the common-envelope ejection. The subsequent outburst is likely due to the gas outflow following a stellar merging event. The inspection of archival HST images taken 22 years before the LRN discovery reveals a faint red source (M_F555W=0.21+-0.14 mag, with F555W-F814W = 2.96+-0.12 mag) at the position of AT 2019zhd, which is the most likely quiescent precursor. The source is consistent with expectations for a binary system including a predominant M5-type star.

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arXiv2020

SN 2018gjx reveals that some SNe Ibn are SNe IIb exploding in dense circumstellar material

We present the data and analysis of SN 2018gjx, an unusual low-luminosity transient with three distinct spectroscopic phases. Phase I shows a hot blue spectrum with signatures of ionised circumstellar material (CSM), Phase II has the appearance of broad SN features, consistent with those seen in a Type IIb supernova at maximum light, and Phase III is that of a supernova interacting with helium-rich CSM, similar to a Type Ibn supernova. This event provides an apparently rare opportunity to view the inner workings of an interacting supernova. The observed properties can be explained by the explosion of a star in an aspherical CSM. The initial light is emitted from an extended CSM (~ 4000 Rsun), which ionises the exterior unshocked material. Some days after, the SN photosphere envelops this region, leading to the appearance of a SN IIb. Over time, the photosphere recedes in velocity space, revealing interaction between the supernova ejecta and the CSM that partially obscures the supernova nebular phase. Modelling of the initial spectrum reveals a surface composition consistent with compact H-deficient Wolf-Rayet and LBV stars. Such configurations may not be unusual, with SNe IIb being known to have signs of interaction so at least some SNe IIb and SNe Ibn may be the same phenomena viewed from different angles or, possibly with differing CSM configurations.

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arXiv2020

An outflow powers the optical rise of the nearby, fast-evolving tidal disruption event AT2019qiz

At 66 Mpc, AT2019qiz is the closest optical tidal disruption event (TDE) to date, with a luminosity intermediate between the bulk of the population and iPTF16fnl. Its proximity allowed a very early detection and triggering of multiwavelength and spectroscopic follow-up well before maximum light. The velocity dispersion of the host galaxy and fits to the TDE light curve indicate a black hole mass $\approx 10^6$ M$_\odot$, disrupting a star of $\approx 1$ M$_\odot$. Comprehensive UV, optical and X-ray data shows that the early optical emission is dominated by an outflow, with a luminosity evolution $L \propto t^2$, consistent with a photosphere expanding at constant velocity ($\gtrsim 2000$ km s$^{-1}$), and a line-forming region producing initially blueshifted H and He II profiles with $v=3000-10000$ km s$^{-1}$. The fastest optical ejecta approach the velocity inferred from radio detections (modelled in a forthcoming companion paper from K.~D.~Alexander et al.), thus the same outflow may be responsible for both the fast optical rise and the radio emission -- the first time this connection has been observed in a TDE. The light curve rise begins $29 \pm 2$ days before maximum light, peaking when the photosphere reaches the radius where optical photons can escape. The photosphere then undergoes a sudden transition, first cooling at constant radius then contracting at constant temperature. At the same time, the blueshifts disappear from the spectrum and Bowen fluorescence lines (N III) become prominent, implying a source of far-UV photons, while the X-ray light curve peaks at $\approx 10^{41}$ erg s$^{-1}$. Assuming that these X-rays are from prompt accretion, the size and mass of the outflow are consistent with the reprocessing layer needed to explain the large optical to X-ray ratio in this and other optical TDEs, possibly favouring accretion-powered over collision-powered outflow models.

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arXiv2020

SN 2016gsd: An unusually luminous and linear type II supernova with high velocities

We present observations of the unusually luminous Type II supernova (SN) 2016gsd. With a peak absolute magnitude of V = $-$19.95 $\pm$ 0.08, this object is one of the brightest Type II SNe, and lies in the gap of magnitudes between the majority of Type II SNe and the superluminous SNe. Its light curve shows little evidence of the expected drop from the optically thick phase to the radioactively powered tail. The velocities derived from the absorption in H$α$ are also unusually high with the blue edge tracing the fastest moving gas initially at 20000 km s$^{-1}$, and then declining approximately linearly to 15000 km s$^{-1}$ over $\sim$100 d. The dwarf host galaxy of the SN indicates a low-metallicity progenitor which may also contribute to the weakness of the metal lines in its spectra. We examine SN 2016gsd with reference to similarly luminous, linear Type II SNe such as SNe 1979C and 1998S, and discuss the interpretation of its observational characteristics. We compare the observations with a model produced by the JEKYLL code and find that a massive star with a depleted and inflated hydrogen envelope struggles to reproduce the high luminosity and extreme linearity of SN 2016gsd. Instead, we suggest that the influence of interaction between the SN ejecta and circumstellar material can explain the majority of the observed properties of the SN. The high velocities and strong H$α$ absorption present throughout the evolution of the SN may imply a circumstellar medium configured in an asymmetric geometry.

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arXiv2020

Design and operation of the ATLAS Transient Science Server

The Asteroid Terrestrial impact Last Alert System (ATLAS) system consists of two 0.5m Schmidt telescopes with cameras covering 29 square degrees at plate scale of 1.86 arcsec per pixel. Working in tandem, the telescopes routinely survey the whole sky visible from Hawaii (above $δ> -50^{\circ}$) every two nights, exposing four times per night, typically reaching $o < 19$ magnitude per exposure when the moon is illuminated and $c < 19.5$ per exposure in dark skies. Construction is underway of two further units to be sited in Chile and South Africa which will result in an all-sky daily cadence from 2021. Initially designed for detecting potentially hazardous near earth objects, the ATLAS data enable a range of astrophysical time domain science. To extract transients from the data stream requires a computing system to process the data, assimilate detections in time and space and associate them with known astrophysical sources. Here we describe the hardware and software infrastructure to produce a stream of clean, real, astrophysical transients in real time. This involves machine learning and boosted decision tree algorithms to identify extragalactic and Galactic transients. Typically we detect 10-15 supernova candidates per night which we immediately announce publicly. The ATLAS discoveries not only enable rapid follow-up of interesting sources but will provide complete statistical samples within the local volume of 100 Mpc. A simple comparison of the detected supernova rate within 100 Mpc, with no corrections for completeness, is already significantly higher (factor 1.5 to 2) than the current accepted rates.

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arXiv2020

The rise and fall of an extraordinary Ca-rich transient -- The discovery of ATLAS19dqr/SN 2019bkc

This work presents the observations and analysis of ATLAS19dqr/SN 2019bkc, an extraordinary rapidly evolving transient event located in an isolated environment, tens of kiloparsecs from any likely host. Its light curves rise to maximum light in $5-6$ d and then display a decline of $Δm_{15} \sim5$ mag. With such a pronounced decay, it has one of the most rapidly evolving light curves known for a stellar explosion. The early spectra show similarities to normal and `ultra-stripped' type Ic SNe, but the early nebular phase spectra, which were reached just over two weeks after explosion, display prominent calcium lines, marking SN 2019bkc as a Ca-rich transient. The Ca emission lines at this phase show an unprecedented and unexplained blueshift of 10,000 -- 12,000 km/s. Modelling of the light curve and the early spectra suggests that the transient had a low ejecta mass of $0.2 - 0.4$ M$_\odot$ and a low kinetic energy of $ (2-4)\times 10^{50}$ erg, giving a specific kinetic energy $\sim1$ [$10^{51}$ erg]/M$_\odot$. The origin of this event cannot be unambiguously defined. While the abundance distribution used to model the spectra marginally favours a progenitor of white dwarf origin through the tentative identification of \ArII, the specific kinetic energy, which is defined by the explosion mechanism, is found to be more similar to an ultra-stripped core-collapse events. SN 2019bkc adds to the diverse range of physical properties shown by Ca-rich events.

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arXiv2020

A detailed non-LTE analysis of LB-1: Revised parameters and surface abundances

LB-1 has recently been proposed to be a binary system at 4 kpc consisting of a B star of 8 Msol and a massive stellar black hole of 70 Msol. This finding challenges our current theories of massive star evolution and formation of BHs at solar metallicity. Our objective is to derive the effective temperature, surface gravity and chemical composition of the B-type component in order to determine its nature and evolutionary status and, indirectly, to constrain the mass of the BH. We use the non-LTE stellar atmosphere code FASTWIND to analyse new and archival high resolution data. We determine (Teff, logg) values of (14000$\pm500$ K, 3.50$\pm0.15$ dex) that, combined with the Gaia parallax, implies a spectroscopic mass, from logg, of $3.2^{+2.1}_{-1.9}$ Msol and an evolutionary mass, assuming single star evolution, of $5.2^{+0.3}_{-0.6}$ Msol. We determine an upper limit of 8 km/s for the projected rotational velocity and derive the surface abundances, finding the star to have a silicon abundance below solar, to be significantly enhanced in nitrogen and iron, and depleted in carbon and magnesium. Complementary evidence derived from a photometric extinction analysis and Gaia yields similar results for Teff and logg and a consistent distance around 2~kpc. We propose that the B star is a slightly evolved main sequence star of 3-5 Msol with surface abundances reminiscent of diffusion in late B/A chemically peculiar stars with low rotational velocities. There is also evidence for CN-processed material in its atmosphere. These conclusions rely critically on the distance inferred from the Gaia parallax. The goodness of fit of the Gaia astrometry also favours a high-inclination orbit. If the orbit is edge-on and the B star has a mass of 3-5 Msol, the mass of the dark companion would be 4-5 Msol, which would be easier to explain with our current stellar evolutionary models.

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