GLUV: A balloon-borne high-cadence ultraviolet monitoring telescope for supernova shock breakouts and exoplanet atmospheres

Routine photometric monitoring at near-ultraviolet wavelengths (< 400 nm) is compromised from the ground due to highly variable atmospheric transmission and cloud cover. The GLUV project will mount a modest sized telescope (200 mm primary) on a series of long-duration high-altitude balloon flights. The wide field camera (∼7 deg 2) will perform high cadence (10-300 second rolling integrations) each night for campaign durations of three to six months. The principle science mission is the early-time detection of supernova shock-breakout at near-ultraviolet wavelengths. Additionally, early design analysis has shown the system is also able to probe the atmospheric composition of exoplanet atmospheres through the combination of UV transit measurements with ground-based measurements at longer wavelengths. In this presentation we consider the specifications for a long-duration balloon platform for such a mission, focusing on the necessary mission requirements (sensitivity, sky coverage, cadence etc.) and the available platform suitability. Particular attention is paid to platform flight altitude and atmospheric transmissio

Cosmological foundations revisited with Pantheon+

We reanalyse the Pantheon+ supernova catalogue to compare a cosmology with non-FLRW evolution, the "timescape cosmology", with the standard $\Lambda$CDM cosmology. To this end, we consider the Pantheon+ supernova catalogue, which is the largest available Type Ia supernova dataset for a geometric comparison between the two models. We construct a covariance matrix to be as independent of cosmology as possible, including independence from the FLRW geometry and peculiar velocity with respect to FLRW average evolution. Within this framework, which goes far beyond most other definitions of "model independence", we introduce new statistics to refine Type Ia supernova (SneIa) light-curve analysis. In addition to conventional galaxy correlation functions used to define the scale of statistical homogeneity we introduce empirical statistics which enables a refined analysis of the distribution biases of SneIa light-curve parameters $\beta c$ and $\alpha x_1$. For lower redshifts, the Bayesian analysis highlights important features attributable to the increased number of low-redshift supernovae, the artefacts of model-dependent light-curve fitting and the cosmic structure through which we observe supernovae. This indicates the need for cosmology-independent data reduction to conduct a stronger investigation of the emergence of statistical homogeneity and to compare alternative cosmologies in light of recent challenges to the standard model. "Dark energy" is generally invoked as a place-holder for "new physics". Our from-first-principles reanalysis of the Pantheon+ catalogue supports future deeper studies of the interplay of matter and nonlinear spacetime geometry, in a data-driven setting. For the first time in 25 years, we find evidence that the Pantheon+ catalogue already contains such a wealth of data that with further reanalysis, a genuine "paradigm shift" may soon emerge. [Abridged]Comment: 23 pages, 14 figures, 3 table

A Phenomenon Resembling Early Superhumps in a New SU UMa-Type Dwarf Nova with a 2-Hour Orbital Period

We investigate K2BS5, an optical transient that we identified in Campaign 13 of the Kepler/K2 archives by the "K2 Background Survey", and classify it as a new SU UMa-type dwarf nova. Using the light curve generated from Kepler's long-cadence observation mode, we analyze the dwarf nova during quiescence and superoutburst. Following 20 days of quiescence at the start of the observation, the system entered a superoutburst lasting 12 days, after which it experienced at least one rebrightening. K2BS5 clearly meets the criteria for an SU UMa star, but at the peak of the superoutburst, it also shows double-wave oscillations consistent with the spectroscopic orbital period, a phenomenon that closely resembles early superhumps in WZ Sge stars. While we do not classify K2BS5 as a WZ Sge system, we discuss how this phenomenon could complicate efforts to use the suspected detection of early superhumps to distinguish SU UMa-type dwarf novae from the recently recognized class of long-orbital-period WZ Sge systems.Comment: 11 pages, 7 figures. Accepted for publication in the Astronomical Journa

K2: Background Survey - The search for undiscovered transients in Kepler/K2 data

The K2 mission of the Kepler Space Telescope offers a unique possibility to examine sources of both Galactic and extragalactic origin with high-cadence photometry. Alongside the multitude of supernovae and quasars detected within targeted galaxies, it is likely that Kepler has serendipitously observed many transients throughout K2. Such events will likely have occurred in background pixels, coincidentally surrounding science targets. Analysing the background pixels presents the possibility to conduct a high-cadence survey with areas of a few square degrees per campaign. We demonstrate the capacity to independently recover key K2 transients such as KSN 2015K and SN 2018oh. With this survey, we expect to detect numerous transients and determine the first comprehensive rates for transients with lifetimes of <1 d.This research was supported by an Australian Government Research Training Program (RTP) Scholarship and utilizes data collected by the K2 missio

A Comprehensive Investigation of Gamma-Ray Burst Afterglows Detected by TESS

Gamma-ray bursts produce afterglows that can be observed across the electromagnetic spectrum and can provide insight into the nature of their progenitors. While most telescopes that observe afterglows are designed to rapidly react to trigger information, the Transiting Exoplanet Survey Satellite (TESS) continuously monitors sections of the sky at cadences between 30 minutes and 200 seconds. This provides TESS with the capability of serendipitously observing the optical afterglow of GRBs. We conduct the first extensive search for afterglows of known GRBs in archival TESS data reduced with the TESSreduce package, and detect 11 candidate signals that are temporally coincident with reported burst times. We classify 3 of these as high-likelihood GRB afterglows previously unknown to have been detected by TESS, one of which has no other afterglow detection reported on the Gamma-ray Coordinates Network. We classify 5 candidates as tentative and the remainder as unlikely. Using the afterglowpy package, we model each of the candidate light curves with a Gaussian and a top hat model to estimate burst parameters; we find that a mean time delay of $740\pm690\,$s between the explosion and afterglow onset is required to perform these fits. The high cadence and large field of view make TESS a powerful instrument for localising GRBs, with the potential to observe afterglows in cases when no other backup photometry is possible.Comment: 17 pages, 7 figures, 5 table

GRB 191016A: A Long Gamma-Ray Burst Detected by TESS

The TESS exoplanet-hunting mission detected the rising and decaying optical afterglow of GRB 191016A, a long Gamma-Ray Burst (GRB) detected by Swift-BAT but without prompt XRT or UVOT follow-up due to proximity to the moon. The afterglow has a late peak at least 1000 seconds after the BAT trigger, with a brightest-detected TESS datapoint at 2589.7 s post-trigger. The burst was not detected by Fermi-LAT, but was detected by Fermi-GBM without triggering, possibly due to the gradual nature of rising light curve. Using ground-based photometry, we estimate a photometric redshift of $z_\mathrm{phot} = 3.29\pm{0.40}$. Combined with the high-energy emission and optical peak time derived from TESS, estimates of the bulk Lorentz factor $\Gamma_\mathrm{BL}$ range from $90-133$. The burst is relatively bright, with a peak optical magnitude in ground-based follow-up of $R=15.1$ mag. Using published distributions of GRB afterglows and considering the TESS sensitivity and sampling, we estimate that TESS is likely to detect $\sim1$ GRB afterglow per year above its magnitude limit.Comment: 10 pages, 7 figures. Accepted for publication in the Astrophysical Journa

High-Resolution Transmission Spectroscopy of the Terrestrial Exoplanet GJ 486b

Terrestrial exoplanets orbiting M-dwarf stars are promising targets for transmission spectroscopy with existing or near-future instrumentation. The atmospheric composition of such rocky planets remains an open question, especially given the high X-ray and ultraviolet flux from their host M dwarfs that can drive atmospheric escape. The 1.3 $R_\oplus$ exoplanet GJ 486b ($T_{\rm{eq}} \sim$ 700 K), orbiting an M3.5 star, is expected to have one of the strongest transmission spectroscopy signals among known terrestrial exoplanets. We observed three transits of GJ 486b using three different high-resolution spectrographs: IRD on Subaru, IGRINS on Gemini-South, and SPIRou on the Canada-France-Hawai'i Telescope. We searched for atmospheric absorption from a wide variety of molecular species via the cross-correlation method, but did not detect any robust atmospheric signals. Nevertheless, our observations are sufficiently sensitive to rule out several clear atmospheric scenarios via injection and recovery tests, and extend comparative exoplanetology into the terrestrial regime. Our results suggest that GJ 486b does not possess a clear H$_2$/He-dominated atmosphere, nor a clear 100% water-vapor atmosphere. Other secondary atmospheres with high mean molecular weights or H$_2$/He-dominated atmospheres with clouds remain possible. Our findings provide further evidence suggesting that terrestrial planets orbiting M-dwarf stars may experience significant atmospheric loss.Comment: Accepted for publication in A

Perfecting our set of spectrophotometric standard DA white dwarfs

We verified for photometric stability a set of DA white dwarfs with Hubble Space Telescope magnitudes from thenear-ultraviolet to the near-infrared and ground-based spectroscopy by using time-spaced observations from theLas Cumbres Observatory network of telescopes. The initial list of 38 stars was whittled to 32final ones, whichcomprise a high-quality set of spectrophotometric standards. These stars are homogeneously distributed around thesky and are all fainter thanr~16.5 mag. Their distribution is such that at least two of them would be available tobe observed from any observatory on the ground at any time at airmass less than 2. Light curves and differentvariability indices from the Las Cumbres Observatory data were used to determine the stability of the candidatestandards. When available, Pan-STARRS1, Zwicky Transient Facility, and TESS data were also used to confirmthe star classification. Our analysis showed that four DA white dwarfs may exhibit evidence of photometricvariability, while afifth is cooler than our established lower temperature limit, and a sixth star might be a binary. Insome instances, due to the presence of faint nearby red sources, care should be used when observing a few of thespectrophotometric standards with ground-based telescopes. Light curves andfinding charts for all the stars areprovided.This study was supportedby NASA through grant O1904 from the Space TelescopeScience Institute, which is operated by AURA, Inc., underNASA contract NAS 5-26555 and the Space Telescope ScienceInstitute. The analysis was also supported by the DDRF grantD0001.82481. E.O. was also partially supported by the NSFthrough grant AST-1815767. R.R. received funding from thepostdoctoral fellowship program Beatriu de Pinós, funded bythe Secretary of Universities and Research(Government ofCatalonia)and by the Horizon 2020 program of research andinnovation of the European Union under the Maria Skło-dowska-Curie grant agreement No. 801370. C.S. is supportedby the US DOE through award DE-SC0007881. This work hasmade use of data from the European Space Agency(ESA)mission Gaia(https://www.cosmos.esa.int/gaia), processedby the Gaia Data Processing and Analysis Consortium(DPAC;https://www.cosmos.esa.int/web/gaia/dpac/consortium).Funding for the DPAC has been provided by nationalinstitutions, in particular the institutions participating in theGaia Multilateral Agreement. This publication makes use ofVOSA, developed under the Spanish Virtual Observatoryproject supported by the Spanish MINECO through grantAyA2017-84089. VOSA has been partially updated by usingfunding from the European Union’s Horizon 2020 Research andInnovation Programme, under grant Agreement No. 776403(EXOPLANETS-A). This work includes data from the AsteroidTerrestrial-impact Last Alert System(ATLAS)project. ATLAS isprimarily funded to search for near-earth asteroids through NASAgrants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575;byproducts of the NEO search include images and catalogs fromthe survey area. The ATLAS science products have been madepossible through the contributions of the University of HawaiiInstitute for Astronomy, the Queen’s University Belfast, the SpaceTelescope Science Institute, and the South African AstronomicalObservatory. G.N. and K.M. gratefully acknowledge support fromNASA under grant 80NSSC20K0453 issued through theNNH18ZDA001N Astrophysics Data Analysis Program(ADAP).Facilities:LCO, HST(WFC3), Gaia, ATLAS, TESS,Pan-STARRS, ZTFPeer ReviewedPostprint (published version

SN 2021zny: an early flux excess combined with late-time oxygen emission suggests a double white dwarf merger event

We present a photometric and spectroscopic analysis of the ultra-luminous and slowly evolving 03fg-like Type Ia SN 2021zny. Our observational campaign starts from $\sim5.3$ hours after explosion (making SN 2021zny one of the earliest observed members of its class), with dense multi-wavelength coverage from a variety of ground- and space-based telescopes, and is concluded with a nebular spectrum $\sim10$ months after peak brightness. SN 2021zny displayed several characteristics of its class, such as the peak brightness ($M_{B}=-19.95$ mag), the slow decline ($\Delta m_{15}(B) = 0.62$ mag), the blue early-time colours, the low ejecta velocities and the presence of significant unburned material above the photosphere. However, a flux excess for the first $\sim1.5$ days after explosion is observed in four photometric bands, making SN 2021zny the third 03fg-like event with this distinct behavior, while its $+313$ d spectrum shows prominent [O I] lines, a very unusual characteristic of thermonuclear SNe. The early flux excess can be explained as the outcome of the interaction of the ejecta with $\sim0.04\:\mathrm{M_{\odot}}$ of H/He-poor circumstellar material at a distance of $\sim10^{12}$ cm, while the low ionization state of the late-time spectrum reveals low abundances of stable iron-peak elements. All our observations are in accordance with a progenitor system of two carbon/oxygen white dwarfs that undergo a merger event, with the disrupted white dwarf ejecting carbon-rich circumstellar material prior to the primary white dwarf detonation.Comment: 19 pages, 16 figures, accepted for publication in MNRA