Time-resolved Photometry Research Articles

Spectro-photometric follow-up of the outbursting AM CVn system ASASSN-21br

ABSTRACT We report on spectroscopic and photometric observations of the AM Canum Venaticorum (AM CVn) system ASASSN-21br, which was discovered in outburst by the All-Sky Automated Survey for Supernovae in 2021 February. The outburst lasted for around three weeks, and exhibited a pronounced brightness dip for $\approx$4 d, during which the spectra showed a sudden transition from emission- to absorption-line dominated. Only $\approx$60 AM CVn systems with derived orbital periods are found in the Galaxy, therefore increasing the sample of AM CVn systems with known orbital periods is of tremendous importance to (1) constrain the physical mechanisms of their outbursts and (2) establish a better understanding of the low-frequency background noise of future gravitational wave surveys. Time-resolved photometry taken during the outburst of ASASSN-21br showed modulation with a period of around 36.65 min, which is likely the superhump or orbital period of the system. Time-resolved spectroscopy taken with the Southern African Large Telescope did not show any sign of periodicity in the He i absorption lines. This is possibly due to the origin of these lines in the outbursting accretion disc, which makes it challenging to retrieve periodicity from the spectral lines. Future follow-up spectral observations during quiescence might allow us better constrain the orbital period of ASASSN-21br.

Evidence for a black hole in the historical X-ray transient A 1524-61 (= KY TrA)

ABSTRACT We present Very Large Telescope spectroscopy, high-resolution imaging, and time-resolved photometry of KY TrA, the optical counterpart to the X-ray binary A 1524-61. We perform a refined astrometry of the field, yielding improved coordinates for KY TrA and the field star interloper of similar optical brightness that we locate 0.64 ± 0.04 arcsec SE. From the spectroscopy, we refine the radial velocity semi-amplitude of the donor star to K2 = 501 ± 52 km s−1 by employing the correlation between this parameter and the full width at half-maximum of the H α emission line. The r-band light curve shows an ellipsoidal-like modulation with a likely orbital period of 0.26 ± 0.01 d (6.24 ± 0.24 h). These numbers imply a mass function f(M1) = 3.2 ± 1.0 M⊙. The KY TrA de-reddened quiescent colour (r − i)0 = 0.27 ± 0.08 is consistent with a donor star of spectral type K2 or later, in case of significant accretion disc light contribution to the optical continuum. The colour allows us to place a very conservative upper limit on the companion star mass, M2 ≤ 0.94 M⊙, and, in turn, on the binary mass ratio, q = M2/M1 ≤ 0.31. By exploiting the correlation between the binary inclination and the depth of the H α line trough, we establish i = 57 ± 13 deg. All these values lead to a compact object and donor mass of $M_1 = 5.8^{+3.0}_{-2.4}$$\, M_\odot$ and M2 = 0.5 ± 0.3 M⊙, thus confirming the black hole nature of the accreting object. In addition, we estimate a distance towards the system of 8.0 ± 0.9 kpc.

The historical active episodes of the disks around γ Cassiopeiae (B0.5 IVe) and 59 Cygni (B1 IVe) revisited

The observations of all known major activity phases of the disks around the classical Be stars γ Cas and 59 Cyg with low-mass companions are comprehensively reviewed and purely qualitatively evaluated again, though taking advantage of new insights gained over the past 25 yr into the physics of Be disks. Both stars have exhibited activity cycles in the violet-to-red (V/R) flux ratio of emission lines with two peaks. This activity is indistinguishable from those of the vast majority of Be stars and so probably were caused by one-armed (m= 1) disk oscillations. The anomalous high-activity phases from 1932 to 1942 in γ Cas and between 1972 and 1976 in 59 Cyg were distinguished fromm= 1 density waves by large variations in the separations of pairs of emission peaks. In two consecutive cycles, shell phases during which the emission peaks were maximally separated alternated with single (blended) emission peaks. The amplitude in peak separation of more than a factor of two implies a high-amplitude variation in the disk aspect angle. When the peaks were blended and the disk was viewed closest to face-on, local maxima in visual brightness probably occurred inγCas, and the visibility of the stellar absorption lines was reduced, as is expected from increased free-bound emission into the line of sight (there is no time-resolved photometry for 59 Cyg from the event in the 1970s). In y Cas, the pre-event V/R variability (pre-event observations of 59 Cyg do not exist) was practically identical tom= 1 variability. In spite of the subsequent rapid rise in amplitude (up to ~4), the V/R variations connected smoothly in phase but may require an explanation involving the 3D structure of the disk. The phasing of single-peak and shell stages relative to the V/R activity was the same in both cycles ofγCas, whereas this is not clear for 59 Cyg. During both high-activity cycles ofγCas, but at different phases, transient additional pairs of emission lines appeared inγCas that were much sharper than the main ones and they also had different peak separations and V/R ratios. In the second instance, their velocities were up to ~+500 km s−1. The extremely rapid excitation of the activity phases and their short duration of only two cycles in both stars may indicate a resonant behavior of an unidentified nature. In both stars, the line emission was strongly developed at the onset of the high-activity phases but it basically disappeared at the end of them, and the disks may have been dynamically destroyed. The atypical disk variations were presumably triggered by enhanced interactions between a disk and companion star. In both systems, there seems to be less evidence for a mass-loss outburst than for a reduced mass-injection rate into the disk. The resulting lower viscous coupling between a disk and star would have facilitated the tilting of the disk.

Optimized temporal binning of comparison star measurements for differential photometry

ABSTRACT Ground-based, high precision observations of the light curves of objects such as transiting exoplanets rely on the application of differential photometry. The flux of the target object is measured relative to a comparison star in the same field, allowing correction for systematic trends in the light curve, mainly due to atmospheric effects including the variation of extinction with airmass. However, the precision of the light curve is then limited by the random noise for the measurements of both the target object and the comparison star. For time-resolved photometry using short exposure times of up to a few tens of seconds, the time-scale of the systematic variations due to atmospheric (or other) effects can be much longer than the cadence of the observations. In this case, the overall signal-to-noise ratio of the observation may be improved significantly by applying some temporal binning to the measurements of the comparison star, before comparison with the target object, without reducing the cadence of the overall light curve. In this paper, we will describe a data reduction pipeline for implementing this method which optimizes the number of frames to be binned for the comparison star, and we present example results for time-resolved photometric data. An example of applying the technique on an exoplanet transit light curve of WASP-166b is presented using four comparison stars of different magnitudes.

X-ray and optical observations of two AM Her type cataclysmic variables: V388 Peg and V1189 Her

We present time-resolved photometry of a polar V388 Peg and a strong candidate polar V1189 Her observed by T100 telescope at TÜBİTAK National Observatory between June–August 2014 and their X-ray observations using Swift archival data obtained in Apr 2014 (V388 Peg) and in Nov 2011 (V1189 Her). These data were supplemented by archival CRTS, ZTF, ASAS-SN, and ATLAS survey data which show frequent changes between high and low states. This study aims to obtain the temporal behaviors of two systems using the optical light curves and the X-ray spectral properties. The orbital periods of systems are corrected using photometric observations. Results obtained from light curves morphology related to accretion region are discussed for each system. V388 Peg shows a higher amplitude (∼1.6 mag) of orbital modulation and smooth structure in T100 optical observations. It is detected in X-rays at an unabsorbed flux (0.3–10 keV) of 3.6×10−12 erg cm−2 s−1 using a simple model of 30 keV bremsstrahlung model and the model-dependent corresponding luminosity is logL=32.3 erg s−1. The X-ray spectra of V1189 Her is fitted by a bremsstrahlung model and the estimated X-ray luminosity is found to be an order of magnitude lower than the 1032 erg s−1 typical of previously known polars. Using Gaia EDR3 distances, the information obtained from the X-ray spectral analysis and MWD∼0.7M⊙ for both sources, we estimate the mass accretion rates of ∼10−11M⊙ yr−1 (V388 Peg) and ∼10−12M⊙ yr−1 (V1189 Her). As a result of our study, it seems that these two systems have lower mass accretion rates than the long-term accretion rate that is driven by secular evolution, and considering their orbital periods this makes them interesting evolutionary test samples for polars.

Are the Hessdalen Lights a Reality, an Illusion, or a Mix of the Two ?

The Hessdalen lights (HLs in the following) are luminous, floating, more or less spherical atmospheric phenomena, with a lifetime of a few seconds to sometimes several minutes. These phenomena are seen in the Hessdalen Valley in Norway for decades. Unfortunately a full understanding of these baffling events is still lacking in spite of solid working scientific projects intended to explain them. This paper tries to improve the situation. It raises the questions where the energy for the creation of the HLs comes from, and what was its nature : (geo) chemical, electric or still other ? We propose a new scenario for the Hessdalen lights. It exploits the recent idea of stable and traversable wormholes whose the potential existence is beginning to be recognized in physics. Even though appearing highly speculative, this hypothesis has not been so far explored elsewhere while it could supply a full description of the wholeness of the phenomenon. On the other side even if the probability that a HL could indeed be a wormhole is may be low, this question should not dismissed out of hand. These theoretical considerations could help to increase knowledge and understanding of both the HLs and the wormholes, drawing mutual enrichment. In other words HLs could betray the presence of hidden wormholes and we must not let slip through our fingers this possibility even if it is very tiny. In this framework we discuss of the stability, the energetics and the oversized dimension of the HLs. In physics the final arbiter is not the theory but the experiment. Thus some “simple” experiments are eventually suggested (high time resolution photometry and magnetic field measurements). Eventually, if the process described is real and after mastering it, there is a free and inexhaustible source of energy that would be derived, a tremendous breakthrough after which we could forget the controlled nuclear fusion. Regarding its structure, the paper is divided in four paragraphs 1, 2,3, 4 independent of each other. Illustrative pictures help to understand the text.

A Multi–Observing Technique Study of the Dynamical Evolution of the Viscous Disk around the Be Star ω CMa

Abstract The observed emission lines of Be stars originate from a circumstellar Keplerian disk, which is generally well explained by the viscous decretion disk model. In an earlier work we performed modeling of the full light curve of the bright Be star ω CMa with the 1D time-dependent hydrodynamic code SINGLEBE and the Monte Carlo radiative transfer code HDUST. We used a V-band light curve that probes the inner disk through four disk formation and dissipation cycles. This new study compares predictions of the same set of model parameters with time-resolved photometry from the near-UV through the mid-IR, comprehensive series of optical spectra, and optical broadband polarimetry, which overall represent a larger volume of the disk. Qualitatively, the models reproduce the trends in the observed data due to the growth and decay of the disk. However, quantitative differences exist, e.g., an overprediction of flux increasing with wavelength, too slow decreases in Balmer emission line strength during disk dissipation, and a discrepancy between the range of polarimetric data and the model. We find that a larger value of the viscosity parameter alone or a disk truncated by a companion star reduces these discrepancies by increasing the dissipation rate in the outer regions of the disk.

Asteroseismology of SZ Lyn using multiband high time resolution photometry from ground and space

ABSTRACT We report the analysis of high temporal resolution ground- and space-based photometric observations of SZ Lyncis, a binary star one of whose components is a high amplitude δ Scuti. UBVR photometric observations were obtained from Mt. Abu Infrared Observatory and Fairborn Observatory; archival observations from the WASP project were also included. Furthermore, the continuous, high-quality light curve from the TESS project was extensively used for the analysis. The well resolved light curve from TESS reveals the presence of 23 frequencies with four independent modes, 13 harmonics of the main pulsation frequency of 8.296943 ± 0.000002 d−1, and their combinations. The frequency 8.296 d−1 is identified as the fundamental radial mode by amplitude ratio method and using the estimated pulsation constant. The frequencies 14.535, 32.620, and 4.584 d−1 are newly discovered for SZ Lyn. Out of these three, 14.535 and 32.620 d−1 are identified as non-radial lower order p modes and 4.584 d−1 could be an indication of a g mode in a δ Scuti star. As a result of frequency determination and mode identification, the physical parameters of SZ Lyn were revised by optimizations of stellar pulsation models with the observed frequencies. The theoretical models correspond to 7500 K ≤ Teff ≤ 7800 K and log(g) = 3.81 ± 0.06. The mass of SZ Lyn was estimated to be close to 1.7–2.0 M⊙ using evolutionary sequences. The period–density relation estimates a mean density (ρ) of 0.1054 ± 0.0016 g cm−3.

A compact X-ray emitting binary in likely association with 4FGL J0935.3+0901

ABSTRACT 4FGL J0935.3+0901 is a γ-ray source detected by the Large Area Telescope (LAT) onboard the Fermi Gamma-Ray Space Telescope. We have conducted detailed analysis of the LAT data for this source and multiwavelength studies of the source field. Its γ-ray emission can be described with a power law (Γ = 2.0 ± 0.2) with an exponential cut-off (Ec = 2.9 ± 1.6 GeV), while the flux shows significant long-term variations. From analysis of archival Neil Gehrels Swift Observatory X-Ray Telescope data, we find only one X-ray source in the LAT’s 2σ error region. Within a $3.7\,{\rm arcsec}$ radius error circle of the X-ray source, there is only one optical object down to r′ ∼ 23 mag. Time-resolved photometry of the optical object indicates a likely 2.5 h periodic modulation, while its spectrum shows double-peaked hydrogen and helium emission lines (similar to those seen in accretion discs in low-mass X-ray binaries). Combining these results, we conclude that we have discovered a compact X-ray emitting binary in likely association with 4FGL J0935.3+0901, i.e. a millisecond pulsar (MSP) binary. We discuss the implication of the optical spectral features: this binary could be a transitional MSP system at a subluminous disc state, although the other possibility, the binary in a rotation-powered state showing the optical emission lines due to intrabinary interaction processes, cannot be excluded. Further observational studies will help to determine detailed properties of this candidate MSP binary and thus clarify its current state.

Time-resolved photometry of the young dipper RX J1604.3-2130A

Context. RX J1604.3-2130A is a young, dipper-type, variable star in the Upper Scorpius association, suspected to have an inclined inner disk, with respect to its face-on outer disk. Aims. We aim to study the eclipses to constrain the inner disk properties. Methods. We used time-resolved photometry from the Rapid Eye Mount telescope and Kepler 2 data to study the multi-wavelength variability, and archival optical and infrared data to track accretion, rotation, and changes in disk structure. Results. The observations reveal details of the structure and matter transport through the inner disk. The eclipses show 5 d quasi-periodicity, with the phase drifting in time and some periods showing increased/decreased eclipse depth and frequency. Dips are consistent with extinction by slightly processed dust grains in an inclined, irregularly-shaped inner disk locked to the star through two relatively stable accretion structures. The grains are located near the dust sublimation radius (~0.06 au) at the corotation radius, and can explain the shadows observed in the outer disk. The total mass (gas and dust) required to produce the eclipses and shadows is a few % of a Ceres mass. Such an amount of mass is accreted/replenished by accretion in days to weeks, which explains the variability from period to period. Spitzer and WISE infrared variability reveal variations in the dust content in the innermost disk on a timescale of a few years, which is consistent with small imbalances (compared to the stellar accretion rate) in the matter transport from the outer to the inner disk. A decrease in the accretion rate is observed at the times of less eclipsing variability and low mid-IR fluxes, confirming this picture. The v sin i = 16 km s−1 confirms that the star cannot be aligned with the outer disk, but is likely close to equator-on and to be aligned with the inner disk. This anomalous orientation is a challenge for standard theories of protoplanetary disk formation.

Probing the Hill Sphere of (486958) 2014 MU69. II. Hubble Space Telescope Fine Guidance Sensors Observations during the 2018 August 4 Stellar Occultation

Abstract We observed the 2018 August 4 stellar occultation by the Kuiper Belt object (486958) 2014 MU69, the first close flyby target of the extended New Horizons mission. Rather than capture a solid-body occultation by the KBO itself, our program aimed to constrain the opacity of rings, moons, or other debris in the nearby environment. We used the Hubble Space Telescope Fine Guidance Sensors (HST FGS) instrument in TRANS F583W mode to collect 40 Hz time resolution photometry of the stellar occultation star for one HST orbit during this observation. We present the results of reduction and calibration of the HST FGS photometry, and set upper limits on τ of ∼0.02–0.08 for rings or other dust opacity within the Hill sphere of (486958) 2014 MU69 at distances ranging from ∼1660 to ∼57,700 km from the main body.

Cloud Atlas: High-contrast Time-resolved Observations of Planetary-mass Companions

Abstract Directly imaged planetary-mass companions offer unique opportunities in atmospheric studies of exoplanets. They share characteristics of both brown dwarfs and transiting exoplanets, and therefore are critical for connecting atmospheric characterizations for these objects. Rotational phase mapping is a powerful technique to constrain the condensate cloud properties in ultra-cool atmospheres. Applying this technique to directly imaged planetary-mass companions will be extremely valuable for constraining cloud models in low mass and surface-gravity atmospheres and for determining the rotation rate and angular momentum of substellar companions. Here, we present Hubble Space Telescope Wide Field Camera 3 near-infrared time-resolved photometry for three planetary-mass companions, AB Pic B, 2M0122B, and 2M1207b. Using two-roll differential imaging and hybrid point-spread function modeling, we achieve sub-percent photometric precision for all three observations. We find tentative modulations (<2σ) for AB Pic B and 2M0122B, but cannot reach conclusive results on 2M1207b due to strong systematics. The relatively low significance of the modulation measurements cannot rule out the hypothesis that these planetary-mass companions have the same vertical cloud structures as brown dwarfs. Our rotation rate measurements, combined with archival period measurements of planetary-mass companions and brown dwarfs, do not support a universal mass-rotation relation. The high precision of our observations and the high occurrence rates of variable low-surface-gravity objects encourage high-contrast time-resolved observations with the James Webb Space Telescope.

A small survey of UV-bright stars around the northern ecliptic pole: seeking new p-mode sdB variables for the TESS mission

Abstract Starting in 2019, the TESS mission will monitor the northern ecliptic pole for 1 year. Data will be collected at 30-minute and 2-minute cadences, and only a limited amount of slots will be reserved for targets requiring a 20- second cadence. Only the 20-second cadence is sufficient to sample p-mode oscillations in sdB stars. From the seismic measurements obtained with the Kepler spacecraft we have gained a wealth of new insights in structural and rotational aspects of mainly g-mode variable sdB stars. Unfortunately only one traditional p-mode sdB variable was found in the main Kepler field. The TESS mission offers the opportunity to obtain more long-time-base coverage of p-mode sdB variables, especially at the ecliptic poles where the time-base will be longest. Thus far, there were only two known (p-mode) sdBVs around the northern ecliptic pole (β > 78): LS Dra and V366 Dra. In this paper we describe our efforts to find more.We compiled a new sample of 76 sdB candidates around the northern ecliptic pole, based on GALEX and optical colours, and we used low-resolution Balmer-line spectroscopy for classification. We identified 39 new sdB stars, of which 29 have characteristics (Teff > 28000 K or a composite spectrum) that may put them in the p-mode instability strip.With our 39 new sdB stars, we augmented the number of known sdB stars in the the northern ecliptic pole area (β> 73) by 46%. Besides these sdB stars, among our spectral classifications are various sdO stars, He-sdB stars, blue horizontal-branch stars, white dwarfs, cataclysmic variables and main sequence B stars. We obtained time-resolved photometry of most of the p-mode sdB candidates, and found one new sdBV, J19384+5824, with a moderately high pulsation amplitude of ≥ 9 mmag.

Nucleus of active asteroid 358P/Pan-STARRS (P/2012 T1)

Context.The dust emission from active asteroids is likely driven by collisions, fast rotation, sublimation of embedded ice, and combinations of these. Characterising these processes leads to a better understanding of their respective influence on the evolution of the asteroid population.Aims.We study the role of fast rotation in the active asteroid 358P (P 2012/T1).Methods.We obtained two nights of deep imaging of 358P with SOAR/Goodman and VLT/FORS2. We derived the rotational light curve from time-resolved photometry and searched for large fragments and debris >8 mm in a stacked, ultra-deep image.Results.The nucleus has an absolute magnitude ofmR= 19.68, corresponding to a diameter of 530 m for standard assumptions on the albedo and phase function of a C-type asteroid. We do not detect fragments or debris that would require fast rotation to reduce surface gravity to facilitate their escape. The 10-h light curve does not show an unambiguous periodicity.

Probing the Hill Sphere of (486958) 2014 MU69: HST FGS Observations during the 2017 July 17 Stellar Occultation

Abstract We observed the 2017 July 17 stellar occultation of HD 168233 by the Kuiper Belt Object (486958) 2014 MU69, the close flyby target of the extended New Horizons mission. Rather than capture a solid body occultation by the KBO itself, our program aimed to constrain the opacity of rings, moons, or other debris in the nearby environment. We used the Hubble Space Telescope Fine Guidance Sensors (HST FGS) instrument in TRANS F583W mode to collect 40 Hz time resolution photometry of the stellar occultation star for two HST orbits during this observation. We present the results of reduction and calibration of the HST FGS photometry, and set upper limits on rings or other dust opacity within the Hill sphere of (486958) 2014 MU69 at distances ranging from ∼20000 km to ∼75000 km from the main body.

Repeated Transient Jets from a Warped Disk in the Symbiotic Prototype Z And: A Link to the Long-lasting Active Phase

Abstract Active phases of some symbiotic binaries survive for a long time, from years to decades. The accretion process onto a white dwarf (WD) sustaining long-lasting activity, and sometimes leading to collimated ejection, is not well understood. We present the repeated emergence of highly collimated outflows (jets) from the symbiotic prototype Z And during its 2008 and 2009–10 outbursts and suggest their link to the current long-lasting (from 2000) active phase. We monitored Z And with high-resolution spectroscopy, multicolor UBVR C—and high time resolution—photometry. The well-pronounced bipolar jets were ejected again during the 2009–10 outburst together with the simultaneous emergence of the rapid photometric variability (Δm ≈ 0.06 mag) on the timescale of hours, showing similar properties as those during the 2006 outburst. These phenomena and the measured disk–jets connection could be caused by the radiation-induced warping of the inner disk due to a significant increase of the burning WD luminosity. Ejection of transient jets by Z And around outburst maxima signals a transient accretion at rates above the upper limit of the stable hydrogen burning on the WD surface, and thus proves the nature of Z And-type outbursts. The enhanced accretion through the disk warping, supplemented by the accretion from the giant’s wind, can keep a high luminosity of the WD for a long time, until depletion of the disk. In this way, the jets provide a link to long-lasting active phases of Z And.

Analytic reflected light curves for exoplanets

The disk-integrated reflected brightness of an exoplanet changes as a function of time due to orbital and rotational motion coupled with an inhomogeneous albedo map. We have previously derived analytic reflected lightcurves for spherical harmonic albedo maps in the special case of a synchronously-rotating planet on an edge-on orbit (Cowan, Fuentes & Haggard 2013). In this paper, we present analytic reflected lightcurves for the general case of a planet on an inclined orbit, with arbitrary spin period and non-zero obliquity. We do so for two different albedo basis maps: bright points ($\delta$-maps), and spherical harmonics ($Y_l^m$-maps). In particular, we use Wigner $D$-matrices to express an harmonic lightcurve for an arbitrary viewing geometry as a non-linear combination of harmonic lightcurves for the simpler edge-on, synchronously rotating geometry. These solutions will enable future exploration of the degeneracies and information content of reflected lightcurves, as well as fast calculation of lightcurves for mapping exoplanets based on time-resolved photometry. To these ends we make available Exoplanet Analytic Reflected Lightcurves (EARL), a simple open-source code that allows rapid computation of reflected lightcurves.

Solar system science with ESAEuclid

Context.The ESAEuclidmission has been designed to map the geometry of the dark Universe. Scheduled for launch in 2020, it will conduct a six-year visible and near-infrared imaging and spectroscopic survey over 15 000 deg2down toVAB~ 24.5. Although the survey will avoid ecliptic latitudes below 15°, the survey pattern in repeated sequences of four broadband filters seems well-adapted to detect and characterize solar system objects (SSOs).Aims.We aim at evaluating the capability ofEuclidof discovering SSOs and of measuring their position, apparent magnitude, and spectral energy distribution. We also investigate how the SSO orbits, morphology (activity and multiplicity), physical properties (rotation period, spin orientation, and 3D shape), and surface composition can be determined based on these measurements.Methods.We used the current census of SSOs to extrapolate the total amount of SSOs that will be detectable byEuclid, that is, objects within the survey area and brighter than the limiting magnitude. For each different population of SSO, from neighboring near-Earth asteroids to distant Kuiper-belt objects (KBOs) and including comets, we compared the expectedEuclidastrometry, photometry, and spectroscopy with the SSO properties to estimate howEuclidwill constrain the SSOs dynamical, physical, and compositional properties.Results.With the current survey design, about 150 000 SSOs, mainly from the asteroid main-belt, should be observable byEuclid. These objects will all have high inclination, which is a difference to many SSO surveys that focus on the ecliptic plane.Euclidmay be able to discover several 104SSOs, in particular, distant KBOs at high declination. TheEuclidobservations will consist of a suite of four sequences of four measurements and will refine the spectral classification of SSOs by extending the spectral coverage provided byGaiaand the LSST, for instance, to 2 microns. Combined with sparse photometry such as measured byGaiaand the LSST, the time-resolved photometry will contribute to determining the SSO rotation period, spin orientation, and 3D shape model. The sharp and stable point-spread function ofEuclidwill also allow us to resolve binary systems in the Kuiper belt and detect activity around Centaurs.Conclusions.The depth of theEuclidsurvey (VAB~ 24.5), its spectral coverage (0.5 to 2.0 μm), and its observation cadence has great potential for solar system research. A dedicated processing for SSOs is being set up within theEuclidconsortium to produce astrometry catalogs, multicolor and time-resolved photometry, and spectral classification of some 105SSOs, which will be delivered as Legacy Science.

MASTER OT J132104.04+560957.8: A Polar with Absorption–Emission Line Reversals

Abstract We present time-resolved photometry and spectroscopy of the recently classified polar MASTER OT J132104.04+560957.8. The spectrum shows a smooth, nonthermal continuum at the time of maximum light, without any individually discernible cyclotron harmonics. Using homogenous cyclotron modeling, we interpret this as cyclotron radiation whose individual harmonics have blended together, and on this basis, we loosely constrain the magnetic-field strength to be less than ∼30 MG. In addition, for about one-tenth of the orbital period, the Balmer and He i emission lines transition into absorption features, with He ii developing an absorption core. We use our observations of this phenomenon to test theoretical models of the accretion curtain and conclude that the H and He i lines are produced throughout the curtain, in contravention of theoretical predictions of separate H and He i line-forming regions. Moreover, a significant amount of He ii emission originates within the accretion curtain, implying that the curtain is significantly hotter than expected from theory. Finally, we comment on the object’s long-term photometry, including evidence that it recently transitioned into a prolonged, exceptionally stable high state following a potentially decades-long low state.

The 2014–2017 outburst of the young star ASASSN-13db

ASASSN-13db is a M5-type star with a protoplanetary disk, the lowest mass star known to experience accretion outbursts. Since its discovery in 2013, it has experienced two outbursts, the second of which started in November 2014 and lasted until February 2017. We use high- and low-resolution spectroscopy and time-resolved photometry from the ASAS-SN survey, the LCOGT and the Beacon Observatory to study the lightcurve and the dynamical and physical properties of the accretion flow. The 2014-2017 outburst lasted for nearly 800 days. A 4.15d period in the lightcurve likely corresponds to rotational modulation of a star with hot spot(s). The spectra show multiple emission lines with variable inverse P-Cygni profiles and a highly variable blueshifted absorption below the continuum. Line ratios from metallic emission lines (Fe I/Fe II, Ti I/Ti II) suggest temperatures of $\sim$5800-6000 K in the accretion flow. Photometrically and spectroscopically, the 2014-2017 event displays an intermediate behavior between EXors and FUors. The accretion rate (\.{M}=1-3$\times$10$^{-7}$M$_\odot$/yr), about 2 orders of magnitude higher than the accretion rate in quiescence, is not significantly different from the accretion rate observed in 2013. The absorption features in the spectra suggest that the system is viewed at a high angle and drives a powerful, non-axisymmetric wind, maybe related to magnetic reconnection. The properties of ASASSN-13db suggest that temperatures lower than those for solar-type stars are needed for modeling accretion in very low-mass systems. Finally, the rotational modulation during the outburst reveals that accretion-related structures settled after the begining of the outburst and can be relatively stable and long-lived. Our work also demonstrates the power of time-resolved photometry and spectroscopy to explore the properties of variable and outbursting stars. (Abridged).