Search

We report the discovery of a new subclass of double-mode RR Lyrae stars in the Large and Small Magellanic Clouds. The sample of 22 pulsating stars have been extracted from the latest edition of the OGLE collection of RR Lyrae variables in the Magellanic System. The stars pulsating simultaneously in the fundamental (F) and first-overtone (1O) modes have distinctly different properties than regular double-mode RR Lyrae variables (RRd stars). The P1O/PF period ratios of our anomalous RRd stars are within a range 0.725-0.738, while "classical" double-mode RR Lyrae variables have period ratios in the range 0.742-0.748. In contrast to the typical RRd stars, in the majority of the anomalous pulsators the F-mode amplitudes are higher than the 1O-mode amplitudes. The light curves associated with the F-mode in the anomalous RRd stars show different morphology than the light curves of, both, regular RRd stars and single-mode RRab stars. Most of the anomalous double-mode stars show long-term modulations of the amplitudes (Blazhko-like effect). Translating the period ratios into the abundance parameter, Z, we find for our stars Z ∈ (0.002, 0.005) - an order of magnitude higher values than typical for RR Lyrae stars. The mass range of the RRd stars inferred from the WI vs. PF diagram is (0.55 - 0.75)M⊚. These parameters cannot be accounted for with single star evolution assuming a Reimers-like mass loss. Much greater mass loss caused by interaction with other stars is postulated. We blame the peculiar pulsation properties of our stars to the parametric resonance instability of the 1O-mode to excitation of the F- and 2O-modes as with the inferred parameters of the stars 2ω1O ≈ ωF + ω2O. [ABSTRACT FROM AUTHOR]

We present the analysis of Blazhko-type modulation in double-mode RR Lyrae (RRd) stars found in the Optical Gravitational Lensing Experiment (OGLE) photometry of the Galactic bulge. Modulation is detected in 15 stars. Most of them have non-typical period ratio of the radial modes. In the Petersen diagram, at a given period of the fundamental mode, they are located significantly below or above the sequence formed by the majority of RRd stars. Multiperiodic modulation is very frequent; two or three modulation periods are detected in eight stars. Modulation periods vary from ∼20 to more than 300 d. Radial mode amplitudes can be modulated by a few to nearly 100 per cent. Both radial modes may be modulated with the same period. More commonly however, dominant modulation for the fundamental mode has different period than dominant modulation for the first overtone. Quite often modulation of only one mode is detected in the data. We find a clear feedback between pulsation amplitude of the dominant mode and mean stellar brightness: the lower the pulsation amplitude, the brighter the star. At phases of low pulsation amplitude, the mode periods are prone to fast changes. All the stars share the common feature: their pulsation properties are non-stationary. Amplitudes and phases of the radial modes vary irregularly on a long time-scale of a few hundred or thousand days. The short-term modulations are also irregular. One of the stars has switched the pulsation mode recently: from single-mode fundamental mode pulsation to RRd state. In other star the non-radial mode with characteristic ∼0.61 period ratio to the first overtone is detected. This non-radial mode is likely modulated with the same period as the radial modes. [ABSTRACT FROM AUTHOR]

We report the discovery of an intriguing triple-mode RR Lyrae star found in the Optical Gravitational Lensing Experiment (OGLE) Galactic bulge collection, OGLE-BLG-RRLYR-24137. In the OGLE catalogue, the star was identified as RRd star – double-mode pulsator, pulsating simultaneously in the fundamental and in the first overtone modes. We find that third mode is excited and firmly detect its period doubling. Period ratios are not far from that expected for triple-mode – fundamental, first and third overtone – pulsation. Unfortunately, we cannot reproduce period ratios of the three modes with a consistent set of pulsation models. Therefore the other interpretation, that additional mode is non-radial, is also likely. [ABSTRACT FROM AUTHOR]

We present a phenomenological analysis of long-period variables (LPVs) in the Large Magellanic Cloud with the aim of detecting pulsation modes associated with different period-luminosity (PL) relations. Among brighter LPVs, we discover a group of triple-mode semi-regular variables with the fundamental, first-overtone, and second-overtone modes simultaneously excited, which fall on PL sequences C, C′, and B, respectively. The mode identification in the fainter red giants is more complicated. We demonstrate that the fundamental-mode pulsators partly overlap with the first-overtone modes. We show a possible range of fundamental mode and first overtone periods in the PL diagram. [ABSTRACT FROM AUTHOR]

ABSTRACT We report the discovery of a period doubling behaviour in a 2.4-d BL Herculis type variable of the Galactic bulge. Another bulge BL Her type star ( P = 2.25 d) is a strong period doubling candidate. Both objects have been identified with the Optical Gravitational Lensing Experiment (OGLE) photometry. Possibility of period doubling in this type of pulsators has been predicted almost 20 yr ago by Buchler & Moskalik. Our finding is the first observational confirmation of their theoretical results. Discovery of the first BL Herculis star showing the period doubling effect motivates a new theoretical investigation with the state-of-the-art convective pulsation codes. We present the results of initial model survey, specifically aimed at studying the observed period-doubled BL Her variable. All of our non-linear models have P = 2.4 d. The computations confirm that the period doubling effect is caused by the 3:2 resonance between the fundamental mode and the first overtone, as indicated by earlier radiative models of Buchler & Moskalik. Comparison of the computed and the observed light curves allows us to constrain the parameters of the star, in particular its metallicity, which appears to be high, Z ≈ 0.01. The recent evolutionary tracks put also constraint on the mass of the star, which is close to M = 0.50 M⊙. [ABSTRACT FROM AUTHOR]

We have performed a frequency analysis of 10 092 δ Scuti-type stars detected in the fourth phase of the Optical Gravitational Lensing Experiment towards the Galactic bulge, which is the most numerous homogeneous sample of δ Scuti stars observed so far. The main goal was to search for stars pulsating in at least two radial modes simultaneously. We have found 3083 candidates for such stars, which is the largest set obtained to date. Among them, 2655 stars pulsate in two radial modes, 414 stars pulsate in three radial modes, and 14 stars pulsate in four radial modes at the same time. We report the identification of 221 δ Scuti stars pulsating in the fundamental mode, first overtone, and third overtone simultaneously. We show the most populated Petersen and Bailey diagrams and discuss statistical properties of the identified frequencies based on this numerous sample. Additionally, we present theoretical predictions of period ratios for δ Scuti stars pulsating in overtones from the fourth to the seventh. [ABSTRACT FROM AUTHOR]

Context. Close binary central stars of planetary nebulae (PNe) must have formed through a common envelope evolution during the giant phase experienced by one of the stars. Transfer of the angular momentum from the binary system to the envelope leads to the shortening of the binary separations from the radius of red giant to the radius of the order of few tenths of AU. Thus, close binary central stars of planetary nebulae are laboratories to study the common envelope phase of evolution. The close binary fraction in the Galaxy has been measured in various sky surveys, but the close binary fraction is not yet well constrained for the Magellanic Clouds, and our results may help the study of common envelope evolution in low-metallicity environments. Aims. This paper presents a continuation of our study of variability in the Magellanic Cloud planetary nebulae on the basis of data from the OGLE survey. Previously, we had analysed the OGLE data in the Small Magellanic Cloud. Here, the study is extended to the Large Magellanic Cloud (LMC). In this paper we search for close binary central stars with the aim to constrain the binary fraction and period distribution in the LMC. Methods. We identified 290 counterparts of PNe in the LMC in the I-band images from the OGLE-III and OGLE-IV surveys. However, the light curves of ten objects were not accessible in the OGLE database, and thus we analysed the time series photometry of 280 PNe. Results. In total, 32 variables were found, but 5 of them turned out to be foreground objects. Another 18 objects show irregular or regular variability that is not attributable to the binarity of their central stars. Their status and the nature of their variability will be verified in the follow-up paper. Nine binary central stars of PNe with periods between 0.24 and 23.6 days were discovered. The obtained fraction for the LMC PNe is 3.3-1.6+2.6% without correcting for incompleteness. This number is significantly lower than the 12–21% derived in the analogous search in the Galactic bulge. We discuss this difference, taking into account observational biases. The lower binary fraction suggests a lower efficiency of the common envelope phase in producing close binaries in the LMC compared to the Galaxy. [ABSTRACT FROM AUTHOR]

Recent studies of massive binaries with putative black hole companions have uncovered a phase of binary evolution that has not been observed before, featuring a bloated stripped star that very recently ceased transferring mass to a main-sequence companion. In this study, we focus on the candidate system VFTS 291, a binary with an orbital period of 108 d and a high semi-amplitude velocity (K 1 = 93.7 ± 0.2 km s−1). Through our analysis of the disentangled spectra of the two components, together with dynamical and evolutionary arguments, we identify a narrow-lined star of ∼1.5–2.5 |$\, \mathrm{M}_\odot$| dominating the spectrum, and an early B-type main-sequence companion of 13.2 ± 1.5 |$\, \mathrm{M}_\odot$|⁠. The low mass of the narrow-lined star, and the high mass ratio, suggest that VFTS 291 is a post-mass-transfer system, with the narrow-lined star being bloated and stripped of its hydrogen-rich envelope, sharing many similarities with other recently discovered stripped stars. Our finding is supported by our detailed binary evolution models, which indicate that the system can be well explained by an initial configuration consisting of an 8.1 |$\, \mathrm{M}_\odot$| primary with an 8 |$\, \mathrm{M}_\odot$| companion in a 7 d orbital period. While some open questions remain, particularly concerning the surface helium enrichment of the stripped star and the rotational velocity of the companion, we expect that high-resolution spectroscopy may help reconcile our estimates with theory. Our study highlights the importance of multi-epoch spectroscopic surveys to identify and characterize binary interaction products, and provides important insights into the evolution of massive binary stars. [ABSTRACT FROM AUTHOR]

We present the analysis of the Blazhko effect – quasi-periodic modulation of pulsation amplitude and/or phase – in the Galactic bulge first overtone RR Lyrae stars (RRc). We used the data gathered during the fourth phase of the Optical Gravitational Lensing Experiment (OGLE). Out of $$10\, 826$$ analysed RRc stars, Blazhko effect was detected in 607 stars which constitute 5.6 per cent of the sample. It is the largest and most homogeneous sample of modulated RRc stars analysed so far. Modulation periods cover a wide range, from slightly above 2 d to nearly 3000 d. Multiperiodic modulation was detected in 47 stars. The appearance of modulation in the frequency domain was studied in detail. Modulation manifests either as close doublets or as equidistant triplets and multiplets centred on radial mode frequency and its harmonics. In a significant fraction (29 per cent) of stars, we have detected the modulation frequency itself, which corresponds to the modulation of the mean stellar brightness. Our search for period doubling effect, that was discovered recently in modulated fundamental mode RR Lyrae stars, and triggered development of new model behind the Blazhko modulation, yielded negative result. In 104 stars, we detected additional signals that could correspond to both radial and non-radial modes. Statistical properties of modulated stars were analysed in detail and confronted with properties of non-modulated stars and of modulated fundamental mode RR Lyrae stars. Our analysis provides constraints for the models to explain the Blazhko phenomenon, which still remains a puzzle more than hundred years after its discovery. [ABSTRACT FROM AUTHOR]

We present a photometric and spectroscopic study of AzV322, an emission line object located in the Small Magellanic Cloud previously classified between O9 and B0. We analyse 17.5 yr of I- and V-band OGLE-II, -III and -IV light curves and find four significant frequencies, viz. f1 = 0.386 549 ± 0.000 003, f2 = 0.101 177 ± 0.000 005, f3 = 0.487 726 ± 0.000 015 and f4 = 0.874 302 ± 0.000 020 cycles d-1. The f1 frequency (period 2.587 00 ± 0.000 02 d) provides the stronger periodogram peak and gives a single wave light curve of full amplitude 0.066 mag in the I band. High-resolution optical spectroscopy confirms the early B-type spectral type and reveals prominent double peak Balmer, Paschen, OI 8446 and He I 5875 emissions. The spectral energy distribution shows significant colour excess towards long wavelengths possibly attributed to free-free emission in a disc -like envelope. Our analysis yields Teff = 23 000 ± 1500 K, log g = 3.0 ± 0.5, M = 16 ± 1 M⊙, R = 31.0 ± 1.1 R⊙ and Lbol = 104.87 ± 0.06 L⊙. AzV322 might be a member of the new class of slowly pulsating B supergiants introduced by Saio et al. and documented by Lefever, Puls & Aerts; however its circumstellar disc makes it an hither to unique object. Furthermore, we notice that an O-C analysis for f1 reveals quasi-cyclic changes for the times of maximum in a time-scale of 20 yr, which might indicate a light-travel time effect in a very wide orbit binary with an undetected stellar component. [ABSTRACT FROM AUTHOR]

We present a study of Optical Gravitational Lensing Experiment light curves of red giant stars exhibiting long secondary periods (LSPs)—an enigmatic phenomenon commonly observed in stars on the upper red giant branch and asymptotic giant branch. We show that the light curves of LSP stars are essentially identical to those of the spotted variables with one dark spot on their photospheres. Such behavior can be explained by the presence of a dusty cloud orbiting the red giant together with a low-mass companion in a close, circular orbit. We argue that the binary scenario is in agreement with most of the observational properties of LSP variables, including non-sinusoidal shapes of their radial velocity curves. [ABSTRACT FROM AUTHOR]

This is the fourth data paper publishing lightcurve survey work of 52 Near Earth Asteroids (NEAs) using 10 telescopes available to the EURONEAR network between 2017 and 2020. Forty six targets were not observed before our runs (88% of the sample) but some of these were targeted during the same oppositions mainly by Brian Warner. We propose new periods for 20 targets (38% of the sample), confirming published data for 20 targets, while our results for 8 targets do not match published data. We secured periods for 15 targets (29% of the sample), candidate periods for 23 objects (44%), tentative periods for 11 asteroids (21%), and have derived basic information about 3 targets (6% of the sample). We calculated the lower limit of the ellipsoid shape ratios a/b for 46 NEAs (including 13 PHAs). We confirmed or suggested 4 binary objects, recommending two of them for follow-up during future dedicated campaigns. [ABSTRACT FROM AUTHOR]

We present a variability study of 4646 massive stars in the Small Magellanic Cloud (SMC) with known spectral types from the catalog of Bonanos et al. (2010) using the light curves from the OGLE-III database. The goal is to exploit the time domain information available through OGLE-III to gain insight into the processes that govern the evolution of massive stars. This variability survey of massive stars with known spectral types is larger than any previous survey by a factor of 7. We find that 60% of our sample (2766 stars) show no significant variability and 40% (1880 stars) exhibit variability distributed as follows: 807 stars display low-amplitude stochastic variability with fluctuations in the I-band of up to 0.05 mag, 443 stars present irregular variability of higher amplitude (76% of these are reported as variables for the first time), 205 are eclipsing binaries (including 101 newly discovered systems), 50 are candidate rotating variables, 126 are classical Cepheids, 188 stars exhibit short-term sinusoidal periodicity (P < 3 days) making them candidate "slowly pulsating B stars" and non-radial Be pulsators, and 61 periodic stars exhibit longer periods. We demonstrate the wealth of information provided in the time domain, by doubling the number of known massive eclipsing binary systems and identifying 189 new candidate early-type Be and 20 Oe stars in the SMC. In addition, we find that ~80% of Be stars are photometrically variable in the OGLE-III time domain and provide evidence that short-term pulsating stars with additional photometric variability are rotating close to their break-up velocity. [ABSTRACT FROM AUTHOR]

BSTRACT We have performed the Fourier decomposition analysis of 11-yr V-band light curves of a carefully selected sample of 100 RR Lyrae variables, detected in the central regions of the Small Magellanic Cloud (SMC), with the Optical Gravitational Lensing Experiment, phases II and III. The sample consisted of 84 fundamental mode pulsators (RRab stars) and 16 first-overtone pulsators (RRc stars). The Fourier decomposition parameters were used to derive metal abundances and distance moduli for these RR Lyrae variables. The average metal abundance of the RRab stars on the new scale of Carretta et al. is found to be 〈[Fe/H]C09〉=−1.62 ± 0.41 dex (std, with a standard error of 0.05 dex). The distance modulus of the SMC was found to be 〈μ〉= 18.90 ± 0.18 (std) from the RRab population in a distance scale where the distance modulus of the Large Magellanic Cloud (LMC) is μLMC= 18.52 ± 0.06 (std). The 1σ line-of-sight depth for the RRab stars was found to be σint= 4.13 ± 0.27 kpc, in good agreement with that estimated from red clump stars and eclipsing binaries. There is a clear indication that the metal-poorer and metal-richer objects in the sample may belong to different dynamical structures. The metal-rich stars have smaller scaleheight and could be part of a disc-like structure, while the more metal poor objects could be part of a much thicker structure, such as an inner halo or a bulge component. [ABSTRACT FROM AUTHOR]

ABSTRACT The intermediate-age Magellanic Cloud clusters NGC 1978 and 419 are each found to contain substantial numbers of pulsating asymptotic giant branch (AGB) stars, both oxygen rich and carbon rich. Each cluster also contains two pulsating AGB stars which are infrared sources with a large mass-loss rate. Pulsation masses have been derived for the AGB variables, from the lowest luminosity O-rich variables to the most evolved infrared sources. It is found that the stars in NGC 1978 have a mass of 1.55 M⊙ early on the AGB while the NGC 419 stars have a mass of 1.87 M⊙ early on the AGB. These masses are in good agreement with those expected from the cluster ages determined by main-sequence turn-off fitting. Non-linear pulsation models fitted to the highly evolved AGB stars show that a substantial amount of mass-loss has occurred during the AGB evolution of these stars. An examination of the observed mass-loss on the AGB, and the AGB tip luminosities, shows that in both clusters the mass-loss rates computed from the formula of Vassiliadis & Wood reproduce the observations reasonably well. The mass-loss rates computed from the formula of Blöcker terminate the AGB in both clusters at a luminosity which is much too low. [ABSTRACT FROM AUTHOR]

We use photometric and spectroscopic observations of the eclipsing binary E32 in the globular cluster 47 Tuc to derive the masses, radii, and luminosities of the component stars. The system has an orbital period of 40.9 d, a markedly eccentric orbit with e  = 0.24, and is shown to be a member of or a recent escaper from the cluster. We obtain |$M_{\rm p} = 0.862\pm 0.005 \, \mathrm{M}_\odot$|⁠ , |$R_{\rm p} = 1.183\pm 0.003 \, \mathrm{R}_\odot$|⁠ , |$L_{\rm p} = 1.65\pm 0.05 \, \mathrm{L}_\odot$| for the primary and |$M_{\rm s} = 0.827\pm 0.005 \, \mathrm{M}_\odot$|⁠ , |$R_{\rm s} = 1.004\pm 0.004 \, \mathrm{R}_\odot$|⁠ , |$L_{\rm s} = 1.14\pm 0.04\, \mathrm{L}_\odot$| for the secondary. Based on these data and on an earlier analysis of the binary V69 in 47 Tuc, we measure the distance to the cluster from the distance moduli of the component stars, and, independently, from a colour – surface brightness calibration. We obtain 4.55 ± 0.03 and 4.50 ± 0.07 kpc, respectively – values compatible within 1 |$\, \sigma$| with recent estimates based on Gaia DR2 parallaxes. By comparing the M – R diagram of the two binaries and the colour–magnitude diagram of 47 Tuc to Dartmouth model isochrones we estimate the age of the cluster to be 12.0 ± 0.5 Gyr, and the helium abundance of the cluster to be Y ≈ 0.25. [ABSTRACT FROM AUTHOR]

We report the discovery of a very young high-mass X-ray binary (HMXB) system associated with the supernova remnant (SNR) MCSNR J0513-6724 in the Large Magellanic Cloud (LMC), using XMM–Newton X-ray observations. The HMXB is located at the geometrical centre of extended soft X-ray emission, which we confirm as an SNR. The HMXB spectrum is consistent with an absorbed power law with spectral index ∼1.6 and a luminosity of 7 × 1033 erg s−1 (0.2–12 keV). Tentative X-ray pulsations are observed with a periodicity of 4.4 s and the OGLE I -band light curve of the optical counterpart from more than 17.5 yr reveals a period of 2.2324 ± 0.0003 d, which we interpret as the orbital period of the binary system. The X-ray spectrum of the SNR is consistent with non-equilibrium shock models as expected for young/less evolved SNRs. From the derived ionization time-scale we estimate the age of the SNR to be <6 kyr. The association of the HMXB with the SNR makes it the youngest HMXB, in the earliest evolutionary stage known to date. An HMXB as young as this can switch on as an accreting pulsar only when the spin period has reached a critical value. Under this assumption, we obtain an upper limit to the magnetic field of <5 × 1011 G. This implies several interesting possibilities including magnetic field burial, possibly by an episode of post-supernova hyper-critical accretion. Since these fields are expected to diffuse out on a time-scale of 103–104 yr, the discovery of a very young HMXB can provide us the unique opportunity to observe the evolution of the observable magnetic field for the first time in X-ray binaries. [ABSTRACT FROM AUTHOR]

We present candidates for non-pulsating stars lying in the classical Cepheid instability strip based on the Optical Gravitational Lensing Experiment (OGLE) photometric maps combined with Strömgren photometry obtained with the 4.1-m Southern Astrophysical Research (SOAR) telescope, and Gaia Data Release 2 data in four fields in the Large Magellanic Cloud. We selected 19 candidates in total. After analysis of their light curves from OGLE surveys we found that all these stars appear to be photometrically stable at the level of a few mmag. Our results show that non-pulsating stars might constitute to about |$21\!-\!30\, {\rm per\, cent}$| of the whole sample of giant stars located in the classical instability strip. Furthermore, we identified potential candidates for classical Cepheids with hot companions based on their Strömgren colours. [ABSTRACT FROM AUTHOR]

We present the analysis of the event OGLE-2017-BLG-1186 from the 2017 Spitzer microlensing campaign. This is a remarkable microlensing event because its source is photometrically bright and variable, which makes it possible to perform an asteroseismic analysis using ground-based data. We find that the source star is an oscillating red giant with average time-scale of ∼9 d. The asteroseismic analysis also provides us source properties including the source angular size (∼27  |$\mu$| as) and distance (∼11.5 kpc), which are essential for inferring the properties of the lens. When fitting the light curve, we test the feasibility of Gaussian processes (GPs) in handling the correlated noise caused by the variable source. We find that the parameters from the GP model are generally more loosely constrained than those from the traditional χ2 minimization method. We note that this event is the first microlensing system for which asteroseismology and GPs have been used to account for the variable source. With both finite-source effect and microlens parallax measured, we find that the lens is likely a ∼0.045 M⊙ brown dwarf at distance ∼9.0 kpc, or a ∼0.073 M⊙ ultracool dwarf at distance ∼9.8 kpc. Combining the estimated lens properties with a Bayesian analysis using a Galactic model, we find a |$\sim 35{{\ \rm per\ cent}}$| probability for the lens to be a bulge object and |$\sim 65{{\ \rm per\ cent}}$| to be a background disc object. [ABSTRACT FROM AUTHOR]

OGLE-LMC-DPV-065 is an interacting binary whose double-hump long photometric cycle remains hitherto unexplained. We analyze photometric time series available in archive data sets spanning 124 yr and present the analysis of new high-resolution spectra. A refined orbital period is found of 10 |${^{\rm d}_{.}}$| 031 6267 ± 0 |${^{\rm d}_{.}}$| 000 0056 without any evidence of variability. In spite of this constancy, small but significant changes in timings of the secondary eclipse are detected. We show that the long period continuously decreases from 350 to 218 d during 13 yr, then remains almost constant for about 10 yr. Our study of radial velocities indicates a circular orbit for the binary and yields a mass ratio of 0.203 ± 0.001. From the analysis of the orbital light curve, we find that the system contains 13.8 and 2.81 M⊙ stars of radii 8.8 and 12.6 R⊙ and absolute bolometric magnitudes −6.4 and −3.0, respectively. The orbit semimajor axis is 49.9 R⊙ and the stellar temperatures are 25 460 K and 9825 K. We find evidence for an optically and geometrically thick disc around the hotter star. According to our model, the disc has a radius of 25 R⊙, central and outer vertical thickness of 1.6 R⊙ and 3.5 R⊙, and temperature of 9380 K at its outer edge. Two shock regions located at roughly opposite parts of the outer disc rim can explain the light-curve asymmetries. The system is a member of the double periodic variables and its relatively high-mass and long photometric cycle make it similar in some aspects to β Lyrae. [ABSTRACT FROM AUTHOR]

Aims. We report on the discovery and follow-up of a peculiar transient, OGLE17aaj, which occurred in the nucleus of a weakly active galaxy. We investigate whether it can be interpreted as a new candidate for a tidal disruption event (TDE). Methods. We present the OGLE-IV light curve that covers the slow 60-day-long rise to maximum along with photometric, spectroscopic, and X-ray follow-up during the first year. Results. OGLE17aaj is a nuclear transient exhibiting some properties similar to previously found TDEs, including a long rise time, lack of colour-temperature evolution, and high black-body temperature. On the other hand, its narrow emission lines and slow post-peak evolution are different from previously observed TDEs. Its spectrum and light-curve evolution is similar to F01004-2237 and AT 2017bgt. Signatures of historical low-level nuclear variability suggest that OGLE17aaj may instead be related to a new type of accretion event in active super-massive black holes. [ABSTRACT FROM AUTHOR]

Aims. Our aim is to obtain high-accuracy measurements of the physical and orbital parameters of two evolved eclipsing binary systems, and to use these measurements to study their evolutionary status. We also aim to derive the distances to the systems by using a surface brightness–colour relation and compare these distances with the measurements provided by Gaia. Methods. We measured the physical and orbital parameters of both systems based on V-band and I-band photometry from OGLE, near-infrared photometry obtained with the NTT telescope and the instrument SOFI, as well as high-resolution spectra obtained at ESO 3.6m/HARPS and Clay 6.5/MIKE spectrographs. The light curves and radial-velocity curves were analysed with the Wilson–Devinney code. Results. We analysed two double-lined eclipsing binary systems OGLE-BLG-ECL-123903 and OGLE-BLG-ECL-296596 from the Optical Gravitational Lensing Experiment (OGLE) catalogue. Both systems have a configuration of two well-detached giants stars. The masses of the components of OGLE-BLG-ECL-123903 are M1 = 2.045 ± 0.027 and M2 = 2.074 ± 0.023 M⊙ and the radii are R1 = 9.540 ± 0.049 and R2 = 9.052 ± 0.060 R⊙. For OGLE-BLG-ECL-296596, the masses are M1 = 1.093 ± 0.015 and M2 = 1.125 ± 0.014 M⊙, while the radii are R1 = 18.06 ± 0.28 and R2 = 29.80 ± 0.33 R⊙. Evolutionary status was discussed based on the isochrones and evolutionary tracks from PARSEC and MESA codes. The ages of the systems were established to be around 1.3 Gyr for the OGLE-BLG-ECL-123903 and 7.7 Gyr for the OGLE-BLG-ECL-296596. We also determined the distance to both systems. For OGLE-BLG-ECL-123903 this is equal to d = 2.95 ± 0.06 (stat.) ±0.07 (syst.) kpc, while for the OGLE-BLG-ECL-296596 it is d = 5.68 ± 0.07 (stat.) ±0.14 (syst.) kpc. This is the first analysis of its kind for these unique evolved eclipsing binary systems. [ABSTRACT FROM AUTHOR]

We have analysed the photometry of 924 type II Cepheids from the Galactic bulge observed by the Optical Gravitational Lensing Experiment. Our goal was to detect and analyse various dynamical phenomena, including, e.g. multimode pulsation, period-doubled pulsation, period- k pulsation, or quasi-periodic modulation of pulsation. First examples of double-mode BL Her-type stars, pulsating simultaneously in the fundamental and first overtone modes are reported. Two double-mode pulsators are not isolated cases however, but form an extension of recently identified class of long-period RRd stars. Period-doubling effect was detected in three BL Her-type stars: two were identified previously and one is a new discovery. Through identification of numerous period-doubled W Vir stars, we show that the appearance of the period-doubling effect, and hence the transition towards RV Tau class, is a smooth process that starts at pulsation periods slightly above 15 d. Interchange of the deep and shallow pulsation cycles is identified in tens of RV Tau variables. One RV Tau star is a strong candidate for period-4 pulsation. Quasi-periodic modulation of pulsation in all sub-classes of type II Cepheids is reported for the first time. Modulation was detected in 16 BL Her and 9 W Vir and in 7 RV Tau stars. In the last case, modulation is the most likely scenario, but two other scenarios, excitation of non-radial modes, or period-4 pulsation, can also account for the observed pulsation pattern. Irregular changes of pulsation amplitude and period on various time-scales are common in all sub-groups of type II Cepheids. [ABSTRACT FROM AUTHOR]

Double Periodic Variables (DPVs) are hot Algols showing a long photometric cycle of uncertain origin. We report the discovery of changes in the orbital light curve of OGLE-LMC-DPV-097 which depend on the phase of its long photometric cycle. During the ascending branch of the long cycle the brightness at the first quadrature is larger than during the second quadrature, during the maximum of the long cycle the brightness is basically the same at both quadratures, during the descending branch the brightness at the second quadrature is larger than during the first quadrature, and during the minimum of the long cycle the secondary minimum disappears. We model the light curve at different phases of the long cycle and find that the data are consistent with changes in the properties of the accretion disc and two disc spots. The disc's size and temperature change with the long-cycle period. We find a smaller and hotter disc at minimum, and larger and cooler disc at maximum. The spot temperatures, locations, and angular sizes also show variability during the long cycle. [ABSTRACT FROM AUTHOR]

Be stars are main-sequence massive stars with emission features in their spectrum, which originates in circumstellar gaseous discs. Even though the viscous decretion disc model can satisfactorily explain most observations, two important physical ingredients, namely the magnitude of the viscosity (α) and the disc mass injection rate, remain poorly constrained. The light curves of Be stars that undergo events of disc formation and dissipation offer an opportunity to constrain these quantities. A pipeline was developed to model these events that use a grid of synthetic light curves, computed from coupled hydrodynamic and radiative transfer calculations. A sample of 54 Be stars from the OGLE survey of the Small Magellanic Cloud (SMC) was selected for this study. Because of the way our sample was selected (bright stars with clear disc events), it likely represents the densest discs in the SMC. Like their siblings in the Galaxy, the mass of the disc in the SMC increases with the stellar mass. The typical mass and angular momentum loss rates associated with the disc events are of the order of ∼10−10 M⊙ yr−1 and ∼5 × 1036 g cm2 s−2, respectively. The values of α found in this work are typically of a few tenths, consistent with recent results in the literature and with the ones found in dwarf novae, but larger than current theory predicts. Considering the sample as a whole, the viscosity parameter is roughly two times larger at build-up (〈αbu〉 = 0.63) than at dissipation (〈αd〉 = 0.26). Further work is necessary to verify whether this trend is real or a result of some of the model assumptions. [ABSTRACT FROM AUTHOR]

We report the discovery of a new gravitationally lensed quasar (double) from the Optical Gravitational Lensing Experiment (OGLE) identified inside the ~670deg2 area encompassing the Magellanic Clouds. The source was selected as one of ~60 'red W1 - W2' mid-infrared objects from WISE and having a significant amount of variability in OGLE for both two (or more) nearby sources. This is the first detection of a gravitational lens, where the discovery is made 'the other way around', meaning we first measured the time delay between the two lensed quasar images of -132 < tAB < -76 d (90 per cent CL), with the median tAB ≈-102 d (in the observer frame), and where the fainter image B lags image A. The system consists of the two quasar images separated by 1.5 arcsec on the sky, with I ≈20.0mag and I ≈19.6mag, respectively, and a lensing galaxy that becomes detectable as I ≈21.5 mag source, 1.0 arcsec from image A, after subtracting the two lensed images. Both quasar images show clear AGN broad emission lines at z=2.16 in the New Technology Telescope spectra. The spectral energy distribution (SED) fitting with the fixed source redshift provided the estimate of the lensing galaxy redshift of z ≈0.9 ± 0.2 (90 per cent CL), while its type is more likely to be elliptical (the SED-inferred and lens-model stellar mass is more likely present in ellipticals) than spiral (preferred redshift by the lens model). [ABSTRACT FROM AUTHOR]

We present a detailed elemental abundance study of 90 F and G dwarf, turn-off, and subgiant stars in the Galactic bulge. Based on high-resolution spectra acquired during gravitational microlensing events, stellar ages and abundances for 11 elements (Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Zn, Y and Ba) have been determined. Four main findings are presented: (1) a wide metallicity distribution with distinct peaks at [Fe=H] = --1:09; --0:63; --0:20; +0:12; +0:41; (2) a high fraction of intermediate-age to young stars where at [Fe=H] > 0 more than 35% are younger than 8 Gyr, and for [Fe=H] ≲ --0:5 most stars are 10 Gyr or older; (3) several episodes of significant star formation in the bulge has been identified: 3, 6, 8, and 11 Gyr ago; (4) tentatively the "knee" in the ff-element abundance trends of the sub-solar metallicity bulge is located at a slightly higher [Fe/H] than in the local thick disk. These findings show that the Galactic bulge has complex age and abundance properties that appear to be tightly connected to the main Galactic stellar populations. In particular, the peaks in the metallicity distribution, the star formation episodes, and the abundance trends, show similarities with the properties of the Galactic thin and thick disks. At the same time, the star formation rate appears to have been slightly faster in the bulge than in the local thick disk, which most likely is an indication of the denser stellar environment closer to the Galactic centre. There are also additional components not seen outside the bulge region, and that most likely can be associated with the Galactic bar. Our results strengthen the observational evidence that support the idea of a secular origin for the Galactic bulge, formed out of the other main Galactic stellar populations present in the central regions of our Galaxy. Additionally, our analysis of this enlarged sample suggests that the (V -- I)0 colour of the bulge red clump should be revised to 1.09. [ABSTRACT FROM AUTHOR]

The Large Synoptic Survey Telescope (LSST) is expected to yield ~107 light curves over the course of its mission, which will require a concerted effort in automated classification. Stochastic processes provide one means of quantitatively describing variability with the potential advantage over simple light-curve statistics that the parameters may be physically meaningful. Here, we survey a large sample of periodic, quasi-periodic and stochastic Optical Gravitational Lensing Experiment-III variables using the damped random walk (DRW; CARMA(1,0)) and quasi-periodic oscillation (QPO; CARMA(2,1)) stochastic process models. The QPO model is described by an amplitude, a period and a coherence time-scale, while the DRWhas only an amplitude and a time-scale.We find that the periodic and quasi-periodic stellar variables are generally better described by a QPO than a DRW, while quasars are better described by the DRW model. There are ambiguities in interpreting the QPO coherence time due to non-sinusoidal light-curve shapes, signal-to-noise ratio, error mischaracterizations and cadence. Higher order implementations of the QPO model that better capture light-curve shapes are necessary for the coherence time to have its implied physical meaning. Independent of physical meaning, the extra parameter of the QPO model successfully distinguishes most of the classes of periodic and quasi-periodic variables we consider. [ABSTRACT FROM AUTHOR]

We present the discovery and first three months of follow-up observations of a currently ongoing unusual transient detected by the Optical Gravitational Lensing Experiment (OGLE-IV) survey, located in the centre of a galaxy at redshift z = 0.1655. The long rise to absolute magnitude of −20.5 mag, slow decline, very broad He and H spectral features make OGLE16aaa similar to other optical/UV tidal disruption events (TDEs).Weak narrow emission lines in the spectrum and archival photometric observations suggest the host galaxy is a weak-line active galactic nucleus, which has been accreting at higher rate in the past. OGLE16aaa, along with SDSS J0748, seems to form a sub-class of TDEs by weakly or recently active supermassive black holes (SMBHs). This class might bridge the TDEs by quiescent SMBHs and flares observed as ‘changing-look quasars’, if we interpret the latter as TDEs. If this picture is true, the previously applied requirement for identifying a flare as a TDE that it had to come from an inactive nucleus, could be leading to observational bias in TDE selection, thus affecting TDE-rate estimations. [ABSTRACT FROM AUTHOR]

Most stellar remnants so far have been found in binary systems, where they interact with matter from their companions. Isolated neutron stars and black holes are difficult to find as they are dark, yet they are predicted to exist in our Galaxy in vast numbers. We explored the OGLE-III data base of 150 million objects observed in years 2001-2009 and found 59 microlensing events exhibiting a parallax effect due to the Earth's motion around the Sun. Combining parallax and brightness measurements from microlensing light curves with expected proper motions in the Milky Way, we identified 13 microlensing events which are consistent with having a white dwarf, neutron star or a black hole lens and we estimated their masses and distances. The most massive of our black hole candidates has 9.3 M⊙ and is at a distance of 2.4 kpc. The distribution of masses of our candidates indicates a continuum in mass distribution with no mass gap between neutron stars and black holes. We also present predictions on how such events will be observed by the astrometric Gaia mission. [ABSTRACT FROM AUTHOR]

We present a statistical analysis of the first four seasons from a 'second-generation' microlensing survey for extrasolar planets, consisting of near-continuous time coverage of 8 deg² of the Galactic bulge by the Optical Gravitational Lens Experiment (OGLE), Microlensing Observations in Astrophysics (MOA), and Wise microlensing surveys. During this period, 224 microlensing events were observed by all three groups. Over 12 per cent of the events showed a deviation from single-lens microlensing, and for ~one-third of those the anomaly is likely caused by a planetary companion. For each of the 224 events, we have performed numerical ray-tracing simulations to calculate the detection efficiency of possible companions as a function of companion-to-host mass ratio and separation. Accounting for the detection efficiency, we find that 55-22+34 per cent of microlensed stars host a snowline planet. Moreover, we find that Neptune-mass planets are ~10 times more common than Jupiter-mass planets. The companion-to-host mass-ratio distribution shows a deficit at q ~ 10-2, separating the distribution into two companion populations, analogous to the stellar-companion and planet populations, seen in radial-velocity surveys around solar-like stars. Our survey, however, which probes mainly lower mass stars, suggests a minimum in the distribution in the super-Jupiter mass range, and a relatively high occurrence of brown-dwarf companions. [ABSTRACT FROM AUTHOR]

We use six years (2003-2008) of Optical Gravitational Lensing Experiment-III microlensing observations to derive the survey detection efficiency for a range of planetary masses and projected distances from the host star. We perform an independent analysis of the microlensing light curves to extract the event parameters and compute the planet detection probability given the data. 2433 light curves satisfy our quality selection criteria and are retained for further processing. The aggregate of the detection probabilities over the range explored yields the expected number of microlensing planet detections. We employ a Galactic model to convert this distribution from dimensionless to physical units, α/au and M⊕. The survey sensitivity to small planets is highest in the range 1-4 au, shifting to slightly larger separations for more massive ones. [ABSTRACT FROM AUTHOR]

The first stars are predicted to have formed within 200 million years after the Big Bang, initiating the cosmic dawn. A true first star has not yet been discovered, although stars with tiny amounts of elements heavier than helium ('metals') have been found in the outer regions ('halo') of the Milky Way. The first stars and their immediate successors should, however, preferentially be found today in the central regions ('bulges') of galaxies, because they formed in the largest over-densities that grew gravitationally with time. The Milky Way bulge underwent a rapid chemical enrichment during the first 1-2 billion years, leading to a dearth of early, metal-poor stars. Here we report observations of extremely metal-poor stars in the Milky Way bulge, including one star with an iron abundance about 10,000 times lower than the solar value without noticeable carbon enhancement. We confirm that most of the metal-poor bulge stars are on tight orbits around the Galactic Centre, rather than being halo stars passing through the bulge, as expected for stars formed at redshifts greater than 15. Their chemical compositions are in general similar to typical halo stars of the same metallicity although intriguing differences exist, including lower abundances of carbon. [ABSTRACT FROM AUTHOR]

Context. The stellar population of the 30 Doradus star-forming region in the Large Magellanic Cloud contains a subset of apparently single, rapidly rotating O-type stars. The physical processes leading to the formation of this cohort are currently uncertain. Aims. One member of this group, the late O-type star VFTS 399, is found to be unexpectedly X-ray bright for its bolometric luminosity - in this study we aim to determine its physical nature and the cause of this behaviour. Methods. To accomplish this we performed a time-resolved analysis of optical, infrared and X-ray observations. Results. We found VFTS 399 to be an aperiodic photometric variable with an apparent near-IR excess. Its optical spectrum demonstrates complex emission profiles in the lower Balmer series and select He I lines - taken together these suggest an OeBe classification. The highly variable X-ray luminosity is too great to be produced by a single star, while the hard, non-thermal nature suggests the presence of an accreting relativistic companion. Finally, the detection of periodic modulation of the X-ray lightcurve is most naturally explained under the assumption that the accretor is a neutron star. Conclusions. VFTS 399 appears to be the first high-mass X-ray binary identified within 30 Dor, sharing many observational characteristics with classical Be X-ray binaries. Comparison of the current properties of VFTS 399 to binary-evolution models suggests a progenitor mass ≳25 M☉ for the putative neutron star, which may host a magnetic field comparable in strength to those of magnetars. VFTS 399 is now the second member of the cohort of rapidly rotating "single" O-type stars in 30 Dor to show evidence of binary interaction resulting in spin-up, suggesting that this may be a viable evolutionary pathway for the formation of a subset of this stellar population. [ABSTRACT FROM AUTHOR]

We present the analysis of the gravitational microlensing event OGLE-2013-BLG-0102. The light curve of the event is characterized by a strong short-term anomaly superposed on a smoothly varying lensing curve with a moderate magnification Amax ~1.5. It is found that the event was produced by a binary lens with a mass ratio between the components of q = 0.13 and the anomaly was caused by the passage of the source trajectory over a caustic located away from the barycenter of the binary. From the analysis of the effects on the light curve due to the finite size of the source and the parallactic motion of the Earth, the physical parameters of the lens system are determined. The measured masses of the lens components are M1 = 0.096 ± 0.013 M⊙ and M2 = 0.012±0.002 M⊙, which correspond to near the hydrogen-burning and deuterium-burning mass limits, respectively. The distance to the lens is 3.04±0.31 kpc and the projected separation between the lens components is 0.80 ± 0.08 AU. [ABSTRACT FROM AUTHOR]

We analyse long-term optical photometry for two Galactic recurrent novae (V745 Sco and V3890 Sgr) and one Large Magellanic Cloud object (Nova LMC 1990b) observed over several years by the Optical Gravitational Lensing Experiment (OGLE) sky survey. We do not find variability with the previously claimed orbital period of V745 Sco. This voids previous findings based on this value (e.g. the distance determination). The quiescence variability of this object is dominated by semiregular pulsations of the red giant secondary (with periods of 136.5 and 77.4 d). The photometry of Nova LMC 1990b reveals an unnoticed eruption in 2010 and eclipse-like variability in quiescence with a period of 1.26432(8) d. The photometric properties make this object very similar to U Sco. Finally, we describe the eruptions of two likely symbiotic novae, V5590 Sgr and OGLE-2011-BLG-1444. The secondary of V5590 Sgr is a Mira star with pulsation period of 236 d. [ABSTRACT FROM PUBLISHER]

Using gravitational microlensing, we detected a cold terrestrial planet orbiting one member of a binary star system. The planet has low mass (twice Earth's) and lies projected at -0.8 astronomical units (AU) from its host star, about the distance between Earth and the Sun. However, the planet's temperature is much lower, <60 Kelvin, because the host star is only 0.10 to 0.15 solar masses and therefore more than 400 times less luminous than the Sun. The host itself orbits a slightly more massive companion with projected separation of 10 to 15 AU. This detection is consistent with such systems being very common. Straightforward modification of current microlensing search strategies could increase sensitivity to planets in binary systems. With more detections, such binary-star planetary systems could constrain models of planet formation and evolution. [ABSTRACT FROM AUTHOR]

Gravitational microlensing events produced by lenses composed of binary masses are important because they provide a major channel for determining physical parameters of lenses. In this work, we analyze the light curves of two binary-lens events, OGLE-2006-BLG-277 and OGLE-2012-BLG-0031, for which the light curves exhibit strong deviations from standard models. From modeling considering various second-order effects, we find that the deviations are mostly explained by the effect of the lens orbital motion. We also find that lens parallax effects can mimic orbital effects to some extent. This implies that modeling light curves of binary-lens events not considering orbital effects can result in lens parallaxes that are substantially different from actual values and thus wrong determinations of physical lens parameters. This demonstrates the importance of routine consideration of orbital effects in interpreting light curves of binary-lens events. It is found that the lens of OGLE-2006-BLG-277 is a binary composed of a low-mass star and a brown dwarf companion. [ABSTRACT FROM AUTHOR]

Observations of accretion disks around young brown dwarfs (BDs) have led to the speculation that they may form planetary systems similar to normal stars. While there have been several detections of planetary-mass objects around BDs (2MASS 1207-3932 and 2MASS 0441-2301), these companions have relatively large mass ratios and projected separations, suggesting that they formed in a manner analogous to stellar binaries. We present the discovery of a planetary-mass object orbiting a field BD via gravitational microlensing, OGLE-2012-BLG-0358Lb. The system is a low secondary/primary mass ratio (0.080 ± 0.001), relatively tightly separated (∼0.87 AU) binary composed of a planetary-mass object with 1.9 ± 0.2 Jupiter masses orbiting a BD with a mass 0.022 M☼. The relatively small mass ratio and separation suggest that the companion may have formed in a protoplanetary disk around the BD host in a manner analogous to planets. [ABSTRACT FROM AUTHOR]

The Galactic bulge shows a double red clump in sightlines at |b| ≳ 5° and –3° ≲ l ≲ 4°. This dump is interpreted as the signature of an X-shaped structure seen almost edge-on. We measure the proper motions of the stars belonging to the closer and the further arms of the X-shaped structure. The intrinsic kinematic properties of the two arms are found by incorporating information taken from the luminosity function. At b = –5°, we find that the proper motion difference between the two arms is a linear function of Galactic longitude for –0.°1 < l < 0.°5, which we interpret as a streaming motion of the stars within the X-shaped structure. A streaming motion was previously reported based on radial velocity data, not the proper motions. The proper motion difference in longitude is constant for –0.°8 < l < –0.°1, which provides an estimate of the bulge rotational speed of 87.9 ± 8.2 km s–1 kpc–1. [ABSTRACT FROM AUTHOR]

The Magellanic Quasars Survey (MQS) has now increased the number of quasars known behind the Magellanic Clouds by almost an order of magnitude. All survey fields in the Large Magellanic Cloud (LMC) and 70% of those in the Small Magellanic Cloud (SMC) have been observed. The targets were selected from the third phase of the Optical Gravitational Lensing Experiment (OGLE-III) based on their optical variability, mid-IR, and/or X-ray properties. We spectroscopically confirmed 758 quasars (565 in the LMC and 193 in the SMC) behind the clouds, of which 94% (527 in the LMC and 186 in the SMC) are newly identified. The MQS quasars have long-term (12 yr and growing for OGLE), high-cadence light curves, enabling unprecedented variability studies of quasars. The MQS quasars also provide a dense reference grid for measuring both the internal and bulk proper motions of the clouds, and 50 quasars are bright enough (I ≲ 18 mag) for absorption studies of the interstellar/intergalactic medium of the clouds. [ABSTRACT FROM AUTHOR]

Aims. We present the analysis of the gravitational microlensing event OGLE-2011-BLG-0251. This anomalous event was observed by several survey and follow-up collaborations conducting microlensing observations towards the Galactic bulge. Methods. Based on detailed modelling of the observed light curve, we find that the lens is composed of two masses with a mass ratio q = 1.9 × 10-3. Thanks to our detection of higher-order effects on the light curve due to the Earth?s orbital motion and the finite size of source, we are able to measure the mass and distance to the lens unambiguously. Results. We find that the lens is made up of a planet of mass 0.53 ± 0.21 MJ orbiting an M dwarf host star with a mass of 0.26 ± 0.11 M☉. The planetary system is located at a distance of 2.57 ± 0.61 kpc towards the Galactic centre. The projected separation of the planet from its host star is d = 1.408 ± 0.019, in units of the Einstein radius, which corresponds to 2.72 ± 0.75 AU in physical units. We also identified a competitive model with similar planet and host star masses, but with a smaller orbital radius of 1.50 ± 0.50 AU. The planet is therefore located beyond the snow line of its host star, which we estimate to be around ~1-1.5 AU. [ABSTRACT FROM AUTHOR]

In the era of precision cosmology, it is essential to determine the Hubble constant to an accuracy of three per cent or better. At present, its uncertainty is dominated by the uncertainty in the distance to the Large Magellanic Cloud (LMC), which, being our second-closest galaxy, serves as the best anchor point for the cosmic distance scale. Observations of eclipsing binaries offer a unique opportunity to measure stellar parameters and distances precisely and accurately. The eclipsing-binary method was previously applied to the LMC, but the accuracy of the distance results was lessened by the need to model the bright, early-type systems used in those studies. Here we report determinations of the distances to eight long-period, late-type eclipsing systems in the LMC, composed of cool, giant stars. For these systems, we can accurately measure both the linear and the angular sizes of their components and avoid the most important problems related to the hot, early-type systems. The LMC distance that we derive from these systems (49.97 ± 0.19 (statistical) ± 1.11 (systematic) kiloparsecs) is accurate to 2.2 per cent and provides a firm base for a 3-per-cent determination of the Hubble constant, with prospects for improvement to 2 per cent in the future. [ABSTRACT FROM AUTHOR]

Aims. We present an analysis of a peculiar supergiant B-type star (VFTS698/Melnick 2/Parker 1797) in the 30 Doradus region of the Large Magellanic Cloud which exhibits characteristics similar to the broad class of B[e] stars. Methods. We analyse optical spectra from the VLT-FLAMES survey, together with archival optical and infrared photometry and X-ray imaging to characterise the system. Results. We find radial velocity variations of around 400 km s-1 in the high excitation Si iv, N iii and He ii spectra, and photometric variability of ∼0.6 mag with a period of 12.7 d. In addition, we detect long-term photometric variations of ∼0.25 mag, which may be due to a longer-term variability with a period of ∼400 d. Conclusions. We conclude that VFTS698 is likely an interacting binary comprising an early B-type star secondary orbiting a veiled, more massive companion. Spectral evidence suggests a mid-to-late B-type primary, but this may originate from an optically-thick accretion disc directly surrounding the primary. [ABSTRACT FROM AUTHOR]

ABSTRACT In this fourth part of the series presenting the Optical Gravitational Lensing Experiment (OGLE) microlensing studies of the dark matter halo compact objects (MACHOs), we describe results of the OGLE-III monitoring of the Small Magellanic Cloud (SMC). Three sound candidates for microlensing events were found and yielded the optical depth τSMC-OIII= 1.30 ± 1.01 × 10−7, consistent with the expected contribution from Galactic disc and SMC self-lensing. We report that event OGLE-SMC-03 is most likely a thick-disc lens candidate, the first of such type found towards the SMC. In this paper we also combined all OGLE Large Magellanic Cloud and SMC microlensing results in order to refine the conclusions on MACHOs. All but one of the OGLE events are most likely caused by the lensing by known populations of stars; therefore, we concluded that there is no need for introducing any special dark matter compact objects in order to explain the observed event rates. Potential black hole event indicates that similar lenses can contribute only about 2 per cent to the total mass of the halo, which is still in agreement with the expected number of such objects. [ABSTRACT FROM AUTHOR]

We present the result of the analysis of the gravitational binary-lensing event OGLE-2005-BLG-018. The light curve of the event is characterized by two adjacent strong features and a single weak feature separated from the strong features. The light curve exhibits noticeable deviations from the best-fit model based on standard binary parameters. To explain the deviation, we test models including various higher-order effects of the motions of the observer, source, and lens. From this, we find that it is necessary to account for the orbital motion of the lens in describing the light curve. From modeling the light curve considering the parallax effect and Keplerian orbital motion, we are able to not only measure the physical parameters but also to find a complete orbital solution of the lens system. It is found that the event was produced by a binary lens located in the Galactic bulge with a distance of 6.7 ± 0.3 kpc from the Earth. The individual lens components with masses 0.9 ± 0.3 M ☉ and 0.5 ± 0.1 M ☉ are separated with a semi-major axis of a = 2.5 ± 1.0 AU and orbiting each other with a period P = 3.1 ± 1.3 yr. This event demonstrates that it is possible to extract detailed information about binary lens systems from well-resolved lensing light curves. [ABSTRACT FROM AUTHOR]

We report the result of the analysis of the light curve of a caustic-crossing binary-lens microlensing event OGLE-2009-BLG-023/MOA-2009-BLG-028. Even though the event was observed solely by survey experiments, we could uniquely determine the mass of the lens and distance to it by simultaneously measuring the Einstein radius and lens parallax. From this, we find that the lens system is composed of M-type dwarfs with masses and located in the Galactic disc with a distance of toward the Galactic bulge direction. The event demonstrates that physical lens parameters of binary-lens events can be routinely determined from future high-cadence lensing surveys and thus microlensing can provide a new way to study Galactic binaries. [ABSTRACT FROM AUTHOR]

ABSTRACT In the third part of the series presenting the Optical Gravitational Lensing Experiment (OGLE) microlensing studies of the dark matter halo compact objects (MACHOs), we describe results of the OGLE-III monitoring of the Large Magellanic Cloud (LMC). This unprecedented data set contains almost continuous photometric coverage over 8 years of about 35 million objects spread over 40 deg2. We report a detection of two candidate microlensing events found with the automated pipeline and an additional two, less probable, candidate events found manually. The optical depth derived for the two main candidates was calculated following a detailed blending examination and detection efficiency determination and was found to be τ= (0.16 ± 0.12) × 10−7. If the microlensing signal we observe originates from MACHOs, then it means their masses are around 0.2 M⊙ and they comprise only f= 3 ± 2 per cent of the mass of the Galactic halo. However, the more likely explanation of our detections does not involve dark matter compact objects at all and relies on the natural effect of self-lensing of LMC stars by LMC lenses. In such a scenario, we can almost completely rule out MACHOs in the subsolar mass range with an upper limit at f < 7 per cent reaching its minimum of f < 4 per cent at M= 0.1 M⊙. For masses around M= 10 M⊙, the constraints on the MACHOs are more lenient with f∼ 20 per cent. Owing to limitations of the survey, there is no reasonable limit found for heavier masses, leaving only a tiny window of mass spectrum still available for dark matter compact objects. [ABSTRACT FROM AUTHOR]

The primary goal of this paper is to provide evidence that can prove true or false the hypothesis that dark matter in the Galactic halo can clump into stellar-mass compact objects. If such objects exist, they would act as lenses to external sources in the Magellanic Clouds, giving rise to an observable effect of microlensing. We present the results of our search for such events, based on data from the second phase of the OGLE survey (1996–2000) towards the Small Magellanic Cloud (SMC). The data set we used comprises 2.1 million monitored sources distributed over an area of 2.4 deg2. We found only one microlensing event candidate, however its poor-quality light curve limited our discussion of the exact distance to the lensing object. Given a single event, taking blending (crowding of stars) into account for the detection-efficiency simulations and deriving the Hubble Space Telescope ( HST)-corrected number of monitored stars, the microlensing optical depth is . This result is consistent with the expected SMC self-lensing signal, with no need to introduce dark matter microlenses. Rejecting the unconvincing event leads to an upper limit on the fraction of dark matter in the form of massive compact halo objects (MACHOs) of per cent for deflector masses around 0.4 and per cent for masses between 0.003 and 0.2 (95 per cent confidence limit). Our result indicates that the Milky Way's dark matter is unlikely to be clumpy and to form compact objects in the subsolar-mass range. [ABSTRACT FROM AUTHOR]