Publication Search Results

ID: 156333
Type: article
Authors: Di Stefano, Rosanne
Abstract: Close-orbit binaries consisting of two compact objects are a centre of attention because of the detection of gravitational-radiation-induced mergers. The creation of close, compact-object binaries involves physical processes that are not yet well understood; there are open questions about the manner in which two compact objects come to be close enough to merge within a Hubble time. Here, we explore an important, and likely common physical process: mass transfer from a third star in a wider, hierarchical orbit. Mass added to the close binary's components can reduce the time to merger and can even change the nature of an accretor, transforming a white dwarf to a neutron star and/or a neutron star to a black hole. Some accreting WDs in close binaries may even explode as Type Ia supernovae. Given the ubiquity of higher order multiples, the evolutionary channels we lay out may be important pathways to gravitational mergers, including Type Ia supernovae. Fortunately, these pathways also lead to testable predictions.

ID: 155692
Type: article
Authors: Di Stefano, Rosanne
Abstract: The Roche lobe formalism describes mass transfer from one star to another. We develop an extension to hierarchical triples, considering the case in which a star donates mass to a companion which is itself a binary. The L1 point moves as the inner binary rotates, and the Roche lobe pulsates with the period of the inner binary. Signatures of mass transfer may therefore be imprinted with the orbital period of the inner binary. For some system parameters, the pulsing Roche lobe can drive mass transfer at high rates. Systems undergoing this type of mass transfer include those with inner binaries consisting of compact objects that will eventually merge, as well as progenitors of Type Ia supernovae.

ID: 158245
Type: article
Authors: D'Orazio, Daniel J.; Di Stefano, Rosanne
Abstract: We explore a unique electromagnetic signature of stellar-mass compact-object binaries long before they are detectable in gravitational waves. We show that gravitational lensing of light emitting components of a compact-object binary, by the other binary component, could be detectable in the nearby Universe. This periodic lensing signature could be detected from present and future X-ray observations, identifying the progenitors of binaries that merge in the LIGO band, and also unveiling populations that do not merge, thus providing a tracer of the compact-object binary population in an enigmatic portion of its life. We argue that periodically repeating lensing flares could be observed for ≲100 ks orbital-period binaries with the future Lynx X-ray mission, possibly concurrent with gravitational wave emission in the LISA band. Binaries with longer orbital periods could be more common and be detectable as single lensing flares, though with reliance on a model for the flare that can be tested by observations of succeeding flares. Non-detection of such events, even with existing X-ray observations, will help to constrain the population of EM bright compact-object binaries.

ID: 156913
Type: article
Authors: Hu, Betty X.; D'Orazio, Daniel J.; Haiman, Zoltán; Smith, Krista Lynne; Snios, Bradford; Charisi, Maria; Di Stefano, Rosanne
Abstract: We examine the light curves of two quasars, motivated by recent suggestions that a supermassive black hole binary (SMBHB) can exhibit sharp lensing spikes. We model the variability of each light curve as due to a combination of two relativistic effects: the orbital relativistic Doppler boost and gravitational binary self-lensing. In order to model each system, we extend previous Doppler plus self-lensing models to include eccentricity. The first quasar is identified in optical data as a binary candidate with a 20-yr period (Ark 120), and shows a prominent spike. For this source, we rule out the lensing hypothesis and disfavour the Doppler-boost hypothesis due to discrepancies in the measured versus recovered values of the binary mass and optical spectral slope. The second source, which we nickname Spikey, is the rare case of an active galactic nucleus identified in Kepler's high-quality, high-cadence photometric data. For this source, we find a model, consisting of a combination of Doppler modulation and a narrow symmetric lensing spike, consistent with an eccentric SMBHB with a mass of $M_{\text{tot}} = 3\times 10^{7} {\, \mathrm{M}_{\odot }}$ , rest- frame orbital period T = 418 d, eccentricity e = 0.5, and seen at an inclination of 8^○ from edge-on. This interpretation can be tested by monitoring Spikey for periodic behaviour and recurring flares in the next few years. In preparation for such monitoring, we present the first X-ray observations of this object taken by the Neil Gehrels Swift Observatory.

ID: 154539
Type: article
Authors: Antoniou, Vallia; Zezas, Andreas; Drake, Jeremy J.; Badenes, Carles; Haberl, Frank; Wright, Nicholas J.; Hong, Jaesub; Di Stefano, Rosanne; Gaetz, Terrance J.; Long, Knox S.; Plucinsky, Paul P.; Sasaki, Manami; Williams, Benjamin F.; Winkler, P. Frank; SMC XVP collaboration
Abstract: We have compiled the most complete census of high-mass X-ray binaries (HMXBs) in the Small Magellanic Cloud with the aim to investigate the formation efficiency of young accreting binaries in its low-metallicity environment. In total, we use 123 X-ray sources with detections in our Chandra X-ray Visionary Program (XVP), supplemented by 14 additional (likely and confirmed) HMXBs identified by Haberl & Sturm that fall within the XVP area, but are neither detected in our survey (nine sources) nor matched with any of the 127 sources identified in the XVP data (five sources). Specifically, we examine the number ratio of the HMXBs [N(HMXBs)] to (a) the number of OB stars, (b) the local star formation rate (SFR), and (c) the stellar mass produced during the specific star formation burst, all as a function of the age of their parent stellar populations. Each of these indicators serves a different role, but in all cases we find that the HMXB formation efficiency increases as a function of time (following a burst of star formation) up to ∼40─60 Myr, and then gradually decreases. The formation efficiency peaks at ∼30─40 Myr with average rates of {\text{}}{{N}}({HMXB})/{SFR}={339}_-83⁺⁷⁸ {({M}_ȯ /{yr})}^-1, and N(HMXB)/M \star =({8.74}_-0.92^+1.0)× {10}^-6 {M}_ȯ ^-1, in good agreement with previous estimates of the average formation efficiency in the broad ∼20─60 Myr age range.

ID: 154436
Type: article
Authors: Kessler, R.; Narayan, G.; Avelino, Arturo; Bachelet, E.; Biswas, R.; Brown, P. J.; Chernoff, D. F.; Connolly, A. J.; Dai, M.; Daniel, S.; Di Stefano, Rosanne; Drout, M. R.; Galbany, L.; González-Gaitán, S.; Graham, M. L.; Hložek, R.; Ishida, E. E. O.; Guillochon, James; Jha, S. W.; Jones, D. O.; Mandel, K. S.; Muthukrishna, D.; O'Grady, A.; Peters, C. M.; Pierel, J. R.; Ponder, K. A.; Prša, A.; Rodney, S.; Villar, Victoria Ashley; LSST Dark Energy Science Collaboration; Transient and Variable Stars Science Collaboration
Abstract: We describe the simulated data sample for the Photometric Large Synoptic Survey Telescope (LSST) Astronomical Time Series Classification Challenge (PLAsTiCC), a publicly available challenge to classify transient and variable events that will be observed by the LSST, a new facility expected to start in the early 2020s. The challenge was hosted by Kaggle, ran from 2018 September 28 to December 17, and included 1094 teams competing for prizes. Here we provide details of the 18 transient and variable source models, which were not revealed until after the challenge, and release the model libraries at https://doi.org/10.5281/zenodo.2612896. We describe the LSST Operations Simulator used to predict realistic observing conditions, and we describe the publicly available SNANA simulation code used to transform the models into observed fluxes and uncertainties in the LSST passbands (ugrizy). Although PLAsTiCC has finished, the publicly available models and simulation tools are being used within the astronomy community to further improve classification, and to study contamination in photometrically identified samples of SN Ia used to measure properties of dark energy. Our simulation framework will continue serving as a platform to improve the PLAsTiCC models, and to develop new models.

ID: 154614
Type: article
Authors: Liu, Jifeng; Zhang, Haotong; Howard, Andrew W.; Bai, Zhongrui; Lu, Youjun; Soria, Roberto; Justham, Stephen; Li, Xiangdong; Zheng, Zheng; Wang, Tinggui; Belczynski, Krzysztof; Casares, Jorge; Zhang, Wei; Yuan, Hailong; Dong, Yiqiao; Lei, Yajuan; Isaacson, Howard; Wang, Song; Bai, Yu; Shao, Yong; Gao, Qing; Wang, Yilun; Niu, Zexi; Cui, Kaiming; Zheng, Chuanjie; Mu, Xiaoyong; Zhang, Lan; Wang, Wei; Heger, Alexander; Qi, Zhaoxiang; Liao, Shilong; Lattanzi, Mario; Gu, Wei-Min; Wang, Junfeng; Wu, Jianfeng; Shao, Lijing; Shen, Rongfeng; Wang, Xiaofeng; Bregman, Joel; Di Stefano, Rosanne; Liu, Qingzhong; Han, Zhanwen; Zhang, Tianmeng; Wang, Huijuan; Ren, Juanjuan; Zhang, Junbo; Zhang, Jujia; Wang, Xiaoli; Cabrera-Lavers, Antonio; Corradi, Romano; Rebolo, Rafael; Zhao, Yongheng; Zhao, Gang; Chu, Yaoquan; Cui, Xiangqun
Abstract: All stellar-mass black holes have hitherto been identified by X-rays emitted from gas that is accreting onto the black hole from a companion star. These systems are all binaries with a black-hole mass that is less than 30 times that of the Sun^1-4. Theory predicts, however, that X-ray-emitting systems form a minority of the total population of star-black-hole binaries^5,6. When the black hole is not accreting gas, it can be found through radial-velocity measurements of the motion of the companion star. Here we report radial-velocity measurements taken over two years of the Galactic B-type star, LB-1. We find that the motion of the B star and an accompanying Hα emission line require the presence of a dark companion with a mass of 68_-13⁺¹¹ solar masses, which can only be a black hole. The long orbital period of 78.9 days shows that this is a wide binary system. Gravitational-wave experiments have detected black holes of similar mass, but the formation of such massive ones in a high- metallicity environment would be extremely challenging within current stellar evolution theories.

ID: 155156
Type: article
Authors: Qiu, Yanli; Soria, Roberto; Wang, Song; Wiktorowicz, Grzegorz; Liu, Jifeng; Bai, Yu; Bogomazov, Alexey; Di Stefano, Rosanne; Walton, Dominic J.; Xu, Xiaojie
Abstract: We investigated the time-variability and spectral properties of the eclipsing X-ray source Circinus Galaxy X-1 (GG X-1), using Chandra, XMM- Newton and ROSAT. We phase-connected the light curves observed over 20 yr, and we obtained a best-fitting period P = (25,970.0 ± 0.1) s ≈ 7.2 hr, and a period derivative \dot{P}/P=(10.2+/- 4.6)× {10}^-7 yr^-1. The X-ray light curve shows asymmetric eclipses, with sharp ingresses and slow, irregular egresses. The eclipse profile and duration vary substantially from cycle to cycle. We show that the X-ray spectra are consistent with a power-law-like component, which is absorbed by neutral and ionized Compton-thin material, and by a Compton- thick, partial-covering medium, which is responsible for the irregular dips. The high X-ray/optical flux ratio rules out the possibility that CG X-1 is a foreground Cataclysmic Variable. In agreement with previous studies, we conclude that it is the first example of a compact ultraluminous X-ray source fed by a Wolf-Rayet star or stripped Helium star. Its unocculted luminosity varies between ≈4 × 10³⁹ erg s^-1 and ≈3 × 10⁴⁰ erg s^-1. Both the donor star and the super-Eddington compact object drive powerful outflows. We suggest that the occulting clouds are produced in the wind- wind collision region and in the bow shock in front of the compact object. Among the rare sample of Wolf-Rayet X-ray binaries, CG X-1 is an exceptional target for studies of supercritical accretion and close binary evolution; it is also a likely progenitor of gravitational wave events.

ID: 146140
Type: article
Authors: D'Orazio, Daniel J.; Di Stefano, Rosanne
Abstract: Nearly 150 massive black hole binary (MBHB) candidates at sub-pc orbital separations have been reported in recent literature. Nevertheless, the definitive detection of even a single such object remains elusive. If at least one of the black holes is accreting, the light emitted from its accretion disc will be lensed by the other black hole for binary orbital inclinations near to the line of sight. This binary self-lensing could provide a unique signature of compact MBHB systems. We show that, for MBHBs with masses in the range 10⁶-10¹⁰ M_⊙ and with orbital periods less than ˜10 yr, strong lensing events should occur in one to 10s of per cent of MBHB systems that are monitored for an entire orbit. Lensing events will last from days for the less massive, shorter period MBHBs to a year for the most massive ˜10 year orbital period MBHBs. At small inclinations of the binary orbit to the line of sight, lensing must occur and will be accompanied by periodicity due to the relativistic Doppler boost. Flares at the same phase as the otherwise average flux of the Doppler modulation would be a smoking gun signature of self-lensing and can be used to constrain binary parameters. For MBHBs with separation ≳100 Schwarzschild radii, we show that finite-sized source effects could serve as a probe of MBH accretion disc structure. Finally, we stress that our lensing probability estimate implies that ˜10 of the known MBHB candidates identified through quasar periodicity should exhibit strong lensing flares.

ID: 143357
Type: article
Authors: Moe, Maxwell; Di Stefano, Rosanne
Abstract: We compile observations of early-type binaries identified via spectroscopy, eclipses, long-baseline interferometry, adaptive optics, common proper motion, etc. Each observational technique is sensitive to companions across a narrow parameter space of orbital periods P and mass ratios q = {M}_{comp}/M ₁. After combining the samples from the various surveys and correcting for their respective selection effects, we find that the properties of companions to O-type and B-type main-sequence (MS) stars differ among three regimes. First, at short orbital periods P ? 20 days (separations a ? 0.4 au), the binaries have small eccentricities e ? 0.4, favor modest mass ratios ? 0.5, and exhibit a small excess of twins q > 0.95. Second, the companion frequency peaks at intermediate periods log P (days) ? 3.5 (a ? 10 au), where the binaries have mass ratios weighted toward small values q ? 0.2-0.3 and follow a Maxwellian "thermal" eccentricity distribution. Finally, companions with long orbital periods log P (days) ? 5.5-7.5 (a ? 200-5000 au) are outer tertiary components in hierarchical triples and have a mass ratio distribution across q ? 0.1-1.0 that is nearly consistent with random pairings drawn from the initial mass function. We discuss these companion distributions and properties in the context of binary-star formation and evolution. We also reanalyze the binary statistics of solar-type MS primaries, taking into account that 30% ± 10% of single-lined spectroscopic binaries likely contain white dwarf companions instead of low-mass stellar secondaries. The mean frequency of stellar companions with q > 0.1 and log P (days) ? 0.5, and exhibit a small excess of twins q > 0.95. Second, the companion frequency peaks at intermediate periods log P (days) ? 3.5 (a ? 10 au), where the binaries have mass ratios weighted toward small values q ? 0.2-0.3 and follow a Maxwellian "thermal" eccentricity distribution. Finally, companions with long orbital periods log P (days) ? 5.5-7.5 (a ? 200-5000 au) are outer tertiary components in hierarchical triples and have a mass ratio distribution across q ? 0.1-1.0 that is nearly consistent with random pairings drawn from the initial mass function. We discuss these companion distributions and properties in the context of binary-star formation and evolution. We also reanalyze the binary statistics of solar-type MS primaries, taking into account that 30% ± 10% of single-lined spectroscopic binaries likely contain white dwarf companions instead of low-mass stellar secondaries. The mean frequency of stellar companions with q > 0.1 and log P (days) ₁,q,P,e)\ne f({M}₁)f(q)f(P)f(e) to the corrected distributions, which can be incorporated into binary population synthesis studies.

ID: 140743
Type: article
Authors: Di Stefano, R.; Ray, A.
Abstract: Globular clusters are ancient stellar populations in compact dense ellipsoids. There is no star formation and there are no core-collapse supernovae, but several lines of evidence suggest that globular clusters are rich in planets. If so, and if advanced civilizations can develop there, then the distances between these civilizations and other stars would be far smaller than typical distances between stars in the Galactic disk, facilitating interstellar communication and travel. The potent combination of long-term stability and high stellar densities provides a globular cluster opportunity. Yet the very proximity that promotes interstellar travel also brings danger, as stellar interactions can destroy planetary systems. We find, however, that large portions of many globular clusters are “sweet spots,” where habitable-zone planetary orbits are stable for long times. Globular clusters in our own and other galaxies are, therefore, among the best targets for searches for extraterrestrial intelligence (SETI). We use the Drake equation to compare the likelihood of advanced civilizations in globular clusters to that in the Galactic disk. We also consider free-floating planets, since wide-orbit planets can be ejected to travel through the cluster. Civilizations spawned in globular clusters may be able to establish self-sustaining outposts, reducing the probability that a single catastrophic event will destroy the civilization. Although individual civilizations may follow different evolutionary paths, or even be destroyed, the cluster may continue to host advanced civilizations once a small number have jumped across interstellar space. Civilizations residing in globular clusters could therefore, in a sense, be immortal.

ID: 136547
Type: article
Authors: Esposito, P.; Israel, G. L.; De Martino, D.; D'Avanzo, P.; Testa, V.; Sidoli, L.; Di Stefano, R.; Belfiore, A.; Mapelli, M.; Piranomonte, S.; Rodríguez Castillo, G. A.; Moretti, A.; D'Elia, V.; Verrecchia, F.; Campana, S.; Rea, N.
Abstract: We report on the discovery of a new X-ray pulsator, Swift J201424.9+152930 (Sw J2014). Owing to its X-ray modulation at 491 s, it was discovered in a systematic search for coherent signals in the archival data of the Swift X-ray Telescope. To investigate the nature of Sw J2014, we performed multiwavelength follow-up observations with space-borne (Swift and XMM-Newton) and ground-based (the 1.5-m Loiano Telescope and the 3.6-m Telescopio Nazionale Galileo) instruments. The X-ray spectrum of Sw J2014 can be described by a hard and highly absorbed (N_H ˜ 5 × 10²² cm^-2) power law (Γ ˜ 1). The optical observations made it possible to single out the optical counterpart to this source, which displays several variable emission lines and total eclipses lasting ≈20 min. Total eclipses of similar length were observed also in X-rays. The study of the eclipses, allowed us to infer a second periodicity of 3.44 h, which we interpret as the orbital period of a close binary system. We also found that the period has not significantly changed over a ˜7 yr timespan. Based on the timing signatures of Sw J2014, and its optical and X-ray spectral properties, we suggest that it is a close binary hosting an accreting magnetic white dwarf. The system is therefore a cataclysmic variable of the intermediate polar type and one of the very few showing deep eclipses.

ID: 137299
Type: article
Authors: Guo, Xinyi; Esin, Ann; Di Stefano, Rosanne; Taylor, Jeffrey
Abstract: Gravitational microlensing events are powerful tools for the study of stellar populations. In particular, they can be used to discover and study a variety of binary systems. A large number of binary lenses have already been found through microlensing surveys and a few of these systems show strong evidence of orbital motion on the timescale of the lensing event. We expect that more binary lenses of this kind will be detected in the future. For binaries whose orbital period is comparable to the event duration, the orbital motion can cause the lensing signal to deviate drastically from that of a static binary lens. The most striking property of such light curves is the presence of quasi-periodic features, which are produced as the source traverses the same regions in the rotating lens plane. These repeating features contain information about the orbital period of the lens. If this period can be extracted, then much can be learned about the lensing system even without performing time-consuming, detailed light-curve modeling. However, the relative transverse motion between the source and the lens significantly complicates the problem of period extraction. To resolve this difficulty, we present a modification of the standard Lomb–Scargle periodogram analysis. We test our method for four representative binary lens systems and demonstrate its efficiency in correctly extracting binary orbital periods.

ID: 138565
Type: article
Authors: Liu, Ji-Feng; Bai, Yu; Wang, Song; Justham, Stephen; Lu, You-Jun; Gu, Wei-Min; Liu, Qing-Zhong; Di Stefano, Rosanne; Guo, Jin-Cheng; Cabrera-Lavers, Antonio; Álvarez, Pedro; Cao, Yi; Kulkarni, Shri
Abstract: The formation of relativistic jets by an accreting compact object is one of the fundamental mysteries of astrophysics. Although the theory is poorly understood, observations of relativistic jets from systems known as microquasars (compact binary stars) have led to a well established phenomenology. Relativistic jets are not expected to be produced by sources with soft or supersoft X-ray spectra, although two such systems are known to produce relatively low-velocity bipolar outflows. Here we report the optical spectra of an ultraluminous supersoft X-ray source (ULS) in the nearby galaxy M81 (M81 ULS-1; refs 9, 10). Unexpectedly, the spectra show blueshifted, broad Ha emission lines, characteristic of baryonic jets with relativistic speeds. These time-variable emission lines have projected velocities of about 17 per cent of the speed of light, and seem to be similar to those from the prototype microquasar SS 433 (refs 11, 12). Such relativistic jets are not expected to be launched from white dwarfs, and an origin from a black hole or a neutron star is hard to reconcile with the persistence of M81 ULS-1’s soft X-rays. Thus the unexpected presence of relativistic jets in a ULS challenges canonical theories of jet formation, but might be explained by a long-speculated, supercritically accreting black hole with optically thick outflows.

ID: 135640
Type: article
Authors: Moe, Maxwell; Di Stefano, Rosanne
Abstract: Early B-type main-sequence (MS) stars (M ₁ ˜ 5-16 M _?) with closely orbiting low-mass stellar companions (q = M ₂/M _{1 refl} ˜ 0.02-0.14 mag). For the 18 definitive MS + pre-MS eclipsing binaries in our sample with good model fits to the observed light-curves, we measure short orbital periods P = 3.0-8.5 days, young ages t ˜ 0.6-8 Myr, and small secondary masses M ₂ ˜ 0.8-2.4 M _? (q ˜ 0.07-0.36). The majority of these nascent eclipsing binaries are still associated with stellar nurseries, e.g., the system with the deepest eclipse ?I ₁ = 2.8 mag and youngest age t = 0.6 ± 0.4 Myr is embedded in the bright H II region 30 Doradus. After correcting for selection effects, we find that (2.0 ± 0.6)% of B-type MS stars have companions with short orbital periods P = 3.0-8.5 days and extreme mass ratios q ˜ 0.06-0.25. This is ˜10 times greater than that observed for solar-type MS primaries. We discuss how these new eclipsing binaries provide invaluable insights, diagnostics, and challenges for the formation and evolution of stars, binaries, and H II regions.

ID: 140485
Type: article
Authors: Moe, Maxwell; Di Stefano, Rosanne
Abstract: We analyze 221 eclipsing binaries (EBs) in the Large Magellanic Cloud with B-type main-sequence (MS) primaries (M₁ ˜ 4–14 {M}_? ) and orbital periods P = 20–50 days that were photometrically monitored by the Optical Gravitational Lensing Experiment. We utilize our three-stage automated pipeline to (1) classify all 221 EBs, (2) fit physical models to the light curves of 130 detached well-defined EBs from which unique parameters can be determined, and (3) recover the intrinsic binary statistics by correcting for selection effects. We uncover two statistically significant trends with age. First, younger EBs tend to reside in dustier environments with larger photometric extinctions, an empirical relation that can be implemented when modeling stellar populations. Second, younger EBs generally have large eccentricities. This demonstrates that massive binaries at moderate orbital periods are born with a Maxwellian “thermal” orbital velocity distribution, which indicates they formed via dynamical interactions. In addition, the age-eccentricity anticorrelation provides a direct constraint for tidal evolution in highly eccentric binaries containing hot MS stars with radiative envelopes. The intrinsic fraction of B-type MS stars with stellar companions q = M₂/M₁ > 0.2 and orbital periods P = 20–50 days is (7 ± 2)%. We find early-type binaries at P = 20–50 days are weighted significantly toward small mass ratios q ˜ 0.2–0.3, which is different than the results from previous observations of closer binaries with P < 20 days. This indicates that early-type binaries at slightly wider orbital separations have experienced substantially less competitive accretion and coevolution during their formation in the circumbinary disk.

ID: 127148
Type: article
Authors: Goodman, Alyssa A.; Pepe, Alberto; Blocker, Alexander W.; Borgman, Christine L.; Cranmer, Kyle; Crosas, Merce; Di Stefano, Rosanne; Gil, Yolanda; Groth, Paul; Hedstrom, Margaret; Hogg, David W.; Kashyap, Vinay; Mahabal, Ashish; Siemiginowska, Aneta; Slavkovic, Aleksandra

ID: 116629
Type: article
Authors: Di Stefano, Rosanne; Matthews, James; Lépine, Sébastien
Abstract: The Einstein rings and proper motions of nearby stars tend to be large. Thus, every year some foreground stars within a few hundred parsecs of Earth induce gravitational lensing events in background stars. In some of these cases, the events may exhibit evidence of planets orbiting the nearby star. In fact, planets can even be discovered during relatively distant passages. Here, we study the lensing signatures associated with planets orbiting nearby high-proper-motion stars. We find the following. (1) Wide-orbit planets can be detected for all distances of closest approach between the foreground and background stars, potentially producing independent events long before and/or after the closest approach. (2) Close-orbit planets can be detected for intermediate distances of closest approach, producing quasiperiodic signatures that may occur days or weeks before and after the stellar-lens event. (3) Planets in the so-called zone for resonant lensing can significantly increase the magnification when the distance of closest approach is small, making the stellar-lens event easier to detect, while simultaneously providing evidence for planets. Because approaches close enough to allow planets to be detected can be predicted, we can plan observing strategies to take advantage of the theoretical framework built in this paper, which describes the sequence of expected effects in terms of a sequence of detection regimes.

ID: 118389
Type: article
Authors: Moe, Maxwell; Di Stefano, Rosanne
Abstract: In order to understand the rates and properties of Type Ia and Type Ib/c supernovae, X-ray binaries, gravitational wave sources, and gamma-ray bursts as a function of galactic environment and cosmic age, it is imperative that we measure how the close binary properties of O- and B-type stars vary with metallicity. We have studied eclipsing binaries with early B main-sequence primaries in three galaxies with different metallicities: the Large and Small Magellanic Clouds (LMC and SMC, respectively) and the Milky Way (MW). The observed fractions of early B stars that exhibit deep eclipses 0.25 ₂/M ₁ > 0.1, (2) an intrinsic orbital period distribution slightly skewed toward shorter periods relative to a distribution that is uniform in log P, (3) a mass-ratio distribution weighted toward low-mass companions, and (4) a small, nearly negligible excess fraction of twins with q > 0.9. Our fitted parameters derived for the MW eclipsing binaries match the properties inferred from nearby, early-type spectroscopic binaries, which further validates our results. There are no statistically significant trends with metallicity, demonstrating that the close binary properties of massive stars do not vary across metallicities –0.7 > 0.1, (2) an intrinsic orbital period distribution slightly skewed toward shorter periods relative to a distribution that is uniform in log P, (3) a mass-ratio distribution weighted toward low-mass companions, and (4) a small, nearly negligible excess fraction of twins with q > 0.9. Our fitted parameters derived for the MW eclipsing binaries match the properties inferred from nearby, early-type spectroscopic binaries, which further validates our results. There are no statistically significant trends with metallicity, demonstrating that the close binary properties of massive stars do not vary across metallicities –0.7 _?) < 0.0 beyond the measured uncertainties.

ID: 116656
Type: article
Authors: Patel, B.; Di Stefano, R.; Nelson, T.; Primini, Francis A.; Liu, J.; Scoles, S.
Abstract: We report on observations of a luminous supersoft X-ray source (SSS) in M31, r1-25, that has exhibited spectral changes to harder X-ray states. We document these spectral changes. In addition, we show that they have important implications for modeling the source. Quasisoft states in a source that has been observed as an SSS represent a newly discovered phenomenon. We show how such state changers could prove to be examples of unusual black hole or neutron star accretors. Future observations of this and other state changers can provide the information needed to determine the nature(s) of these intriguing new sources.