The majority of all stars are born in binary or multiple star sys- tems, and a significant fraction of these will interact at some point in their lives. Once the more massive star leaves the main sequence, and depending on the initial conditions, dynamically unstable mass transfer or a tidal instability may force the system to enter the common envelope (CE) phase (see Taam & Ricker 2006, for more details). In this phase a gaseous envelope around the entire binary forms, and friction within the envelope signif- icantly reduces the binary separation. Since Paczy´nski (1976) it is generally believed that virtually all close compact binary sys- tems ranging from X-ray binaries to double white dwarf binaries or cataclysmic variables, to name a few, have formed through CE evolution. Willems & Kolb (2004) performed comprehen- sive binary population synthesis studies for white dwarf/main
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We present a long-term programme for timing the eclipses of white dwarfs in close binaries to measure apparent and/or real variations in their orbital periods. Our programme includes 67 close binaries, both detached and semi-detached and with M-dwarfs, K-dwarfs, brown dwarfs or white dwarfs secondaries. In total, we have ob- served more than 650 white dwarf eclipses. We use this sample to search for orbital period variations and aim to identify the underlying cause of these variations. We find that the probability of observing orbital period variations increases significantly with the observational baseline. In particular, all binaries with baselines exceeding 10 yrs, with secondaries of spectral type K2 – M5.5, show variations in the eclipse arrival times that in most cases amount to several minutes. In addition, among those with baselines shorter than 10 yrs, binaries with late spectral type (>M6), brown dwarf or white dwarf secondaries appear to show no orbital period variations. This is in agreement with the so-called Applegate mechanism, which proposes that magnetic cy- cles in the secondary stars can drive variability in the binary orbits. We also present new eclipse times of NN Ser, which are still compatible with the previously published circumbinary planetary system model, although only with the addition of a quadratic term to the ephemeris. Finally, we conclude that we are limited by the relatively short observational baseline for many of the binaries in the eclipse timing programme, and therefore cannot yet draw robust conclusions about the cause of orbital period variations in evolved, white dwarf binaries.
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A central hypothesis in the theory of cataclysmic variable (CV) evolution is the need to explain the observed lack of accreting systems in the 2–3 h orbital period range, known as the period gap. The standard model, disrupted magnetic braking (DMB), reproduces the gap by postulating that CVs transform into inconspicuous detached white dwarf (WD) plus main sequence systems, which no longer resemble CVs. However, observational evidence for this standard model is currently indirect and thus this scenario has attracted some criticism throughout the last decades. Here, we perform a simple but exceptionally strong test of the existence of detached CVs (dCVs). If the theory is correct, dCVs should produce a peak in the orbital period distribution of detached close binaries consisting of a WD and an M4–M6 secondary star. We measured six new periods which brings the sample of such binaries with known periods below 10 h to 52 systems. An increase of systems in the 2–3 h orbital period range is observed. Comparing this result with binary population models, we find that the observed peak cannot be reproduced by post-common envelope binaries (PCEBs) alone and that the existence of dCVs is needed to reproduce the observations. Also, the WD mass distribution in the gap shows evidence of two populations in this period range, i.e. PCEBs and more massive dCVs, which is not observed at longer periods. We therefore conclude that CVs are indeed crossing the gap as detached systems, which provides strong support for the DMB theory.
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Liebert et al. (2005b) spotted another oddity about MWDs, namely that not a single MWD has been found in any of the > 1600 known (wide and close) WD plus M-dwarf binaries (Silvestri et al. 2007; Heller et al. 2009; Rebassa-Mansergas et al. 2009) – contrasting the large fre- quency of interacting MWD plus M-dwarf binaries, i.e. magnetic cataclysmic variables, which make up 25% of all known CVs (Wickramasinghe & Ferrario 2000). This mo- tivated Tout et al. (2008) to outline a very different sce- nario for the origin of MWDs, in which dynamos during the common envelope evolution of close binaries generate strong magnetic fields in the core of the WD progenitor. During the common envelope of a WD plus M-dwarf bi- nary, the separation shrinks, leading primarily to two differ- ent possible outcomes. If the two stars avoid merging, they leave the common envelope as a short-period binary that will relatively rapidly start mass transfer as a magnetic cata- clysmic variable. In fact, a number of such systems, magnetic pre-cataclysmic variables, are known (Reimers et al. 1999; Reimers & Hagen 2000; Szkody et al. 2003; Schmidt et al. 2007; Schwope et al. 2009). Alternatively, the two stars may coalesce, forming a single MWD, which will typically be more massive than non-magnetic field WDs.
There is a possible weak trend of decreasing MF with increasing density in all separation ranges, including, unexpectedly, the 19– 100 au unprocessed range. In the 62–620 au range, the separation distributions of all our young regions are similar to each other and to the field. In the wider 19–774 au range the separation distributions of the five young regions are similar to each other, but significantly different to the field due to an excess of close binaries ( 50 au, roughly twice what is found in the field). The only range in which there is any statistical evidence for variations between separation distributions of individual young clusters is in the 19–100 au range. We reiterate here that we are considering a limited range of binary separations of 19–774 au in which we can compare different regions. In particular, we only have data for all regions in a very limited range of 62–620 au (set by the observational constraints of the ONC sample of Reipurth et al. 2007).
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stellar components did not interact and thus evolved like single stars, and close binaries that suﬀered from dynamically unsta- ble mass transfer, i.e. PCEBs. We are carrying out a dedicated follow-up observing program with the aim to identify the PCEBs among the SDSS WDMS binaries, and to determine their bi- nary parameters. First results on individual systems have been published by Schreiber et al. (2008); Rebassa-Mansergas et al. (2008); Pyrzas et al. (2009); Nebot Gómez-Morán et al. (2009); Schwope et al. (2009). Here we present radial velocity studies of 670 SDSS WDMS binaries, identify 205 strong PCEB candi- dates and determine the fraction of PCEBs among WDMS bina- ries, henceforth N PCEB /N WDMS , as a function of secondary mass.
We report on the search for new eclipsing white dwarf plus main-sequence (WDMS) bi- naries in the light curves of the Catalina surveys. We use a colour selected list of almost 2000 candidate WDMS systems from the Sloan Digital Sky Survey, specifically designed to identify WDMS systems with cool white dwarfs and/or early M type main-sequence stars. We iden- tify a total of 17 eclipsing systems, 14 of which are new discoveries. We also find 3 candidate eclipsing systems, 2 main-sequence eclipsing binaries and 22 non-eclipsing close binaries. Our newly discovered systems generally have optical fluxes dominated by the main-sequence components, which have earlier spectral types than the majority of previously discovered eclipsing systems. We find a large number of ellipsoidally variable binaries with similar pe- riods, near 4 hours, and spectral types M2–3, which are very close to Roche-lobe filling. We also find that the fraction of eclipsing systems is lower than found in previous studies and likely reflects a lower close binary fraction among WDMS binaries with early M-type main- sequence stars due to their enhanced angular momentum loss compared to fully convective late M type stars, hence causing them to become cataclysmic variables quicker and disappear from the WDMS sample. Our systems bring the total number of known detached, eclipsing WDMS binaries to 71.
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The K2 microlensing campaign, scheduled for 83 days beginning 2016 April, will provide a unique opportunity for space-based microlensing without the need for ground-based alerts, and hence with a greater chance that caustic entrances will be monitored from space. This advantage ( relative to Spitzer ) is balanced by the fact that these events will be drawn from a relatively small area, albeit one with close to a peak surface density of microlensing events. From the standpoint of making binary-lens mass measurements, and BH-binary mass measurements in particular, we note that it is exceptionally important that this entire area be monitored from the ground at high cadence and as continuously as possible. For example, even extremely faint stars can give rise to brie ﬂ y bright caustic crossings that can be effectively monitored by Kepler with its 30 minute cadence, even if the majority of the light curve cannot. However, only if the corresponding caustic crossings are monitored from the ground well enough to effectively model the light curve, will this result in accurate mass measurements. A very aggressive attitude toward continuous coverage will be especially important toward the beginning of the campaign when individual southern sites can observe the bulge for only ﬁ ve hours per night.
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The binary system Cyg X-3 is one of the brightest Galactic X-ray sources, displaying high and low states and rapid variability in X-rays. In addition to being a powerful X-ray source, Cyg X-3 is seen in the infrared and is a strong and variable radio source. It is also the strongest radio source among X-ray binaries (Fig. 1) and shows both huge radio outbursts and relativistic jets. The radio-activity is closely linked with the X-ray emission and the different X-ray states [7, 8]. Cygnus X-3 is a high mass X-ray binary and microquasar, with a compact object, which is either a neutron star or may be a black hole, and a companion
In this thesis I have presented high precision studies of a number of eclipsing white dwarf binaries. I have used a combination of spectroscopy and photometry to de- termine the physical parameters of both stars. I have shown that there is a de- generacy between the scaled radii of both stars and the orbital inclination which makes model-independent mass-radius measurements difficult. However, I have also shown that there are a number of methods that can be used to break this degen- eracy. First, I presented two systems (NN Ser and SDSS J0857+0342) where the degeneracy was broken by measuring the depth of the secondary eclipse. Then I presented two more systems (GK Vir and SDSS J1212-0123) where the degeneracy was broken by measuring the gravitational redshift of the white dwarf and using it as a prior constraint to the light curve fit. Our project to measure precise parameters in eclipsing PCEBs serendipitously lead to the discovery of only the second known eclipsing double white dwarf binary (CSS 41177). I presented the first data for this system and identified it as a double-lined spectroscopic binary, currently the only known double-lined eclipsing white dwarf binary, and thus a prime target for future high-precision studies.
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The increase of the sample of sources has also enlarged the covered parameter space. New sources sometimes display rare phenomena and properties that were thought to be unique for a certain class turned out to be more common. For instance, more and more faint tran- sient and faint persistent low–mass X–ray binaries have been identified (Muno et al. 2003a, King & Wijnands 2006, in’t Zand et al. 2007, and Wijnands 2008 these proceedings). In addition, there have been several new long–duration transients (e.g. the transient Z source XTE J1701–462; Homan et al. 2007), of which two sources still remain active today (the black hole candidate SWIFT J1753.5–0127, see Figure 1, and the accretion powered ms X–ray pul- sar HETE J1900.1–2455; Galloway et al. 2007). Two new groups of high–mass X–ray binaries have recently been identified. The obscured high–mass X–ray binaries (e.g. IGR J16318– 4848; Walter et al. 2003) and the supergiant fast X–ray binaries (e.g. Sguera et al. 2005, Negueruela et al. 2007).
In Chapter 4, we consider the dynamics of spinning compact binaries using the post-Newtonian (PN) equations with spin [9–11], which are essentially a perturbative expansion of full general relativ- ity in v/c; these equations allow us to consider the case of comparable masses. We use a Hamiltonian formulation that is particularly well-suited to the study of the PN system as a nonlinear dynamical system. The post-Newtonian Hamiltonian we use contains several spin interaction terms, including the traditional spin-orbit coupling and a spin-spin coupling term, as well as mass monopole/spin- induced quadrupole interaction terms that are of the same order as the spin-spin coupling . Since these quadrupole terms are too large to neglect, we specialize in the present case to binary black holes, for which the quadrupole terms are known exactly. (In neutron stars these terms depend on the poorly known equation of state. 3 ) We pay particularly close attention to quasi-circular orbits (“as circular as possible,” given nonvanishing spins). These are particularly relevant for gravitational wave generation and detection, since many binary systems are expected to circularize due to grav- itational radiation reaction  (and, moreover, stay circular ). In Chapter 4, we also consider the extreme mass-ratio limit of the PN equations, in order to make contact with the Papapetrou approximation considered in Chapters 2 and 3.
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BH-BH binaries, and their waveforms, are described by parameters including the masses M, μ, spins, orbital parameters, etc. The parameter space expands for BH- neutron star (NS) binaries — a key target for detection in 2016  — as tidal interactions also play an important role [6,7]. Semianalytic models, such as the EOB model, allow for much finer-grained coverage of parameter space than would be possible with (computationally expensive) NR simulations alone. In addition, effective models can bring physical insight [8 – 10]. For real-time data analysis it may be necessary to blend effective models with surrogate/ emulator models [11 – 13] and careful analysis of modeling uncertainties .
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limit our ability to understand important external dynamical factors that dictate LMXB forma- tion in GCs. First, observations of GC-LMXBs with HST and Chandra have been confined to the central parts of nearby elliptical galaxies. This restricted field of view overlooks the pres- ence of GCs at large distances from the core. It also prevents us from determining how vary- ing GC orbits influence LMXB formation. Therefore, blue metal-poor GCs would be largely excluded from any analysis since mostly red metal-rich GCs are found close to the galactic centre. As a result of the GC metallicity distribution and the limited field of view, the sizes of GCs would also be skewed to those that are brightest and found in galaxy centres. Coinciden- tally, one of the more detailed studies of the GC-LMXB connection of a rich GC population was completed by Paolillo et al. via wide-field, high spatial resolution observations with HST and Chandra in the nearby massive elliptical galaxy NGC 1399. Their results were similar to previous groups, where LMXBs were found to favour GCs that are massive, compact, red, and bright. They also found the highest fraction of GC-LMXBs of all early-type galaxies, which was dependent on galactocentric distance. They calculated the cumulative X-ray luminosity function (XLF) of LMXBs in both the field and in GCs (more to follow in Section 1.4). The XLF is a measure of the number of sources per luminosity interval while the cumulative XLF measures the number of sources brighter than a given luminosity L. Investigating the shape of the XLF in relation to fundamental parameters such as galactocentric distance allows us to ob- tain insight into the metallicity e ff ect (Peacock et al., 2010) and by extension the formation and evolution of these systems. It also forms the observational basis for X-ray population synthesis models.
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In the case of the remaining two systems, NN Ser and UZ For, our dynamical simulations reveal that the candidate companions would move on orbits that are stable on long timescales. As a result, the existence of companions in those systems cannot be ruled out on the basis of their current dynamical evolution alone. As a result, we have begun a program of study investigating whether planets that formed with the host binary would be able to survive the binary’s post-main sequence evolution before being emplaced on the orbits proposed around the PCEB. In the case of NN Ser, our study suggests that it is highly implausible that planets could survive the transition from the main sequence to the PCEB state for architectures that would result in the proposed planetary system (). However, recent observations () have added further data to the archive for NN Ser, an analysis of which strengthens the conclusion that the observed periodic variation in eclipse timings for that system is a real effect – and that therefore the unseen companion hypothesis remains strongly supported by the observational data 9 . If the proposed planets do exist, then that system will prove a fascinating test-bed for models of exoplanet formation and evolution, and also for the fine details of post-main sequence evolution of close binary star systems.
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Challenging the Euro centrism of much of psychoanalysis, Ghent writes, “In the West surrender has meant "defeat." In the East it has meant transcendence, liberation. In the West "ego", as used in the vernacular, has meant one's strength, rationality, a very close relative, until recently, of one's self. In the East "ego" has meant maya, (dream, the illusion of one's self)…” Ghent also makes reference to gurus “which permits the disciple to yield, surrender false self, and therein have a chance at finding himself… The "ego," false self, "mind" wants to argue; the guru won't argue. He knows that all engagement at this level reinforces the strength of the "ego" (false self)…we are so impressed by our "ego," we need to find something or someone who so totally transcends our experience, whose presence is so total and affirming that we will take a chance on surrendering… He is an excuse, an ally for true self to come forth.” While Ghent likens the guru to the analyst, one could also think of the Dominant in terms of the enabling role that she/he/they perform.
Given the energetics, burst rate and implied fluences, the coalescence, or merger, of two bound neutron stars (NSs) is the leading mechanism whereby gamma-ray bursts are thought to arise (Paczy´ nski 1986; Goodman 1986; Eichler et al. 1989; Narayan et al. 1992). One quantifiable prediction of the NS– NS merger hypothesis is the spatial distribution of GRBs (and GRB afterglow) with respect to their host galaxies. Conventional wisdom, using the relatively long–lived Hulse-Taylor binary pulsar as a model, is that such mergers can occur quite far ( ∼ > 100 kpc) outside of a host galaxy. Observed pulsar (PSR) binaries with a NS companion provide the only direct constraints on such populations, but the observations are biased both toward long lived systems, and systems that are close to the Galactic plane. The merger rate of NS–NS binaries has been discussed both in the context of gravitational wave- detection and GRBs (e.g., Phinney 1991; Narayan et al. 1991; Narayan et al. 1992; Tutukov & Yungelson 1994; Lipunov et al. 1995). Recently Fryer et al. (1998); Lipunov et al. (1997); Portegies Zwart & Spreeuw (1996) studied the effect of asymmetric kicks on birthrates of NS–NS binaries, but did not quantify the spatial distribution of such binaries around their host galaxies. Tutukov & Yungelson (1994) discussed the spatial distribution of NS–NS mergers but neglected asymmetric kicks and the effect of a galactic potential in their simulations. Only Portegies Zwart & Yungelson (1998) have discussed the maximum travel distance of merging neutron stars including asymmetric supernovae kicks.
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We have presented the evolution of angular momentum and orbital period changes between the component spins and the orbit in close double white dwarf binaries undergoing mass transfer through direct impact accretion over a broad range of orbital parameter space. This work improves upon similar earlier studies in a number of ways: First, we calculate self-consistently the angular momentum of the orbit at all times. This includes gravitational, tides and mass transfer effects in the orbital evolution of the component structure models, and allow the Roche lobe radius of the donor star and the rotational angular velocities of both components to vary, and account for the exchange of angular momentum between the spins of the white dwarfs and the orbit. Second, we investigate the mass transfer by modeling the ballistic motion of a point mass ejected from the center of the donor star through the inner La- grangian point. Finally, we ensure that the angular momentum is conserved, which requires the donor star spin to vary self-consistently. With these im- provements, we calculate the angular momentum and orbital period changes of the orbit and each binary component across the entire parameter space of direct impact double white dwarf binary systems. We find a significant de- crease in the amount of angular momentum removed from the orbit during mass transfer, as well as cases where this process increases the angular mo- mentum and orbital period of the orbit at the expense of the spin angular momentum of the donor and accretor. We find that our analysis yields an in- crease in the predicted number of stable systems compared to that in the pre- vious studies, survive the onset of mass transfer, even if this mass transfer is initially unstable. In addition, as a consequence of the tidal coupling, systems that come into contact near the mass transfer instability boundary undergo a phase of mass transfer with their orbital period.
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Each target spectrum was cross-correlated against an RV standard star best matched to its SpT and observed on the same night. We used the IRAF 8 fxcor routine ( Wyatt 1985; Fitzpatrick 1993 ) to obtain RVs and search for single- and double-lined spectroscopic binaries ( SB1s and SB2s, respec- tively ) . The standards and their published RVs are listed in Table 3. We used orders 39, 40, 43, 44, 46, 48, 49, 51, and 53, which ranged from 6400 to 8900 Å , for the cross-correlation to avoid telluric absorption lines and very low S / N regions. The RVs reported in Table 2 are the averages of those measured for each order weighted by the average S / N in each order. Table 2 also lists the standard star used, and Li I and H α EWs ( see
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Figure 5 shows that magnetic fields in PREPs cluster around 60–65 MG, with one clear exception, SDSS J103100.6+202832.2 at 42 MG, and another likely exception, SDSS J120615.73 + 510047.0 at 108 MG. While it is tempting to speculate about an evolutionary link between high magnetic fields on the one hand and long orbital periods / cool white dwarfs on the other hand, care must be taken about selection e ﬀ ects. The tenous, low-temperature plasmas in PREPs have radiative power only in the first few cyclotron harmonics. The cyclotron fundamental is likely optically thick (although not observed yet), and the fourth harmonic in several of the known cases is optically thin and rather diﬃcult to detect. A straightforward detection by spectroscopic means as done in the past is more or less easily feasable, if the second or third harmonic lies between 4500 Å and ∼8000 Å, corresponding to B = 45 . . . 120 MG. The object SDSS J103100.6+202832.2 is slightly below this range. Its fortunate discovery was possible due to its rather high power in harmonics higher than the third while SDSS J120615.73+510047.0 is possibly close to the upper limit.