adsabs/FOCAL huggingface dataset:
In addition to the above problem in the UV spectroscopy of disk-dominated systems, X-ray data of CVs have also been in strong disagreement with theoretical expectations for more than three decades (Ferland et al. 1982). The culprit has been the difficult to study boundary layer (BL) between the accretion disk and the WD star. About half of the disk accretion energy (in the form of kinetic energy) is expected to be dissipated in the BL between the Keplerian disk and slowly rotating stellar surface (Pringle 1981). Because of its small size, the BL was predicted to emit in the X-ray band: at low accretion rates (when the WD is dominant in the UV), the BL was expected to be optically thin and emit hard X-rays (Pringle & Savonije 1979; Tylenda 1981; Narayan & Popham 1993); at large accretion rates, typical of NLs in a high state and DNe in outburst, the BL was expected to be optically thick and emit soft X-rays (Pringle 1977; Narayan & Popham 1993; Popham & Narayan 1995). Systems in the low state indeed reveal optically thin hard X-ray emission (Szkody et al. 2002; Pandel et al. 2005; Mukai et al. 2009). However, systems in a state of high mass accretion often do not show an optically thick soft X-ray component; instead, many exhibit optically thin hard X-ray emission (Patterson & Raymond 1985a, 1985b; Mauche et al. 1995; van Teeseling et al. 1996; Baskill et al. 2005; Balman et al. 2014), with an X-ray luminosity much smaller than expected, i.e., much smaller than the disk luminosity. While optically thin hard X-ray emission from high mass accretion rate systems was unexpected, it is, however, not especially inconsistent with the theoretical work: optically thin BLs can occur in high mass accretion rate systems, since the transition to being optically thin depends not only on the mass accretion rate, but also on the WD mass, the WD rotation rate, and the (unknown) alpha viscosity parameter (Popham & Narayan 1995). Simulations of optically thin BLs (Narayan & Popham 1993; Popham 1999) show that the inner edge of the Keplerian (and optically thick) disk starts at an actual radius R0 = Rwd + δBL, where the size δBL of the BL is of the order of the stellar radius Rwd: δBL ∝ Rwd (the BL is actually geometrically thick). The direct consequence of having an optically thin and geometrically thick BL is that the optically thick Keplerian disk will appear to have an inner hole of size δBL (possibly of the order of the radius of the WD Rwd). Two decades ago, it had already been pointed out that optically thin BL can explain the inner hole observed in circumstellar disks around young stellar objects (T Tauri stars; Godon 1996). In other words, optically thin BLs are consistent not only with the X-ray data, but also with the UV data, as truncated optically thick disks produce a UV continuum with a shallow slope in better agreement with the UV observations than non-truncated disks.
Citation: Mauche et al. 1995
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We have employed a simple grey-body fitting method to model the infrared SED of individual sources. In the LMC, which has the most numerous list of LBVs, large amounts of dust are observed (∼10−3 − 10−2 M⊙), similar to Galactic LBVNe. We stacked the infrared images of the LMC LBVs and extracted the photometry of the resulting source, detected up to 160−250 μm. The integrated SED from the stacks resembles that of LBVs with a strong ionised stellar wind and an extended dusty nebula. The SED can be fitted with only two components: a power-law describing the free-free spectrum of ionised stellar winds and the stellar photosphere, and a single-component grey-body for the dust. For the grey-body we adopted two different values of the κ parameter, including the value determined by Gordon et al. (2014) to fit the integrated ISM SED of the LMC. A significant contribution to the stack SED comes from a few sources, the most important one is RMC143. This was already identified as a massive nebula (Agliozzo et al. 2019). We obtain an integrated present dust mass of 



0
.

11

−
0.03


+
0.06




M
⊙



$ 0.11^{+0.06}_{-0.03}\,M_{\odot} $


. We have repeated a similar analysis on the sample of AGBs and RSGs by Riebel et al. (2012). We obtain a detection in the stacked images only when considering the “extreme”-AGBs. We find that the integrated 160 μm emission of 1342 extreme-AGBs is of the same order of magnitude as that of 18 LBVs. The integrated dust mass from these sources is 



1
.

2

−
0.4


+
0.3


×

10

−
4




M
⊙



$ 1.2^{+0.3}_{-0.4}\times 10^{-4}\,M_{\odot} $


. We do not find any correlation between the dust masses and the stellar luminosities. This could be due to the fact that such stars have different evolutionary histories or that the dust production mechanism does not depend on the initial mass of the star. Most likely we are also unable to detect the lowest-mass nebulae. To estimate the total dust mass produced by LBVs in the LMC during its full lifetime, we consider two cases: constant number of LBVs across time and a case accounting for IMF and SFH. The uncertainty on the duration of LBV phase in the first case, or on the population of LBVs in the second case, add a significant uncertainty in the total mass produced by LBVs.
Citation: Agliozzo et al. 2019
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Comparison to a sample of inactive galaxies is also key to demonstrating that an observed merger fraction is actually related to AGN activity. Large samples of inactive galaxies are observed at all merger stages, so it is clear that a major merger alone is not a sufficient condition for quasar activity. Thus, to conclusively demonstrate that mergers are an important channel for quasar fueling, we would need to observe an enhancement to the merger fraction in quasar hosts relative to a matched sample of inactive galaxies. Several studies of lower-luminosity AGN host morphologies with inactive control samples have been conducted in HST extragalactic survey fields (e.g., Grogin et al. 2005; Gabor et al. 2009; Cisternas et al. 2011; Schawinski et al. 2011; Kocevski et al. 2012; Böhm et al. 2013; Villforth et al. 2014). In particular, we designed our study methodology following Cisternas et al. (2011), who used visual classification to compare strong distortion signatures in moderate-luminosity X-ray selected AGN hosts to a comparison sample of inactive galaxies in the redshift range z = 0.3–1.0. They found no significant enhancement to the merger fraction of AGN hosts relative to inactive galaxies, demonstrating that the majority of cosmic black hole mass accretion at 





, i.e., in AGN with inferred SMBH masses 





 (Vestergaard & Osmer 2009), is not merger driven. How do we then reconcile this result with the results from the red quasar and radio galaxy studies? One possibility is that certain sub-classes of AGN may be preferentially merger driven, even though the bulk of all objects are not. In particular, a downsizing trend has been observed, such that near the peak of quasar activity at z = 2, higher-mass SMBHs dominate the cosmic mass accretion (





, Vestergaard & Osmer 2009). It is possible that forming these most massive black holes requires major mergers, as a particularly efficient gas transport mechanism, and that the declining major merger rate of galaxies is one of the driving forces behind this downsizing trend.
Citation: Villforth et al. 2014
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For an informed opinion on the expected fluxes and durations of short transients, we have to understand coherent emission processes. What we know from incoherent emission physics is that the intrinsic brightness temperature of sources is likely limited to 1011 − 12 K (e.g. Kellermann & Pauliny-Toth 1969; Singal 1986, where brightness temperature is defined as the value of T in the Rayleigh–Jeans formula that yields the correct flux of the source). Sources having brightness temperatures above about 1012 K must emit coherently, have their emission relativistically boosted, or both. However, we understand the processes underlying such sources very poorly, and so in this paper, we shall take the approach of expecting a wide range of known and unknown types of source, and exploring as much of parameter space as our experiment allows. One important thing to note, that is particularly relevant here, is that most of those coherent emitters for which we know the properties of the radio spectrum have quite steep spectra, typically going as ν−2 or even ν−3 (see e.g. Melrose 2009; Lorimer & Kramer 2012), in contrast with a typical ν−0.8 for optically thin synchrotron emission. This means that low-frequency instruments such as LOFAR may be intrinsically at an advantage to find coherent emitters (in addition to having larger fields of view). While known coherent transients have mostly been found in beam-formed searches and last milliseconds to seconds, more recently fast transients have been discovered in low-frequency image plane surveys. For example, the sources GCRT J1745-3009 (Hyman et al. 2005) and GCRT J1746-2757 (Hyman et al. 2002) were detected at 330 MHz with the VLA, while GCRT J1742-3001 (Hyman et al. 2009) was discovered at 235 MHz with the Giant Metrewave Radio Telescope. These sources showed bright flares lasting from minutes to a few hours. More recently, the low-frequency radio transient ILT J225347+862146 (Stewart et al. 2016) was discovered at 60 MHz with LOFAR, lasting about 10 min. The only significant population of transient radio sources previously known in this duration range are relatively nearby and low-luminosity flare stars, having fluxes of about 1 Jy at 1.2 GHz (Osten & Bastian 2006).
Citation: Lorimer & Kramer 2012
Citation function: Motivation; Motivation
The hard X-ray band has been crucial to studying some of the most powerful blazars (see e.g., Tavecchio et al. 2000; Massaro et al. 2004a, 2004b, 2006; Donato et al. 2005). More recently, the outstanding sensitivity of the Nuclear Spectroscopic Telescope Array (NuSTAR, 3–79 keV, Harrison et al. 2013) has enabled us to find and study some of the most distant and luminous ones (e.g., Sbarrato et al. 2013; Tagliaferri et al. 2015; Ajello et al. 2016; Paliya et al. 2016; Sbarrato et al. 2016; Marcotulli et al. 2017). Harboring highly relativistic jets pointed closely at the observer (θV  1/Γ, θV being the viewing angle and Γ the bulk Lorentz factor, Γ ∼ 10−15, Urry & Padovani 1995), this subclass of the Active Galactic Nuclei (AGNs) is home to some of the most energetic particle acceleration and radiation processes known in astrophysics. The boost in flux ascribed to relativistic beaming, arising from the peculiar orientation of the jets, renders them visible at redshifts well beyond z = 2 (the farthest blazar detected so far is at z = 5.47, Romani et al. 2004), making them extraordinary beacons with which to explore the early universe. Their typical double-hump spectral energy distribution (SED) spans the whole electromagnetic spectrum and is shaped by the nonthermal processes occurring in the jets. Relativistic electrons, spiraling along the magnetic field lines, undergo both synchrotron and inverse Compton (IC) processes. The first produces a peak in the SED located between infrared and X-ray frequencies. The second instead results in a peak located between X- and γ-ray energies. If the electrons interact with a source of low-energy photons within the jet, this is referred to as Synchrotron Self Compton (SSC, e.g., Ghisellini & Maraschi 1989), whereas if the photons are external to the jet (i.e., the accretion disk, the torus, and/or the broad line region, BLR), it is referred to as External Compton process (EC, e.g., Sikora et al. 1994). Based on their optical spectra, blazars are usually classified either as BL Lacertae objects (BL Lacs) or flat spectrum radio quasars (FSRQs), the first showing weak or no emission lines, the second showing broad (EW > 5 Å) ones. Following Abdo et al. (2010a), these sources can also be classified according to the position of the synchrotron peak (





), with low-, intermediate-, and high-synchrotron peak (LSP, ISP, HSP) blazars having, respectively, 





 Hz, 





 Hz, and 





 Hz. FSRQs usually belong to the LSP class and at the high-luminosity end of such subclass are the so-called “MeV blazars,” whose high-energy peak falls in (or close to) the MeV band. With bolometric luminosities exceeding 1048 erg s−1, these are among the most powerful objects in the universe. In fact, they host powerful relativistic jets (Ghisellini et al. 2014), are usually found at high-redshift (z > 2, e.g., Ajello et al. 2009; Ghisellini et al. 2010; Ackermann et al. 2017; Marcotulli et al. 2017), and typically host billion solar mass black holes (e.g., Ghisellini et al. 2010; Paliya et al. 2017a).
Citation: Romani et al. 2004
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The physical nature of the X-structures themselves is also an open question, although significant progress has already been made in this direction. The spatial resolution of the models was not very good in the first numerical studies of 3D bar structure (≈103−104 particles represented the disc component). X-structures were only distinguished in unsharp masked images constructed from such models. Some authors (Friedli & Pfenniger 1990; Pfenniger & Friedli 1991) therefore suggested that X-structures are akin to an optical illusion due to the tendency of eyes to perceive the intensity gradients instead of actual intensity values. However, with an increase of spatial resolution, it became evident that the X-structures are real density enhancements that can be observed even without unsharp masking processing (see Smirnov & Sotnikova 2018 for many representative examples). The question then is why such density enhancements are observed. Studies of orbit composition of B/PS bulges and X-structures in different numerical and analytical models (Patsis, Skokos & Athanassoula 2002; Quillen 2002; Quillen et al. 2014; Patsis & Katsanikas 2014a; Parul, Smirnov & Sotnikova 2020) showed that an X-shape is observed in these models due to a tendency of a star to spend more time near turning points of its trajectory. More specifically, 3D bars are constituted by different types of periodic, quasi-periodic, and sticky chaotic orbits (Pfenniger 1984; Pfenniger & Friedli 1991; Patsis et al. 2002; Skokos, Patsis & Athanassoula 2002; Patsis & Katsanikas 2014a,b; Patsis & Harsoula 2018; Patsis & Athanassoula 2019). Stars that move along such orbits spend different periods of time in different parts of their trajectories. For example, stars moving along banana-shaped orbits (Pfenniger & Friedli 1991) spend more time at the highest points of their trajectory (Patsis et al. 2002; Patsis & Katsanikas 2014a). Therefore, the bulk of such orbits produces a density profile with visible density enhancements at the highest points of such orbits. For an X-structure to be observed, these density enhancements should be aligned along an almost straight line for orbits with different apocentric distances. This is indeed the case for the realistic bar potential (Patsis et al. 2002; Quillen 2002; Quillen et al. 2014; Patsis & Katsanikas 2014a). Parul et al. (2020) showed that the orbits of a more complicated morphology than banana-shaped orbits can build an X-structure in a similar manner. In general, cited works showed that X-structures and B/PS bulges are produced by the same orbits. They are not constituted by different types of orbits like, for example, disc and classical bulge components. The open question that has to be answered in the upcoming studies is what types of orbits are actually presented in real galaxies.
Citation: Quillen 2002; Quillen 2002
Citation function: Background; Background; Background
Lempo–Paha—Hiisi is a trans-Neptunian hierarchical triple system composed of a tight inner binary with components of similar size and an outer companion about half their size orbiting 10 times further away (Trujillo & Brown 2002; Benecchi et al. 2010). All large trans-Neptunian objects like Pluto have multiple small moons, but Lempo’s structure is unique in the Solar system. The place and timing of its origin are still a subject of debate (Nesvorný, Youdin & Richardson 2010; Correia 2018). In contrast to the Lempo system, all other known triples in the Solar system have their orbits almost regularly spaced, with one component much smaller than the others, with the most distant component being the largest (Johnston 2020). Unveiling the possible origin of the Lempo system is relevant, as the architecture of multiples holds clues to their formation story and the conditions prevailing in the primitive outer Solar system, but its origin is still unclear. Capture theories proposed so far failed to reproduce the orbital characteristics of observed binaries, especially the distribution of their orbital inclinations (Nesvorný et al. 2010; Brunini 2020). Also, triple formation requires multiple captures, a very unlikely event. Gravitational collapse of pebble clouds in a turbulent gas disc would be efficient in producing binaries and, in some particular conditions, can also produce triple systems. However, such triples do not seem to match the orbital structure of Lempo–Paha–Hiisi (Nesvorný et al. 2010). The non-detection of triple systems in the cold classical Kuiper belt, where the number of known binaries is much higher than in the resonant populations (Noll et al. 2020), argues against this formation mechanism of triple systems. The fragile dynamical stability of Lempo–Paha–Hiisi (Correia 2018) also casts doubt on the place and time of its origin, leading to speculations about a possible recent formation at the place in which it currently resides.
Citation: Johnston 2020
Citation function: Differences
The Nançay Radio Telescope (NRT) is a meridian telescope equivalent to a 94 m parabolic dish located near Orléans (France). Owing to its design, the NRT can track objects with declinations δ >  −39° for approximately one hour around culmination, and is thus well suited for the long-term timing of pulsars, for example, for the study of individual objects (see, e.g., Cognard et al. 2017; Octau et al. 2018, for recent examples) or for searching low-frequency gravitational waves from supermassive black hole binaries, using pulsar timing arrays (PTAs; see, e.g., Desvignes et al. 2016). With the goal of identifying new exotic pulsar systems or highly stable MSPs suitable for PTA studies, the SPAN512 pulsar survey (Desvignes et al. 2013; Octau et al. 2016; Desvignes et al., in prep.) was conducted between 2012 and 2018 at the NRT. As part of this survey, new pulsars were searched for at intermediate Galactic latitudes (3.5 °  |b|  5°) and away from the inner Galaxy (Galactic longitudes 74 °  l   150°). Observations were conducted at 1.4 GHz with 0.5 MHz frequency channels over a total bandwidth of 512 MHz and a fine time resolution of 64 μs, to be sensitive to faint and distant MSPs. We used PRESTO pulsar searching routines (Ransom et al. 2002) to search the data for pulsars with dispersion measures (DMs) up to 1800 pc cm−3, and a moderate acceleration search in the Fourier domain (the zmax parameter was set to 50 in PRESTO analyses) to be sensitive to pulsars in binary systems. Searches for periodic signals in the data from this survey so far led to the discovery of one “ordinary” (i.e., non-millisecond) pulsar, PSR J2048+49, and two MSPs, PSRs J2055+3829 and J2205+6012. Details on the survey, the data analysis, and the discovered pulsars will be reported in Desvignes et al. (in prep.). In the present paper we report on the results from the timing of PSR J2055+3829, an MSP in an eclipsing BW system, and from the analysis of the radio eclipses of the pulsar. In Sect. 2 we describe the radio timing observations and the results from the analysis of the timing data. In Sect. 3 we present observations of eclipses of PSR J2055+3829 at 1.4 GHz, and analyses of the data taken around superior conjunction of the pulsar. In the following section (Sect. 4), we present comparisons of the mass function distributions for eclipsing and non-eclipsing BWs, and for Galactic disk and globular cluster BWs. Finally, Sect. 5 summarizes our findings.
Citation: Octau et al. 2016
Citation function: Motivation; Motivation
In addition to the unconstrained optimization problems of (11) and (12), many CS-based approaches consider constrained forms of the analysis and synthesis models, which are, respectively, given by 
(14)
\begin{eqnarray*}
\min _{\boldsymbol{x}} \Vert \boldsymbol {\mathsf {\Psi }}^\dagger {\boldsymbol{x}}\Vert _1, \quad {\rm s.t.} \ \ \Vert {\boldsymbol{y}}-\boldsymbol {\mathsf {\Phi }} {\boldsymbol{x}}\Vert _2^2 \le \epsilon
\end{eqnarray*}
and 
(15)
\begin{eqnarray*}
\min _{\boldsymbol{a}} \Vert {\boldsymbol{a}}\Vert _1, \quad {\rm s.t.} \ \ \Vert {\boldsymbol{y}}-\boldsymbol {\mathsf {\Phi }}\boldsymbol {\mathsf {\Psi }} {\boldsymbol{a}}\Vert _2^2 \le \epsilon ,
\end{eqnarray*}
where ε is an upper-bound related to the noise level present in . CS approaches based on constrained optimization problems, solved via convex optimization techniques, have been applied broadly in RI imaging (Wiaux et al. 2009a,b; McEwen & Wiaux 2011; Li et al. 2011a,b; Carrillo et al. 2012, 2014; Onose et al. 2016; Pratley et al. 2018). These techniques have shown promising results, with improvements in terms of image fidelity and flexibility compared to traditional approaches such as clean-based methods and MEM. For these constrained regularization approaches, parallel implementation structures have also been explored (Carrillo et al. 2014; Onose et al. 2016). Compared with the unconstrained analysis and synthesis models, constrained approaches are parametrized by ε (related to noise level) which controls the error of the reconstruction explicitly; in contrast, unconstrained models use regularization parameter μ to impose a tradeoff between the prior and data fidelity. The constrained approach therefore avoids the problem of unknown regularization parameter μ, replacing it with the problem of estimating the noise bound ε. The latter can be performed in a principled manner by noting that for Gaussian noise the ℓ2 norm data fidelity term follows a χ2 distribution with 2M degrees of freedom (see e.g. Carrillo et al. 2012). While constrained problems do not afford a straightforward Bayesian interpretation, the constrained and unconstrained models are closely related (Nikolova 2016).
Citation: Li et al. 2011a
Citation function: Background; Compare/Contrast