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We study the evolution and gravitational wave emission of white dwarf–black hole accreting binaries with a semianalytical model. These systems will evolve across the mHz gravitational wave frequency band and potentially be detected by the Laser Interferometer Space Antenna (LISA). We identify new universal relations for this class of binaries, which relate the component masses to the gravitational wave frequency and its first derivative. Combined with the high precision measurements possible with LISA, these relations could allow us to infer the component masses and the luminosity distance of the source. LISA has therefore the potential to detect and characterize a virtually unexplored binary population. [ABSTRACT FROM AUTHOR]

Observations indicate that most massive galaxies contain a supermassive black hole, and theoretical studies suggest that when such galaxies have a major merger, the central black holes will form a binary and eventually coalesce. Here we discuss two spectral signatures of such binaries that may help distinguish them from ordinary active galactic nuclei. These signatures are expected when the mass ratio between the holes is not extreme and the system is fed by a circumbinary disk. One such signature is a notch in the thermal continuum that has been predicted by other authors; we point out that it should be accompanied by a spectral revival at shorter wavelengths and also discuss its dependence on binary properties such as mass, mass ratio, and separation. In particular, we note that the wavelength λn at which the notch occurs depends on these three parameters in such a way as to make the number of systems displaying these notches ; longer wavelength searches are therefore strongly favored. A second signature, first discussed here, is hard X-ray emission with a Wien-like spectrum at a characteristic temperature ∼100 keV produced by Compton cooling of the shock generated when streams from the circumbinary disk hit the accretion disks around the individual black holes. We investigate the observability of both signatures. The hard X-ray signal may be particularly valuable as it can provide an indicator of black hole merger a few decades in advance of the event. [ABSTRACT FROM AUTHOR]

Interacting galaxies often have complexes of hundreds of young stellar clusters of individual masses ∼104-106M☼ in regions that are a few hundred parsecs across. These cluster complexes interact dynamically, and their coalescence is a candidate for the origin of some ultracompact dwarf galaxies. Individual clusters with short relaxation times are candidates for the production of intermediate-mass black holes of a few hundred solar masses, via runaway stellar collisions prior to the first supernovae in a cluster. It is therefore possible that a cluster complex hosts multiple intermediate-mass black holes that may be ejected from their individual clusters due to mergers or binary processes, but bound to the complex as a whole. Here we explore the dynamical interaction between initially free-flying massive black holes and clusters in an evolving cluster complex. We find that, after hitting some clusters, it is plausible that the massive black hole will be captured in an ultracompact dwarf forming near the center of the complex. In the process, the hole typically triggers electromagnetic flares via stellar disruptions, and is also likely to be a prominent source of gravitational radiation for the advanced ground-based detectors LIGO and VIRGO. We also discuss other implications of this scenario, notably that the central black hole could be considerably larger than expected in other formation scenarios for ultracompact dwarfs. [ABSTRACT FROM AUTHOR]

Simultaneous, precise measurements of the mass M and radius R of neutron stars can yield uniquely valuable information about the still uncertain properties of cold matter at several times the density of nuclear matter. One method that could be used to measure M and R is to analyze the energy-dependent waveforms of the X-ray flux oscillations seen during some thermonuclear bursts from some neutron stars. These oscillations are thought to be produced by X-ray emission from hotter regions on the surface of the star that are rotating at or near the spin frequency of the star. Here we explore how well M and R could be determined by generating and analyzing, using Bayesian techniques, synthetic energy-resolved X-ray data that we produce assuming a future space mission having 2-30 keV energy coverage and an effective area of 10 m2, such as the proposed Large Observatory for X-Ray Timing or Advanced X-Ray Timing Array missions. We find that waveforms from hot spots within 10° of the rotation equator usually constrain both M and R with an uncertainty of about 10%, if there are 106 total counts from the spot, whereas waveforms from spots within 20° of the rotation pole provide no useful constraints. The constraints we report can usually be achieved even if the burst oscillations vary with time and data from multiple bursts must be used to obtain 106 counts from the hot spot. This is therefore a promising method to constrain M and R tightly enough to discriminate strongly between competing models of cold, high-density matter. [ABSTRACT FROM AUTHOR]

Recent studies of accretion onto supermassive black hole binaries suggest that much, perhaps most, of the matter eventually accretes onto one hole or the other. If so, then for binaries whose inspiral from ~1 pc to ~10-3-10-2 pc is driven by interaction with external gas, both the binary orbital axis and the individual black hole spins can be reoriented by angular momentum exchange with this gas. Here we show that, unless the binary mass ratio is far from unity, the spins of the individual holes align with the binary orbital axis in a time ~few-100 times shorter than the binary orbital axis aligns with the angular momentum direction of the incoming circumbinary gas; the spin of the secondary aligns more rapidly than that of the primary by a factor ~(m1/m2)1/2 > 1. Thus the binary acts as a stabilizing agent, so that for gas-driven systems, the black hole spins are highly likely to be aligned (or counteraligned if retrograde accretion is common) with each other and with the binary orbital axis. This alignment can significantly reduce the recoil speed resulting from subsequent black hole merger. [ABSTRACT FROM AUTHOR]

One of the main results of the Fermi Gamma-Ray Space Telescope is the discovery of γ-ray selected pulsars. The high magnetic field pulsar, PSR J0007+7303 in CTA1, was the first ever to be discovered through its γ-ray pulsations. Based on analysis of two years of Large Area Telescope (LAT) survey data, we report on the discovery of γ-ray emission in the off-pulse phase interval at the ~6σ level. The emission appears to be extended at the ~2σ level with a disk of extension ~0°.6. level. The flux from this emission in the energy range E ≥ 100 MeV is F100 = (1.73 ± 0.40stat ± 0.18sys) x 10-8 photons cm-2 s-1 and is best fitted by a power law with a photon index of Γ = 2.54 ± 0.14stat ± 0.05sys. The pulsed γ-ray flux in the same energy range is F100 = (3.95 ± 0.07stat ± 0.30sys) x 10-7 photons cm-2 s-1 and is best fitted by an exponentially cutoff power-law spectrum with a photon index of Γ = 1.41 ± 0.23stat ± 0.03sys and a cutoff energy Ec = 4.04 ± 0.20stat ± 0.67sys GeV. We find no flux variability either at the 2009 May glitch or in the long-term behavior. We model the γ-ray light curve with two high-altitude emission models, the outer gap and slot gap, and find that the preferred model depends strongly on the assumed origin of the off-pulse emission. Both models favor a large angle between the magnetic axis and observer line of sight, consistent with the nondetection of radio emission being a geometrical effect. Finally, we discuss how the LAT results bear on the understanding of the cooling of this neutron star. [ABSTRACT FROM AUTHOR]

We have identified the presence of large-scale, low-frequency dynamo cycles in a long-duration, global, magnetohydrodynamic (MHD) simulation of black hole accretion. Such cycles have previously been seen in local shearing box simulations, but we discuss their evolution over 1500 inner disk orbits of a global n/4 disk wedge spanning two orders of magnitude in radius and seven scale heights in elevation above/below the disk midplane. The observed cycles manifest themselves as oscillations in azimuthal magnetic field occupying a region that extends into a low-density corona several scale heights above the disk. The cycle frequencies are 10-20 times lower than the local orbital frequency, making them potentially interesting sources of low-frequency variability when scaled to real astrophysical systems. Furthermore, power spectra derived from the full time series reveal that the cycles manifest themselves at discrete, narrowband frequencies that often share power across broad radial ranges. We explore possible connections between these simulated cycles and observed low-frequency quasi-periodic oscillations (LFQPOs) in galactic black hole binary systems, finding that dynamo cycles have the appropriate frequencies and are located in a spatial region associated with X-ray emission in real systems. Derived observational proxies, however, fail to feature peaks with rms amplitudes comparable to LFQPO observations, suggesting that further theoretical work and more sophisticated simulations will be required to form a complete theory of dynamo-driven LFQPOs. Nonetheless, this work clearly illustrates that global MHD dynamos exhibit quasi-periodic behavior on timescales much longer than those derived from test particle considerations. [ABSTRACT FROM AUTHOR]

The use of infrared spectra to determine molecular abundances of icy astronomical objects and to study their chemistry requires laboratory measurements of reference spectra and related quantities, such as the index of refraction (n) and density (ρ) of candidate ices. Here we present new n and ρ measurements on ices involving over 30 C-, H-, and O-containing compounds, both acyclic and cyclic, representing seven chemical families. We examine the results in a way that is rare in the astrochemical literature, namely one in which data from an ice formed from molecules of a particular chemical family are compared to measurements on another member of the same family, such as of a homologous series or a pair of isomers. Apart from the intrinsic usefulness of the n and ρ data, a structure-based comparison can help establish trends and identify possibly spurious results. As liquid-phase data sometimes are used in low-temperature astrochemical work in the absence of solid-phase measurements, we compare our new ice results to those for the corresponding room-temperature liquids. We emphasize the use of our n and ρ data to compute the molar refraction (RM) for each of our ices, and how the resulting RM values compare to those expected from molecular structures. The use of calculated RM values and measured n values to calculate ice densities, in the absence of direct measurements, is also addressed. [ABSTRACT FROM AUTHOR]

We report on a search for high-energy counterparts to fast radio bursts (FRBs) with the Fermi Gamma-ray Burst Monitor, Fermi Large Area Telescope, and the Neil Gehrels Swift Observatory Burst Alert Telescope. We find no significant associations for any of the 23 FRBs in our sample, but report upper limits to the high-energy fluence for each on timescales of 0.1, 1, 10, and 100 s. We report lower limits on the ratio of the radio to high-energy fluence, fr/fγ, for timescales of 0.1 and 100 s. We discuss the implications of our non-detections on various proposed progenitor models for FRBs, including analogs of giant pulses from the Crab pulsar and hyperflares from magnetars. This work demonstrates the utility of analyses of high-energy data for FRBs in tracking down the nature of these elusive sources. [ABSTRACT FROM AUTHOR]

The discovery of quasi-periodic brightness oscillations (QPOs) in the X-ray emission accompanying the giant flares of the soft gamma-ray repeaters SGR 1806–20 and SGR 1900+14 has led to intense speculation about their nature and what they might reveal about the interiors of neutron stars. Here we take a fresh look at the giant flare data for SGR 1806–20, and in particular we analyze short segments of the post-peak emission using a Bayesian procedure, which has not previously been applied to these data. We find at best weak evidence that any QPO persists for more than ∼1 s; instead, almost all the data are consistent with a picture in which there are numerous independently excited modes that decay within a few tenths of a second. This has interesting implications for the rapidity of decay of the QPO modes, which could occur by the previously suggested mechanism of coupling to the MHD continuum. The strongest QPOs favor certain rotational phases, which might suggest special regions of the crust or of the magnetosphere. We also find several previously unreported QPOs in these data, which may help in tracking down their origin. [ABSTRACT FROM AUTHOR]

As gravitational wave instrumentation becomes more sensitive, it is interesting to speculate about subtle effects that could be analyzed using upcoming generations of detectors. One such effect that has great potential for revealing the properties of very dense matter is fluid oscillations of neutron stars. These have been found in numerical simulations of the hypermassive remnants of double neutron star mergers and of highly eccentric neutron star orbits. Here we focus on the latter and sketch out some ideas for the production, gravitational-wave detection, and analysis of neutron star oscillations. These events will be rare (perhaps up to several tens per year could be detected using third-generation detectors such as the Einstein Telescope or the Cosmic Explorer), but they would have unique diagnostic power for the analysis of cold, catalyzed, dense matter. Furthermore, these systems are unusual in that analysis of the tidally excited f-modes of the stars could yield simultaneous measurements of their masses, moments of inertia, and tidal Love numbers, using the frequency, damping time, and amplitude of the modes. They would thus present a nearly unique opportunity to test the I-Love-Q relation observationally. The analysis of such events will require significant further work in nuclear physics and general relativistic nonlinear mode coupling, and thus we discuss further directions that will need to be pursued. For example, we note that for nearly grazing encounters, numerical simulations show that the energy delivered to the f-modes may be up to two orders of magnitude greater than predicted in the linear theory. [ABSTRACT FROM AUTHOR]

In order to understand the rate of merger of stellar mass black hole binaries (BHBs) by gravitational wave (GW) emission it is important to determine the major pathways to merger. We use numerical simulations to explore the evolution of BHBs inside the radius of influence of supermassive black holes (SMBHs) in galactic centers. In this region, the evolution of binaries is dominated by perturbations from the central SMBH. In particular, as first pointed out by Antonini and Perets, the Kozai–Lidov mechanism trades relative inclination of the BHB to the SMBH for eccentricity of the BHB, and for some orientations can bring the BHB to an eccentricity near unity. At very high eccentricities, GW emission from the BHB can become efficient, causing the members of the BHB to coalesce. We use a novel combination of two N-body codes to follow this evolution. We are required to simulate small systems to follow the behavior accurately. We have completed 400 simulations that range from ∼300 stars around a 103 black hole to ∼4500 stars around a 104 black hole. These simulations are the first to follow the internal orbit of a binary near an SMBH while also following the changes to its external orbit self-consistently. We find that this mechanism could produce mergers at a maximum rate per volume of ∼100 Gpc−3 yr−1 or considerably less if the inclination oscillations of the binary remain constant as the BHB inclination to the SMBH changes, or if the binary black hole fraction is small. [ABSTRACT FROM AUTHOR]

The IceCube collaboration has reported the first detection of high-energy astrophysical neutrinos, including ∼50 high-energy starting events, but no individual sources have been identified. It is therefore important to develop the most sensitive and efficient possible algorithms to identify the point sources of these neutrinos. The most popular current method works by exploring a dense grid of possible directions to individual sources, and identifying the single direction with the maximum probability of having produced multiple detected neutrinos. This method has numerous strengths, but it is computationally intensive and because it focuses on the single best location for a point source, additional point sources are not included in the evidence. We propose a new maximum likelihood method that uses the angular separations between all pairs of neutrinos in the data. Unlike existing autocorrelation methods for this type of analysis, which also use angular separations between neutrino pairs, our method incorporates information about the point-spread function and can identify individual point sources. We find that if the angular resolution is a few degrees or better, then this approach reduces both false positive and false negative errors compared to the current method, and is also more computationally efficient up to, potentially, hundreds of thousands of detected neutrinos. [ABSTRACT FROM AUTHOR]

The Neutron star Interior Composition ExploreR (NICER) is expected to launch in 2017 and will gather X-ray data on neutron stars and other high-energy sources from a berth on the International Space Station. Its prime scientific goal is to measure the masses and radii of non-accreting neutron stars via fits to the energy-dependent waveforms produced by the rotation of hot spots on their surfaces. These measurements will provide valuable input to theoretical models of cold matter beyond nuclear density. Here we propose that PSR J1614−2230, despite its low count rate, is a promising source to observe with NICER. The reason is that XMM-Newton observations suggest that the fractional oscillation amplitude from PSR J1614−2230 could be high enough that this star cannot be very compact. We show that if we analyze 0.5 Ms of NICER data and 0.1 Ms of nearby off-source data and combine that analysis with the known mass of this star, we would find a robust lower limit to the radius with a statistical uncertainty of only km. We also show that even if there is an unmodeled nonthermal component modulated at the pulsation frequency, good statistical fits could rule out significant biases. The low count rate will make reliable upper limits on the radius difficult, but the lower limit could rule out some equations of state that are currently being discussed. This analysis would require a good estimate of the non-source background, so Chandra observations of the vicinity of PSR J1614−2230 would be helpful. [ABSTRACT FROM AUTHOR]

Broad Fe K emission lines have been widely observed in the X-ray spectra of black hole systems as well as in neutron star systems. The intrinsically narrow Fe K fluorescent line is generally believed to be part of the reflection spectrum originating in an illuminated accretion disk which is broadened by strong relativistic effects. However, the nature of the lines in neutron star low-mass X-ray binaries (LMXBs) has been a matter of debate. We therefore obtained the longest, high-resolution X-ray spectrum of a neutron star LMXB to date with a 300 ks Chandra High Energy Transmission Grating Spectrometer (HETGS) observation of Serpens X-1. The observation was taken under the “continuous clocking” mode, and thus was free of photon pile-up effects. We carry out a systematic analysis and find that the blurred reflection model fits the Fe line of Serpens X-1 significantly better than a broad Gaussian component does, implying that the relativistic reflection scenario is much preferred. Chandra HETGS also provides a highest spectral resolution view of the Fe K region and we find no strong evidence for additional narrow lines. [ABSTRACT FROM AUTHOR]

We describe sophisticated new Bayesian analysis methods that make it possible to estimate quickly the masses and radii of rapidly rotating, oblate neutron stars by fitting oblate-star waveform models to energy-resolved observations of the waveforms of X-ray burst oscillations produced by such stars. We find that a 25% variation of the temperature of the hot spot in the north-south direction does not significantly bias estimates of the mass M and equatorial radius derived by fitting a model that assumes a uniform-temperature spot. Our results show that fits of oblate-star waveform models to waveform data can simultaneously determine M and with 1σ uncertainties ≲7% if (1) the star's rotation rate is ≳ 600 Hz; (2) the spot center and observer's sightline are both within of the star's rotational equator; and (3) counts are collected during burst oscillations that have a fractional rms amplitude ≳10%. A fractional amplitude of ∼10% is realistic, and the accepted NICER and proposed LOFT and AXTAR space missions could collect counts from a single star by combining data from many X-ray bursts from the star. Uncertainties ≲7% are small enough to improve substantially our understanding of cold, ultradense matter. [ABSTRACT FROM AUTHOR]

Among the many intriguing aspects of optically discovered tidal disruption events (TDEs) is that their temperatures are lower than predicted and that the temperature does not evolve as rapidly with a decreasing fallback rate as would be expected in standard disk theory. We show that this can be explained qualitatively using an idea proposed by Laor & Davis in the context of normal active galactic nuclei: that larger accretion rates imply stronger winds and thus that the accretion rate through the inner disk only depends weakly on the inflow rate at the outer edge of the disk. We also show that a reasonable quantitative agreement with data requires that, as has been suggested in recent papers, the characteristic radius of the tidal stream is approximately equal to the semimajor axis of the most bound orbit of the debris rather than twice the pericenter distance, which would be expected from circularization without rapid angular momentum redistribution. If this explanation is correct, it suggests that the evolution of TDEs may test both non-standard disk theory and the details of the interactions of the tidal stream. [ABSTRACT FROM AUTHOR]

If binaries consisting of two ∼100 M☼ black holes exist, they would serve as extraordinarily powerful gravitational-wave sources, detectable to redshifts of z ∼ 2 with the advanced LIGO/Virgo ground-based detectors. Large uncertainties about the evolution of massive stars preclude definitive rate predictions for mergers of these massive black holes. We show that rates as high as hundreds of detections per year, or as low as no detections whatsoever, are both possible. It was thought that the only way to produce these massive binaries was via dynamical interactions in dense stellar systems. This view has been challenged by the recent discovery of several ≳ 150 M☼ stars in the R136 region of the Large Magellanic Cloud. Current models predict that when stars of this mass leave the main sequence, their expansion is insufficient to allow common envelope evolution to efficiently reduce the orbital separation. The resulting black hole-black hole binary remains too wide to be able to coalesce within a Hubble time. If this assessment is correct, isolated very massive binaries do not evolve to be gravitational-wave sources. However, other formation channels exist. For example, the high multiplicity of massive stars, and their common formation in relatively dense stellar associations, opens up dynamical channels for massive black hole mergers (e.g., via Kozai cycles or repeated binary-single interactions). We identify key physical factors that shape the population of very massive black hole-black hole binaries. Advanced gravitational-wave detectors will provide important constraints on the formation and evolution of very massive stars. [ABSTRACT FROM AUTHOR]

The brightest ultraluminous X-ray source currently known, HLX-1, has been observed to undergo five outburst cycles. The periodicity of these outbursts, and their high inferred maximum accretion rates of ∼few × 10–4M☼ yr–1, naturally suggest Roche lobe overflow at the pericenter of an eccentric orbit. It is, however, difficult for the Roche lobe overflow model to explain the apparent trend of decreasing decay times over the different outbursts while the integrated luminosity also drops. Thus, if the trend is real rather than simply being a reflection of the complex physics of accretion disks, a different scenario may be necessary. We present a speculative model in which, within the last decade, a high-mass giant star had most of its envelope tidally stripped by the ∼104 – 5M☼ black hole in HLX-1, and the remaining core plus low-mass hydrogen envelope now feeds the hole with a strong wind. This model can explain the short decay time of the disk, and could explain the fast decrease in decay time if the wind speed changes with time. A key prediction of this model is that there will be excess line absorption due to the wind; our analysis does in fact find a flux deficit in the ∼0.9-1.1 keV range that is consistent with predictions, albeit at low significance. If this idea is correct, we also expect that within years to dacades the bound material from the original disruption will return and will make HLX-1 a persistently bright source. [ABSTRACT FROM AUTHOR]

Massive black holes have been discovered in all closely examined galaxies with high velocity dispersion. The case is not as clear for lower-dispersion systems such as low-mass galaxies and globular clusters. Here we suggest that above a critical velocity dispersion ∼40 km s–1, massive central black holes will form in relaxed stellar systems at any cosmic epoch. This is because above this dispersion primordial binaries cannot support the system against deep core collapse. If, as previous simulations show, the black holes formed in the cluster settle to produce a dense subcluster, then given the extremely high densities reached during core collapse the holes will merge with each other. For low velocity dispersions and hence low cluster escape speeds, mergers will typically kick out all or all but one of the holes due to three-body kicks or the asymmetric emission of gravitational radiation. If one hole remains, it will tidally disrupt stars at a high rate. If none remain, one is formed after runaway collisions between stars, and then it tidally disrupts stars at a high rate. The accretion rate after disruption is many orders of magnitude above Eddington. If, as several studies suggest, the hole can accept matter at that rate because the generated radiation is trapped and advected, then it will grow quickly and form a massive central black hole. [ABSTRACT FROM AUTHOR]