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The merger of stellar-mass black holes (BHs) is not expected to generate detectable electromagnetic (EM) emission. However, the gravitational wave (GW) events GW150914 and GW170104, detected by the Laser Interferometer Gravitational Wave Observatory (LIGO) to be the result of merging, ~60 solar mass binary black holes (BBHs), each have claimed coincident gamma-ray emission. Motivated by the intriguing possibility of an EM counterpart to BBH mergers, we construct a model that can reproduce the observed EM and GW signals for GW150914- and GW170104-like events, from a single-star progenitor. Following Loeb (2016), we envision a massive, rapidly rotating star within which a rotating bar instability fractures the core into two overdensities that fragment into clumps which merge to form BHs in a tight binary with arbitrary spin-orbit alignment. Once formed, the BBH inspirals due to gas and gravitational-wave drag until tidal forces trigger strong feeding of the BHs with the surrounding stellar-density gas about 10 seconds before merger. The resulting giga-Eddington accretion peak launches a jet that breaks out of the progenitor star and drives a powerful outflow that clears the gas from the orbit of the binary within one second, preserving the vacuum GW waveform in the LIGO band. The single-progenitor scenario predicts the existence of variability of the gamma-ray burst, modulated at the ~0.2 second chirping period of the BBH due to relativistic Doppler boost. The jet breakout should be accompanied by a low-luminosity supernova. Finally, because the BBHs of the single progenitor model do not exist at large separations, they will not be detectable in the low frequency gravitational wave band of the Laser Interferometer Space Antenna (LISA). Hence, the single-progenitor BBHs will be unambiguously discernible from BBHs formed through alternate, double-progenitor evolution scenarios., Comment: Published in Physical Review D

It has been suggested that non-relativistic outflows from quasars can naturally account for the missing component of the extragalactic $\gamma$-ray background and explain the cumulative neutrino background through pion decay in collisions between protons accelerated by the outflow shock and interstellar protons. Here we show that the same quasar outflows are capable of accelerating protons to energies of $\sim 10^{20}$ eV during the early phase of their propagation. The overall quasar population is expected to produce a cumulative ultra high energy cosmic ray flux of $\sim10^{-7}\,\rm GeV\,cm^{-2}s^{-1}sr^{-1}$ at $E_{\rm CR}\gtrsim10^{18}$ eV. The spectral shape and amplitude is consistent with recent observations for outflow parameters constrained to fit secondary $\gamma$-rays and neutrinos without any additional parameter tuning. This indicates that quasar outflows simultaneously account for all three messengers at their observed levels., Comment: Submitted for publication, 5 pages, 3 figures

The precise of short gamma ray bursts (SGRBs) remains an important open question in relativistic astrophysics. Increasingly, observational evidence suggests the merger of a binary compact object system as the source for most SGRBs, but it is currently unclear how to distinguish observationally between a binary neutron star progenitor and a black hole-neutron star progenitor. We suggest the quasiperiodic signal of jet precession as an observational signature of SGRBs originating in mixed binary systems, and quantify both the fraction of mixed binaries capable of producing SGRBs and the distributions of precession amplitudes and periods. The difficulty inherent in disrupting a neutron star outside the horizon of a stellar mass black hole biases the jet precession signal towards low amplitude and high frequency. Precession periods of $\ensuremath{\sim}0.01--0.1\text{ }\text{ }\mathrm{s}$ and disk-black hole spin misalignments $\ensuremath{\sim}10\ifmmode^\circ\else\textdegree\fi{}$ are generally expected, although sufficiently high viscosity may prevent the accumulation of multiple precession periods during the SGRB. The precessing jet will naturally cover a larger solid angle in the sky than would standard SGRB jets, enhancing observability for both prompt emission and optical afterglows.

[Abridged] The spatially averaged global spectrum of the redshifted 21cm line has generated much experimental interest, for it is potentially a direct probe of the Epoch of Reionization and the Dark Ages. Since the cosmological signal here has a purely spectral signature, most proposed experiments have little angular sensitivity. This is worrisome because with only spectra, the global 21cm signal can be difficult to distinguish from foregrounds such as Galactic synchrotron radiation, as both are spectrally smooth and the latter is orders of magnitude brighter. We establish a mathematical framework for global signal data analysis in a way that removes foregrounds optimally, complementing spectra with angular information. We explore various experimental design trade-offs, and find that 1) with spectral-only methods, it is impossible to mitigate errors that arise from uncertainties in foreground modeling; 2) foreground contamination can be significantly reduced for experiments with fine angular resolution; 3) most of the statistical significance in a positive detection during the Dark Ages comes from a characteristic high-redshift trough in the 21cm brightness temperature; and 4) Measurement errors decrease more rapidly with integration time for instruments with fine angular resolution. We show that if observations and algorithms are optimized based on these findings, an instrument with a 5 degree beam can achieve highly significant detections (greater than 5-sigma) of even extended (high Delta-z) reionization scenarios after integrating for 500 hrs. This is in contrast to instruments without angular resolution, which cannot detect gradual reionization. Abrupt ionization histories can be detected at the level of 10-100's of sigma. The expected errors are also low during the Dark Ages, with a 25-sigma detection of the expected cosmological signal after only 100 hrs of integration., 34 pages, 30 figures. Replaced (v2) to match accepted PRD version (minor pedagogical additions to text; methods, results, and conclusions unchanged). Fixed two typos (v3); text, results, conclusions etc. completely unchanged

(Abridged) The extragalactic background light (EBL) observed at multiple wavelengths is a promising tool to probe the nature of dark matter since it might contain a significant contribution from gamma-rays produced promptly by dark matter annihilation. Additionally, the electrons and positrons produced in the annihilation give energy to the CMB photons to populate the EBL with X-rays and gamma-rays. We here create full-sky maps of the radiation from both of these contributions using the high-resolution Millennium-II simulation. We use upper limits on the contributions of unknown sources to the EBL to constrain the intrinsic properties of dark matter using a model-independent approach that can be employed as a template to test different particle physics models (including those with a Sommerfeld enhancement). These upper limits are based on observations spanning eight orders of magnitude in energy (from soft X-rays measured by CHANDRA to gamma-rays measured by Fermi), and on expectations for the contributions from blazars and star forming galaxies. To exemplify this approach, we analyze a set of benchmark Sommerfeld-enhanced models that give the correct dark matter abundance, satisfy CMB constraints, and fit the cosmic ray spectra measured by PAMELA and Fermi without any contribution from local subhalos. We find that these models are in conflict with the EBL constraints unless the contribution of unresolved subhalos is small and the annihilation signal dominates the EBL. We conclude that provided the collisionless cold dark matter paradigm is accurate, even for conservative estimates of the contribution from unresolved subhalos and astrophysical backgrounds, the EBL is at least as sensitive a probe of these types of scenarios as the CMB. Our results disfavor an explanation of the positron excess measured by PAMELA based only on dark matter annihilation in the smooth Galactic halo., 18 pages, 7 figures, accepted for publication in Physical Review D. New Appendix D and other minor additions

Observations of the frequency dependence of the global brightness temperature of the redshifted 21 cm line of neutral hydrogen may be possible with single dipole experiments. In this paper, we develop a Fisher matrix formalism for calculating the sensitivity of such instruments to the 21 cm signal from reionization and the dark ages. We show that rapid reionization histories with duration $\ensuremath{\Delta}z\ensuremath{\lesssim}2$ can be constrained, provided that local foregrounds can be well modeled by low order polynomials. It is then shown that observations in the range $\ensuremath{\nu}=50--100\text{ }\text{ }\mathrm{MHz}$ can feasibly constrain the $\mathrm{Ly}\ensuremath{\alpha}$ and x-ray emissivity of the first stars forming at $z\ensuremath{\sim}15--25$, provided that systematic temperature residuals can be controlled to less than 1 mK. Finally, we demonstrate the difficulty of detecting the 21 cm signal from the dark ages before star formation.

Mergers of gas-rich galaxies lead to black hole binaries that coalesce as a result of dynamical friction on the ambient gas. Once the binary tightens to $\ensuremath{\lesssim}{10}^{3}$ Schwarzschild radii, its merger is driven by the emission of gravitational waves (GWs). We show that this transition occurs generically at orbital periods of $\ensuremath{\sim}1\char21{}10\text{ }\text{ }\mathrm{years}$ and an orbital velocity $v$ of a few thousand $\mathrm{km}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$, with a very weak dependence on the supply rate of gas ($v\ensuremath{\propto}{\stackrel{\ifmmode \dot{}\else \textperiodcentered \fi{}}{M}}^{1/8}$). Therefore, as binaries enter their GW-dominated inspiral, they inevitably induce large periodic shifts in the broad emission lines of any associated quasar(s). The probability of finding a binary in tighter configurations scales as ${v}^{\ensuremath{-}8}$ owing to their much shorter lifetimes. Narrow-band monitoring of the broad emission lines of quasars on time scales of months to decades can set a lower limit on the expected rate of GW sources for the Laser Interferometer Space Antenna.

We derive the generally covariant equations describing the propagation of waves with an arbitrary dispersion relation in a nonuniform, nondissipative medium. The back-reaction of the waves on the medium is expressed in terms of the wave energy-momentum tensor. The formalism is based on variations of the Lagrangian of the system with respect to the wave amplitude and phase and the particle orbits. The Lagrangian approach is considered in detail in the context of a cold, unmagnetized plasma. It is shown that the "inertial" mass of a photon in a plasma, namely the plasma frequency, is also its gravitational mass. Extremely precise experiments are needed to measure the gravitational "free fall" of phonons, plasmons, or photons in laboratory media. Finally, we indicate how the formalism can be extended to hot magnetized plasmas.

When a source emits a gravity-wave (GW) pulse over a short period of time, the leading edge of the GW signal is redshifted more than the inner boundary of the pulse. The GW pulse is distorted by the gravitational effect of the self-energy residing in between these shells. We illustrate this distortion for GW pulses from the final plunge of black hole (BH) binaries, leading to the evolution of the GW profile as a function of the radial distance from the source. The distortion depends on the total GW energy released and the duration of the emission, scaled by the total binary mass, M. The effect should be relevant in finite box simulations where the waveforms are extracted within a radius of, Comment: accepted for publication in Physical Review D

Cosmic microwave background (CMB) anisotropies probe the primordial density field at the edge of the observable Universe. There is a limiting precision (``cosmic variance'') with which anisotropies can determine the amplitude of primordial mass fluctuations. This arises because the surface of last scatter (SLS) probes only a finite two-dimensional slice of the Universe. Probing other SLSs observed from different locations in the Universe would reduce the cosmic variance. In particular, the polarization of CMB photons scattered by the electron gas in a cluster of galaxies provides a measurement of the CMB quadrupole moment seen by the cluster. Therefore, CMB polarization measurements toward many clusters would probe the anisotropy on a variety of SLSs within the observable Universe, and hence reduce the cosmic-variance uncertainty., Comment: 6 pages, RevTeX, with two postscript figures

One of the best motivated hypotheses in cosmology states that most of the matter in the universe is in the form of weakly-interacting massive particles that decoupled early in the history of the universe and cooled adiabatically to an extremely low temperature. Nevertheless, the finite temperature and horizon scales at which these particles decoupled imprint generic signatures on their small scales density fluctuations. We show that the previously recognized cut-off in the fluctuation power-spectrum due to free-streaming of particles at the thermal speed of decoupling, is supplemented by acoustic oscillations owing to the initial coupling between the cold dark matter (CDM) and the radiation field. The power-spectrum oscillations appear on the scale of the horizon at thermal decoupling which corresponds to a mass scale of \~10^{-4}*(T_d/10MeV)^{-3} solar masses for a CDM decoupling temperature T_d. The suppression of the power-spectrum on smaller scales by the acoustic oscillations is physically independent from the free-streaming effect, although the two cut-off scales are coincidentally comparable for T_d~10MeV and a particle mass of M~100GeV. The initial conditions for recent numerical simulations of the earliest and smallest objects to have formed in the universe, need to be modified accordingly. The smallest dark matter clumps may be detectable through gamma-ray production from particle annihilation, through fluctuations in the event rate of direct detection experiments, or through their tidal gravitational effect on wide orbits of objects near the outer edge of the solar system., Physical Review D, in press

[Abridged] The polarization of the CMB is widely recognized as a potential source of information about primordial gravitational waves. The gravitational wave contribution can be separated from the dominant CMB polarization created by density perturbations because it generates both E and B polarization modes, whereas the density perturbations create only E polarization. The limits of our ability to measure gravitational waves are thus determined by statistical and systematic errors, foregrounds, and nonlinear evolution effects such as lensing of the CMB. Usually it is assumed that most foregrounds can be removed because of their frequency dependence, however Thomson scattering of the CMB quadrupole by electrons in the Galaxy or nearby structures shares the blackbody frequency dependence of the CMB. If the optical depth from these nearby electrons is anisotropic, the polarization generated can include B modes even without tensor perturbations. We estimate this effect for the Galactic disk and nearby extragalactic structures, and find that it contributes to the B polarization at the level of ~(1--2)x10^-4��K per logarithmic interval in multipole L for L, 10 pages, 4 figures, to be submitted to Phys. Rev. D

The relaxation of the spin of an initially polarized relativistic neutral particle in a stochastic electromagnetic field is analyzed covariantly and found to have a simple expression in terms of the helicity. The relaxation rate, to leading order in the magnetic moment {mu}, is given by {mu}{sup 2} times a correlation function of the electromagnetic fields. The correlation integral, however, is zero unless there are excitations with a phase velocity matching the velocity of the particle. Thus, for blackbody radiation in a vacuum there is no spin relaxation to this order, as would be expected from a corpuscular point of view. Hence, polarized cosmic rays or neutrinos are only depolarized by background radiation to order {mu}{sup 4}, i.e., by magnetic Compton scattering. However, in a medium {mu}{sup 2} relaxation is possible. We examine the case of an ultrarelativistic plasma at a temperature {ital T}, where coupling to the longitudinal modes yields typical relaxation rates of order {mu}{sup 2}T{sup 3}. Since this effect is able to turn a left-handed into a right-handed neutrino, the standard cosmological nucleosynthesis bounds lead to an upper limit of several times 10{sup {minus}11}{mu}{sub B} on the magnetic (or electric) dipole moment of at least two neutrinomore » species.« less