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

We describe a directed search for continuous gravitational waves in data from the sixth initial LIGO science run. The target was the nearby globular cluster NGC 6544 at a distance of ≈2.7 kpc. The search * Deceased. SEARCH FOR CONTINUOUS GRAVITATIONAL WAVES …PHYSICAL REVIEW D 95, 082005 (2017) 082005-5 covered a broad band of frequencies along with first and second frequency derivatives for a fixed sky position. The search coherently integrated data from the two LIGO interferometers over a time span of 9.2 days using the matched-filtering F -statistic. We found no gravitational-wave signals and set 95% confidence upper limits as stringent as 6.0 × 10 −25 on intrinsic strain and 8.5 × 10 −6 on fiducial ellipticity. These values beat the indirect limits from energy conservation for stars with characteristic spindown ages older than 300 years and are within the range of theoretical predictions for possible neutron-star ellipticities. An important feature of this search was use of a barycentric resampling algorithm which substantially reduced computational cost; this method is used extensively in searches of Advanced LIGO and Virgo detector data.

Gravitational waves (GWs) are produced by colliding particles through the gravitational analogue of electromagnetic bremsstrahlung. We calculate the contribution of free-free emission in the radiation-dominated Universe to the stochastic GW background. We find that the energy density of the resulting GW radiation is heavily dependent on the number of elementary particles, Ntot, and the maximum initial temperature, Tmax. We rule out Ntot NSM for Tmax ∼ T Planck ≈ 10 19 GeV and Ntot 10 13 × NSM for Tmax ∼ 10 16 GeV, where NSM is the number of particles in the Standard Model. In the case of inflation, existing cosmological data constrain Tmax 10 16 GeV. However, alternative models to inflation such as bouncing cosmologies allow for Tmax near T Planck . At the energy scales we are considering, the extra number of particles arise naturally in models of extra dimensions.

We consider collisions between stars moving near the speed of light around supermassive black holes (SMBHs), with mass M • 10 8 M , without being tidally disrupted. The overall rate for collisions taking place in the inner ∼ 1 pc of galaxies with M • = 10 8 , 10 9 , 10 10 M are Γ ∼ 5, 0.07, 0.02 yr −1 , respectively. We further calculate the differential collision rate as a function of total energy released, energy released per unit mass lost, and galactocentric radius. The most common collisions will release energies on the order of ∼ 10 49 − 10 51 erg, with the energy distribution peaking at higher energies in galaxies with more massive SMBHs. Depending on the host galaxy mass and the depletion timescale, the overall rate of collisions in a galaxy ranges from a small percentage to several times larger than that of core-collapse supernovae (CCSNe) for the same host galaxy. In addition, we show example light curves for collisions with varying parameters, and find that the peak luminosity could reach or even exceed that of superluminous supernovae (SLSNe), although with light curves with much shorter duration. Weaker events could initially be mistaken for low-luminosity supernovae. In addition, we note that these events will likely create streams of debris that will accrete onto the SMBH and create accretion flares that may resemble tidal disruption events (TDEs).