A wide variety of astrophysical and cosmological sources are expected to contribute to a stochastic gravitational-wave background. Following the observations of GW150914 and GW151226, the rate and mass of coalescing binary black holes appear to be greater than many previous expectations. As a result, the stochastic background from unresolved compact binary coalescences is expected to be particularly loud. We perform a search for the isotropic stochastic gravitational-wave background using data from Advanced LIGO's first observing run. The data display no evidence of a stochastic gravitational-wave signal. We constrain the dimensionless energy density of gravitational waves to be Ω0 < 1.7 × 10 −7 with 95% confidence, assuming a flat energy density spectrum in the most sensitive part of the LIGO band (20 − 86 Hz). This is a factor of ∼33 times more sensitive than previous measurements. We also constrain arbitrary power-law spectra. Finally, we investigate the implications of this search for the background of binary black holes using an astrophysical model for the background. The recent detections of binary black hole (BBH) coalescences by Advanced LIGO [32,33] suggest that the Universe may be rich with coalescing BBHs. While events like GW150914 and GW151226 are loud enough to be clearly detected, we expect there to be many more events that are too far away to be individually resolved and that contribute to the background. Since this BBH population originates from sources that are too distant to be individually detected, the stochastic search probes a distinct population of binaries compared to nearby sources [34]. The background from these binaries provides complementary information to individually resolved binary coalescences [35].In this Letter, we report on the search for an isotropic background using data from Advanced LIGO's first observing run O1. We search for the background by crosscorrelating data streams from the two separate LIGO detectors and looking for a coherent signal. We find no evidence for the background and place the best upper limits to date on the energy density of the background in the LIGO frequency band. We also update the impli-

We present the result of searches for gravitational waves from 200 pulsars using data from the first observing run of the Advanced LIGO detectors. We find no significant evidence for a gravitational-wave signal from any of these pulsars, but we are able to set the most constraining upper limits yet on their gravitational-wave amplitudes and ellipticities. For eight of these pulsars, our upper limits give bounds that are improvements over the indirect spin-down limit values. For another 32, we are within a factor of 10 of the spin-down limit, and it is likely that some of these will be reachable in future runs of the advanced detector. Taken as a whole, these new results improve on previous limits by more than a factor of two.

Parameter estimates of GW150914 were obtained using Bayesian inference, based on three semi-analytic waveform models for binary black hole coalescences. These waveform models differ from each other in their treatment of black hole spins, and all three models make some simplifying assumptions, notably to neglect sub-dominant waveform harmonic modes and orbital eccentricity. Furthermore, while the models are calibrated to agree with waveforms obtained by full numerical solutions of Einstein's equations, any such calibration is accurate only to some non-zero tolerance and is limited by the accuracy of the underlying phenomenology, availability, quality, and parameter-space coverage of numerical simulations. This paper complements the original analyses of GW150914 with an investigation of the effects of possible systematic errors in the waveform models on estimates of its source parameters. To test for systematic errors we repeat the original Bayesian analyses on mock signals from numerical simulations of a series of binary configurations with parameters similar to those found for GW150914. Overall, we find no evidence for a systematic bias relative to the statistical error of the original parameter recovery of GW150914 due to modeling approximations or modeling inaccuracies. However, parameter biases are found to occur for some configurations disfavored by the data of GW150914: for binaries inclined edge-on to the detector over a small range of choices of polarization angles, and also for eccentricities greater than ∼0.05. For signals with higher signal-to-noise ratio than GW150914, or in other regions of the binary parameter space (lower masses, larger mass ratios, or higher spins), we expect that systematic errors in current waveform models may impact gravitational-wave measurements, making more accurate models desirable for future observations.

We employ gravitational-wave radiometry to map the gravitational waves stochastic background expected from a variety of contributing mechanisms and test the assumption of isotropy using data from Advanced LIGO's first observing run. We also search for persistent gravitational waves from point sources with only minimal assumptions over the 20 -1726 Hz frequency band. Finding no evidence of gravitational waves from either point sources or a stochastic background, we set limits at 90% confidence. For broadband point sources, we report upper limits on the gravitational wave energy flux per unit frequency in the range Fα,Θ(f ) < (0.1 − 56) × 10 −8 erg cm −2 s −1 Hz −1 (f /25 Hz) α−1 depending on the sky location Θ and the spectral power index α. For extended sources, we report upper limits on the fractional gravitational wave energy density required to close the Universe of Ω(f, Θ) < (0.39−7.6)×10−8 sr −1 (f /25 Hz) α depending on Θ and α. Directed searches for narrowband gravitational waves from astrophysically interesting objects (Scorpius X-1, Supernova 1987 A, and the Galactic Center) yield median frequency-dependent limits on strain amplitude of h0 < (6.7, 5.5, and 7.0) × 10 −25 respectively, at the most sensitive detector frequencies between 130 -175 Hz. This represents a mean improvement of a factor of 2 across the band compared to previous searches of this kind for these sky locations, considering the different quantities of strain constrained in each case.Introduction.-A stochastic gravitational-wave background (SGWB) is expected from a variety of mechanisms [1][2][3][4][5]. Given the recent observations of binary black hole mergers GW150914 and GW151226 [6,7], we expect the SGWB to be nearly isotropic [8] and dominated [9] by compact binary coalescences [10][11][12]. The LIGO and Virgo Collaborations have pursued the search for an isotropic stochastic background from LIGO's first observational run [13]. Here, we adopt an eyes-wide-open philosophy and relax the assumption of isotropy in order to allow for the greater range of possible signals. We search for an anisotropic background, which could indicate a richer, more interesting cosmology than current models. We present the results of a generalized search for a stochastic signal with an arbitrary angular distribution mapped over all directions in the sky.Our search has three components. First, we utilize a broadband radiometer analysis [14,15], optimized for detecting a small number of resolvable point sources. This method is not applicable to extended sources. Second, we employ a spherical harmonic decomposition [16,17], which can be employed for point sources but is better suited to extended sources. Last, we carry out a narrowband radiometer search directed at the sky position of three astrophysically interesting objects: Scorpius X-1 (Sco X-1) [18,19], Supernova 1987 [20,21], and the Galactic Center (GC) [22].These three search methods are capable of detecting a wide range of possible signals with only minimal assumptions about the signal morphology. ...

6We present the results from an all-sky search for short-duration gravitational waves in the data of the first run of the Advanced LIGO detectors between September 2015 and January 2016. The search algorithms use minimal assumptions on the signal morphology, so they are sensitive to a wide range of sources emitting gravitational waves. The analyses target transient signals with duration ranging from milliseconds to seconds over the frequency band of 32 to 4096 Hz. The first observed gravitational-wave event, GW150914, has been detected with high confidence in this search; the other known gravitational-wave event, GW151226, falls below the search's sensitivity. Besides GW150914, all of the search results are consistent with the expected rate of accidental noise coincidences. Finally, we estimate rate-density limits for a broad range of non-BBH transient gravitational-wave sources as a function of their gravitational radiation emission energy and their characteristic frequency. These rate-density upper-limits are stricter than those previously published by an order-of-magnitude.