We report the detection of new binary black hole merger events in the publicly available data from the second observing run of Advanced LIGO and Advanced Virgo (O2). The mergers were discovered using ...the new search pipeline described in Venumadhav et al. Phys. Rev. D 100, 023011 (2019) and are above the detection thresholds as defined in Abbott et al. (LIGO Scientific and Virgo Collaborations) Phys. Rev. X 9, 031040 (2019).. Three of the mergers (GW170121, GW170304, GW170727) have inferred probabilities of being of astrophysical origin pastro>0.98. The remaining three (GW170425, GW170202, GW170403) are less certain, with pastro ranging from 0.5 to 0.8. The newly found mergers largely share the statistical properties of previously reported events, with the exception of GW170403, the least secure event, which has a highly negative effective spin parameter χeff. The most secure new event, GW170121 (pastro>0.99), is also notable due to its inferred negative value of χeff, which is inconsistent with being positive at the ≈95.8% confidence level. The new mergers nearly double the sample of gravitational wave events reported from O2 and present a substantial opportunity to explore the statistics of the binary black hole population in the Universe. The number of detected events is not surprising since we estimate that the detection volume of our pipeline may be larger than that of other pipelines by as much as a factor of 2 (with significant uncertainties in the estimate). The increase in volume is larger when the constituent detectors of the network have very different sensitivities, as is likely to be the case in current and future runs.
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The recent detection of gravitational waves indicates that stellar-mass black hole binaries are likely to be a key population of sources for forthcoming observations. With future upgrades, ...ground-based detectors could detect merging black hole binaries out to cosmological distances. Gravitational-wave bursts from high redshifts (z≳1) can be strongly magnified by gravitational lensing due to intervening galaxies along the line of sight. In the absence of electromagnetic counterparts, the mergers’ intrinsic mass scale and redshift are degenerate with the unknown magnification factor μ. Hence, strongly magnified low-mass mergers from high redshifts appear as higher-mass mergers from lower redshifts. We assess the impact of this degeneracy on the mass-redshift distribution of observable events for generic models of binary black hole formation from normal stellar evolution, Pop III star remnants, or a primordial black hole population. We find that strong magnification (μ≳3) generally creates a heavy tail of apparently massive mergers in the event distribution from a given detector. For LIGO and its future upgrades, this tail may dominate the population of intrinsically massive, but unlensed mergers in binary black hole formation models involving normal stellar evolution or primordial black holes. Modeling the statistics of lensing magnification can help account for this magnification bias when testing astrophysical scenarios of black hole binary formation and evolution.
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We perform a statistical inference of the astrophysical population of binary black hole (BBH) mergers observed during the first two observing runs of Advanced LIGO and Advanced Virgo, including ...events reported in the GWTC-1 and IAS catalogs. We derive a novel formalism to fully and consistently account for events of arbitrary significance. We carry out a software injection campaign to obtain a set of mock astrophysical events subject to our selection effects, and use the search background to compute the astrophysical probabilities pastro of candidate events for several phenomenological models of the BBH population. We emphasize that the values of pastro depend on both the astrophysical and background models. Finally, we combine the information from individual events to infer the rate, spin, mass, mass-ratio and redshift distributions of the mergers. The existing population does not discriminate between random spins with a spread in the effective spin parameter, and a small but nonzero fraction of events from tidally torqued stellar progenitors. The mass distribution is consistent with one having a cutoff at ... M⊙ , while the mass ratio favors equal masses; the mean mass ratio q > 0.67 . The rate shows no significant evolution with redshift. We show that the merger rate restricted to BBHs with a primary mass between 20–30 M , and a mass ratio q > 0.5 , and at z ~ 0.2 , is 1.5–5.3 Gpc−3 yr−1 (90% c.l.); these bounds are model independent and a factor of ∼ 3 tighter than that on the local rate of all BBH mergers, and hence are a robust constraint on all progenitor models. Including the events in our catalog increases the Fisher information about the BBH population by ∼ 47 % , and tightens the constraints on population parameters. (ProQuest: ... denotes formula omitted.)
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In this paper, we report on the construction of a new and independent pipeline for analyzing the public data from the first observing run of Advanced LIGO for mergers of compact binary systems. The ...pipeline incorporates different techniques and makes independent implementation choices in all its stages including the search design, the method to construct template banks, the automatic routines to detect bad data segments ("glitches") and to insulate good data from them, the procedure to account for the nonstationary nature of the detector noise, the signal-quality vetoes at the single-detector level and the methods to combine results from multiple detectors. Our pipeline enabled us to identify a new binary black hole merger GW151216 in the public LIGO data. This paper serves as a bird's eye view of the pipeline's important stages. Full details and derivations underlying the various stages will appear in accompanying papers.
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We report a new binary black hole merger in the publicly available LIGO first observing run (O1) data release. The event has a false alarm rate of one per six years in the detector-frame chirp-mass ...range Mdet∈20,40M⊙ in a new independent analysis pipeline that we developed. Our best estimate of the probability that the event is of astrophysical origin is Pastro~0.71. The estimated physical parameters of the event indicate that it is the merger of two massive black holes, Mdet=31−3+2M⊙ with an effective spin parameter, χeff=0.81−0.21+0.15, making this the most highly spinning merger reported to date. It is also among the two highest redshift mergers observed so far. The high aligned spin of the merger supports the hypothesis that merging binary black holes can be created by binary stellar evolution.
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The intergalactic medium is expected to be at its coldest point before the formation of the first stars in the Universe. Motivated by recent results from the EDGES experiment, we revisit the standard ...calculation of the kinetic temperature of the neutral gas through this period. When the first ultraviolet (UV) sources turn on, photons redshift into the Lyman lines of neutral hydrogen and repeatedly scatter within the Lyman-α line. They heat the gas via atomic recoils, and, through the Wouthuysen-Field effect, set the spin temperature of the 21-cm hyperfine (spin-flip) line of atomic hydrogen in competition with the resonant cosmic microwave background (CMB) photons. We show that the Lyman-α photons also mediate energy transfer between the CMB photons and the thermal motions of the hydrogen atoms. In the absence of x-ray heating, this new mechanism is the major correction to the temperature of the adiabatically cooling gas (∼10% at z=17), and is several times the size of the heating rate found in previous calculations. We also find that the effect is more dramatic in nonstandard scenarios that either enhance the radio background above the CMB or invoke new physics to cool the gas in order to explain the EDGES results. The coupling with the radio background can reduce the depth of the 21-cm absorption feature by almost a factor of 2 relative to the case with no sources of heating, and prevent the feature from developing a flattened bottom. As an inevitable consequence of the UV background that generates the absorption feature, this heating should be accounted for in any theoretical prediction.
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The distribution of effective spin χeff, a parameter that encodes the degree of spin-orbit alignment in a binary system, has been widely regarded as a robust discriminator between the isolated and ...dynamical formation pathways for merging binary black holes. Until the recent release of the GWTC-2 catalog, such tests have yielded inconclusive results due to the small number of events with measurable nonzero spins. In this work, we study the χ eff distribution of the binary black holes detected in the LIGO-Virgo O1–O3a observing runs. Our focus is on the degree to which the χeff distribution is symmetric about χeff = 0 and whether the data provide support for a population of negative- χ eff systems. We find that the χeff distribution is asymmetric at 95% credibility, with an excess of aligned-spin binary systems (χeff >0) over antialigned ones. Moreover, we find that there is no evidence for negative- χeff systems in the current population of binary black holes. Thus, based solely on the χeff distribution, dynamical formation is disfavored as being responsible for the entirety of the observed merging binary black holes, while isolated formation remains viable. We also study the mass distribution of the current binary black hole population, confirming that a single truncated power-law distribution in the primary source-frame mass, m1s, fails to describe the observations. Instead, we find that the preferred models have a steep feature at m1s ~ 40 M⊙ consistent with a step and an extended, shallow tail to high masses.
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Recent observations of lensed galaxies at cosmological distances have detected individual stars that are extremely magnified when crossing the caustics of lensing clusters. In idealized cluster ...lenses with smooth mass distributions, two images of a star of radius R approaching a caustic brighten as and reach a peak magnification before merging on the critical curve. We show that a mass fraction ( ) in microlenses inevitably disrupts the smooth caustic into a network of corrugated microcaustics and produces light curves with numerous peaks. Using analytical calculations and numerical simulations, we derive the characteristic width of the network, caustic-crossing frequencies, and peak magnifications. For the lens parameters of a recent detection and a population of intracluster stars with , we find a source-plane width of for the caustic network, which spans on the image plane. A source star takes years to cross this width, with a total of crossings, each one lasting for with typical peak magnifications of . The exquisite sensitivity of caustic-crossing events to the granularity of the lens-mass distribution makes them ideal probes of dark matter components, such as compact halo objects and ultralight axion dark matter.
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