[en] The LIGO and Virgo detectors are sensitive to a variety of noise sources, such as instrumental artifacts and environmental disturbances. The Stochastic Transient Analysis Multi-detector Pipeline has been developed to search for long-duration (t ≥ 1 s) gravitational-wave (GW) signals. This pipeline can also be used to identify environmental noise transients. Here, we present an algorithm to determine when long-duration noise sources couple into the interferometers, as well as identify what these noise sources are. We analyze the cross-power between a GW strain channel and an environmental sensor, using pattern recognition tools to identify statistically significant structure in cross-power time-frequency maps. We identify interferometer noise from airplanes, helicopters, thunderstorms and other sources. Examples from LIGO's sixth science run, S6, and Virgo's third scientific run, VSR3, are presented. (paper)

[en] We present an algorithm for the identification of transient noise artifacts (glitches) in cross-correlation searches for long gravitational-wave (GW) transients lasting seconds to weeks. The algorithm utilizes the auto-power in each detector as a discriminator between well-behaved stationary noise (possibly including a GW signal) and non-stationary noise transients. We test the algorithm with both Monte Carlo noise and time-shifted data from the LIGO S5 science run and find that it removes a significant fraction of glitches while keeping the vast majority (99.6%) of the data. We show that this cleaned data can be used to observe GW signals at a significantly lower amplitude than can otherwise be achieved. Using an accretion disk instability signal model, we estimate that the algorithm is accidentally triggered at a rate of less than 10^-5% by realistic signals, and less than 3% even for exceptionally loud signals. We conclude that the algorithm is a safe and effective method for cleaning the cross-correlation data used in searches for long GW transients. (paper)

[en] The first two months of the third Advanced LIGO and Virgo observing run (2019 April-May) showed that distant gravitational-wave (GW) events can now be readily detected. Three candidate mergers containing neutron stars (NS) were reported in a span of 15 days, all likely located more than 100 Mpc away. However, distant events such as the three new NS mergers are likely to be coarsely localized, which highlights the importance of facilities and scheduling systems that enable deep observations over hundreds to thousands of square degrees to detect the electromagnetic counterparts. On 2019 May 10 02:59:39.292 UT the GW candidate S190510g was discovered and initially classified as a binary neutron star (BNS) merger with 98% probability. The GW event was localized within an area of 3462 deg², later refined to 1166 deg² (90%) at a distance of 227 ± 92 Mpc. We triggered Target-of-Opportunity observations with the Dark Energy Camera (DECam), a wide-field optical imager mounted at the prime focus of the 4 m Blanco Telescope at Cerro Tololo Inter-American Observatory in Chile. This Letter describes our DECam observations and our real-time analysis results, focusing in particular on the design and implementation of the observing strategy. Within 24 hr of the merger time, we observed 65% of the total enclosed probability of the final skymap with an observing efficiency of 94%. Here, we identified and publicly announced 13 candidate counterparts. S190510g was reclassified 1.7 days after the merger, after our observations were completed, with a "BNS merger" probability reduced from 98% to 42% in favor of a "terrestrial classification.

[en] The observation of a compact object with a mass of 2.50–2.67M _⊙ on 2019 August 14, by the LIGO Scientific and Virgo collaborations (LVC) has the potential to improve our understanding of the supranuclear equation of state. While the gravitational-wave analysis of the LVC suggests that GW190814 likely was a binary black hole system, the secondary component could also have been the heaviest neutron star observed to date. We use our previously derived nuclear-physics-multimessenger astrophysics framework to address the nature of this object. Based on our findings, we determine GW190814 to be a binary black hole merger with a probability of >99.9%. Even if we weaken previously employed constraints on the maximum mass of neutron stars, the probability of a binary black hole origin is still ∼81%. Furthermore, we study the impact that this observation has on our understanding of the nuclear equation of state by analyzing the allowed region in the mass–radius diagram of neutron stars for both a binary black hole or neutron star–black hole scenario. We find that the unlikely scenario in which the secondary object was a neutron star requires rather stiff equations of state with a maximum speed of sound $c_s⩾<!--\; ???\; -->\sqrt{0.6}$ times the speed of light, while the binary black hole scenario does not offer any new insight.

[en] Searches for gravitational waves (GWs) traditionally focus on persistent sources (e.g., pulsars or the stochastic background) or on transients sources (e.g., compact binary inspirals or core-collapse supernovae), which last for time scales of milliseconds to seconds. We explore the possibility of long GW transients with unknown waveforms lasting from many seconds to weeks. We propose a novel analysis technique to bridge the gap between short O(s)''burst'' analyses and persistent stochastic analyses. Our technique utilizes frequency-time maps of GW strain cross power between two spatially separated terrestrial GW detectors. The application of our cross power statistic to searches for GW transients is framed as a pattern recognition problem, and we discuss several pattern-recognition techniques. We demonstrate these techniques by recovering simulated GW signals in simulated detector noise. We also recover environmental noise artifacts, thereby demonstrating a novel technique for the identification of such artifacts in GW interferometers. We compare the efficiency of this framework to other techniques such as matched filtering.

[en] Soft gamma repeaters and anomalous x-ray pulsars are thought to be magnetars, neutron stars with strong magnetic fields of order $\sim <!--\; ???\; -->$ ${10}^{13}$–${10}^{15}\mathrm{g}\mathrm{a}\mathrm{u}\mathrm{s}\mathrm{s}$. These objects emit intermittent bursts of hard x-rays and soft gamma rays. Quasiperiodic oscillations in the x-ray tails of giant flares imply the existence of neutron star oscillation modes which could emit gravitational waves powered by the magnetar’s magnetic energy reservoir. We describe a method to search for transient gravitational-wave signals associated with magnetar bursts with durations of 10 s to 1000 s of seconds. The sensitivity of this method is estimated by adding simulated waveforms to data from the sixth science run of Laser Interferometer Gravitational-wave Observatory (LIGO). We find a search sensitivity in terms of the root sum square strain amplitude of $h_{\mathrm{r}\mathrm{s}\mathrm{s}}=1.3\times <!--\; ??\; -->{10}^{-<!--\; ???\; -->21}{\mathrm{H}\mathrm{z}}^{-<!--\; ???\; -->1/2}$ for a half sine-Gaussian waveform with a central frequency $f_0=150$ Hz and a characteristic time $\mathrm{\tau <!--\; ??\; -->}=400$ s. This corresponds to a gravitational wave energy of $E_{\mathrm{G}\mathrm{W}}=4.3\times <!--\; ??\; -->{10}^{46}\mathrm{e}\mathrm{r}\mathrm{g}$, the same order of magnitude as the 2004 giant flare which had an estimated electromagnetic energy of $E_{\mathrm{E}\mathrm{M}}=\sim <!--\; ???\; -->1.7\times <!--\; ??\; -->{10}^{46}(d/8.7\mathrm{k}\mathrm{p}\mathrm{c}){}^2\mathrm{e}\mathrm{r}\mathrm{g}$, where d is the distance to SGR 1806-20. We present an extrapolation of these results to Advanced LIGO, estimating a sensitivity to a gravitational wave energy of $E_{\mathrm{G}\mathrm{W}}=3.2\times <!--\; ??\; -->{10}^{43}\mathrm{e}\mathrm{r}\mathrm{g}$ for a magnetar at a distance of $1.6$ kpc. These results suggest this search method can probe significantly below the energy budgets for magnetar burst emission mechanisms such as crust cracking and hydrodynamic deformation. (paper)

[en] The recent discovery of merging black holes suggests that a stochastic gravitational-wave background is within reach of the advanced detector network operating at design sensitivity. However, correlated magnetic noise from Schumann resonances threatens to contaminate observation of a stochastic background. In this paper, we report on the first effort to eliminate intercontinental correlated noise from Schumann resonances using Wiener filtering. Using magnetometers as proxies for gravitational-wave detectors, we demonstrate as much as a factor of two reduction in the coherence between magnetometers on different continents. While much work remains to be done, our results constitute a proof-of-principle and motivate follow-up studies with a dedicated array of magnetometers. (paper)

[en] We report the discovery of ZTF J2243+5242, an eclipsing double white dwarf binary with an orbital period of just 8.8 minutes, the second known eclipsing binary with an orbital period of less than 10 minutes. The system likely consists of two low-mass white dwarfs and will merge in approximately 400,000 yr to form either an isolated hot subdwarf or an R Coronae Borealis star. Like its 6.91 minute counterpart, ZTF J1539+5027, ZTF J2243+5242 will be among the strongest gravitational-wave sources detectable by the space-based gravitational-wave detector the Laser Space Interferometer Antenna (LISA) because its gravitational-wave frequency falls near the peak of LISA's sensitivity. Based on its estimated distance of $d={2425}_{-<!--\; ???\; -->114}^{+108}\mathrm{p}\mathrm{c}$, LISA should detect the source within its first few months of operation and achieve a signal-to-noise ratio of 63 ± 7 after 4 yr. We find component masses of $M_A={0.323}_{-<!--\; ???\; -->0.047}^{+0.065}$ and $M_B={0.335}_{-<!--\; ???\; -->0.054}^{+0.052}{M}_{\odot <!--\; ???\; -->}$, radii of $R_A={0.0298}_{-<!--\; ???\; -->0.0012}^{+0.0013}$ and $R_B={0.0275}_{-<!--\; ???\; -->0.0013}^{+0.0012}{R}_{\odot <!--\; ???\; -->}$, and effective temperatures of $T_A={26,300}_{-<!--\; ???\; -->900}^{+1700}$ and $T_B={19,200}_{-<!--\; ???\; -->900}^{+1500}\mathrm{K}$. We determine all of these properties and the distance to this system using only photometric measurements, demonstrating a feasible way to estimate parameters for the large population of optically faint (r > 21 m _AB) gravitational-wave sources that the Vera Rubin Observatory and LISA should identify.

[en] We report the discovery of the first short-period binary in which a hot subdwarf star (sdOB) filled its Roche lobe and started mass transfer to its companion. The object was discovered as part of a dedicated high-cadence survey of the Galactic plane named the Zwicky Transient Facility and exhibits a period of P = 39.3401(1) minutes, making it the most compact hot subdwarf binary currently known. Spectroscopic observations are consistent with an intermediate He-sdOB star with an effective temperature of $T_{\mathrm{e}\mathrm{f}\mathrm{f}}$ = 42,400 ± 300 K and a surface gravity of $\mathrm{l}\mathrm{o}\mathrm{g}(g)$ = 5.77 ± 0.05. A high signal-to-noise ratio GTC+HiPERCAM light curve is dominated by the ellipsoidal deformation of the sdOB star and an eclipse of the sdOB by an accretion disk. We infer a low-mass hot subdwarf donor with a mass M _sdOB = 0.337 ± 0.015 $M_{\odot <!--\; ???\; -->}$ and a white dwarf accretor with a mass M _WD = 0.545 ± 0.020 $M_{\odot <!--\; ???\; -->}$. Theoretical binary modeling indicates the hot subdwarf formed during a common envelope phase when a 2.5–2.8 $M_{\odot <!--\; ???\; -->}$ star lost its envelope when crossing the Hertzsprung gap. To match its current $P_{\mathrm{o}\mathrm{r}\mathrm{b}}$, $T_{\mathrm{e}\mathrm{f}\mathrm{f}}$, $\mathrm{l}\mathrm{o}\mathrm{g}(g)$, and masses, we estimate a post–common envelope period of $P_{\mathrm{o}\mathrm{r}\mathrm{b}}$ ≈ 150 minutes and find that the sdOB star is currently undergoing hydrogen shell burning. We estimate that the hot subdwarf will become a white dwarf with a thick helium layer of ≈0.1 $M_{\odot <!--\; ???\; -->}$, merge with its carbon/oxygen white dwarf companion after ≈17 Myr, and presumably explode as a thermonuclear supernova or form an R CrB star.

[en] Using photometry collected with the Zwicky Transient Facility, we are conducting an ongoing survey for binary systems with short orbital periods ($P_{\mathrm{b}}<<!--\; <\; -->1\mathrm{h}\mathrm{r})$ with the goal of identifying new gravitational-wave sources detectable by the upcoming Laser Interferometer Space Antenna (LISA). We present a sample of 15 binary systems discovered thus far, with orbital periods ranging from 6.91 to 56.35 minutes. Of the 15 systems, seven are eclipsing systems that do not show signs of significant mass transfer. Additionally, we have discovered two AM Canum Venaticorum systems and six systems exhibiting primarily ellipsoidal variations in their lightcurves. We present follow-up spectroscopy and high-speed photometry confirming the nature of these systems, estimates of their LISA signal-to-noise ratios, and a discussion of their physical characteristics.