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NASA NTRS2019

Swift-XRT Follow-up of Gravitational-wave Triggers in the Second Advanced LIGO/Virgo Observing Run

The Neil Gehrels Swift Observatory carried out prompt searches for gravitational-wave (GW) events detected by the LIGO/Virgo Collaboration (LVC) during the second observing run ("O2"). Swift performed extensive tiling of eight LVC triggers, two of which had very low false-alarm rates (GW170814 and the epochal GW170817), indicating a high confidence of being astrophysical in origin; the latter was the first GW event to have an electromagnetic counterpart detected. In this paper we describe the follow-up performed during O2 and the results of our searches. No GW electromagnetic counterparts were detected; this result is expected, as GW170817 remained the only astrophysical event containing at least one neutron star after LVC's later retraction of some events. A number of X-ray sources were detected, with the majority of identified sources being active galactic nuclei. We discuss the detection rate of transient X-ray sources and their implications in the O2 tiling searches. Finally, we describe the lessons learned during O2 and how these are being used to improve the Swift follow-up of GW events. In particular, we simulate a population of gamma-ray burst afterglows to evaluate our source ranking system's ability to differentiate them from unrelated and uncataloged X-ray sources. We find that ≈60%–70% of afterglows whose jets are oriented toward Earth will be given high rank (i.e., "interesting" designation) by the completion of our second follow-up phase (assuming that their location in the sky was observed), but that this fraction can be increased to nearly 100% by performing a third follow-up observation of sources exhibiting fading behavior.

N. J. Klingler↗
NASA NTRSDate not supplied

Searching for GRB Counterparts to GW Events from the Third Gravitational Wave Observing Run with Fermi-GBM and Swift-BAT

We present Fermi Gamma-ray Burst Monitor (Fermi-GBM) and Swift Burst Alert Telescope (Swift-BAT) observations of gravitational-wave (GW) events detected during the third LIGO/Virgo/KAGRA Collaboration observing run (O3). Using Fermi-GBM on-board triggers as well as sub-threshold searches in both Fermi-GBM and Swift-BAT ground data, we search for coincident gamma-ray transients associated with the GW events. No new joint events beyond GRB 170817A and GW170817 were found. We therefore calculate flux upper limits for the associated gamma-ray luminosity of each GW event. Using the lack of electromagnetic (EM) emission from binary black hole (BBH) mergers, we begin to constrain theoretical models of any such emission.

Joshua Wood↗
NASA NTRS2019

Transient Astrophysics Probe: White Paper

The Transient Astrophysics Probe (TAP) is a wide-field multi-wavelength transient mission proposed for flight starting in the late 2020s. The mission instruments include unique ``Lobster-eye'' imaging soft X-ray optics that allow an approximately 1600-degrees-squared Field of View (FoV); a high sensitivity, 1-degree-squared FoV soft X-ray telescope; a 1-degree-squared FoV Infrared telescope with bandpass 0.6 to 3 microns; and a set of 8 NaI gamma-ray detectors. TAP's most exciting capability will be the observation of tens per year of X-ray and Infrared counterparts of gravitational waves (GWs) involving stellar-mass black holes and neutron stars detected by LIGO (Laser Interferometer Gravitational-Wave Observatory ) / Virgo / KAGRA (Kamioka (Japan) Gravitational Wave Detector) / LIGO-India, and possibly several per year X-ray counterparts of GWs from supermassive black holes, detected by LISA (Laser Interferometer Space Antenna) and Pulsar Timing Arrays. TAP will also discover hundreds of X-ray transients related to compact objects, including tidal disruption events, supernova shock breakouts, and Gamma-Ray Bursts from the epoch of reionization.

Camp, Jordan↗
NASA NTRS2019

The NANOGrav 11 Yr Data Set: Limits on Gravitational Waves from Individual Supermassive Black Hole Binaries

Observations indicate that nearly all galaxies contain supermassive black holes at their centers. When galaxies merge, their component black holes form SMBH binaries (SMBHBs), which emit low-frequency gravitational waves (GWs) that can be detected by pulsar timing arrays. We have searched the North American Nanohertz Observatory for Gravitational Waves 11 yr data set for GWs from individual SMBHBs in circular orbits. As we did not find strong evidence for GWs in our data, we placed 95% upper limits on the strength of GWs from such sources. At f(gw) = 8 nHz, we placed a sky-averaged upper limit of h(0) < 7.3(3) × 10(exp −15). We also developed a technique to determine the significance of a particular signal in each pulsar using "dropout" parameters as a way of identifying spurious signals. From these upper limits, we ruled out SMBHBs emitting GWs f(gw) = 8 nHz within 120 Mpc for M = 10(exp 9) Solar Mass, and within 5.5 Gpc for M= 10(exp 10) Solar Mass at our most sensitive sky location. We also determined that there are no SMBHBs with M > 1.6 x 10(exp 9) Solar Mass emitting GWs with f(gw) = 2.8–317.8 nHz in the Virgo Cluster. Finally, we compared our strain upper limits to simulated populations of SMBHBs, based on galaxies in the Two Micron All-Sky Survey and merger rates from the Illustris cosmological simulation project, and found that only 34 out of 75,000 realizations of the local universe contained a detectable source.

Aggarwal, K.↗
NASA NTRS2011

Gravitational Wave Astrophysics: Opening the New Frontier

The gravitational wave window onto the universe is expected to open in ~ 5 years, when ground-based detectors make the first detections in the high-frequency regime. Gravitational waves are ripples in spacetime produced by the motions of massive objects such as black holes and neutron stars. Since the universe is nearly transparent to gravitational waves, these signals carry direct information about their sources such as masses, spins, luminosity distances, and orbital parameters through dense, obscured regions across cosmic time. This article explores gravitational waves as cosmic messengers, highlighting key sources, detection methods, and the astrophysical payoffs across the gravitational wave spectrum. Keywords: Gravitational wave astrophysics; gravitational radiation; gravitational wave detectors; black holes.

Centrella, Joan↗
NASA NTRS2023

Tensor-to-scalar ratio forecasts for extended LiteBIRD frequency configurations

LiteBIRD is a planned JAXA-led cosmic microwave background (CMB) B -mode satellite experiment aiming for launch in the late 2020s, with a primary goal of detecting the imprint of primordial inflationary gravitational waves. Its current baseline focal-plane configuration includes 15 frequency bands between 40 and 402 GHz, fulfilling the mission requirements to detect the amplitude of gravitational waves with the total uncertainty on the tensor-to-scalar ratio, δr , down to δr < 0.001. A key aspect of this performance is accurate astrophysical component separation, and the ability to remove polarized thermal dust emission is particularly important. In this paper we note that the CMB frequency spectrum falls off nearly exponentially above 300 GHz relative to the thermal dust spectral energy distribution, and a relatively minor high frequency extension can therefore result in even lower uncertainties and better model reconstructions. Specifically, we compared the baseline design with five extended configurations, while varying the underlying dust modeling, in each of which the High-Frequency Telescope (HFT) frequency range was shifted logarithmically toward higher frequencies, with an upper cutoff ranging between 400 and 600 GHz. In each case, we measured the tensor-to-scalar ratio r uncertainty and bias using both parametric and minimum-variance component-separation algorithms. When the thermal dust sky model includes a spatially varying spectral index and temperature, we find that the statistical uncertainty on r after foreground cleaning may be reduced by as much as 30–50% by extending the upper limit of the frequency range from 400 to 600 GHz, with most of the improvement already gained at 500 GHz. We also note that a broader frequency range leads to higher residuals when fitting an incorrect dust model, but also it is easier to discriminate between models through higher χ 2 sensitivity. Even in the case in which the fitting procedure does not correspond to the underlying dust model in the sky, and when the highest frequency data cannot be modeled with sufficient fidelity and must be excluded from the analysis, the uncertainty on r increases by only about 5% for a 500 GHz configuration compared to the baseline.

U. Fuskeland↗
NASA NTRS2011

Gravitational Wave Astrophysics: Opening the New Frontier

The gravitational wave window onto the universe is expected to open in approx. 5 years, when ground-based detectors make the first detections in the high-frequency regime. Gravitational waves are ripples in spacetime produced by the motions of massive objects such as black holes and neutron stars. Since the universe is nearly transparent to gravitational waves, these signals carry direct information about their sources - such as masses, spins, luminosity distances, and orbital parameters, through dense, obscured regions across cosmic time. This article explores gravitational waves as cosmic messengers, highlighting key sources, detection methods, and the astrophysical payoffs across the gravitational wave spectrum.

Centrella, Joan↗
NASA NTRS2011

Gravitational Wave Astrophysics: Opening the New Frontier

The gravitational wave window onto the universe is expected to open in approximately 5 years, when ground-based detectors make the first detections in the high-frequency regime. Gravitational waves are ripples in spacetime produced by the motions of massive objects such as black holes and neutron stars. Since the universe is nearly transparent to gravitational waves, these signals carry direct information about their sources - such as masses, spins, luminosity distances, and orbital parameters through dense, obscured regions across cosmic time. This talk will explore gravitational waves as cosmic messengers, highlighting key sources, detection methods, and the astrophysical payoffs across the gravitational wave spectrum.

Centrella, Joan↗
NASA NTRS1974

Gravitational-wave bursts from the nuclei of distant galaxies and quasars: Proposal for detection using Doppler tracking of interplanetary spacecraft

Supermassive black holes which exist in the nuclei of many quasars and galaxies are examined along with the collapse which forms these holes and subsequent collisions between them which produce strong, broad-band bursts of gravitational waves. Such bursts might arrive at earth as often as 50 times per year--or as rarely as once each 300 years. The detection of such bursts with dual-frequency Doppler tracking of interplanetary spacecraft is considered.

Thorne, K. S.↗
NASA NTRS2019

Multi-messenger Astrophysics with Pulsar Timing Arrays: Astro2020 Science White Paper

Pulsar timing arrays (PTAs) are on the verge of detecting low-frequency gravitational waves (GWs)from supermassive black hole binaries (SMBHBs). With continued observations of a large sampleof millisecond pulsars, PTAs will reach this major milestone within the next decade. Already,SMBHB candidates are being identied by electromagnetic surveys in ever-increasing numbers;upcoming surveys will enhance our ability to detect and verify candidates, and will be instrumentalin identifying the host galaxies of GW sources. Multi-messenger (GW and electromagnetic) obser-vations of SMBHBs will revolutionize our understanding of the co-evolution of SMBHs with theirhost galaxies, the dynamical interactions between binaries and their galactic environments, and thefundamental physics of accretion. Multi-messenger observations can also make SMBHBs `standardsirens' for cosmological distance measurements out to z ~ 0.5 LIGO has already ushered in break-through insights in our knowledge of black holes. The multi-messenger detection of SMBHBs withPTAs will be a breakthrough in the years 2020-2030 and beyond, and prepare us for LISA to helpcomplete our views of black hole demographics and evolution at higher redshifts.

Kelley, L. Z.↗
NASA NTRS1976

Gravitational-wave bursts from the nuclei of distant galaxies and quasars - Proposal for detection using Doppler tracking of interplanetary spacecraft

It is argued that the collapse which forms the supermassive black holes that are likely to exist in the nuclei of many quasars and galaxies, and collisions between those holes, should produce strong broadband bursts of gravitational waves. The mean time between such gravitational-wave bursts at earth is analyzed in terms of the present Hubble expansion rate, the present deceleration parameter of the universe, the redshift z = 2.5 (at which most of the bursts are assumed to have been generated), the present number density of 'centers' where the bursts originated, the mean number of bursts generated in each center during its active life, and the speed of light. The analysis shows that the range from one week to 300 years is reasonable for the time between bursts, although it is admitted that hardly any bursts at all is also reasonable. Expected characteristics of the bursts are deduced, and it is shown that the best detector for these bursts will probably be Doppler tracking of one or more interplanetary spacecraft.

Thorne, K. S.↗
NASA NTRS2003

Possible Class of Nearby Gamma-Ray Burst/Gravitational Wave Sources

A possible subclass of gamma-ray bursts - those with few, wide pulses, spectral lags of order one to several seconds, and soft spectra - has been identified. Their Log[N]-Log[Fp] distribution approximates a -3/2 power-law, suggesting homogeneity and relatively nearby sources. These mostly dim bursts account for approximately 50% of the BATSE sample of long bursts near that instrument s trigger threshold, suggesting that this subluminous class constitutes a more common variety than the more familiar burst sources which lie at truly cosmological distances. Theoretical scenarios predicted such a class, motivated by their exemplar GRB 980425 (SN 1998bw) lying at a distance of approximately 38 Mpc. The observations are explained by invoking off-axis viewing of the GRB jet and/or bulk Lorentz factors of order a few. Long-lag bursts show a tendency to concentrate near the Supergalactic Plane with a quadrupole moment of -0.10 plus or minus 0.04, similar to that for SNe type Ib/c within the same volume. The rate of the observed subluminous bursts is of order 1/4 that of SNe Ib/c. Evidence for a sequential relationship between SNe Ib/c and GRBs is critiqued for two cases, as simultaneity of the SN and GRB events may be important for detection of the expected gravitational wave signal; at most, SN to GRB delays appear to be a few days. SN asymmetries and ultrarelativistic GRB jets suggest the possibility of rapid rotation in the pre-collapse objects, a primary condition required for highly nonaxisymmetric SN collapse to produce strong gravitational waves.

Norris, Jay P.↗
NASA NTRS2002

All-sky search for gravitational waves

We present a set of general tools to search for multiparameter gravitational-wave signals buried in the noise of a detector.

gravitational waves signal detection↗
NASA NTRS2008

LISA Pathfinder

USA Pathfinder is a space mission dedicated to demonstrating technology for the Laser Interferometer Space Antenna (LISA). LISA is a joint ESA/NASA mission to detect low-frequency gravitational waves on the 0.0001 to 0.1 Hz frequency band. LISA is expected to observe 100's of merging massive black hole binaries out z-15, tens of thousands of close compact binary systems in the Milky Way, merging intermediate-mass black hole binaries, tens of stellar-mass black holes falling into supermassive black holes in galactic centers, and possibly other exotic sources. Several critical LISA technologies have not been demonstrated at the requisite level of performance. In spaceflight, and some fight hardware cannot be tested in a 1-g environment. Hence, the LISA Pathfinder mission is being implemented to demonstrate these critical LISA technologies in a relevant flight environment. LISA Pathfinder mimics one arm of the LISA constellation by shrinking the 5-million-kilometer armlength down to a few tens of centimeters. The experimental concept is to measure the relative separation between two test masses nominally following their own geodesics, and thereby determine the relative residual acceleration between them near 1 mHz, about a decade above the lowest frequency required by LISA. To implement such a concept, disturbances on the test masses must be kept very small by many design features, but chiefly by "drag-free" flight. A drag-free spacecraft follows a free-falling test mass which it encloses, but has no mechanical connection to. The spacecraft senses it's orientation and separation with respect to the proof mass, and its propulsion system is commanded to keep the spacecraft centered about the test mass. Thus, the spacecraft shields the test mass from most external influences, and minimizes the effect of force gradients arising from the spacecraft, and acting on the test mass. LISA Pathfinder will compare the geodesic of one test mass against that of the other. Only a metrology system based on interferometry can achieve the displacement sensitivity. Interferometers monitor the separation of both test masses with a sensitivity comparable to that required by LISA, and using the same technologies. LISA Pathfinder is scheduled to be launched in the first half of 1020 to a Lissajous orbit around the first Sun-Earth Lagrange point, L1. In addition to a complete European technology package (the LISA Technology Package, or LTP), LISA Pathfinder will also carry thrusters and software, known as ST-7, a part of NASA's New Millennium Program.

Stebbins, Robin↗
NASA NTRS2010

A LISA Interferometry Primer

A key challenge for all gravitational wave detectors in the detection of changes in the fractional difference between pairs of test masses with sufficient precision to measure astrophysical strains with amplitudes on the order of approx.10(exp -21). ln the case of the five million km arms of LISA, this equates to distance measurements on the ten picometer level. LISA interferometry utilizes a decentralized topology, in which each of the sciencecraft houses its own light sources, detectors, and electronics. The measurements made at each of the sciencecraft are then telemetered to ground and combined to extract the strain experienced by the constellation as a whole. I will present an overview of LISA interferometry and highlight some of the key components and technologies that make it possible.

Thorpe, James Ira↗
NASA NTRS2011

CFRP Dimensional Stability Investigations for Use on the LISA Mission Telescope

The Laser Interferometer Space Antenna (LISA) is a mission designed to detect low frequency gravitational-waves. In order for LISA to succeed in its goal of direct measurement of gravitational waves, many subsystems must work together to measure the distance between proof masses on adjacent spacecraft. One such subsystem, the telescope, plays a critical role as it is the laser transmission and reception link between spacecraft. Not only must the material that makes up the telescope support structure be strong, stiff and light, but it must have a dimensional stability of better than 1 pm Hz(exp -1/2) at 3 mHz and the distance between the primary and the secondary mirrors must change by less than 2.5 micron over the mission lifetime. CFRP is the current baseline materiaL however, it has not been tested to the pico-meter level as required by the LISA mission. In this paper we present dimensional stability results, outgassing effects occurring in the cavity and discuss its feasibility for use as the telescope spacer for the LISA spacecraft.

Sanjuan, J.↗
NASA NTRS2017

Drag-Free Control and Drag Force Recovery of Small Satellites

Drag-free satellites provide autonomous precision orbit determination, accurately map the static and time varying components of Earth's mass distribution, aid in our understanding of the fundamental force of gravity, and will ultimately open up a new window to our universe through the detection and observation of gravitational waves. At the heart of this technology is a gravitational reference sensor, which (a) contains and shields a free-floating proof mass from all non-gravitational forces, and (b) precisely measures the position of the test mass inside the sensor. Thus, both test mass and spacecraft follow a pure geodesic in spacetime. By tracking the position of a low Earth orbiting drag-free satellite we can directly determine the detailed shape of geodesics and through analysis, the higher order harmonics of the Earths geopotential. This paper explores two different drag-free control systems on small satellites. The first drag-free control system is a continuously compensated single thruster 3-unit CubeSat with a suspension-free spherical proof-mass. A feedback control system commands the thruster and Attitude and Determination Control System to fly the tender spacecraft with respect to the test mass. The spheres position is sensed with a LED-based differential optical shadow sensor, its electric charge controlled by photoemission using UV LEDs, and the spacecraft position is maintained with respect to the sphere using an ion electrospray propulsion system. This configuration is the most fuel-efficient drag-free system possible today. The second drag-free control system is an electro-statically suspended cubical proof-mass that is operated with a low duty cycle, limiting suspension force noise over brief, known time intervals on a small GRACE-II -like satellite. The readout is performed using a laser interferometer, which is immune to the dynamic range limitations of voltage references. This system eliminates the need for a thruster, enabling drag-free control systems for passive satellites. In both cases, the test mass position, GPS tracking data, and commanded actuation, either thrust or suspension system, can be analyzed to estimate the 3-axis drag forces acting on the satellite. The data produces the most precise maps of upper atmospheric drag forces and with additional information, detailed models that describe the dynamics of the upper atmosphere and its impact on all satellites that orbit the Earth. This paper highlights the history, applications, design, laboratory technology development and highly detailed simulation results of each control system.

Small Satellites↗