GW asteroseismology

Probing mass-radius relation of protoneutron stars from gravitational-wave asteroseismology

ABSTRACT: The gravitational-wave (GW) asteroseismology is a powerful technique for extracting interior information of compact objects. In this work, we focus on spacetime modes, the so-called w-modes, of GWs emitted from a proto-neutron star (PNS) in the postbounce phase of core-collapse supernovae. Using results from recent three-dimensional supernova models, we study how to infer the properties of the PNS based on a quasi-normal mode analysis in the context of the GW asteroseismology. We find that the w1-mode frequency multiplied by the PNS radius is expressed as a linear function with respect to the ratio of the PNS mass to the PNS radius. This relation is insensitive to the nuclear equation of state (EOS) employed in this work. Combining with another universal relation of the f-mode oscillations, we point out that the time dependent mass-radius relation of the PNS can be obtained by observing both the f- and w1-mode GWs simultaneously. Our results suggest that the simultaneous detection of the two modes could provide a new probe into finite-temperature nuclear EOS that predominantly determines the PNS evolution.


Mystery solved: cluster Abell 2146

Mystery solved: discovery of extended radio emission in the merging galaxy cluster Abell 2146

ABSTRACT: Abell 2146 (z = 0.232) is a massive galaxy cluster currently undergoing a spectacular merger in the plane of the Sky with a bullet-like morphology. It was the first system in which both the bow and upstream shock fronts were detected at X-ray wavelengths (Mach ~ 2), yet deep GMRT 325 MHz observations failed to detect extended radio emission associated with the cluster as is typically seen in such systems. We present new, multi-configuration 1 – 2 GHz Jansky Very Large Array (VLA) observations of Abell 2146 totalling 16 hours of observations (4 hours in B-array, 10 hours in C-array and 2 hours in D-array). These data reveal for the first time the presence of an extended, faint radio structure associated with Abell 2146. The structure extends over 850 kpc in length and appears to harbour multiple components, one associated with the upstream shock which we classify as a radio relic and one associated with the subcluster core (the bullet) which is consisted as being a radio halo bounded by the bow shock. The newly detected structures have some of the lowest radio powers detected thus far in any cluster (P_1.4 GHz,halo = 2.4 +/- 0.2 * 10^23 W/Hz and P_1.4 GHz,relic = 2.2 +/- 0.2 * 10^23 W/Hz). The flux measurement of the halo, as well as its morphology, also suggest that the halo was recently created (~0.3 Gyr after core passage) and that it has entered a phase in which the radio halo emission is rising with time. This is consistent with the dynamical state of the cluster. These observations demonstrate the capacity of the upgraded VLA to detect extremely faint and extended radio structures. Based on these observations, we predict that many more radio relics and radio halos in merging clusters should be detected by future radio facilities such as the Square Kilometre Array (SKA).


A Mission to Alpha Centauri

The Andromeda Study: A Femto-Spacecraft Mission to Alpha Centauri

ABSTRACT: This paper discusses the physics, engineering and mission architecture relating to a gram-sized interstellar probe propelled by a laser beam. The objectives are to design a fly-by mission to Alpha Centauri with a total mission duration of 50 years travelling at a cruise speed of 0.1c. Furthermore, optical data from the target star system is to be obtained and sent back to the Solar system. The main challenges of such a mission are presented and possible solutions proposed. The results show that by extrapolating from currently existing technology, such a mission would be feasible. The total mass of the proposed spacecraft is 23g and the space-based laser infrastructure has a beam power output of 15GW. Rurther exploration of the laser – spacecraft tradespace and associated technologies are necessary.


Earth-mass planets around YZ Cet

The HARPS search for southern extra-solar planets XLII. A system of Earth-mass planets around the nearby M dwarf YZ Cet

ABSTRACT: Exoplanet surveys have shown that systems with multiple low-mass planets on compact orbits are common. Except for few cases, however, the masses of these planets are generally unknown. At the very end of the main sequence, host stars have the lowest mass and hence offer the largest reflect motion for a given planet. In that context, we monitored the low-mass (0.13 Msun) M dwarf YZ Cet (GJ 54.1, HIP 5643) intensively and obtained both radial velocities and stellar-activity indicators derived from both spectroscopy and photometry. We find strong evidence that it is orbited by at least three planets in compact orbits (P=1.97, 3.06, 4.66 days), with the inner two near a 2:3 mean-motion resonance. The minimum masses are comparable to that of Earth (Msini=0.75+-0.13, 0.98+-0.14, and 1.14+-0.17 Mearth) and also the lowest masses measured by radial velocity so far. We note the possibility for an even lower-mass, fourth planet with Msini=0.472+-0.096 Mearth at P=1.04 days. An n-body dynamical model is used to put further constraints on the system parameters. At 3.6 parsecs, YZ Cet is the nearest multi-planet system detected to date.


Rocket Lab Post-Flight Analysis

Rocket Lab Completes Post-Flight Analysis


Rocket Lab has completed an internal review of data from its May 25 test flight of its Electron rocket. The review found the launch had to be terminated due to an independent contractor’s ground equipment issue, rather than an issue with the rocket. Rocket Lab’s investigation board has identified the root causes and corrective actions.

The Federal Aviation Administration, the primary body responsible for licensing the launch, has overseen Rocket Lab’s comprehensive investigation and will review the findings.

Rocket Lab’s engineers have spent the last two months working through an extensive fault tree analysis to ensure all factors that may have influenced the outcome of the launch were thoroughly evaluated. The investigation involved the review of over 25,000 channels of data collected during the flight in addition to extensive testing at Rocket Lab facilities in California and New Zealand.

Rocket Lab’s investigation team determined the launch, named ‘It’s a Test’, was terminated due to a data loss time out, which was caused by misconfiguration of telemetry equipment owned and operated by a third-party contractor who was supporting the launch from Rocket Lab’s Launch Complex 1.

Four minutes into the flight, at an altitude of 224 km, the equipment lost contact with the rocket temporarily and, according to standard operating procedures, range safety officials terminated the flight. Data, including that from Rocket Lab’s own telemetry equipment, confirmed the rocket was following a nominal trajectory and the vehicle was performing as planned at the time of termination.

“We have demonstrated Electron was following its nominal trajectory and was on course to reach orbit,” said Peter Beck, Rocket Lab CEO. “While it was disappointing to see the flight terminated in essence due to an incorrect tick box. We can say we tested nearly everything, including the flight termination system. We were delighted with the amount of data we were able to collect during an exceptional first test launch.

Rocket Lab’s telemetry systems provided data verifying Electrons capabilities and providing us with high confidence ahead of our second test flight. The call to terminate a launch would be tough for anyone, and we appreciated the professionalism of the flight safety officials involved.”

The telemetry data loss that led to the termination of the flight has been directly linked to a key piece of equipment responsible for translating radio signals into data used by safety officials to track the vehicle performance. It was discovered a contractor failed to enable forward error correction on this third-party device causing extensive corruption of received position data. The failure was first indicated by the fact that Rocket Lab’s own equipment did not suffer similar data loss during launch. Further confirmation of the cause was demonstrated when replaying raw radio-frequency data – recorded on launch day – through correctly configured equipment also resolved the problem.

The fix for the issue is simple and corrective procedures have been put in place to prevent a similar issue in future. No major changes to the Electron launch vehicle hardware have been required and the company has authorized the production of four additional launch vehicles as it prepares for commercial operations ahead of the test flight program. Rocket Lab’s second Electron rocket, named ‘Still Testing’, is undergoing final checks and preparations ahead of being shipped to Rocket Lab Launch Complex 1 shortly.


Relativistic Astronomy

Relativistic Astronomy

ABSTRACT: The Breakthrough Initiatives are a program of scientific and technological exploration, probing some big questions of life in the universe. Among them is the “Breakthrough Starshot” program, which aims at proving the concept of developing unmanned space flight (probe) at a good fraction of the speed of light, c. Such a probe is designated to reach nearby stellar systems such as Alpha Centauri within decades, allowing humankind to explore extra-solar systems for the first time. The first prototype “Sprites” of 3.5 cm x 3.5 cm chips weighing just 4 grams each, which are the precursors to eventual “starChip” probes, have been recently launched to a low-earth orbit. Here we point out that due to the relativistic effects, trans-relativistic cameras serve as natural lenses and spectrographs while traveling in space, allowing humankind to study the astrophysical objects in a unique manner and to conduct precise tests on special relativity. Launching trans-relativistic cameras would mark the beginning of “relativistic astronomy”.


4 planets around tau Ceti

Color difference makes a difference: four planet candidates around tau Ceti

ABSTRACT: The removal of noise typically correlated in time and wavelength is one of the main challenges for using the radial velocity method to detect Earth analogues. We analyze radial velocity data of tau Ceti and find robust evidence for wavelength dependent noise. We find this noise can be modeled by a combination of moving average models and “differential radial velocities”. We apply this noise model to various radial velocity data sets for tau Ceti, and find four periodic signals at 20.0, 49.3, 160 and 642 d which we interpret as planets. We identify two new signals with orbital periods of 20.0 and 49.3 d while the other two previously suspected signals around 160 and 600 d are quantified to a higher precision. The 20.0 d candidate is independently detected in KECK data. All planets detected in this work have minimum masses less than 4M with the two long period ones located around the inner and outer edges of the habitable zone, respectively. We find that the instrumental noise gives rise to a precision limit of the HARPS around 0.2 m/s. We also find correlation between the HARPS data and the central moments of the spectral line profile at around 0.5 m/s level, although these central moments may contain both noise and signals. The signals detected in this work have semi-amplitudes as low as 0.3 m/s, demonstrating the ability of the radial velocity technique to detect relatively weak signals.

Dark Energy Survey (DES)

Survey finds galaxy clumps stirred up by dark energy

Daniel Clery | Science  11 Aug 2017

The biggest study so far of large-scale cosmic structure—a survey of 26 million galaxies that traced the forces shaping the universe over deep time—has revealed the dominating effect of dark energy, the mysterious pressure expanding space faster and faster. The survey finds about as much dark energy as the gold standard in assessing this cosmic recipe: precision maps of the nearly 14-billion-year-old cosmic microwave background (CMB) that forms the backdrop of the universe. But the new survey and other techniques are now allowing astronomers to watch how dark energy plays out over time. Their goal: to see whether it changes.

Dark energy was discovered 2 decades ago, when astronomers found that supernovae in distant galaxies were farther away than they should have been if the universe were expanding at a stately, steady pace. A bizarre picture of the universe emerged, in which only about 5% of its current mass comes from the atoms and photons we see. Another 25% is “dark matter,” invisible mass that helps pull galaxies together into clusters and filaments but doesn’t otherwise interact with ordinary matter. The remaining 70% is dark energy, doing its part to rebuff gravity’s clumping tendencies.

The CMB maps responsible for that picture, from missions such as Europe’s Planck satellite, provide only a snapshot of the universe in its infancy, before dark energy grew, along with space itself. To chart how dark energy evolved over time in its cosmic tug-of-war with dark matter, astronomers are pursuing four main techniques. They have continued to look back in time for ever-more-distant supernovae, although there are nagging uncertainties about the regularity of the stellar explosions and whether they can serve as reliable beacons. They also have developed a powerful new technique based on baryon acoustic oscillations (BAOs), ripples in the density of normal matter that trace their heritage back to sound waves in the roiling gas of the early universe. The ripples, now preserved as peaks in the density of galaxies at intervals of 490 million light-years, provide a yardstick for measuring how fast space is expanding.

The DES is focusing on two other techniques for gauging how space is stretching. One is a statistical measurement of how “clumpy” galaxy clusters are; the other is weak gravitational lensing, a measure of how the observed shapes of distant galaxies are distorted by the gravity of matter—both visible and dark—between them and Earth. The density and distribution of that matter, in turn, can reveal the stretching of space.

Because weak lensing smears the shape of each galaxy by just a percent or two, such surveys are challenging from the ground, where the atmosphere introduces its own distortions. But DES astronomers equipped the 4-meter Víctor M. Blanco Telescope in Chile with a 570-megapixel camera—large enough to capture an image 14 times the area of the full moon in one 90-second exposure. The project’s first observing season, from August 2013 to February 2014, amassed enough galaxies to see the imprint of dark energy, every bit as potent as the Planck results predicted.

So far, the prevailing cosmological model holds that dark energy is an intrinsic property of empty space, described by adding a fixed term to Albert Einstein’s theory of general relativity called the cosmological constant. This version of dark energy would generate the same force everywhere, throughout the history of the universe. Although the DES’s first results agree with that picture, to a few percent, some astronomers were hoping they would veer off and point to a more exotic explanation. Contenders include a new force field that might vary in time and space, akin to the Higgs field that gives particles mass. Another possibility is that, at cosmological scales, general relativity breaks down and a new theory of gravity is required. Advocates of such theories still hold out hope that the DES and other surveys, as they push farther out across space and time, will point to one of these alternatives.

They may not have to wait long. By next year the DES will have gathered 5 years of data. Investigators will analyze 300 million galaxies and thousands of supernovae going back 7 billion years. An expanded version of BOSS—eBOSS—is underway, and it will be joined next year by the Dark Energy Spectroscopic Instrument on a 4-meter telescope in Arizona. In 2022, the 8.4-meter Large Synoptic Survey Telescope (LSST) in Chile is due to begin supplying data for all four kinds of dark energy probes. And weak lensing surveys will eventually move to space for a clearer view of the distorted galaxy shapes. Euclid will launch in 2020. And later next decade, NASA’s Wide Field Infrared Survey Telescope may take up the search.


Pb isotopic ages of chondrules

Early formation of planetary building blocks inferred from Pb isotopic ages of chondrules

ABSTRACT: The most abundant components of primitive meteorites (chondrites) are millimeter-sized glassy spherical chondrules formed by transient melting events in the solar protoplanetary disk. Using Pb-Pb dates of 22 individual chondrules, we show that primary production of chondrules in the early solar system was restricted to the first million years after formation of the Sun and that these existing chondrules were recycled for the remaining lifetime of the protoplanetary disk. This is consistent with a primary chondrule formation episode during the early high-mass accretion phase of the protoplanetary disk that transitions into a longer period of chondrule reworking. An abundance of chondrules at early times provides the precursor material required to drive the efficient and rapid formation of planetary objects via chondrule accretion.