## From European Space Agency: “ESA sets clock by distant spinning stars”

From European Space Agency

24 December 2018

ESA’s technical centre in the Netherlands has begun running a pulsar-based clock. The ‘PulChron’ system measures the passing of time using millisecond-frequency radio pulses from multiple fast-spinning neutron stars.

Operating since the end of November, this pulsar-based timing system is hosted in the Galileo Timing and Geodetic Validation Facility of ESA’s ESTEC establishment, at Noordwijk in the Netherlands, and relies on ongoing observations by a five-strong array of radio telescopes across Europe.

Pulsar encased in supernova bubble

Neutron stars are the densest form of observable matter in the cosmos, formed out of the collapsed core of exploding stars. Tiny in cosmic terms, on the order of a dozen kilometres in diameter, they still have a higher mass than Earth’s Sun.

A pulsar is a type of rapidly rotating neutron star with a magnetic field that emits a beam of radiation from its pole. Because of their spin – kept steady by their extreme density – pulsars as seen from Earth appear to emit highly regular radio bursts – so much so that in 1967 their discoverer, UK astronomer Jocelyn Bell Burnell, initially considered they might be evidence of ‘little green men’.

Women in STEM – Dame Susan Jocelyn Bell Burnell

Dame Susan Jocelyn Bell Burnell, discovered pulsars with radio astronomy. Jocelyn Bell at the Mullard Radio Astronomy Observatory, Cambridge University, taken for the Daily Herald newspaper in 1968. Denied the Nobel.

Dame Susan Jocelyn Bell Burnell 2009

Dame Susan Jocelyn Bell Burnell (1943 – ), still working from http://www. famousirishscientists.weebly.com

ESTEC

“PulChron aims to demonstrate the effectiveness of a pulsar-based timescale for the generation and monitoring of satellite navigation timing in general, and Galileo System Time in particular,” explains navigation engineer Stefano Binda, overseeing the PulChron project.

“A timescale based on pulsar measurements is typically less stable than one using atomic or optical clocks in the short term but it could be competitive in the very long term, over several decades or more, beyond the working life of any individual atomic clock.

“In addition, this pulsar time scale works quite independently of whatever atomic clock technology is employed – it doesn’t rely on switches between atomic energy states but the rotation of neutron stars.”

PulChron sources batches of pulsar measurements from the five 100-m class radio telescopes comprising the European Pulsar Timing Array – the Westerbork Synthesis Radio Telescope in the Netherlands, Germany’s Effelsberg Radio Telescope, the Lovell Telescope in the UK , France’s Nancay Radio Telescope and the Sardinia Radio Telescope in Italy.

Westerbork Synthesis Radio Telescope, an aperture synthesis interferometer near World War II Nazi detention and transit camp Westerbork, north of the village of Westerbork, Midden-Drenthe, in the northeastern Netherlands

MPIFR/Effelsberg Radio Telescope, in the Ahrgebirge (part of the Eifel) in Bad Münstereifel, Germany

Lovell Telescope, Jodrell Bank

Nancay decametric radio telescope located in the small commune of Nançay, two hours’ drive south of Paris, France

Sardinia Radio Telescope based in Pranu Sanguni, near Sant’Andrea Frius and San Basilio, about 35 km north of Cagliari (Sardinia, Italy).

This multinational effort monitors 18 highly precise pulsars in the European sky to search out any timing anomalies, potential evidence of gravitational waves – fluctuations in the fabric of spacetime caused by powerful cosmic events.

For PulChron, these radio telescope measurements are used to steer the output of an active hydrogen maser atomic clock with equipment based in the Galileo Timing and Geodetic Validation Facility – combining its extreme short- and medium-term stability with the longer-term reliability of the pulsars. A ‘paper clock’ record is also generated out of the measurements, for subsequent post-processing checks.

Atomic clocks at ESTEC

ESA established the Timing and Geodetic Validation Facility in the early days of the Galileo programme, first to prepare for ESA’s two GIOVE test satellites and then in support of the world-spanning Galileo system, based on ‘Galileo System Time’ which needs to remain accurate to a few billionths of a second. The Facility continues to serve as an independent yardstick of Galileo performance, linked to monitoring stations across the globe, as well as a tool for anomaly investigation.

Stefano adds: “The TGVF provided a perfect opportunity to host the PulChron because it is capable of integrating such new elements with little effort, and has a long tradition in time applications, having been used even to synchronise time and frequency offset of the Galileo satellites themselves.”

PulChron setup

PulChron’s accuracy is being monitored down to a few billionths of a second using ESA’s adjacent UTC Laboratory, which harnesses three such atomic hydrogen maser clocks plus a trio of caesium clocks to produce a highly-stable timing signal, contributing to the setting of Coordinated Universal Time, UTC – the world’s time.

The gradual diversion of pulsar time from ESTEC’s UTC time can therefore be tracked – anticipated at a rate of around 200 trillionths of a second daily.

This project is supported through ESA’s Navigation Innovation and Support Programme (NAVISP), applying ESA’s hard-won expertise from Galileo and Europe’s EGNOS satellite augmentation system to new satellite navigation and – more widely – positioning, navigation and timing challenges.

PulChron is being led for ESA by GMV in the UK in collaboration with the University of Manchester and the UK’s NPL National Physical Laboratory.

five-ways-keep-your-child-safe-school-shootings

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The European Space Agency (ESA), established in 1975, is an intergovernmental organization dedicated to the exploration of space, currently with 19 member states. Headquartered in Paris, ESA has a staff of more than 2,000. ESA’s space flight program includes human spaceflight, mainly through the participation in the International Space Station program, the launch and operations of unmanned exploration missions to other planets and the Moon, Earth observation, science, telecommunication as well as maintaining a major spaceport, the Guiana Space Centre at Kourou, French Guiana, and designing launch vehicles. ESA science missions are based at ESTEC in Noordwijk, Netherlands, Earth Observation missions at ESRIN in Frascati, Italy, ESA Mission Control (ESOC) is in Darmstadt, Germany, the European Astronaut Centre (EAC) that trains astronauts for future missions is situated in Cologne, Germany, and the European Space Astronomy Centre is located in Villanueva de la Cañada, Spain.

## From astrobites: “The Predictor of Pulsar Timing”

Astrobites

Aug 10, 2016
Michael Zevin

Title: Upper Limits on the Isotropic Gravitational Radiation Background from Pulsar Timing Analysis
Authors: R. W Hellings and G. S. Downs
First Author’s Institution: Jet Propulsion Laboratory, California Institute of Technology
Paper Status: Published in The Astrophysical Journal (1983)

The gravitational-wave Universe has at long last been unlocked: LIGO’s recent observations of gravitational radiation from binary black hole mergers have given astronomers a new means to observe the cosmos.

“Caltech/MIT Advanced aLigo Hanford, WA, USA installation

Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

With the influx of papers that pursued these detections over the past half year, it is easy to assume that gravitational-wave physics is a research topic exclusive to recent times. In reality, this research field has a rich history, and the methods used to detect gravitational waves were developed over many decades. For example, the idea of using interferometers like LIGO for gravitational-wave detection got its roots in the 1960s, and took nearly half a century to evolve from an idea to an observation. Today’s Astrobite will take us back to a classic astrophysical paper that delivered a crucial result for the future of gravitational-wave astronomy.

Ground-based interferometers are not the only way that scientists are working to detect the ripples of spacetime. Analogous to light, gravitational waves come in a range of frequencies, and require unique methods and technologies to unveil the diverse array of astrophysical sources. LIGO and other ground-based interferometers are sensitive to high-frequency gravitational waves (from about 10 Hz to 1000 Hz). Though improving instrumentation and adding more detectors to the network will increase the sensitivity of LIGO in this frequency range, ground-based interferometers will always be insensitive to gravitational waves below 10 Hz due to the rumbling and grumbling of the planet on which they sit.

The full spectrum of gravitational-wave emission. The line in the middle of the image shows the periods of gravitational waves that are believed to be generated by associated objects and events at the top of the image, and the bottom of the image illustrates techniques used to detect gravitational waves at these different frequencies. Credit: NASA/J. I. Thorpe.

Pulsar Timing Arrays (PTAs) provide one method for detecting lower-frequency gravitational waves, from about 10^{-9} – 10^{-6} Hz. Pulsars are rapidly-rotating neutron stars that act as lighthouses, beaming a pulse of radiation at Earth once per rotation. The rotations of many pulsars are incredibly stable, allowing the repeated pulses of radiation to be used as a precise astrophysical clock. PTAs utilize high precision measurements of pulsar pulse periods to detect gravitational waves. A pulsar and Earth can be thought of as two ends of a gravitational-wave antenna, where the relative motion of Earth and the pulsar (say, by the stretching and squeezing of space from a passing gravitational wave) is monitored by observing the Doppler shift of pulse arrival times. This Doppler shift therefore acts to alter the period of the pulses received at Earth. PTAs can be leveraged to detect a stochastic background of supermassive black hole mergers – the culmination of gravitational waves from many such events occurring in the distant universe.

By subtracting the observed pulse arrival times with the expected arrival times (known as timing residuals), pulsars can provide information about the stretching and squeezing of space caused by a gravitational wave (known as the strain) at Earth and at the pulsar. However, this is relatively useless on its own due to the inherent noise in each pulsar’s pulses. The real strength of PTAs comes from comparing the timing residuals of pulsars in different parts of the sky, since data from all the pulsars in the array will carry common information about the gravitational-wave strain at Earth. PTAs rely on cross-correlation of pulsar data, which essentially is the comparison of timing residuals from one pulsar with residuals of other pulsars in the array. By cross-correlating residuals of multiple pulsars at a given time, one can potentially detect a gravitational-wave signal by leveraging the common influence of background gravitational waves against unwanted, uncorrelated noise.

In the early 1980s, Hellings and Downs dug into the equations for correlating pulsar timing residuals. They found that, by assuming the stochastic gravitational waves come from all directions (i.e. isotropic) and the combination of background gravitational waves no longer carry any polarization information, the cross-correlation of timing residuals goes from being a very nasty integral to a mildly nasty integral. The solution gives the correlation in timing residuals, which is solely a function of pulsar angular separation:

$2$

where \alpha_{ij} is the residual correlation of two pulsars (labelled i and j), and \gamma_{ij} is the angle between the two pulsars. Plotting this function over angular separation gives the famous Hellings and Downs curve.

Hellings and Downs curve. This curve gives the pulse arrival time correlation as a function of angular separation in the sky. Credit: my mediocre plotting skills.

This curve gives the arrival time correlation of PTA pulsars – telling us exactly what we should see when we cross-correlate timing residuals of pulsars in the presence of a stochastic gravitational-wave background!

One thing I found particularly fascinating in this classic paper is that Hellings and Downs decreased the upper limit of gravitational wave energy density from a stochastic background by over 5 orders of magnitude, using their correlation analysis with just 4 pulsars! (For reference, the diameter of the Earth is about 5 orders of magnitude smaller than the distance between the Earth and the Sun). Though the Hellings and Downs curve has yet to be uncovered using PTAs, continued investigation keeps lowering the upper-limit on the energy density of the stochastic gravitational-wave background. This in turn reveals information about supermassive black holes in the universe and how often (if ever) they merge. Since the 1980s, this upper limit down has been brought down by another 4 orders of magnitude at the gravitational-wave frequencies which PTAs are sensitive, and many believe that the stochastic background should be detected by PTAs in the near future.

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