National Time Service Center. CAS Time Standard and Ensemble Pulsar Time Scale Ding Chen, George & Bill, Dick, PPTA team 2011 年 5 月 9 日, Beijing

Outline  Time Standard  TAI and UTC  Pulsar Time Scale algorithm simulation real data  Problem and future improvements  Summary

Unit of time (SI)The second is the duration of periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the Cesium 133 atom. Unit of time (SI) :The second is the duration of periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the Cesium 133 atom.

Log (  y (  )) Log (  ), seconds day 1 month Hydrogen Maser Rubidium Quartz Cesium The Stabilities of different Frequency Scales

Ensemble Atomic Time Scale  TAI calculation is done each month  BIPM firstly computes a free atomic scale: EAL, from around 400 clocks all over the world, to get the optimal high 1-month stability. --AlGOS: weighted average algorithm. --An average of N identical clocks may be more stable than individual one clock -- Time comparison to same laboratory (PTB) of different laboratories (time transfer GPS CV) -- EAL is stable but may have some values shift to SI second.  Every month, primary frequency standard (PFS) are used to estimate the TAI from the frequency correction of EAL in order to be more closer to SI second.

The weights of different Lab(k) in TAI NTSC USNO NICT NIST

Time Links and Comparation

TAI and UTC: leap second  Coordinated Universal Time (UTC), maintained by the BIPM, is the time scale that forms the basis for the coordinated dissemination of standard frequencies and time signals. The UTC scale is adjusted by the insertion of leap seconds to ensure approximate agreement with the time derived from the rotation of the Earth.  Physical realizations of UTC – named UTC (k) – are maintained in national metrology institutes or observatories contributing with their clock data to the BIPM.  The dates of leap seconds of UTC are decided and announced by the International Earth Rotation and Reference Systems Service (IERS), which is responsible for the determination of Earth rotation parameters and the maintenance of the related celestial and terrestrial reference systems.

TT(BIPM)  TAI is computed in real time and will not be updated even an error is discovered, so it is not optimal.  Therefore the BIPM computes a post-processed time scale TT(BIPM)  Each new version TT(BIPMxx) updates and replaces the previous one. – Post-processed using all available PFS data. – Complete re-processing starting 1993 (change of algorithm). – Monthly estimation of the data are smoothed and integrated to obtain TT(BIPMxx).  Significant and time-varying frequency difference between TAI and TT(BIPM) integrates to more than 100 ns/yr, so TAI should not been used as a long-term reference.

Ensemble time scale: aiming at and beyond  More clocks for time keeper, a 100-fold increase in clock number would be needed to reach  New clock technologies: Cs, Rb fountain, Light clock : clocks, each with ≈ 5× month provide 3-4× for the ensemble time scale)  Long term stability (more than 3 months or one year) ~ or beyond : Combined with a independent more stable time scale (Pulsar Time Scale is a good candidate)

Ensemble Pulsar Time Scale  The arrival time of each Pulsar ‘k’ which is its date in PT K, to be actually measured based on atomic clock, which is date in TAI. So we can obtain R k =TAI-PT k. which is the timing residuals.  Ensemble Pulsar Time Scale(EPT) :  PPTA is the best project to establish the new independent ensemble time scale which will take contribution to both GW detection and BIPM.

Statistics for Clock Stabilities  Allan Deviation: (Allan, D 1987),  SigmaZ: (Matsakis, Taylor et al. 1997) Fittng the data by X(R)=c 0 +c 1 (R-R 0 )+c 2 (R- R 0 )2 +c 3 (R-R 0 ) 3 with minimizing, τ=2 -n ×T, n=1,2,3,4,5

The observatory clock  Parkes hydrogen maser -> GPS -> TT(TAI) Parkes-GPSEffelsberg-GPS

What happens if irregularities exist in time-scale? TT(TAI)-TT(BIPM2010)(George’s slide)

New Technique  Define clock function to be simple Fourier expansion:  (note: can use other functional forms if needed)  Carry out a standard least-squares fit of pulsar timing model parameters + f(t) as usual, except:  simultaneously fit to multiple pulsars  use measurement of the covariance in the residuals for a given pulsar as part of the least-squares-fit fit (to deal with timing noise)

Final result (real data) PT(PPTA)-TT(TAI) and TT(BIPM2010)-TT(TAI) 1s1s

New Clock Reference for Pulsar Timing L: TAI-TT(BIPM2010); R: TAI-TT(ePT) JUMP –f e-07 MJD(20cm_fptm) H-OH_cpsr2m H-OH_cpsr2n Ref:MULTI_cpsr2n MULTI_fptm cm_fb -40 MULTI_cpsr2m

Future work  We will produce the first detailed ensemble pulsar time scale based on international PTA  Combining our data with observations from Europe, USA and China will allow us to make a significant improvement in our time scale  We will publish this new time scale and hope that BIPM will use the pulsar timescale to check/correct long-term timing irregularities CSIRO. Gravitational wave detection

Some suspicions  Consistency  Precision  Validity  Application

Summary  The long term stability of ensemble atomic time scale is  Pulsars, being macroscopic astrophysical objects, are completely unrelated to atomic clocks and are governed largely by unrelated physical principles. The high-precision pulsar timing project like PPTA makes it possible to establish the new independent pulsar time scale.  A time standard based upon the rotation of pulsars should be independent of one based on atomic clocks.  The rotation of millisecond pulsars is expected to remain stable for billions of years, so they can provide a very long-term standard. More algorithms are expected to derive the intrinsic ensemble pulsar time scale.  The new interesting and useful applications of EPT like corrected clock for GW detection and new independent time scale for BIPM are on the way.

Thank You!