Applying clock comparison methods to pulsar timing observations

Frequency metrology outperforms any other branch of metrology in accuracy (parts in 10−16) and small fluctuations (<10−17). In turn, among celestial bodies, the rotation speed of millisecond pulsars (MSP) is by far the most stable (<10−18). Therefore, the precise measurement of the time of arrival (TOA) of pulsar signals is expected to disclose information about cosmological phenomena, and to enlarge our astrophysical knowledge. Related to this topic, Pulsar Timing Array (PTA) projects have been developed and operated for the last decades. The TOAs from a pulsar can be affected by local emission and environmental effects, in the direction of the propagation through the interstellar medium or universally by gravitational waves from super massive black hole binaries. These effects (signals) can manifest as a low-frequency fluctuation over time, phenomenologically similar to a red noise. While the remaining pulsar intrinsic and instrumental background (noise) are white. This article focuses on the frequency metrology of pulsars. From our standpoint, the pulsar is an accurate clock, to be measured simultaneously with several telescopes in order to reject the uncorrelated white noise. We apply the modern statistical methods of time-and-frequency metrology to simulated pulsar data, and we show the detection limit of the correlated red noise signal between telescopes.

An experiment of determining the geopotential difference using two hydrogen atomic clocks

<p>According to general relativity theory, one may determine the geopotential difference between two arbitrary stations by comparing there-located clocks’ running rates. In this study, we provide experimental results of the geopotential determination based on the time elapse comparison between two hydrogen atomic clocks, one fixed clock  and one portable clock , using the common view satellite time transfer (CVSTT) technique. We compared the portable clock  located at Jiugongshan Time Frequency Station (JTFS) with the fixed clock  located at Luojiashan Time Frequency Station (LTFS) for 30 days. The two stations are separated by a geographic distance of around 240 km with height difference around 1230 m. Then the clock  was transported (without stopping its running status) to LTFS and compared with clock  for zero-baseline calibration for 15 days. The clock-comparison-determined geopotential difference between JTFS and LTFS is determined. Results show that the clock-comparison-determined result deviates from the EGM20080-determined result by about 2322±1609 m<sup>2</sup>s<sup>-2</sup>, equivalent to 237±164  m in height, in consistence with the stability of the hydrogen atomic clocks applied in the experiments (at the level of 10<sup>-15</sup>/day).</p><p>This study is supported by NSFCs (grant Nos. 41721003, 41631072, 41874023, 41804012, 41429401, 41574007) and Natural Science Foundation of Hubei Province of China (grant No. 2019CFB611).</p>