Shanghai Astronomical Observatory Research Articles

Design and experiment of onboard laser time transfer in Chinese Beidou navigation satellites

High-precision time synchronization between satellites and ground stations plays the vital role in satellite navigation system. Laser time transfer (LTT) technology is widely recognized as the highest accuracy way to achieve time synchronization derived from satellite laser ranging (SLR) technology. Onboard LTT payload has been designed and developed by Shanghai Astronomical Observatory, and successfully applied to Chinese Beidou navigation satellites. By using the SLR system, with strictly controlling laser firing time and developing LTT data processing system on ground, the high precise onboard laser time transfer experiment has been first performed for satellite navigation system in the world. The clock difference and relative frequency difference between the ground hydrogen maser and space rubidium clocks have been obtained, with the precision of approximately 300ps and relative frequency stability of 10E−14. This article describes the development of onboard LTT payload, introduces the principle, system composition, applications and LTT measuring results for Chinese satellite navigation system.

Fast estimation and analysis of the inter-frequency clock bias for Block IIF satellites

The inter-frequency bias of PRN25 was noticed by the scientific community and considered to be caused by thermal variations. The inter-frequency bias leads to an apparent inter-frequency clock bias (IFCB), which could be obtained using the difference of two ionosphere-free phase combinations (L1/L2 and L1/L5). We present an efficient approach derived from the epoch-differenced strategy for fast estimation of IFCBs for Block IIF satellites. For the analysis, data from 32 stations from the IGS network spanning 10 months (DOY 213, 2011---153, 2012) are processed. The processing times show that the epoch-differenced method is more efficient than the undifferenced one. In order to study the features of IFCB, a harmonic analysis is performed by using a FFT (fast Fourier transformation), and significant periodic variations with the periods of 12, 6 and 8 h are noticed. The fourth-order period is determined by comparing the performances of the model with different periods. After determination, a harmonics-based function of order 4 is used to model the IFCB, and the single-day amplitudes and phases are estimated for the 10 months from a least squares fit. Based on the estimated results, the characterization of IFCB is discussed. The algorithm is incorporated into the MGPSS software developed at SHAO (Shanghai Astronomical Observatory, Chinese Academy of Sciences) and used to monitor the IFCB variations of GPS and COMPASS systems in near real time.

The use of laser ranging to measure space debris

Space debris is a major problem for all the nations that are currently active in space. Adopting high-precision measuring techniques will help produce a reliable and accurate catalog for space debris and collision avoidance. Laser ranging is a kind of real-time measuring technology with high precision for space debris observation. The first space-debris laser-ranging experiment in China was performed at the Shanghai Observatory in July 2008 with a ranging precision of about 60–80 cm. The experimental results showed that the return signals from the targets with a range of 900 km were quite strong, with a power of 40W (2J at 20 Hz) using a 10ns pulse width laser at 532nm wavelength. The performance of the preliminary laser ranging system and the observed results in 2008 and 2010 are also introduced.

Rapid variability in S5 0716+714

S5 0716+714 is one of the targets in our long term blazar monitoring program carried out with the 1.56-m telescope at Sheshan Station of Shanghai Astronomical Observatory, China. We report a very rapid variability of ΔI = 0.611 ± 0.102 mag over 3.6 min detected in our monitoring program during the period from December 2000 to March 2007. The rapid variability suggests that the mass for the central black hole is log(M/M⊙) = 7.68–8.38.

Optical Monitoring of 3C 66A in 1994–2008

3C 66A is one of the most interesting blazar. Our monitoring was carried out with a 1.56-m telescope of the Shanghai Astronomical Observatory (SHAO) from 13 December 1994 to 9 November 2008. Some peaks and gradual brightening of the source up to three times were observed.

脉冲微波式氢原子钟初步结果

Adequate coherent pulsed microwave input into microwave cavity of atomic clock can induce Ramsey fringes of the atomic transition. By which the line width of atomic clock can be greatly reduced and the performance might be improved. This method has been applied in the POP rubidium clock. Shanghai Astronomical Observatory, Chinese Academy of Sciences has studied the application of this method in passive hydrogen maser. The electrical circuit for the pulsed microwave frequency sweeping has been designed, and the Ramsey fringes of hydrogen atom have been observed, which means that this method for passive hydrogen maser is feasible. We plan to optimize the parameter of the system and microwave pulse to further reduce the line width. And the designed full-scale servo circuit realizes the pulsed microwave hydrogen maser. Finally the system performance will be measured and optimized.

Mars Express tracking and orbit determination trials with Chinese VLBI network

With strong support from European Space Agency (ESA), Shanghai Astronomical Observatory (SHAO) organized a tracking and orbit determination trails using Chinese VLBI Network (CVN) to track Mars Express, the first Mars probe launched by ESA. Using a high-resolution VLBI software correlator and Doppler measurement system developed in-house, two sets of tracking data, VLBI and Doppler, were acquired. The trials represent the first successful foray held in China to track a probe about 360 million kilometers away from the Earth. The tracking data are analyzed using a Mars satellite orbit determination software system developed at SHAO. The results show that the accuracy of 5 s integrated three-way-Doppler data is about 0.3 mm/s, or roughly the same accuracy as ESA’s tracking data. Position discrepancies between the Doppler-based orbit solution of 8 h arc-length (about 1 orbital revolution) and ESA’s reconstructed orbit are of the order of several hundred meters. In preparing for the Russia-China co-sponsored Mars exploration mission Phobos-Grunt-YingHuo, simulations were carried out to evaluate the achievable orbital accuracy levels and the contributions of VLBI and Doppler data respectively. Results show that Doppler data provide better orbit accuracy, so that for VLBI to be able to provide kilometer level orbit solutions, the accuracy of VLBI measurement needs to be improved by at least one order of magnitude.

Optical observations of Scorpius X-1 in 2008: Evidence of rapid variability

Optical observations of Sco X-1 with exposure times around 0.5 s were performed with the 1.56 m telescope of the Shanghai Astronomical Observatory in 2008. With these observations, we studied the fluctuation of the optical flux on three short timescales (5, 10, 100 s). We found that the standard deviations of fluctuations on these three timescales differ from those of Gaussian distributions by 7 Sigma, 5 Sigma and 3 Sigma, respectively. The result suggests that the variations in the intensity on these short timescales differ from pure statistical fluctuations, which, we suggest, is due to the intrinsic variability in the optical emission.

First CCD photometry for the contact binary VZ Trianguli

First CCD photometry is presented for the eclipsing binary VZ Trianguli, observed at the Sheshan Station of Shanghai Astronomical Observatory in 2008. Using the Wilson–Devinney Code, the photometric solution of VZ Tri was first deduced from the R-band observations. The results show that VZ Tri is an A-subtype late-type contact binary, with a mass ratio of q=0.350(±0.004) and a low contact degree of f=27.9%(±1.0%). Based on all available light minimum times covering over 40 years, it is found that the orbital period shows a long-term decrease at a rate of dP/dt=−1.52(±0.03)×10−7 d yr−1, suggesting that VZ Tri is undergoing mass transfer from the more massive component to the less massive component, accompanied with angular momentum loss. With period decreasing, the inner and outer critical Roche lobes will shrink, and then cause the contact degree to increase. Therefore, the weak-contact binary VZ Tri with decreasing period may evolve into a deep-contact configuration.

OPTICAL OBSERVATIONS OF 3C 273 FROM 2000 TO 2008

The quasar 3C 273 is one of the widely studied quasars that is also a selected target in our monitoring program. In this paper, we present the optical V, R, and I photometric observations done with the 1.56 m telescope at the Shanghai Astronomical Observatory from 2000 to 2008. Our monitoring results show 1118 sets of observations (258 sets for the V band, 414 sets for the R band, and 446 sets for the I band). The observations indicate that (1) the variations in the three bands are Delta V = 1.363 +/- 0.088 mag from V = 12.204 +/- 0.062 to V = 13.567 +/- 0.062, Delta R = 1.299 +/- 0.023 mag from R = 12.014 +/- 0.014 to R = 13.313 +/- 0.013, and Delta I = 1.041 +/- 0.088 mag from I = 11.628 +/- 0.046 to I = 12.669 +/- 0.075. Compared with the result by Toone, who presented the yearly averaged light curve during the period of 1980-2004 and found that the source shows a slow brightening with = 12.95 in 1996-1997 to = 12.5 in 2003-2004, our observations show that it is variable and both a state brighter than the yearly averaged brightness of = 12.5 and a state fainter than the yearly averaged brightness of = 12.95 were detected in our observing period. Intra-day variation timescales are from 13 minutes to 245 minutes. (2) The color indices are variable with V-R from -0.27 to 0.68, V-I from 0.11 to 1.14, and R-I from 0.14 to 0.86. The average values are = 0.18 +/- 0.10, = 0.64 +/- 0.14, and = 0.46 +/- 0.11.

PHOTOMETRIC MONITORING OF OJ 287 FROM 2002 TO 2007

OJ 287 is one of the best studied blazars. It is also a prime target in our long-term blazar monitoring program carried out with the 1.56 m telescope at the Sheshan Station of the Shanghai Astronomical Observatory, China. Here, we present optical photometry results of V, R, and I passbands of the BL Lacertae object OJ 287 (0851+202) carried out from 2002 January to 2007 February. The largest variations of ΔV = 1.96 mag (14.01-15.97 mag), ΔR = 2.36 mag (13.39-15.75 mag), and ΔI = 1.95 mag (12.79-14.74 mag) were detected in our observations. Intraday variation timescales of ~10 minutes to 2 hr were detected in optical passbands.

The long-term multiband optical observations and colour index for the quasar 3C 273

The long baseline simultaneous multiband (BVRI) observations of blazar 3C 273 are presented. We have made 758 optical multiband observations with the Yunnan Astronomical Observatory (YAO) 103 cm and Shanghai Astronomical Observatory (SHAO) 156 cm from 2003 January to 2005 April. In our observational campaign, the average colour indices are B - V = 0.21, V - R= 0.26 and R - I = 0.31. The mean magnitudes in B, V, R and I are 12.895, 12.698, 12.441 and 12.139, respectively. The source is in a steady state over observational campaign. The correlations between colour index and brightness are discussed. We find that the correlations of B - V versus B, V - R versus V and R - I versus R have significant negative correlation, while B - V versus V, V - R versus R and R - I versus I have positive correlation. These strong correlations imply that the spectrum becomes bluer (flatter) when the source becomes brighter, and redden (softer) when the source fades both in intraday and long-term variability. The spectral evolution trends of 3C 273 are consistent with those of BL Lac objects.

Photon counting module for laser time transfer via Earth orbiting satellite

For the project of the Laser Time Transfer (LTT) we have developed the photon counting detector package designed to synchronize the ground and space based clocks by laser pulses. The device flying modules were constructed at the Shanghai Observatory, China, and they were tested for operation in a space environment in 2006. Numerous ground indoor tests of time synchronization of the rubidium clock were performed before the satellite launch. The device was launched onboard the Chinese experimental navigation satellite Compass-M1 on 13 April 2007 to a high altitude orbit of 21,500 km. The world's first space clock synchronization by means of laser pulses was carried out from the satellite laser station Changchun, Jilin, China, 1 August 2007. The results of the space born detector package pre-launch tests along with the first in-flight operation results are presented.

CCD PHOTOMETRIC MONITORING OF THE BL LACERTAE OBJECT S5 0716+714 FROM 2001 TO 2006

We present the optical photometries (B, V, R, and I) of the BL Lac object S5 0716+714 (z > 0.3), which were observed with the 1 m telescope at Yunnan Astronomical Observatory and the 1.56 m telescope at Shanghai Astronomical Observatory from 2001 February to 2006 April. Our results show a significant rapid variation and exhibit a short time variability of ΔI = 0.60 mag in 28 minutes, ΔR = 1.58 mag in 23 minutes, ΔV = 1.06 mag in 10 minutes, and ΔB = 1.36 mag in 33 minutes. We find a strong correlation between the V – R color index and the magnitude in 292 minutes. Our CCD photometry of the S5 0716+714 showed they are in good similitude with Raiteri et al., and Qian et al. presented the optical variability magnitude and light curves from 1994 to 2001.

Prototype of space-borne LTT module and its ground tests

In order to develop the technique of Laser Time Transfer (LTT), Shanghai Astronomical Observatory has built a prototype of space-borne LTT module. The performance of the LTT module and the results of ground tests are discussed in the paper. The average precision of time difference between two rubidium clocks measured by laser pulses is 196 ps, and the uncertainty of measurement for the relative frequency differences is 1.2×10−13/2800 s.

Optical CCD Photometry of the Variability of the BLLacertae Object ON231 in a Low State

Abstract We present optical photometry ($B, V, R$, and $I$) of the BL Lac object ON231 (1219$+$285, $z$$=$0.102), which was observed with the 1 m telescope at Yunnan Astronomical Observatory and the 1.56 m telescope at Shanghai Astronomical Observatory from 2003 May to 2006 May. Our results indicate that (i) there is a short time variability of ${\Delta} I =$ 1.04 mag during 25 minutes; (ii) there is a strong correlation between the $R-I$ color and the $R$ magnitude in 72.2 hours; and (iii) our optical variability is consistent with those of Tosti et al. (2002, A&A, 395, 11).

Long-term Variability Properties of Mkn501

Optical photometric observations at the I and R wavebands were carried out towards Mkn501 using the 1.56 meter optical telescope of Shanghai Astronomical Observatory at Sheshan. Combining our new observations with the published historical data, we have obtained the light curves of Mkn501 at the optical, infrared and radio wavebands with a time coverage of nearly 80 years. The relationship between the light variability and the color index is discussed, it is found that a strong correlation exists between the color indices ( B − V) and ( B − R), with the correlation coefficient reaching r = 0.73. The correlations of multi-band light variabilities are analyzed by the DCF method, it is found that certain positive correlations of B-band light variability with the 4.8 GHz and infrared light variabilities exist. And the spectral analysis on the B-band light curve with the CLEANest method indicates that the light curve of Mkn501 contains probably two periodical components of (10.06 ± 0.04) yr and (21.60 ± 0.17) yr.

OPTICAL MONITORING OF 3C 390.3 FROM 1995 TO 2004 AND POSSIBLE PERIODICITIES IN THE HISTORICAL LIGHT CURVE

We report V, R, and I band CCD photometry of the radio galaxy 3C 390.3 obtained with the 1.56-m telescope of the Shanghai Astronomical Observatory from March 1995 to August 2004. Combining these data with data from the literature, we have constructed a historical light curve from 1894 to 2004 and searched for periodicities using the CLEANest program. We find possible periods of 8.30+-1.17, 5.37+-0.49, 3.51+-0.21, and 2.13+-0.08 years.

CCD Photometry and Optical Variability of the BL Lacertae Object H0323+022

We present optical photometry (B, V, R, and I) of the BL Lac object H0323+022 (z = 0.147) for the years 2000-2003. These were observed in the high-luminosity state with the 1 m telescope at the Yunnan Astronomical Observatory and the 1.56 m telescope at the ShangHai Astronomical Observatory. Our results show significant rapid variation and exhibit a short time variability of 19 minutes in the I band. We find a strong correlation between the B − V color index and the magnitude in the V band. Our CCD photometry of H0323+022 show good agreement with Bai et al., who find optical variability in the I and R magnitudes of about ΔI = 0.80 mag and ΔR = 0.63 mag.

Mass Segregation in Very Young Open Clusters: A Case Study of NGC 2244 and NGC 6530

We derive the proper motions, membership probabilities, and velocity dispersions of stars in the regions of the young (~2-4 Myr old) open clusters NGC 2244 (the central cluster in the Monoceros R2 association) and NGC 6530 (the dominant cluster in the Sgr OB1 association) from photographic plate material obtained at Shanghai Astronomical Observatory, with time baselines of 34 and 87 yr, respectively. Both clusters show clear evidence of mass segregation, but they do not exhibit any significant velocity-mass (or, equivalently, velocity-luminosity) dependence. This provides strong support for the suggestion that the observed mass segregation is at least partially due to the way in which star formation has proceeded in these complex star-forming regions (primordial mass segregation). Based on arguments related to the clusters' published initial mass functions, in conjunction with our new measurements of their internal velocity dispersions (~35 and 8 km s-1 for NGC 2244 and NGC 6530, respectively), we provide strong arguments in favor of the dissolution of NGC 2244 on very short timescales, while we speculate that NGC 6530 may be more stable against the effects of internal two-body relaxation. However, this latter object may well be destroyed by the strong tidal field prevalent at its location in the Galactic plane in the direction of the Galactic center.