Development and performance test of dynamic simulation system for X-ray pulsar navigation

Effect of time profile of laser pulse on the excitation of sound signals in a liquid with a series of laser pulses

The time-resolved soft x-ray spectrometer (TSXS) aboard on the X-ray Pulsar Navigation Test Satellite is an x-ray timing spectrometer covering the energy range of 0.5 to 10 keV. It is China’s first focusing x-ray telescope launched into space orbit. The optical system of TSXS is an x-ray grazing incidence focusing system with a field of view 15 arc min, which is nested with 4 parabolic mirrors with a focal length of 1150 mm. The focal plane detector of TSXS uses a silicon drift detector. From April to June 2016, ground calibration was carried out on TSXS, including the optical axis determination, calibration of energy linearity and energy resolution, calibration of time resolution and photon arrival time accuracy, and calibration of mirrors’ reflectivity. After the launch on November 10, 2016, the in-orbit calibration and performance verification of the telescope was carried out, including the optical axis determination, the performance of energy response, the performance of time accuracy, the calibration of effective area, and the evaluation of telescope sensitivity. After calibration and verification on the ground and in orbit, the photon energy measurement error of the telescope is better than 0.5% at energies above 1.5 keV, the energy resolution is better than 156 eV at 6.4 keV, the time resolution is <1 μs, the photon arrival time measurement accuracy is <302 ns, and the telescope in-orbit background is <4.16 ± 1.42 × 10 − 3 photons / s (0.5 to 3 keV, 40°N to 40°S, not including South Atlantic Anomaly). The telescope has an in-orbit observation sensitivity of 2.09 × 10 − 3 photons / cm2 / s / keV (0.5 to 3 keV, T = 1000 s, and nσ = 5).

In order to improve the navigation accuracy of the X-ray pulsar navigation system, in this paper we propose a constant fraction timing method based on the low-pass filter to measure the arrival time of the X-ray pulse in X-ray pulsar navigation. According to the technical scheme, the timing accuracies and dead times of original peaking timing and the improved constant fraction timing are measured. Experimental results indicate that the timing accuracy and the dead time of the peaking timing system are 18 ns and 4750 ns, the timing accuracy and the dead time of the constant fraction timing system are 0.78 ns and 105 ns. The timing accuracy and the dead time of constant fraction timing system are significantly improved compared with those of the peaking timing system. In the X-ray pulsar navigation system, the cumulative pulse profile of the X-ray pulse is constructed in the two different timing systems through measuring the arrival time of the X-ray photon. Experimental results indicate that compared with using the peaking timing system, the cumulative pulse profile of the X-ray pulse using the improved constant fraction timing system is improved obviously, therefore, the navigation accuracy could be improved by using the constant fraction timing system.

The photon arrival times detected by COS-B around the direction of the Crab pulsar have been analyzed by an adaptive cluster method in order to search for short bursts of gamma-ray emission from the source. As a result, two clusters of 3 and 6 photon arrival times within 264 s and 1813s respectively were found with the corresponding residual phases having close correlation with the peaks of the Crab pulsed emission. The comparison with a similar analysis performed by using photon arrival times not correlated with the Crab direction shows that the events have chance occurrence probabilities of about 10-3 and therefore may be consistent with (rare) random fluctuations of the background. Finally, 3 σ upper limits on the existence of bursts of emission from the Crab are given.

BATSE detection of a new hard x‐ray pulsar is reported. The source was first observed on 14 Jul 1993, reached a maximum intensity on 25 Jul 1993, then declined smoothly until it became undetectable on 16 Aug 1993. The BATSE location was adequate to permit OSSE, ASCA, and ROSAT observations, leading to an improved source location. The observed period at the solar system barycenter was 93.548±0.002s, 93.5665±0.0005s, and 93.541±0.004 on 15 Jul 1993, 23 Jul 1993, and 10 Aug 1993, respectively. The source is detected between ∼20 keV and 160 keV, with a spectrum fit by an optically thin thermal bremsstrahlung form, having a kT of 25.4±2.1 keV during the rise to maximum intensity, decreasing monotonically during the remainder of the outburst to 17.1±2.5 keV on 5 Aug 1993. The pulse profile in this energy range has a single broad peak, with the maximum occurring later at higher energies. The pulse profile, intensity history, and spectral behavior observed by BATSE are reported.

In this paper, the method of pulse phase tracking for X-ray pulsar based spacecraft navigation is expanded to accommodate signals arriving from pulsars with lower flux than the Crab Pulsar. Spacecraft position and velocity estimates can be calculated using phase and frequency estimates of an observed pulsar signal. This allows for nearly continuous updates of vehicle motion estimates over short time frames and eliminates the need to time transfer all of the arriving photons to an inertial reference at the solar system barycenter. Previous work in pulse phase tracking uses a maximum likelihood estimator (MLE) for initial phase and a second-order digital phase-locked loop (DPLL) with a one second block size. This technique accurately tracks phase and Doppler frequency using simulated photons modeling the output of the Crab Pulsar. This method breaks down when considering pulsars that have a lower flux. The main difficulty is balancing the need to keep DPLL block sizes short with the fact that very few photons arrive in any given block of the phase tracking algorithm. Most X-ray pulsars have fluxes that are multiple orders of magnitude lower than the Crab Pulsar. Two phase tracking methods for low flux X-ray pulsars are hypothesized and tested. The first involves removing the dynamics from the signal each block using estimates of the detectors velocity and acceleration. This parabolic phase model is used to compute an MLE for initial phase. The output is fed into a third-order DPLL to give estimates for the next block. The second method uses a three parameter MLE that is derived to estimate the phase, frequency, and frequency derivative of a pulsar signal. These estimates are sent to a frequency locked-loop assisted phase locked-loop. Both methods allow for longer blocks which is essential unless the detector area or background rejection is improved. The two methods are validated and compared on their ability to track detectors undergoing constant acceleration along the line-of-sight to a pulsar and to track a portion of the Cassini cruise trajectory using PSR B1821-24 and PSR B1937+21.

In the course of a programme to study optical pulsars, we have observed at different occasions PSR0540-69, the 50 msec pulsar in the LMC. Like the Crab pulsar, it has been detected at X-rays, optical and radio and its braking index (∼2) determined. In this communication we report some results obtained on this pulsar, in particular the presence of narrow features in the pulse profile and compare it with a nearly simultaneous X-ray observation. The observations were carried out at the 3.6m telescope at La. Silla observatory, Chile. The instrument consisted on a single channel photometer coupled with a. photomultiplier (GaAs photocathode). The sampling frequency was 10 kHz and the output of the photon counting system was recorded sequentially on magnetic tape. A set of filters and diaphragms were available. A standard analysis was then performed. After corrections to the solar system barycenter, the data were analyzed and the best period determined using the test (Buccheri et al, 1983).

The problem of optimal reorientation of spacecraft orbit is considered in quaternion formulation. Control (jet thrust vector orthogonal to the plane of the orbit) is limited in magnitude. It is necessary to minimize the costs of characteristic velocity for the process of reorientation of the spacecraft orbit. To describe the motion of the spacecraft center of mass quaternion differential equation of the orientation of the orbital coordinate system was used. The actual special case of the problem, when the spacecraft's orbit is circular and control equals to its maximum (in modulus) value on adjacent parts of active spacecraft motion (there are also parts of passive motion of the spacecraft), was considered. Various iterative methods (Newton's method, gradient descent method, etc.) are often used to solve boundary value problems obtained as a result of the maximum principle. There are no formulas for finding unknown initial values of conjugate variables. Original evolutionary algorithm for finding the trajectories of spacecraft fastest flights is built. In this case the lengths of the sections of the spacecraft motion are unknown. This method does not require any information about the unknown initial values of conjugate variables. The high speed of operation of the proposed evolutionary algorithm is achieved through the use of existing in this case, a known analytical solution of equations of the problem. Examples of numerical solution of the problem for the case when the difference between the initial and final orientations of the spacecraft's orbit equals to a few (or tens of) degrees in angular measure, are given. Also the case was considered when the final orientation of the spacecraft's orbit corresponds to one of the satellites of Russian GLONASS orbital grouping. The graphs of components of the quaternion of orientation of the orbital coordinate system, the deviation of the current position of the spacecraft's orbit to the required and optimal control are drawn. Specific features and regularities of the process of optimum reorientation of the spacecraft's orbit are given.

We present the spectrum and the pulse profile of the Crab Pulsar in the near-ultraviolet (1600-3200 Å) observed with the Space Telescope Imaging Spectrograph (STIS) during the Hubble Space Telescope (HST) second Servicing Mission Orbital Verification (SMOV) period. The two-dimensional near-ultraviolet Multianode Microchannel Array (NUV MAMA) was used in time-tag mode with a 2''×2'' aperture and the low-dispersion grating, G230L, to obtain a cube with axes of slit position, wavelength, and time. The observation-derived pulse period is consistent with radio measurements, and the pulse profile agrees well with previous NUV broadband measurements by the High Speed Photometer. The pulsar spectrum includes the 2200 Å dust absorption feature, plus several interstellar absorption lines. Dereddening the spectrum using the Savage-Mathis model with E(B-V)=0.55±0.05 leads to a good fit to a power law with slope αν = -0.3±0.2. The spectra of the main pulse, the interpulse, and the individual rising and falling edges are similar to the total spectrum within the limits of photon statistics. The four pulse profiles produced by breaking the spectrum into 400 Å bins show the pulse profile to be stable across the NUV spectral range. Histogram analysis reveals no evidence for the superpulses seen at radio wavelengths. The interstellar absorption-line equivalent widths of Mg I, Mg II, and Fe II are lower than expected based on the implied H I column density from E(B-V)=0.5. While several explanations are possible, additional studies will be necessary to narrow the options.

Attitude stability analysis of a dual-spin spacecraft in halo orbits

Third and fourth generation light sources have revolutionized the research in many scientific and technological disciplines. New scientific challenges impose the construction of cutting-edge performance machines and experimental stations. In this context, off-the-shelf detection systems severely constrain the achievable results. These reasons motivated the ReDSoX research project, aiming to explore new solutions related to energy resolving imagers based on Silicon Drift Detectors (SDD), which are among the most employed acquisition devices in X-ray fluorescence spectroscopy. The main goal of the project is to develop novel versatile detection systems able to cover a large photoemission solid angle, being easily adaptable to the needs of different X-ray spectroscopy beamlines and ready to cope with high photon count-rates in order to exploit all the power of new light sources. Research efforts yielded two detector systems, dedicated to different experimental needs. The first system is composed of 32 SDD elements arranged on 4 monolithic sensors and covers a total non-collimated area of 1230 mm2. Such device is optimized for detecting low-energy photons in the 200 – 4000 eV energy range. The second detector consists of a matrix of 64 SDD elements disposed on 8 monolithic arrays covering an overall non-collimated active area of 576 mm2 and operating over an energy range between 4 and 30 keV. Both systems are highly integrated and can either be operated as an apparent large area single detector or as a multi-element detector, collecting information separately from each single element in order to enable spatially and angularly resolved advanced studies. The performances of the two detector systems have been studied at the TwinMic and XAFS beamlines (Elettra Sincrotrone Trieste, Italy), respectively. Recent results obtained during these measurements are presented and discussed.

We present an analysis of the highly variable accreting X-ray pulsar 3A 1954+319 using 2005-2009 monitoring data obtained with INTEGRAL and Swift. This considerably extends the pulse period history and covers flaring episodes in 2005 and 2008. In 2006 the source was identified as one of only a few known symbiotic X-ray binaries (SyXBs), i.e., systems composed of a neutron star accreting from the inhomogeneous medium around an M-giant star. The extremely long pulse period of 5.3 hr is directly visible in the 2008 INTEGRAL-ISGRI outburst light curve. The pulse profile is double peaked and generally not significantly energy dependent although there is an indication of possible softening during the main pulse. During the outburst a strong spin-up of -1.8 10^(-4) hr hr^(-1) occurred. Between 2005 and 2008 a long-term spin-down trend of 2.1 10^-5 hr hr^(-1) was observed for the first time for this source. The 3-80 keV pulse peak spectrum of 3A 1954+319 during the 2008 flare could be well described by a thermal Comptonization model. We interpret the results within the framework of a recently developed quasi-spherical accretion model for SyXBs.

We report on the MAXI GSC X-ray monitoring of the Crab nebula and pulsar during the GeV gamma-ray flare on 2010 September detected by AGILE and Fermi-LAT. There were no significant variations on the pulse-phase-averaged and pulsed fluxes during the gamma-ray flare. The pulse profile also showed no significant change during the period. The upper limits on the variation of the pulse-phase-averaged and pulsed fluxes on MJD 55457.5–55462.5 in the 4–10 keV band are 1% and 19% at the 90% confidence limit of the statistical uncertainty, respectively. Here, the measured fluxes include a 2% systematic uncertainty at the 1-σ limit due to the error on the instrument calibration. The lack of variations in the pulsed component in multi-wavelength range (radio, soft X-ray, hard X-ray, and gamma-ray) supports the nebula origin for the gamma-ray flare.

We study non-thermal emission process in pulsar magnetospheric accelerator (outer gap). We discuss emission process in optical through γ-ray bands. We solve the dynamics in the outer gap in the magnetic meridional plane, which includes the rotation and magnetic axes. We calculate the spectrum of the curvature and synchrotron processes. Using a threedimensional geometrical model, we compute pulse profile and polarization characteristics. We compare the model results with the observations of the Vela pulsar and the Crab pulsar. We demonstrate that the observed multi-peak structure of the pulse profile observed by RXTE can be reproduced by the present study with the outward and inward emissions from the outer gap accelerator. The calculated polarization characteristics are in general consistent with the Crab optical data. © 2009 American Institute of Physics.

This paper addresses the use of the position difference between the reference satellite and the target spacecraft to improve X-ray pulsar navigation (XPNAV) for Earth orbit spacecraft. This is achieved by first installing an X-ray detector on the reference satellite whose position is accurately known. The position measurement error of the reference satellite, known as the correction value, is sent to the spacecraft through the X-ray communication (XCOM) link. It is hoped that the accuracy of the spacecraft state measurement can be improved by offsetting common errors of measurement. X-ray ranging observation between the reference satellite and the target spacecraft, obtained from XCOM, can accomplish high precision in distance measurements, which can supply precise information for XPNAV. A novel pulse position modulation (PPM) polarization encode and modulation mode is used to achieve difference time transmission and range measurement simultaneously. Through the information fusion of the difference timing observation and the ranging observation, the positioning accuracy of the spacecraft is improved further. With the aim of estimating the spacecraft’s errors in location and speed, an adaptive divided difference filter (ADDF) is applied to eliminate nonlinearity. Several simulation cases are designed to verify the proposed method. Numerical simulations show that, compared with the traditional timing observation, the difference timing and ranging method can improve the position estimation accuracy by 27% and the velocity estimation accuracy by 22%.