T. Inoue

Time assignment system and its performance aboard the Hitomi satellite

Abstract. Fast timing capability in x-ray observation of astrophysical objects is one of the key properties for the ASTRO-H (Hitomi) mission. Absolute timing accuracies of 350 or 35u2009u2009μs are required to achieve nominal scientific goals or to study fast variabilities of specific sources. The satellite carries a GPS receiver to obtain accurate time information, which is distributed from the central onboard computer through the large and complex SpaceWire network. The details of the time system on the hardware and software design are described. In the distribution of the time information, the propagation delays and jitters affect the timing accuracy. Six other items identified within the timing system will also contribute to absolute time error. These error items have been measured and checked on ground to ensure the time error budgets meet the mission requirements. The overall timing performance in combination with hardware performance, software algorithm, and the orbital determination accuracies, etc. under nominal conditions satisfies the mission requirements of 35u2009u2009μs. This work demonstrates key points for space-use instruments in hardware and software designs and calibration measurements for fine timing accuracy on the order of microseconds for midsized satellites using the SpaceWire (IEEE1355) network.

Simulating Cherenkov Telescope Array observation of RX J1713.7–3946

We perform simulations of Cherenkov Telescope Array (CTA) observations of a young supernova remnant RX J1713.7-3946. This target is not only one of the brightest sources ever discovered in very high-energy gamma rays but also well observed in other wavebands. In X-rays, the emission is dominated by synchrotron radiation, which links directly to the existence of high-energy electrons. Radio observations of CO and HI gas have revealed a highly inhomogeneous medium surrounding the SNR, such as clumpy molecular clouds. Therefore gamma rays from hadronic interactions are naturally expected. However, the spectrum in GeV energy range measured by Fermi/LAT indicates more typical of leptonic emission from accelerated electrons. Despite lots of multi-wavelength information, the competing interpretations have led to much uncertainty in the quest of unraveling the true origin of the gamma-ray emission from RX~J1713.7--3946. CTA will achieve highest performance ever in sensitivity, angular resolution, and energy resolution. We estimate CTA capability to examine the emission mechanisms of the gamma rays through simulated spatial distribution, spectra, and their time variation.

Design of the time assignment system for ASTRO-H and its performance before launch

The ASTRO-H, which will be launched in 2015, is the sixth in a series of Japanese X-ray satellites. It is an international mission led by JAXA in collaboration with NASA and ESA, aiming to observe astrophysical objects in the X-ray band from 0.5 to 600 keV. One of the important scientific goals is to understand physical processes in the extreme environments of active and variable astrophysical objects, such as black holes, neutron stars, binary star, and active galactic nuclei. Therefore, a fast timing capability is a key requirement for the mission. According to numerical estimates of scientific performance, absolute times of X-ray events are required to have an accuracy of 300 μs to achieve minimum scientific goals and an accuracy of 30 μs is desired as a goal. The satellite carries a GPS receiver to get the accurate time information, which is distributed from the central computer on board through the large-and-complex SpaceWire network. Distributions of time information are shared in the same lines used for communications of telemetry and commands, and thus propagation delays and jitters affect the timing accuracy of the payload instruments. Further six items are identified as sources of timing errors and are measured on ground to be used in the calibration by off-line software. The time-assignment tasks in the off-line software packages are designed to be common for all the scientific instruments, although the hardware designs for finer timing resolutions are different by the instruments. Measurements of propagation delays in the flight configuration on ground and in-orbit calibration plans are described. The detail description will be submitted to the IEEE TNS paper in near future. This work demonstrates a good example of care points for space-use instruments in the hardware-and-software designs and calibration measurements in order to achieve a fine timing resolution at the micro second order with the middle-sized satellites using the SpaceWire (IEEE1355) network.