China's Hard X-ray Modulation Telescope (HXMT), the country's first X-ray observatory — not to mention its first astronomical satellite — headed to orbit last week.
China entered the orbital X-ray astronomy game last week as a Long March 4B rocket roared to life, hoisting the nation's first astronomical observatory into space.
The launch occurred from China's Jiuquan space center on Thursday, June 15th at 3:00 UT / 11:00 p.m. EDT on June 14th. HXMT was renamed Huiyan (Chinese for “insight”) shortly after launch. Three other smaller Earth-observing satellites, including Argentina's NuSat-3 and two of the Chinese company Orbita's Zhuhai satellites also hitched a ride to space along with the observatory.
To study the high-energy universe, you have to get up above the Earth's atmosphere, which absorbs cosmic X-rays on their way to our planet's surface. In fact, it was brief sounding-rocket flights that carried out our very first X-ray observations of the Sun, using captured Nazi V-2 rockets that were launched from the White Sands missile range in 1949. Now, several X-ray observatories circle Earth, including NASA's Chandra, NuSTAR, and Swift telescopes, as well as the European Space Agency's XMM-Newton.
China's HXMT joins this cohort to study X-ray sources such as supermassive black holes in distant galaxies, as well as stellar-mass black holes and pulsars in our own galaxy. HMXT will also survey the sky across the galactic plane on the hunt for new, short-lived X-ray sources, mapping out the cosmic X-ray background in the process. HXMT sees in gamma rays as well: It can catch gamma-ray bursts in the range of 3 million electron-volts (3 MeV). The mission could work in tandem with China's new Five-hundred-meter Aperture Spherical radio Telescope (FAST) to study the properties of millisecond pulsars, in hopes of using these sources as navigational beacons for deep-space missions.
HXMT followed a long road to the launch pad. The mission was first proposed 1993 and moved ahead into the early development stage in 2000. Originally, China hoped to launch the observatory in 2010, but funding constraints pushed the mission over three of China's Five Year Plans for development to 2017.
The Tsinghua University, the Ministry of Science and Technology of China, and the Chinese Academy of Sciences' Institute for High-Energy Physics (IHEP) collaborate in operating the telescope. The HXMT team has released a statement on their website on June 19th, which reports that the spacecraft is healthy and its low and medium energy detectors have successfully switched on.
HXMT joins China's Dark Matter Particle Explorer (DAMPE) as the second premier astronomical observatory in low-Earth orbit.
Want to catch a glimpse yourself? HXMT is in a 43°-inclined, 536- by 546-kilometer orbit, going around the Earth once every 95 minutes. This orbit means that — like Hubble and Hitomi — the observatory will be visible to naked-eye satellite observers from about latitude 45°N to 45°S. Heavens-Above is a great place to check for local passes: HXMT's NORAD ID is (42758) 2017-034A.
How the Hard X-ray Modulating Telescope "Sees" X-rays
HXMT couples an incredible range of energy coverage, from 1 to 250 keV, with wide sky coverage. To achieve this, the observatory uses three detectors with overlapping energy regimes: low energies (1–15 keV), medium energies (5–30 keV), and high energies (20–250 keV). The high-energy detector is the main science instrument and has the greatest sensitivity of the three detectors. Another instrument, designed to help calibrate the high-energy detector, can also detect gamma-ray bursts between 300 keV and 3,000 keV in energy.
Rather than using complex and expensive nested mirrors to focus the X-rays, HXMT employs an innovative focusing technique, filtering out all X-rays except those that are coming in parallel to a specified direction. Thanks to this method, the telescope doesn't have to narrow its field of view, making it an ideal instrument for large sky surveys. The trade-off for its broad energy and sky coverage is its sensitivity (that is, the faintest source it can pick up), which is on par with the now-retired Rossi Timing X-ray Explorer (RXTE).
The Future of X-ray Astronomy
The European Space Agency (ESA) plans to collaborate with the Chinese observatory, enabling comparisons between observations from HXMT and ESA's XMM-Newton. In a 2016 interview, HXMT Principle Investigator Zhang Shuangnan also mentions possible collaboration with NASA's NuSTAR team.
Perhaps the mission will usher in a new era of international scientific collaboration along with a new era for X-ray astronomy.