Tsinghua used small commercial satellites to fill the field that had been left in white for more than 40 years, and appeared on the cover of nature sub-magazine

In 1975, the first satellite in human history with an X-ray polarization mission was launched by NASA, and the results of the crab nebula were encouraging. Unexpectedly, “the beginning is the peak”, the field into a long period of stagnation and waiting. “That was the first and last time. Feng Wei, a professor in the Department of Astronomy at Tsinghua University, lamented in an interview with the News.

Imaging, energy, time change, cosmic source X-ray research is thriving, only polarization, as one of the basic properties of photons, but seems to have been forgotten.

On May 11, Feng’s team and collaborators reported that the X-ray polarization detector equipped with the Aurora Project had been observing the satellite for a year, detecting soft X-ray polarization signals from the Crab Nebula and pulsars (a type of neutron star), and for the first time detected changes in the X-ray polarization signal during pulsar rotation mutation and recovery, indicating that the pulsar’s magnetic field had changed during this process.

The results, published in the form of a cover paper in Nature Astronomy, mark the reopening of the astronomical soft X-ray polarization detection window, which has been stalled for more than 40 years due to technical difficulties.

Tsinghua used small commercial satellites to fill the field that had been left in white for more than 40 years, and appeared on the cover of nature sub-magazine

Nature Astronomy Cover Image

This cover paper is doomed not to repeat the fate of “the coming is the peak”, but the prelude to the great music. The technology adopted by the Aurora Project will be applied to the Enhanced X-ray Timing and Polarization Observatory (eXTP) in China’s next generation of major scientific projects.

On the other hand, the encounter between China’s astronomical frontier research and low-cost commercial cubic stars is also worth paying attention to. The Aurora Project is headed by Tsinghua University, and tianyi Research Institute, a commercial satellite company, as the general of satellite engineering.

Polarization of “forgotten”

Polarization is one of the basic properties of photons, and the polarization filter is like a fence in a specific direction, allowing only the photons in the corresponding polarization direction to pass through. When watching a 3D movie, the glasses use this principle, selecting one part of the light into the left eye and the other part of the light entering the right eye to form a 3D image.

Compared to the visible light we are familiar with, the wavelength of X-rays is very short. Although invisible to the naked eye, it is useful in astronomy. There are objects in the universe (such as black holes, neutron stars, etc.) that emit almost no visible light, but emit “bright” X-rays and reveal important information about their magnetic field and the geometry of celestial bodies.

Since the 1960s, humans have been able to detect X-ray energy, time changes and other information through X-ray telescopes, but have been slow to solve the technical problems of X-ray polarization detection, Feng said. It’s funny, but it’s hard, and that’s the tone of the field.

The satellite, which the U.S. launched, was based on Thomson/Compton scattering or Prague diffraction, and was inefficient, the equivalent of photons in “selection conditions” that were so harsh that very few parts could be captured, resulting in poor statistics and sensitivity. As a result, the target was the Very bright, polarized and strong Crab Nebula with X-rays.

“The Crab Nebula is just so special that you can’t detect anything else, just like you can see Yao Ming more than two meters high in the crowd. Feng said, figuratively.

As a result, the field has been in a blank phase for more than 40 years after the discovery of the Crab Nebula.

Who will start the “new window”?

Feng and his collaborators used a new generation of photoelectric-based detection methods / For energy is thousands of electron volts x-rays, their main mechanism of action with matter is photoelectric effect, photons are absorbed, energy outside the nucleus of a binding electron excited into a free electron. The direction in which electrons are accelerated is related to the direction of electric field vibration of the incident photons, i.e. the direction of polarization.

“Just like you kick a ball, the ball is most likely to fly out along the direction of your kick, electrons have the greatest probability of shooting along the direction of the incident photon polarization, there is a minimum probability of vertical shooting in the direction of polarization, the azimuth is cos2 distribution.” If we can measure the trace of electrons in the detector and calculate the direction of electron emission, we can effectively measure the X-ray polarization. Feng wrote in a popular science article.

Although the method of detecting X-ray polarization with photoelectric effects was proposed by the Italian team in 2001 and attracted the attention of several research teams in Europe and the United States, satellite projects missed opportunities for various reasons.

This “new window” is finally China’s turn to open.

Feng began studying polarization measurements in 2009, spent two or three years on principle validation, and two or three years for technical optimization, before he began to consider really bringing the satellite to the sky.

Meet the Commercial Cube Star

In 2017, when Feng and his collaborators have acquired a mature version of the probe, it was the rise of commercial cubic stars in China.

The so-called cubic star, is a low-cost microsatellite using international standards, divided by “U”, 1U (Unit) cube star volume is 10 cm x 10 cm x 10 cm, can also form 2U, 3U, 6U or even larger cubic stars. The Aurora Project core detector is only the size of a matchbox and is ideal for passengers above.

Tsinghua used small commercial satellites to fill the field that had been left in white for more than 40 years, and appeared on the cover of nature sub-magazine

Map of the structure of the Aurora Project’s cubic stars and probes

Driven by this idea, Feng’s team made the first version of the space payload development, and within a year completed a tense commissioning and calibration, and finally carried on the Tianyi Research Institute independently developed a 10 kg microsatellite platform.

On October 29, 2018, the Aurora Project was launched from Jiuquan aboard the “Copper River One” Cub Star.

On July 23, 2019, it captured the polarization signal change satofing of the Crab Nebula pulsar in a rotation mutation.

Gu Yidong, chief designer of the manned space engineering application system, said: “The Aurora Project has successfully measured the polarization signals of crab nebulae and pulsars using commercial cubic stars, obtained the important results of x-ray polarization of pulsars over time, and broke out an innovative way to carry out low-cost space astronomy research, which is of great significance for promoting the development of space science in universities.” “

As an observation-driven science, the development of astronomy depends to a large extent on new methods and means of flight observation.

In the case of X-ray polarization detection, the United States had used sounding rocket observations to try to harvest scientific results in just a few minutes of exposure before NASA launched the probe. From July 1968 to February 1971, three attempts were made in 31 months, culminating in the third measurement of the polarization of the Crab Nebula.

In 1975, NASA’s OSO-8 satellite was launched, and the exposure was less than the same length as a sounding rocket, and the results were much more beautiful.

However, the astronomical satellites of that era were very demanding of satellite platforms, generally large satellites of tons, cost hundreds of millions of dollars, and the research and development cycle was long. Many scientists are deterred from the theoretical validation stage.

Although limited by the weight of the load, microsatellites cannot completely replace large satellites, but they can undoubtedly complement large satellites well, completing pre-positioning verification, as the Aurora Project is the future flagship project of the international X-ray astronomy industry, eXTP.

“This is the first time that a tiny satellite developed in China has been featured in one of the world’s top scientific journals, ” said Yang Feng, founder and CEO of Tianyi Research Institute. The emergence of microsatellites in China in recent years has provided more low-cost possibilities for flight verification of new detection techniques and methods. “