The Insight-HXMT satellite has made new discoveries? You need to know the man behind the scenes

Recently, my country's Insight-HXMT satellite team has once again significantly refreshed the world record for direct measurement of the highest energy cyclotron absorption line and the strongest magnetic field in the universe. Scientists said that the magnetic field strength on the surface of the neutron star corresponding to this observation exceeded 1.6 billion Tesla, exceeding the cosmic magnetic field of about 1 billion Tesla directly measured in 2020.

The strong observation capability of the Insight-HXMT satellite is inseparable from its powerful data analysis capability. How does the Insight-HXMT satellite collect information? And how does it process it? Let's continue reading.

Artistic image of the Insight-HXMT satellite observing an accreting pulsar

(Image source: Institute of High Energy Physics, Chinese Academy of Sciences)

1. How does the HXMT telescope satellite collect data?

We already know that the satellite carries three telescope payloads: a high-energy telescope, a medium-energy telescope, and a low-energy telescope. However, in terms of data analysis methods, there is no essential difference between the three telescopes. Therefore, we will take the high-energy telescope as an example to further introduce the scientific data analysis of the hard X-ray modulation telescope.

I believe everyone knows the CT used in hospitals. The Chinese name of CT is computer tomography. This is very similar to the scanning observation mode of the hard X-ray modulation telescope.

CT machine

(Photo source: veer photo gallery)

The difference is that CT places the sample (such as a patient) at the center, and the radiation source and detector rotate around the sample. The resulting image reflects the absorption or transmittance of radiation at different locations of the sample slice. In a geometric sense, the scanning of the telescope is equivalent to scanning from the inside of the sample (such as the Milky Way). The resulting image reflects the brightness distribution of the sample projected onto a unit sphere along the line of sight.

Oh, that's not right. I saw in the news that this telescope can't form an image directly! So how does it form an image?

When X-ray photons from celestial bodies hit a sodium iodide crystal, the crystal flashes

(Photo source: Institute of High Energy Physics, Chinese Academy of Sciences)

A collimator is essentially a hollow tube made of a material that X-rays cannot easily penetrate. When the collimator is facing the X-ray source, the most X-ray photons can pass through the collimator and reach the detector; when the collimator is slightly deflected, the X-ray photon count rate (the number per unit time) decreases; when the collimator deflection angle is large enough, X-ray photons cannot enter through the aperture of the collimator, but can only scatter on the inner wall or penetrate from the outer wall to reach the detector, and the count rate is significantly reduced.

Therefore, we use the collimator to rotate and scan and record the angle between the collimator's direction and the initial direction at each moment, while recording the moment when each photon reaches the detector. In this way, we complete the scanning observation of the X-ray source and obtain the observation data - the time series of photon arrival events and detector status.

X-ray image (Image source: Insight—HXMT)

2. How to process the collected data?

Next, we divide the time axis into equally spaced time periods and add up the number of photons that arrive in each time period. In other words, the number of photons that arrive in each time period is a function of the direction of the detector. To generalize, the observed data can be expressed as a function of the detector state.

X-ray telescope counting spectrum

(Image source: Insight—HXMT)

Before launch, scientists calibrate the characteristics of the detector (such as detection efficiency, spatial response, energy response, etc.) through calibration experiments on the ground, and compare them with the theoretical model to verify whether the theoretical model is comprehensive and accurate. After launch, calibration experiments are carried out in orbit to continuously revise the model. In this way, the detector response can generally be regarded as a known function with a certain degree of uncertainty. Among them, the detector response and observation data are the key to inferring the target characteristics.

It can be understood in this way. Suppose there is a small town with a swimming pool. The "observation data" is the ticket sales of the town's swimming pool that we record every day. The "detector response" mainly describes the correspondence between the actual number of people entering the swimming pool and the ticket sales. We need to infer from this information the "target features" such as changes in the number of members in the swimming enthusiast group and changes in exercise intensity.

In actual work, scientists have adopted some specific methods to solve specific problems not covered by the above simplified situations.

For example, although the telescope is equipped with detectors specifically to measure the background, scientists still designed a variety of on-orbit experimental schemes and data analysis methods to estimate the background during the telescope detection process, taking into account the background of different components and the individual differences between detectors, such as the small town swimming pool model mentioned above. For another example, the actual modulation equation is high in dimension and complexity, so the computational requirements for demodulation are very prominent. Therefore, we designed an adaptive fast algorithm based on machine learning, compressed sensing and other technologies.

These seemingly disorganized data condense the scientific research efforts of several generations and contain cosmic secrets to be discovered. How many surprises can the Insight-HXMT satellite bring us? Perhaps we cannot accurately answer this question for the time being, but in the vast and mysterious universe, we firmly believe that there are still many mysteries waiting for humans to unlock. As the eyes of mankind looking into space, the Insight-HXMT satellite will carry our curiosity and move deeper into the universe!

Produced by: Science Popularization China

Author: China Science Expo

Producer: China Science Expo