Significance mosaic maps around the supernova remnant Cassiopeia A in Equatorial J2000 coordinates as seen (panels from left to right) w...
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| Significance mosaic maps around the supernova remnant Cassiopeia A in Equatorial J2000 coordinates as seen (panels from left to right) with INTEGRAL/JEMX in the range of 3 – 10 keV and INTEGRAL/IBIS in three energy bands: 20 – 60 keV, 60 – 90 keV and 90 – 200 keV. Credit: Wei Wang, Zhuo Li, 2016. |
Located about 11,000 light years away, Cassiopeia A is the brightest extrasolar radio source in the sky at frequencies above 1 GHz. It is bright in the electromagnetic spectrum, which makes it a unique laboratory for studying high-energy phenomena in supernova remnants.
To peek inside these phenomena, two Chinese astronomers have recently studied a 10-year dataset of Cassiopeia A observations provided by ESA's INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) spacecraft. By using these data, Wei Wang of the Chinese Academy of Sciences and Zhuo Li of the Peking University detected an X-ray emission from this source above 100 keV for the first time.
"We first detected the emission from Cassiopeia A above 100 keV, and found the spectrum has a power-law feature up to 220 keV without cutoff," Wang told Phys.org.
Cassiopeia A has a thermal emission in soft X-ray bands 0.1-10 keV, but in hard X-ray bands above 10 keV, it shows a non-thermal emission. Hard X-ray observations on this supernova remnant can also study the hard X-ray lines at 67.9 and 78.4 keV coming from the decays of radioactive 44Titanium (Ti) – a short-lived radioactive isotope with a half-life of 59 years.
"Two 44Ti emission lines at 68 and 78 keV were detected in Cassiopeia A," Wang said.
The scientists took measures to explain the physical origin of the non-thermal emission above 100 keV from the studied supernova remnant. One of the most plausible explanations offered by them is that there may exist a higher magnetic field in some small region in the remnant or the magnetic field might increase downstream with the distance away from the shock front.
"The magnetic field increases downstream with the distance away from the shock front, so that electrons may produce higher energy photons when flowing downstream. But so far there is no support from theory and observation for magnetic field increasing downstream," the paper reads.
Other hypotheses include synchrotron radiation of the secondary relativistic electrons that originate in the hadronic process and the radiation from the asymmetrical supernova explosion.
According to the research, recent direct imaging observations of the 44Ti emission in Cassiopeia A could confirm the asymmetrical explosion theory. The studies suggested an intermediate asymmetry in this core-collapse supernova as the 44Ti is extended along the jet axis seen in X-rays. Thus, Cassiopeia A may be a very special case of supernova explosions in our galaxy, and produced by an asymmetric or a relatively more energetic explosion.
The researchers underlined how much complex and time-consuming is their study. They also revealed plans for future observations.
"The data analysis is complicated and needs much time. It took about two years to finish the data analysis work. Some theoretical work will go on and new observations by the future Hard X-ray Modulation Telescope (HXMT) will be carried out," Wang said.
HXMT, scheduled for launch this year, is a Chinese astronomy spacecraft. It is designed to scan the galactic plane to find new transient sources and to monitor the known variable sources, and also to observe X-ray binaries to study the dynamics and emission mechanism in strong gravitational or magnetic fields.
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