An X-Ray Map Reveals the Bones of the Universe

By Allison Kubo

On June 19, the eRosita instrument aboard the Russian-German “Spectrum-Roentgen-Gamma” (SRG) mission finished cataloging more that 1 million high energy x-ray sources more than had ever been recorded before this study. The image above shows our sky illuminated in x-rays, the range of the electromagnetic spectrum which is far more energetic than visible light. The red colors indicate low energies (0.3-0.6 keV), green for intermediate range (0.6-1 keV), blue for high energy sources (1-2.3 keV). Along the middle line of the ellipse, we see the Milky Way Galaxy which appears as only high energy sources; this is due to the fact that abundant dust and particulates of the Milky Way which we can visually see spread across our night sky. Bright splashes of yellow and green indicate high energy events such as supernovae and outbursts from supermassive blackholes. The white dots throughout the image are just under a million x-ray sources.

Although you may be familiar with the stunning images from the Hubble Space Telescope which looks in visible light, the rest of the spectrum holds valuable information about our galaxy and the universe. Radio astronomy which utilizes long-wavelength spectra has been used since 1932 when Karl Jansky was investigating what was interfering with transatlantic radio signals. Radio waves are able to pierce our atmosphere so we are able to observe them from the Earth’s surface such as the Atacama Large Millimeter Array. X-rays, however, are not able to pierce Earth’s atmosphere and thus must be observed from space or very high altitude. Thus the first observations of extra-solar x-ray sources did not occur until the space program in the 1960s.

The Crab Nebula supernova in different wavelengths in the electromagnetic spectrum, each illustrate different features. Image Credit: High Energy Focusing Telescope (HEFT), NASA’s Scientific Ballooning Program

Instruments like eROSITA observe most violent events in the universe around us. X-rays are short-wavelength energetic waves emitted when gas is heated to millions of degrees. X-rays can be emitted when gas is compressed or accelerated. Emissions can come from solar flares as a star dies tremendous supernovae compresses gas in the shockwave. The leftovers of dead stars, either neutron stars (which are so dense that one sugar cube of neutron star weighs more than all humans on Earth) or black holes can also be seen in the x-ray spectrum. Black holes do not actually emit x-rays, they actually are black because all electromagnetic radiation is sucked into them, but material gathered into the singularity does signal in the x-ray spectrum as it spins and generates magnetic fields. Neutron stars and black holes act as one member in an “X-rays binaries” or a binary star system consisting of the accelerator with high gravity and the donor which provides gas which is superheated during its acceleration toward the neutron star or black hole.

An x-ray binary where a black hold pulls material into emitting x-ray waves. Image Credit: NASA/ESA

The sun too puts off x-rays although weakly. X-rays are used to study an interesting conundrum in heliophysics. The outer layer of the sun, the corona, is much hotter than the rest of the sun sitting at 1-20 x 106 K compared to the average temperatures of the sun which is approximately 5,570 K. X-ray emissions of solar flares can be used to study the magnetic field and its effect on coronal heating.

Finally, this new map of the solar system could be key to understanding dark matter. First seen in the Chandra X-ray observatory in 2012 by Jee et al., colliding galaxies show a clear separation in x-ray emission and mass distribution. It is theorized that this is due to dark matter which causes gravitational lensing, the bending, and shearing of light. This is strong measurable evidence of dark matter. The full-sky survey of eROSITA will provide a huge data set of x-ray sources to study and may shed some “light” on dark matter.

Colliding galaxies show the dissociation of x-ray (pink) and gravitation (blue) which may indicate dark matter.  Image Credit: X-RAY: NASA/CXC/UVIC./A.MAHDAVI ET AL. OPTICAL/LENSING: CFHT/UVIC./A. MAHDAVI ET AL. (TOP LEFT); X-RAY: NASA/CXC/UCDAVIS/W.DAWSON ET AL.; OPTICAL: NASA/ STSCI/UCDAVIS/ W.DAWSON ET AL. (TOP RIGHT); ESA/XMM-NEWTON/F. GASTALDELLO (INAF/ IASF, MILANO, ITALY)/CFHTLS (BOTTOM LEFT); X-RAY: NASA, ESA, CXC, M. BRADAC (UNIVERSITY OF CALIFORNIA, SANTA BARBARA), AND S. ALLEN (STANFORD UNIVERSITY) (BOTTOM RIGHT))