SRG/eROSITA discovers X-ray emission from a record luminous “cow” in the sky
AT2020mrf could have remained one of many hundreds of ordinary optical transients that are regularly detected by the Asteroid Terrestrial-impact Last Alert System (ATLAS). However, in the same place in the sky, and at about same time, the eROSITA telescope aboard the SRG orbital X-ray observatory, discovered an unusual new X-ray source, called SRGe J154754.2+443907. It soon became clear that scientists had found a unique cosmic object. They witnessed the birth of a new magnetar — a neutron star with the superstrong magnetic field – or of a black hole in a distant galaxy.
In July 2020, soon after the start of the second all-sky survey, the eROSITA telescope aboard the SRG orbital X-ray observatory discovered a new source in a place where no X-rays have been detected so far. Inspection of the database of optical transients revealed that about forty days earlier, in the same place, the Zwicky Transient Facility (ZTF) and the Asteroid Terrestrial-impact Last Alert System (ATLAS) registered what appeared to be an ordinary optical transient, which was called AT2020mrf. AT2020mrf was originally classified as a Type II supernova which erupts when a massive star collapses at the end of its life. But the eROSITA discovery of X-ray emission and the shape of the optical light curve of the source fundamentally changed the initial classification. It became clear that astrophysicists came upon an even more interesting object.
There is a class of optical transients associated with supernova explosions, which are characterized by fast light curves and a blue excess in their spectra, the so-called Fast Blue Optical Transients (FBOTs). They are difficult to study because their brightness falls off quickly. But among them there is a mysterious and rare subclass, the so-called AT2018cow-like objects. The names of optical transients detected by the ATLAS facility (hence the letters "AT" in the name) are given according to the year of detection (in this case 2018), followed by a combination of several letters generated by a computer. In this case letters happened to form the English word “cow” - hence the astronomers now call this class –“cow-like” objects. Prior to the discovery of SRGe J154754.2+443907, only four such objects were known; the SRG/eROSITA source became the fifth.
"Cows" are characterized by a record-high luminosity (reaching 1043 erg/s at the peak), which is about 1000 times brighter than an ordinary type II supernova. Such a luminosity cannot be explained by the decay of radioactive Nickel-56 and requires an alternative source of energy.
SRGe J154754.2+443907 was discovered by the eROSITA team while searching for events of tidal disruption of a star by a supermassive black hole. However, soon it became clear that the researchers were dealing with something else. This triggered a multiwavelength observational campaign to study the new source from radio to X-rays, which confirmed that SRGe J154754.2+443907 is the fifth cow-like object. The multiwavelength observations involved the 10-metre Keck telescope, the VLA and GMRT radio telescopes, as well as the Chandra, XMM-Newton and Swift orbital X-ray observatories. The program was coordinated by a PhD student from Caltech, Yuhan Yao.
The eROSITA telescope observed this object shortly after the peak of the light curve. These observations have shown that AT2020mrf/SRGe J154754.2+443907 is the brightest known "cow", with a luminosity of over ~2 x 1043 erg/s. Such a luminosity could be powered by a young, rapidly spinning (with a period of about 10 milliseconds) neutron star with a magnetic field on the order of 1014 Gauss – a so-called magnetar –, or by a newly born black hole accreting material of the progenitor star in the super-critical regime. In either case, the scientists have witnessed the birth of a relativistic, compact object during the explosion of a massive star.
The new “cow” has already faded away while many questions remained unanswered. To clarify the nature of these sources and understand the physical mechanisms that determine their behavior, scientists need to find and study more objects of this class in detail. The on-going SRG/eROSITA all sky survey will make an important contribution to this work.