Ultra-bright X-ray sources found in two old galaxies. New natural phenomenon?

It would seem that the visible region of the universe is well studied. We know many types of stars and other astronomical objects. But even here there is a place for absolutely mysterious and inexplicable phenomena. How else to call super-bright X-ray flashes, which are seen near the galaxies NGC 4636 and NGC 5128 .

Astrophysicists say directly that they have not seen anything like it. This may be a completely new natural phenomenon.

According to the official classification, flares of this kind belong to ultra-bright X-ray sources (ULX). Their story began in 2005, when a scientific paper on ULX was published near the galaxy NGC 4697 . Near NGC 4697, only two short flashes of ULX were detected. No one understood what caused this strange phenomenon.
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What do scientists do when they encounter a strange and inexplicable phenomenon? That's right, they collect more information. Need more observational data. Perhaps, it will be possible to find certain patterns and put forward some theories.

Over the years, the ULX near the galaxy NGC 4697 was seen only twice: once in 2003 and once in 2007. Astronomer Jimmy Irwin (Jimmy Irwin) from the University of Alabama (USA) with colleagues set the task to find the same objects in other regions of the Universe. They began to systematically study the archived data collected by the Chandra Space X-ray Observatory. The search was concentrated on 70 galaxies as the most suitable candidates. Having studied thousands of observational data, scientists managed to find something similar . Two more of the same ultra-bright X-ray sources resembling the signal from NGC 4697.

The first of them was revealed in the archived observational data for February 2003 near the galaxy NGC 4636, approximately 46 million light years from Earth.

The second source appeared five times from 2007 to 2014 around the galaxy NGC 5128, about 14 million light years from Earth.

At first glance it may seem that the sources found are quite rare. In fact, it is not. The fact is that Chandra targets concrete galaxies quite rarely, for example, once a year or every few years. Say, the same galaxy NGC 4697, in which the first signal was seen, was observed only five times by the time the scientific work was published in 2005. Chandra. The same with the number of observations of two new galaxies, near which ULX was detected.

This is what ULX looks like near the galaxy NGC 5128, based on the data visualization results conducted by NASA and the authors of the research.



The same image in a more visual animated form .

When NGC 4697 was observed in both cases, the signal brightness increased about 90 times in about one minute. No optical signal source is observed at these coordinates. But if we assume that the source is at the same distance as the galaxy, then the peak luminosity of the flashes exceeds 10 ³⁹ erg per second, that is, this source radiated an energy flow of 2,500,000 times more than the Sun (our star radiates 3.86 · 10 ³³ erg in the same unit of time).

If the same assumptions about the distance to the sources are true for the two new ULXs, then the scientists estimated the peak luminosity of the first one at 9 · 10 ⁴⁰ erg per second, and the second one flashed five times with a peak luminosity of 10 ⁴⁰ erg / s.

The closest phenomenon in X-ray power is magnetars, neutron stars with an extremely strong magnetic field (up to 10 ¹¹ T), first discovered in 1998. But the flashes of magnetars in the X-ray range fade out in a few seconds or tens of seconds. ULX flashes last much longer. They reach a peak in almost a minute, and die out within an hour.

Scientists suggest that ultra-bright X-ray sources may be somehow related to binary systems. This is either a black hole or a neutron star gravitationally associated with a companion star. When circling around a common center of mass and the interaction of objects in a binary system with each other, strange phenomena such as x-ray flashes may appear. For example, if a black hole or a neutron star accretes the substance of a companion star. If you do not take into account the periods of outbreaks, then in quiet periods, these binary systems look quite normal. That is, flashes do not reflect fatally on their condition.

The unusual nature of ULX lies in the fact that they are located in elliptical galaxies, that is, in old stellar formations, unlike magnetars, abnormal X-ray pulsars or sources of soft repetitive gamma-ray bursts that produce repeated flashes of similar brightness. It turns out that the sources of outbreaks are hardly new stars. If these are old stars - and they flare up so brightly, there is definitely something unusual here.

"These are truly extraordinary flashes," said Peter Maksym, co-author of a research paper from the Harvard-Smithsonian Center for Astrophysics (USA). “For a short period, one of the sources has become one of the brightest ULXs that has ever been seen in an elliptical galaxy.” Indeed, amazing.

Astronomers still have plenty of time to observe. Clearly, the flashes around NGC 4697 were not random. This happens around other galaxies. Maybe the Chandra space observatory can be sent to the old galaxies a little more often if there is an incomprehensible activity going on there?

The scientific article was published on October 19, 2016 in the journal Nature (doi: 10.1038 / nature19822).