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Ancient X-ray radiation from the center of Milky Way rivaled the Sun

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Posted December 19, 2014

Thanks to modern astronomy, we quite reliably know that the center of our galaxy, known as Sagittarius A*, contains a supermassive black hole. Astrophysicists say this object is now in inactive state, but it certainly was not like that all the earlier time. And as Sagittarius A* is the closets supermassive black hole known to humanity, located 8 kpc from the Solar System, the Earth had certainly feel the after-effects of the processes that took place in the center of Milky Way.

NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR, has captured these first, focused views of the supermassive black hole at the heart of our galaxy in high-energy X-ray light. The background image, taken in infrared light, shows the location of our Milky Way's humongous black hole, called Sagittarius A*, or Sgr A* for short. In the main image, the brightest white dot is the hottest material located closest to the black hole, and the surrounding pinkish blob is hot gas, likely belonging to a nearby supernova remnant. Image credit: NASA

NASA’s Nuclear Spectroscopic Telescope Array, or NuSTAR, has captured these first, focused views of the supermassive black hole at the heart of our galaxy in high-energy X-ray light. The background image, taken in infrared light, shows the location of our Milky Way’s humongous black hole, called Sagittarius A*, or Sgr A* for short. In the main image, the brightest white dot is the hottest material located closest to the black hole, and the surrounding pinkish blob is hot gas, likely belonging to a nearby supernova remnant. Image credit: NASA

A distance of 8 kpc surely sounds not so small. Not every object in our galaxy can be observed over such scales, let alone ability to produce any tangible physical impact. However, supermassive black holes are another story. These monstrous objects exert immensely powerful gravitational fields, suck any surrounding matter, and by doing that generate radiation streams that may pierce galactic-scale distances.

‘Our own’ Sagittarius A* (Sgr A*) is not an exception. In a paper published on arXiv.org this week, scientists Xian Chen and Pau Amaro-Seoane from the Max Planck Institute for Gravitational physics argue that in ancient times the X-ray radiation produced by the supermassive black hole contained in the center of our galaxy could be strong enough to ‘outshine’ X-rays emitted by the Sun.

Chandra image of Sgr A* and the surrounding region. Image credit: NASA/CXC/MIT/F. Baganoff, R. Shcherbakov et al.

Chandra image of Sgr A* and the surrounding region. Image credit: NASA/CXC/MIT/F. Baganoff, R. Shcherbakov et al.

Until this century, scientists hypothesized that supermassive black holes operating in an active state were not common in the Universe. This class of objects is also known as active galactic nuclei (AGN), and they were thought to be triggered by galaxy mergers. But there is no evidence suggesting that Milky Way galaxy could have experienced  any substantial merger in the past 10 billion years.

However, this notion has changed when recent studies suggested that AGNs may emerge in Milky-Way-like galaxies relatively frequently, about once every 107-108 years per galaxy, while each AGN lasts approximately 105 years.  Such data indicates that supermassive black holes may be activated not only by galaxy mergers, but by other processes as well, such as gravitational instability, explain the authors of the current study. They also note that the most recent outburst of Sagittarius A* occurred merely 2-8 million years ago and this can be completely explained using modern astrophysical models.

In their work, the scientists assessed the relative importance of the X-ray radiation from Sgr A* by comparing the irradiances on Earth from the Sun and from the Sgr A* in its active state. The distribution of spectral energy was computed using the observational data collected from a sample of unobscured AGNs (Jin et al. 2012). Two scenarios were investigated: 1) assuming Sgr A* is in active state; 2) assuming Sgr A* is inactive, but X-ray activity is generated by a tidal disruption of a nearby star.

X-ray irradiances on Earth from different sources. The four solid lines show the irradiances from the AGN in the Galactic Center, and the increasing thickness corresponds to a increase of luminosity. The three dashed curves are for the sun (Peres et al. 2000), and from top to bottom correspond to the irradiances of an X-class solar flare, the maximum state during the solar cycle, and the state during solar minimum. Image courtesy of the researchers.

X-ray irradiances on Earth from different sources. The four solid lines show the irradiances from the AGN in the Galactic Center, and the increasing thickness corresponds to a increase of luminosity. The three dashed curves are for the sun (Peres et al. 2000), and from top to bottom correspond to the irradiances of an X-class solar flare, the maximum state during the solar cycle, and the state during solar minimum. Image courtesy of the researchers.

When analyzing the first scenario, the obtained results indicated that the active AGN at the center of our galaxy could easily deliver stronger X-ray radiation compared to the Sun while it is in the solar minimum state. The relative solar radiation becomes stronger when the Sun enters its maximum state during the solar cycle and especially during the X-class solar flare. However, for the photons in the energy band starting from 102 keV and above (soft gamma ray band) the irradiance coming from the AGN at the center of our galaxy could be brighter than the Sun, even when the Sun is undergoing an X-class solar flare.

The events of tidal flares investigated under the second scenario can temporary enhance the luminosity of supermassive black hole. Such events happen approximately one in 104-105 years, when a star gets too close to black hole to be disrupted by its gravitational field, and this could have happened (at least theoretically) in case of last outburst from Sgr A*. The team compared the spectral irradiance of AGN with that of a tidal flare, assuming a solar-type star being disrupted by Sgr A*. The results of calculations indicated that such tidal flare during the first few weeks could be as bright as an AGN.

Comparing spectral irradiances from a tidal flare and an AGN. The four solid lines with increasing thickness depict the irradiances of the tidal flare (14; 28; 90; 365) days after the initial outburst. The four dashed curves are for the AGN, adopted from the figure above. Image courtesy of the researchers.

Comparing spectral irradiances from a tidal flare and an AGN. The four solid lines with increasing thickness depict the irradiances of the tidal flare (14; 28; 90; 365) days after the initial outburst. The four dashed curves are for the AGN, adopted from the figure above. Image courtesy of the researchers.

“In the past 30 years several distant (soft) gamma-ray bursts (GRBs) had induced similar level of irradiation on Earth, during which disturbances of the ionosphere had been detected. The disturbance of the ionosphere by Sgr A* may be more serious, because tidal flares and AGNs last for a much longer time, ranging from a few weeks to as lons as 105 years”, the authors of the study say. Fortunately, they note, a total dose of hard X-rays injected into the Earth’s atmosphere would be much less (and thus much less lethal) compared to a hypothetical scenario of a nearby supernova burst, and the biosphere of Earth could probably easily adjust to new circumstances.

Previous studies have also shown that X-ray irradiation of certain parameters can drive chemical reactions in dense molecular clouds and increase the abundance of organic molecules in protoplanetary disks from which planets originate. “The ancient outbursts from Sgr A* may have played an important role in shaping the habitable environment in the solar system, as well as in other places throughout the Milky Way”, conclude the scientists.

Written by Alius Noreika

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