Uptick in Galactic Core Activity: Is There a Cause for Alarm?

 

Swift X-ray Observations of the Galactic Center as of September 15, 2015

Swift X-ray Observations of the Galactic Center as of September 15, 2015

 

Posted September 2015, P. LaViolette

If we look at recent data from the Swift X-ray Observatory (see above), there does not appear to be much cause for alarm for activity at the galactic core.  The X-ray output from Sagittarius A* is seen to be relatively constant and hugging the long-term activity baseline (dotted line).  However a recent report by a consortium of astronomers states that within the past year, there has been a ten-fold increase in the rate of bright flares from Sgr A*. This increase happened soon after mid 2014 when the G2 cloud (star) made its close approach to Sgr A*.  These new results were obtained by combining the data from three different X-ray space telescopes (Chandra, Newton, and Swift).

http://arxiv.org/abs/1507.02690

http://www.nasa.gov/mission_pages/chandra/milky-way-s-black-hole-shows-signs-of-increased-chatter.html

 As of November 2014, the end of the study's data analysis period, Sgr A*  was producing one bright X-ray flare about every  day.  It does not state what the situation has been during the ten months following up to the present.  This increase, however, is not apparent on the Swift data graph shown above.  This is likely because this charts daily averages of X-ray intensity, whereas flares would be seen only in the unaveraged data.  That is, each flare typically lasts only about an hour and would become smoothed out in a daily average.

The upsurge in core activity after G2's passage of its orbital pericenter suggests that material was tidally stripped off by the core's gravitational field and that this material was of sufficiently small size to be able to decelerate under the action of the galactic core ion wind such that it was able to loop around into a trajectory that brought it crashing onto the core.  This looping trajectory is illustrated in the diagram below which I had drawn and posted in May 2014: http://etheric.com/computer-simulation-binary-star-g2-cloud-orbit/.  Stripped material that was too massive would not be decelerated sufficiently to make this looping orbit, its momentum carrying it on outward away from the galactic core.  Material too fine would interact strongly with the galactic core wind and would be blown outward.  So only material in a certain size range will survive the journey inward to eventually crash onto our Galaxy's Mother star.

G2 cloud trajectory showing hypothetical path for both the primary star and smaller debris accreted due to hydrodynamic drag

G2 cloud trajectory showing hypothetical path for both the primary star and smaller debris accreted due to hydrodynamic drag

So, at this point, is there any cause for alarm given the quiescent appearance of the Swift data?  That is not certain at this point.  Because flaring activity has picked up dramatically since summer of 2014, we should be keeping a close watch on the core's X-ray activity.  But since there has been no increase in the day-averaged X-ray output, as indicated in the Swift data, things currently do not look so serious that we should be alarmed.

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