The Gamma Ray Bubbles In The Milky Way - Linked To Super Massive Black Hole?

Two images of the gamma ray bubbles in our galaxy as recorded by the Large Area Telescope during 50 months of observations. (From Physics Today, July, p. 61)

Question: If a stellar black hole, i.e. as one member of a binary system, is capable of pulling the outer gaseous envelope of its companion off and generating x-rays in the process (by which we can confirm its existence) is it possible the supermassive black hole at the center of our galaxy could emit much more powerful gamma rays – as matter accretes onto it?

This is a question researchers Anna Franckowiak and Stephen Funk  (of SLAC) seek to answer with their research as proposed in their article, ‘Giant Gamma –Ray Bubbles in the Milky Way’ (Physics Today, July, p. 60). The authors refer to gamma rays recorded by the Large Array Telescope (LAT) aboard NASA’s Fermi Gamma Ray Space Telescope as an indicator, revealing two giant structures (“Fermi bubbles”) that appear to emerge from the galactic center. These bubbles “extend 39,000 light years above and below the galactic center and have well-defined edges.”. In addition, the intensity of the radiation doesn’t vary across their expanse.

As usual with a new concept in astrophysics the proof will be in essentially explaining away all other possibilities and leaving the Fermi bubbles as the only viable hypothesis. At present this isn’t possible because the origin of these ‘bubbles’ can be explained without recourse to the supermassive black hole.   If the bubbles can be tied exclusively to the supermassive black hole at the galactic center then the way is paved for “better understanding of the energy output of the enigmatic massive object at the center of our galaxy.”

As most physics students and aficionados are aware, gamma rays are the highest energy photons. Generally, they earn their keep by exposing the highest energy processes in the cosmos which are most often predicated on relativistic charged particles. (I.e. those traveling near the speed of light, c).  Since the atmosphere blocks them, fortunately for us, special detectors have to be dispatched into space on satellites.

A few words are in order for the LAT and its capabilities: It is sensitive to gammas rays in the energy range of a few tens of MeV to a few hundred GeV. (Recall here, MeV refers to one million electron volts. Recall the energy conversion: 

1.6 x 10^-19 J = 1 eV  so that:

1 MeV =  10⁶  eV (1.6 x 10^-19 J /  eV )  =      1.6 x 10^-13 J 

Similarly for 1 GeV, the joule equivalent is:

1 GeV =  10 ⁹  eV (1.6 x 10^-19 J /  eV )  =      1.6 x 10^-10 J 

The LAT has a field of about 20 percent of the sky at any given moment and continuously scans the whole sky with unprecedented precision. (The images shown were taken over a 50 month time frame.)

If the reader examines panel (a) in the top graphic, the sky as recorded by the LAT at energies above 6.4 GeV is seen.  As the authors note (ibid.):

“Clearly visible are the galactic interstellar emission and point sources well within and beyond the galaxy. Less evident, but still visible, is an additional component of gamma ray emission above and below the galactic center, extending perpendicular to the galactic plane.”

In panel b, meanwhile, “the symmetrical Fermi bubbles stand out once all known sources of gamma rays in and outside the galaxy are subtracted and the Milky Way Band is masked. “

The authors note that the alternative explanations still hold court and it may be difficult to ultimately tie the Fermi bubbles to the supermassive central black hole. The primary reason for that is that the latter is currently in a “quiet phase ” – meaning the rate of material accretion onto it is low, so that gamma ray generation would be low. Franckowiak and Funk acknowledge that the supermassive hole can go through quiet and active phases and point to “x-ray light echoing off interstellar gas clouds” as evidence for “a prior episode of intense eruption that occurred a century ago.” (One alternative hypothesis is that a burst of star formation in dense gas clouds occurred in the vicinity of the galactic center.)

In any case, we will have to let the extended investigations play out and see if the Fermi bubble link to the galactic black hole is real or just a mirage.  The physical processes for such generation are at least well known and may exhibit one of two forms: 1) Inverse Compton scattering, i.e. in which relativistic electrons collide with low energy photons and boost them to gamma ray energies, or (2) high energy protons interact with the nucleons of interstellar gas to produce pions (pi mesons, neutral particles) that immediately decay into two gamma rays each.

The stage is set and perhaps a renewed “active” phase may soon arrive. In that case all eyes will be (via LAT) on these Fermi bubbles and also whether they have companion microwave bubbles.

Stay tuned!