For over a decade, through the ingenious tracking of stellar orbits in the galactic centre, we’ve known that a supermassive black hole weighing the equivalent of several million solar masses is lurking at the centre of our galaxy. But this discovery, while offering us the tantalising opportunity to study these enigmatic objects in our own back yard, raises many more questions.
One persisting puzzle has been the low luminosity of the black hole. The balance of inward and outward forces around an accreting object place a limit on its luminosity (the Eddington luminosity). As the SgrA* black hole is located in a densely populated part of our galaxy, we’d expect it to be accreting actively and therefore pretty bright. Instead, its luminosity is just 0.000001% of the Eddington limit – a factor of 10^8 lower. Yusef-Zadeh & Wardle give a good discussion of this phenomenon in this recent paper.
This has led astronomers to wonder: has our black hole always been this dim, or are we just seeing it in an unusually quiet phase, or are we missing something more fundamental? With some very cool X-ray observations stretching over almost a decade, Gabriele Ponti and colleagues have been able to uncover evidence of a very strong flare in SgrA*’s past, several hundred years ago.
The central few hundred parsec of our Galaxy is a hotbed of activity: as well as the black hole it harbours many young massive clusters of stars, sites of ongoing star formation, and much of the molecular content of the entire Galaxy in a number of massive molecular clouds. Strong energetic X-ray emission from the iron Kα spectral line at 6.4 keV in the galactic centre was first observed to follow the distribution of the molecular clouds in 1993, reported by Sunyaev & Churazov.
Molecular clouds, full of gas and dust, are often sites of active star formation, with complex light signatures. But this kind of diffuse X-ray emission is not a typical feature. The authors suggested that an energetic source external to the clouds, whose radiation was scattered by the molecular material, could be responsible for the observed X-ray emission.Two main candidates for such a source were posited: the black hole at the centre of the galaxy, whose existence had at that time not yet been proven (Sunyaev & Churazov, 1998); and low energy cosmic ray electrons heating the molecular material (Yusef-Zadeh et al, 2002).
Sunyaev & Churazov’s very elegant paper of 1998 outlines the black hole flare scenario as mechanism behind the X-ray emission from the GC molecular clouds. They predict the characteristics of the iron K spectral line and its apparent evolution over time as seen from Earth. Interestingly, the projection of the flare travelling through the clouds as seen from Earth would show the peak emission moving at superluminal speed – faster than the light travel-time. So this is not actual superluminal travel, but the geometry of the situation just makes it seem so to an observer here on Earth.
And now, after this long-winded story, Ponti and colleagues report that their X-ray observations with the XMM-Newton satellite spanning 8 years show pretty much what Sunyaev & Churazov predicted more than a decade ago. The brightest emission spot in the above image is located at the the ellipse labelled MC1 in the November 2004 panel. By April 2007, the peak has shifted into cloud no. 1 of the structure they term the “Bridge”. By April 2009, the brightest spot is in the Bridge’s cloud 3. Assuming that this complex lies at the same distance as the galactic centre itself – it probably lies somewhat further from us than that – this means that in the space of 2-4 years, the emission peak has travelled at least 15 lightyears. That’s quite clearly superluminal.
The superluminal motion, the authors argue, effectively rules out sources of high-energy radiation internal to the clouds, as well as the low-energy cosmic ray scenario. They suggest that a strong flare ending around 100 years ago, is the most likely mechanism causing this sudden X-ray brightening in the nearby molecular clouds. At the time of this outburst, SgrA* would be around 1000 times brighter than at present, showing that at least at some point in the Galaxy’s recent future the black hole was a much more active accreter.
On our short human timescales, the Universe beyond our own Solar System is essentially static. Very few phenomena vary before our eyes; when they do, it’s always exciting. Being able to see the echo of a hundred-year old bright flare from SgrA* travelling through this molecular material is pretty awesome. The 1998 paper by Sunyaev & Churazov also showed great foresight, as well as insight, as to the capabilities of the next generation of X-ray observatories. The paper essentially gives observers a template for what signatures to look for, and it describes the theory and reasoning incredibly well (it’s almost like they knew some blogger would be writing about this with little background knowledge of X-ray astronomy!) So as well as a great observational result, this story also struck me as an elegant demonstration of how theory and observation can conspire over long timescales, slowly but ultimately unstoppably, to give us a new understanding of our Universe.
Gabriele Ponti, Regis Terrier, Andrea Goldwurm, Guillaume Belanger, & Guillaume Trap (2010). Discovery of a superluminal Fe K echo at the Galactic Center: The glorious past of Sgr A* preserved by molecular clouds ApJ arXiv: 1003.2001v1
Sunyaev, R., & Churazov, E. (1998). Equivalent width, shape and proper motion of the iron fluorescent line emission from molecular clouds as an indicator of the illuminating source X-ray flux history Monthly Notices of the Royal Astronomical Society, 297 (4), 1279-1291 DOI: 10.1046/j.1365-8711.1998.01684.x
F. Yusef-Zadeh, C. Law, & M. Wardle (2002). The Origin of X-ray Emission from a Galactic Center Molecular Cloud: Low
Energy Cosmic Ray Electrons Arxiv arXiv: astro-ph/0202442v1
F. Yusef-Zadeh, & M. Wardle (2010). The Underluminous Nature of Sgr A* Arxiv arXiv: 1003.1519v1