Awaiting activity of the Milky Way supermassive black hole

Supermassive black holes lurk at the centers of every Galaxy. Our own Milky Way harbors a black hole of approximately 4 million Solar masses, whose electromagnetic counterpart is known as Sagittarius A* (Sgr A*). Most surprisingly, its luminosity is about 9 orders of magnitude lower than the maximum brightness that a black hole of this mass can reach. Nevertheless, observational features such as the gigantic “Fermi Bubbles” and “light echoes” from molecular clouds near the Galactic center suggest that Sgr A* has not always been dormant, but instead evidences a wild and glorious past.

We may now find ourselves at the dawn of a reactivation phase of our supermassive black hole, which is foreshadowed by the discovery of a cold gas cloud (a.k.a “G2”) that is on a collision course with Sgr A* and is predicted to impact in late 2013 or early 2014. The cloud may become disrupted due to tidal forces and parts of the shredded gas could then be accreted onto the black hole. However, whether this interaction leads to fireworks remains to be seen. Right from the start of G2’s reported discovery in early 2012, there has been ongoing discussion regarding the nature, origin, and hence the faith of this tantalizing gas cloud that seems to have come out of nowhere. It remains uncertain as to whether G2 harbors a central object (e.g., a young star or a binary) that is keeping the cloud gravitationally bound. If so, G2 may survive its doom-trail, keeping any observable effects on the emission of Sgr A* to a bare minimum.

Astronomers all over the world are at the ready in case Sgr A* becomes revived, armed with monitoring campaigns utilizing ground-based and space-based facilities, and target-of-opportunity programs covering the entire electromagnetic spectrum. And so they should: this has the potential to be a unique, once-in-a-lifetime opportunity to observe a disruption event in our own backyard and have an unprecedented view of the feeding process of our Galactic nucleus. Me and my co-workers occupy a front seat and are in place to follow this historic event at infrared and X-ray wavelengths.

We have recently embarked on a monitoring program employing the infrared-imager FourStar mounted on 6.5-m Magellan-Baade telescope, located at the Las Campanas Observatory in Chili. Between July and October, as long as the Galactic center is observable from this site, we are monitoring our Galactic nucleus nearly weekly using in the infra-red J, H and Ks wavebands. This allows us to detect any possible changes in the infrared emission of Sgr A*, which might signal an enhancement of the accretion flow due to the shredded gas cloud.

Our Magellan infrared campaign is complemented by intensive X-ray monitoring. Utilizing the unique flexibility of the Swift satellite, we observe the center of our Galaxy every day with the onboard X-ray telescope. This program has been running since 2006, and has provided us with valuable insight into the long-term X-ray behavior of the supermassive black hole. This serves as an important calibration point to assess if, and how, the X-ray properties of Sgr A* change as a result of its interaction with G2. Moreover, Swift is the only observatory that can accommodate daily X-ray observations, and may therefore turn out to be the first to detect any action and thereby serve as a trigger for other observatories.

Given the uniqueness of this astronomical event and the broad scientific and public interest, we have set up an automated reduction and analysis pipeline for the daily X-ray observations obtained with Swift. New data is downloaded the instant that it becomes available; generally this is within a mere 3 hours after an observation was taken. Quick-look images and light curves are then produced and immediately uploaded onto a website (, followed by an instant e-mail notice distributed to subscribers. This allows the scientific community to optimally benefit and promptly respond, in case our Galactic nucleus awakens.

Our Swift Monitoring Campaign website:

The dedicated wiki-page about the gas cloud “G2”: MPE

Selection of press:

NRC news item, 2014 March (Dutch news paper)

BBC science news, 2014 January

NY Times science news, 2014 January

NASA/Swift press release, 2014 January

HEAPOW, 2014 January

Michigan Astronomy feature, 2014 January, 2013 September (Dutch science site)

An artist impression of an Active Galactic Nucleus (AGN). Credit: ESA/NASA, the AVO project and Paolo Padovani.

An artist impression of an Active Galactic Nucleus (AGN).
Credit: ESA/NASA, the AVO project and Paolo Padovani.