Is the Milky Way’s Black Hole Feeding?

The answer is a tantalizing maybe. Astronomers are investigating whether an increase in the number of flares from Sgr A* is due to the recent close passage of a dusty object known as G2.

Milky Way Center in X-rays

The Milky Way center imaged in X-rays. Sgr A*, Milky Way's supermassive black hole, is the white splotch in the center of the image.
NASA / CXC / MPE / G. Ponti et al

You would be forgiven if you had forgotten all about G2. This mysterious dusty object made a splash in 2012 when it was discovered heading straight for an encounter with the Milky Way's central supermassive black hole. Astronomers waited with baited breath (and telescopes tuned to wavelengths from radio to X-rays) as the object made its closest approach in early 2014.

They weren't sure what to expect. The Milky Way's central black hole, also known as Sgr A*, typically feeds on at most five times the Moon's mass in a year. Initial mass measurements of G2 promised a meal of three times Earth's mass (so almost 250 Moon masses). That suggested fireworks were in order, though it was unclear whether all that mass would make it into the black hole.

G2 and Sgr A*

This artist's conception shows how G2 might have caused increased flaring as it passed by the black hole in early 2014.
NASA / CXC / M. Weiss

But radio, submillimeter, and X-ray observatories saw nothing unusual during G2's closest approach. No blast of X-rays as G2 plowed through the hot gas surrounding the black hole, no increase in radio emission from the beginning of a jet, nada. Even infrared telescopes, the ones who had initially discovered G2 and followed its progress during its approach, gave conflicting results.

Now, Gabriele Ponti (Max Planck Institute for Extraterrestrial Physics, Germany) and colleagues are taking the long view. Analyzing 15 years of X-ray data, they find that things have indeed changed since G2's approach. Typically, they find, the black hole emits bright belches of X-rays every 10 days or so. But since 2014, about six months after G2's closest approach to the black hole, these bright X-ray flares have upped their frequency, occurring every day or so.

The question is, did this increase in the number of flares occur because of G2's close passage? Did the tail of dust and gas trailing off this mysterious object interact with the hot gas in the black hole's vicinity? Or is the frequency of flares increasing randomly, as astronomers have witnessed in other quiescent, stellar-mass black holes? The answer to that crucial question isn't in yet — it'll require more monitoring.

For more details on the study, see the Harvard-Smithsonian's Center for Astrophysics press release:

Milky Way's Black Hole Shows Signs of Increased Chatter

Three orbiting X-ray space telescopes have detected an increased rate of X-ray flares from the usually quiet giant black hole at the center of our Milky Way galaxy after new long-term monitoring. Scientists are trying to learn whether this is normal behavior that was unnoticed due to limited monitoring, or these flares are triggered by the recent close passage of a mysterious, dusty object.
Read more . . .

Gabriele Ponti et al. "Fifteen years of XMM-Newton and Chandra monitoring of Sgr A*: Evidence for a recent increase in the bright flaring rate." Accepted for publication in the Monthly Notices of the Royal Astronomical Society.

One thought on “Is the Milky Way’s Black Hole Feeding?

  1. Howard RitterHoward Ritter

    Baited breath? Like from eating minnows? Or WORMS?? It’s “bated”, meaning held back or restrained. Related to “abate”, etc.
    But an interesting and informative article; thank you. We were all a little let down by G2’s fizzle. Maybe it was “holding back” a surprise for us, a sort of…”abated” display.

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